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The 12th edition of Guyton and Hall Textbook of Medical Physiology continues this bestselling title's long tradition as one of the world's favorite physiology textbooks. The immense success of this book is due to its description of complex physiologic principles in language that is easy to read and understand. Now with an improved color art program, thorough updates reflecting today's medicine and science, this textbook is an excellent source for mastering essential human physiology knowledge.

  • Learn and remember vital concepts easily thanks to short, easy-to-read, masterfully edited chapters and a user-friendly full-color design.
  • See core concepts applied to real-life situations with clinical vignettes throughout the text.
  • Discover the newest in physiology with updates that reflect the latest advances in molecular biology, cardiovascular, neurophysiology and gastrointestinal topics.
  • Visualize physiologic principles clearly with over 1000 bold, full-color drawings and diagrams.
  • Distinguish core concepts from more in-depth material with a layout that uses gray shading to clearly differentiate between "need-to-know" and "nice-to-know" information.


United States of America
Cardiac dysrhythmia
White blood cell
Vitamin D
Functional disorder
Spinal cord
Hearing (sense)
Somatosensory system
Myocardial infarction
Circulatory collapse
Thyroid hormone
Excitation-contraction coupling
Membrane channel
Gastrointestinal physiology
Protein metabolism
Cell physiology
Lipid metabolism
Ventilation (physiology)
Renal blood flow
Apex beat
Protein S
Muscle contraction
Digestive disease
Shock Treatment
Congenital heart defect
Enterprise application integration
Cerebral circulation
Eye disease
Acute kidney injury
Pulmonary hypertension
Renal function
Blood flow
Pulmonary circulation
Receptor (biochemistry)
Membrane potential
Oxygen therapy
Extracellular fluid
Interstitial fluid
Adrenal medulla
Cardiac muscle
Parathyroid hormone
Pulmonary edema
Carbohydrate metabolism
Body water
Pineal gland
Heart failure
Heart sounds
Heart valve
Further education
Limbic system
Pleural cavity
Respiratory failure
Ventricular fibrillation
Growth hormone
Organ transplantation
Autonomic nervous system
Action potential
Coronary circulation
Salt water
Tissue (biology)
Cellular respiration
Basal ganglia
Alcohol dehydrogenase
Altitude sickness
Human gastrointestinal tract
Blood cell
Angina pectoris
Ischaemic heart disease
Peptic ulcer
Cerebral cortex
Clinical neurophysiology
Diabetes mellitus
Chemical synapse
Protein biosynthesis
Nervous system
Mind control
Ion channel
Fatty acid
Cerebrospinal fluid
Cell membrane
Carbon dioxide
Amino acid
Adénosine triphosphate


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Date de parution 19 juillet 2010
Nombre de lectures 4
EAN13 9781437726749
Langue English
Poids de l'ouvrage 7 Mo

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Guyton and Hall Textbook of Medical Physiology
Twelfth Edition

John E. Hall, Ph.D.
Arthur C. Guyton Professor and Chair, Department of Physiology and Biophysics
Associate Vice Chancellor for Research, University of Mississippi Medical Center, Jackson, Mississippi
Front matter
Guyton and Hall Textbook of Medical Physiology

Guyton and Hall Textbook of Medical Physiology
John E. Hall, Ph.D. , Arthur C. Guyton Professor and Chair, Department of Physiology and Biophysics, Associate Vice Chancellor for Research, University of Mississippi Medical Center, Jackson, Mississippi

ISBN: 978-1-4160-4574-8
International Edition: 978-0-8089-2400-5
Copyright © 2011, 2006, 2000, 1996, 1991, 1986, 1981, 1976, 1966, 1961, 1956 by Saunders, an imprint of Elsevier Inc.
All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permissions may be sought directly from Elsevier’s Rights Department: phone: (+1) 215 239 3804 (US) or (+44) 1865 843830 (UK); fax: (+44) 1865 853333; e-mail: . You may also complete your request on-line via the Elsevier website at .

Knowledge and best practice in this field are constantly changing. As new research and experience broaden our knowledge, changes in practice, treatment, and drug therapy may become necessary or appropriate. Readers are advised to check the most current information provided (i) on procedures featured or (ii) by the manufacturer of each product to be administered, to verify the recommended dose or formula, the method and duration of administration, and contraindications. It is the responsibility of the practitioner, relying on his or her experience and knowledge of the patient, to make diagnoses, to determine dosages and the best treatment for each individual patient, and to take all appropriate safety precautions. To the fullest extent of the law, neither the Publisher nor the Author assume any liability for any injury and/or damage to persons or property arising out of or related to any use of the material contained in this book.
The Publisher
Library of Congress Cataloging-in-Publication Data
Hall, John E. (John Edward), 1946-
Guyton and Hall textbook of medical physiology / John Hall. – 12th ed.
p. ; cm.
Rev. ed. of: Textbook of medical physiology. 11th ed. c2006.
Includes bibliographical references and index.
ISBN 978-1-4160-4574-8 (alk. paper)
1. Human physiology. 2. Physiology, Pathological. I. Guyton, Arthur C. II.
Textbook of medical physiology. III. Title. IV. Title: Textbook of medical physiology.
[DNLM: 1. Physiological Phenomena. QT 104 H1767g 2011]
QP34.5.G9 2011
Publishing Director: William Schmitt
Developmental Editor: Rebecca Gruliow
Editorial Assistant: Laura Stingelin
Publishing Services Manager: Linda Van Pelt
Project Manager: Frank Morales
Design Manager: Steve Stave
Illustrator: Michael Schenk
Marketing Manager: Marla Lieberman
Printed in the United States of America
Last digit is the print number: 9 8 7 6 5 4 3 2 1
My Family
For their abundant support, for their patience and understanding, and for their love
Arthur C. Guyton
For his imaginative and innovative research
For his dedication to education
For showing us the excitement and joy of physiology
And for serving as an inspirational role model

John E. Hall
The first edition of the Textbook of Medical Physiology was written by Arthur C. Guyton almost 55 years ago. Unlike most major medical textbooks, which often have 20 or more authors, the first eight editions of the Textbook of Medical Physiology were written entirely by Dr. Guyton, with each new edition arriving on schedule for nearly 40 years. The Textbook of Medical Physiology , first published in 1956, quickly became the best-selling medical physiology textbook in the world. Dr. Guyton had a gift for communicating complex ideas in a clear and interesting manner that made studying physiology fun. He wrote the book to help students learn physiology, not to impress his professional colleagues.
I worked closely with Dr. Guyton for almost 30 years and had the privilege of writing parts of the 9th and 10th editions. After Dr. Guyton’s tragic death in an automobile accident in 2003, I assumed responsibility for completing the 11th edition.
For the 12th edition of the Textbook of Medical Physiology , I have the same goal as for previous editions—to explain, in language easily understood by students, how the different cells, tissues, and organs of the human body work together to maintain life.
This task has been challenging and fun because our rapidly increasing knowledge of physiology continues to unravel new mysteries of body functions. Advances in molecular and cellular physiology have made it possible to explain many physiology principles in the terminology of molecular and physical sciences rather than in merely a series of separate and unexplained biological phenomena.
The Textbook of Medical Physiology , however, is not a reference book that attempts to provide a compendium of the most recent advances in physiology. This is a book that continues the tradition of being written for students. It focuses on the basic principles of physiology needed to begin a career in the health care professions, such as medicine, dentistry and nursing, as well as graduate studies in the biological and health sciences. It should also be useful to physicians and health care professionals who wish to review the basic principles needed for understanding the pathophysiology of human disease.
I have attempted to maintain the same unified organization of the text that has been useful to students in the past and to ensure that the book is comprehensive enough that students will continue to use it during their professional careers.
My hope is that this textbook conveys the majesty of the human body and its many functions and that it stimulates students to study physiology throughout their careers. Physiology is the link between the basic sciences and medicine. The great beauty of physiology is that it integrates the individual functions of all the body’s different cells, tissues, and organs into a functional whole, the human body. Indeed, the human body is much more than the sum of its parts, and life relies upon this total function, not just on the function of individual body parts in isolation from the others.
This brings us to an important question: How are the separate organs and systems coordinated to maintain proper function of the entire body? Fortunately, our bodies are endowed with a vast network of feedback controls that achieve the necessary balances without which we would be unable to live. Physiologists call this high level of internal bodily control homeostasis . In disease states, functional balances are often seriously disturbed and homeostasis is impaired. When even a single disturbance reaches a limit, the whole body can no longer live. One of the goals of this text, therefore, is to emphasize the effectiveness and beauty of the body’s homeostasis mechanisms as well as to present their abnormal functions in disease.
Another objective is to be as accurate as possible. Suggestions and critiques from many students, physiologists, and clinicians throughout the world have been sought and then used to check factual accuracy as well as balance in the text. Even so, because of the likelihood of error in sorting through many thousands of bits of information, I wish to issue a further request to all readers to send along notations of error or inaccuracy. Physiologists understand the importance of feedback for proper function of the human body; so, too, is feedback important for progressive improvement of a textbook of physiology. To the many persons who have already helped, I express sincere thanks.
A brief explanation is needed about several features of the 12th edition. Although many of the chapters have been revised to include new principles of physiology, the text length has been closely monitored to limit the book size so that it can be used effectively in physiology courses for medical students and health care professionals. Many of the figures have also been redrawn and are in full color. New references have been chosen primarily for their presentation of physiologic principles, for the quality of their own references, and for their easy accessibility. The selected bibliography at the end of the chapters lists papers mainly from recently published scientific journals that can be freely accessed from the PubMed internet site at . Use of these references, as well as cross-references from them, can give the student almost complete coverage of the entire field of physiology. The effort to be as concise as possible has, unfortunately, necessitated a more simplified and dogmatic presentation of many physiologic principles than I normally would have desired. However, the bibliography can be used to learn more about the controversies and unanswered questions that remain in understanding the complex functions of the human body in health and disease.
Another feature is that the print is set in two sizes. The material in large print constitutes the fundamental physiologic information that students will require in virtually all of their medical activities and studies.
The material in small print is of several different kinds: first, anatomic, chemical, and other information that is needed for immediate discussion but that most students will learn in more detail in other courses; second, physiologic information of special importance to certain fields of clinical medicine; and, third, information that will be of value to those students who may wish to study particular physiologic mechanisms more deeply.
I wish to express sincere thanks to many persons who have helped to prepare this book, including my colleagues in the Department of Physiology and Biophysics at the University of Mississippi Medical Center who provided valuable suggestions. The members of our faculty and a brief description of the research and educational activities of the department can be found at the web site: . I am also grateful to Stephanie Lucas and Courtney Horton Graham for their excellent secretarial services, to Michael Schenk and Walter (Kyle) Cunningham for their expert artwork, and to William Schmitt, Rebecca Gruliow, Frank Morales, and the entire Elsevier Saunders team for continued editorial and production excellence.
Finally, I owe an enormous debt to Arthur Guyton for the great privilege of contributing to the Textbook of Medical Physiology, for an exciting career in physiology, for his friendship, and for the inspiration that he provided to all who knew him.
Table of Contents
Front matter
UNIT I: Introduction to Physiology: The Cell and General Physiology
Chapter 1: Functional Organization of the Human Body and Control of the “Internal Environment”
Chapter 2: The Cell and Its Functions
Chapter 3: Genetic Control of Protein Synthesis, Cell Function, and Cell Reproduction
UNIT II: Membrane Physiology, Nerve, and Muscle
Chapter 4: Transport of Substances Through Cell Membranes
Chapter 5: Membrane Potentials and Action Potentials
Chapter 6: Contraction of Skeletal Muscle
Chapter 7: Excitation of Skeletal Muscle: Neuromuscular Transmission and Excitation-Contraction Coupling
Chapter 8: Excitation and Contraction of Smooth Muscle
UNIT III: The Heart
Chapter 9: Cardiac Muscle; The Heart as a Pump and Function of the Heart Valves
Chapter 10: Rhythmical Excitation of the Heart
Chapter 11: The Normal Electrocardiogram
Chapter 12: Electrocardiographic Interpretation of Cardiac Muscle and Coronary Blood Flow Abnormalities: Vectorial Analysis
Chapter 13: Cardiac Arrhythmias and Their Electrocardiographic Interpretation
UNIT IV: The Circulation
Chapter 14: Overview of the Circulation; Biophysics of Pressure, Flow, and Resistance
Chapter 15: Vascular Distensibility and Functions of the Arterial and Venous Systems
Chapter 16: The Microcirculation and Lymphatic System: Capillary Fluid Exchange, Interstitial Fluid, and Lymph Flow
Chapter 17: Local and Humoral Control of Tissue Blood Flow
Chapter 18: Nervous Regulation of the Circulation, and Rapid Control of Arterial Pressure
Chapter 19: Role of the Kidneys in Long-Term Control of Arterial Pressure and in Hypertension: The Integrated System for Arterial Pressure Regulation
Chapter 20: Cardiac Output, Venous Return, and Their Regulation
Chapter 21: Muscle Blood Flow and Cardiac Output During Exercise; the Coronary Circulation and Ischemic Heart Disease
Chapter 22: Cardiac Failure
Chapter 23: Heart Valves and Heart Sounds; Valvular and Congenital Heart Defects
Chapter 24: Circulatory Shock and Its Treatment
UNIT V: The Body Fluids and Kidneys
Chapter 25: The Body Fluid Compartments: Extracellular and Intracellular Fluids; Edema
Chapter 26: Urine Formation by the Kidneys: I. Glomerular Filtration, Renal Blood Flow, and Their Control
Chapter 27: Urine Formation by the Kidneys: II. Tubular Reabsorption and Secretion
Chapter 28: Urine Concentration and Dilution; Regulation of Extracellular Fluid Osmolarity and Sodium Concentration
Chapter 29: Renal Regulation of Potassium, Calcium, Phosphate, and Magnesium; Integration of Renal Mechanisms for Control of Blood Volume and Extracellular Fluid Volume
Chapter 30: Acid-Base Regulation
Chapter 31: Diuretics, Kidney Diseases
UNIT VI: Blood Cells, Immunity, and Blood Coagulation
Chapter 32: Red Blood Cells, Anemia, and Polycythemia
Chapter 33: Resistance of the Body to Infection: I. Leukocytes, Granulocytes, the Monocyte-Macrophage System, and Inflammation
Chapter 34: Resistance of the Body to Infection: II. Immunity and Allergy
Chapter 35: Blood Types; Transfusion; Tissue and Organ Transplantation
Chapter 36: Hemostasis and Blood Coagulation
UNIT VII: Respiration
Chapter 37: Pulmonary Ventilation
Chapter 38: Pulmonary Circulation, Pulmonary Edema, Pleural Fluid
Chapter 39: Physical Principles of Gas Exchange; Diffusion of Oxygen and Carbon Dioxide Through the Respiratory Membrane
Chapter 40: Transport of Oxygen and Carbon Dioxide in Blood and Tissue Fluids
Chapter 41: Regulation of Respiration
Chapter 42: Respiratory Insufficiency—Pathophysiology, Diagnosis, Oxygen Therapy
UNIT VIII: Aviation, Space, and Deep-Sea Diving Physiology
Chapter 43: Aviation, High Altitude, and Space Physiology
Chapter 44: Physiology of Deep-Sea Diving and Other Hyperbaric Conditions
UNIT IX: The Nervous System: A. General Principles and Sensory Physiology
Chapter 45: Organization of the Nervous System, Basic Functions of Synapses, and Neurotransmitters
Chapter 46: Sensory Receptors, Neuronal Circuits for Processing Information
Chapter 47: Somatic Sensations: I. General Organization, the Tactile and Position Senses
Chapter 48: Somatic Sensations: II. Pain, Headache, and Thermal Sensations
UNIT X: The Nervous System: B. The Special Senses
Chapter 49: The Eye: I. Optics of Vision
Chapter 50: The Eye: II. Receptor and Neural Function of the Retina
Chapter 51: The Eye: III. Central Neurophysiology of Vision
Chapter 52: The Sense of Hearing
Chapter 53: The Chemical Senses—Taste and Smell
UNIT XI: The Nervous System: C. Motor and Integrative Neurophysiology
Chapter 54: Motor Functions of the Spinal Cord; the Cord Reflexes
Chapter 55: Cortical and Brain Stem Control of Motor Function
Chapter 56: Contributions of the Cerebellum and Basal Ganglia to Overall Motor Control
Chapter 57: Cerebral Cortex, Intellectual Functions of the Brain, Learning, and Memory
Chapter 58: Behavioral and Motivational Mechanisms of the Brain—The Limbic System and the Hypothalamus
Chapter 59: States of Brain Activity—Sleep, Brain Waves, Epilepsy, Psychoses
Chapter 60: The Autonomic Nervous System and the Adrenal Medulla
Chapter 61: Cerebral Blood Flow, Cerebrospinal Fluid, and Brain Metabolism
UNIT XII: Gastrointestinal Physiology
Chapter 62: General Principles of Gastrointestinal Function—Motility, Nervous Control, and Blood Circulation
Chapter 63: Propulsion and Mixing of Food in the Alimentary Tract
Chapter 64: Secretory Functions of the Alimentary Tract
Chapter 65: Digestion and Absorption in the Gastrointestinal Tract
Chapter 66: Physiology of Gastrointestinal Disorders
UNIT XIII: Metabolism and Temperature Regulation
Chapter 67: Metabolism of Carbohydrates, and Formation of Adenosine Triphosphate
Chapter 68: Lipid Metabolism
Chapter 69: Protein Metabolism
Chapter 70: The Liver as an Organ
Chapter 71: Dietary Balances; Regulation of Feeding; Obesity and Starvation; Vitamins and Minerals
Chapter 72: Energetics and Metabolic Rate
Chapter 73: Body Temperature Regulation, and Fever
UNIT XIV: Endocrinology and Reproduction
Chapter 74: Introduction to Endocrinology
Chapter 75: Pituitary Hormones and Their Control by the Hypothalamus
Chapter 76: Thyroid Metabolic Hormones
Chapter 77: Adrenocortical Hormones
Chapter 78: Insulin, Glucagon, and Diabetes Mellitus
Chapter 79: Parathyroid Hormone, Calcitonin, Calcium and Phosphate Metabolism, Vitamin D, Bone, and Teeth
Chapter 80: Reproductive and Hormonal Functions of the Male (and Function of the Pineal Gland)
Chapter 81: Female Physiology Before Pregnancy and Female Hormones
Chapter 82: Pregnancy and Lactation
Chapter 83: Fetal and Neonatal Physiology
UNIT XV: Sports Physiology
Chapter 84: Sports Physiology
Introduction to Physiology: The Cell and General Physiology
CHAPTER 1 Functional Organization of the Human Body and Control of the “Internal Environment”
The goal of physiology is to explain the physical and chemical factors that are responsible for the origin, development, and progression of life. Each type of life, from the simple virus to the largest tree or the complicated human being, has its own functional characteristics. Therefore, the vast field of physiology can be divided into viral physiology, bacterial physiology, cellular physiology, plant physiology, human physiology, and many more subdivisions.

Human Physiology
In human physiology, we attempt to explain the specific characteristics and mechanisms of the human body that make it a living being. The very fact that we remain alive is the result of complex control systems, for hunger makes us seek food and fear makes us seek refuge. Sensations of cold make us look for warmth. Other forces cause us to seek fellowship and to reproduce. Thus, the human being is, in many ways, like an automaton, and the fact that we are sensing, feeling, and knowledgeable beings is part of this automatic sequence of life; these special attributes allow us to exist under widely varying conditions.

Cells as the Living Units of the Body
The basic living unit of the body is the cell. Each organ is an aggregate of many different cells held together by intercellular supporting structures.
Each type of cell is specially adapted to perform one or a few particular functions. For instance, the red blood cells, numbering 25 trillion in each human being, transport oxygen from the lungs to the tissues. Although the red cells are the most abundant of any single type of cell in the body, there are about 75 trillion additional cells of other types that perform functions different from those of the red cell. The entire body, then, contains about 100 trillion cells.
Although the many cells of the body often differ markedly from one another, all of them have certain basic characteristics that are alike. For instance, in all cells, oxygen reacts with carbohydrate, fat, and protein to release the energy required for cell function. Further, the general chemical mechanisms for changing nutrients into energy are basically the same in all cells, and all cells deliver end products of their chemical reactions into the surrounding fluids.
Almost all cells also have the ability to reproduce additional cells of their own kind. Fortunately, when cells of a particular type are destroyed, the remaining cells of this type usually generate new cells until the supply is replenished.

Extracellular Fluid—The “Internal Environment”
About 60 percent of the adult human body is fluid, mainly a water solution of ions and other substances. Although most of this fluid is inside the cells and is called intracellular fluid, about one third is in the spaces outside the cells and is called extracellular fluid. This extracellular fluid is in constant motion throughout the body. It is transported rapidly in the circulating blood and then mixed between the blood and the tissue fluids by diffusion through the capillary walls.
In the extracellular fluid are the ions and nutrients needed by the cells to maintain cell life. Thus, all cells live in essentially the same environment—the extracellular fluid. For this reason, the extracellular fluid is also called the internal environment of the body, or the milieu intérieur, a term introduced more than 100 years ago by the great 19th-century French physiologist Claude Bernard.
Cells are capable of living, growing, and performing their special functions as long as the proper concentrations of oxygen, glucose, different ions, amino acids, fatty substances, and other constituents are available in this internal environment.

Differences Between Extracellular and Intracellular Fluids
The extracellular fluid contains large amounts of sodium, chloride, and bicarbonate ions plus nutrients for the cells, such as oxygen, glucose, fatty acids, and amino acids. It also contains carbon dioxide that is being transported from the cells to the lungs to be excreted, plus other cellular waste products that are being transported to the kidneys for excretion.
The intracellular fluid differs significantly from the extracellular fluid; for example, it contains large amounts of potassium, magnesium, and phosphate ions instead of the sodium and chloride ions found in the extracellular fluid. Special mechanisms for transporting ions through the cell membranes maintain the ion concentration differences between the extracellular and intracellular fluids. These transport processes are discussed in Chapter 4 .

“Homeostatic” Mechanisms of the Major Functional Systems

The term homeostasis is used by physiologists to mean maintenance of nearly constant conditions in the internal environment. Essentially all organs and tissues of the body perform functions that help maintain these relatively constant conditions. For instance, the lungs provide oxygen to the extracellular fluid to replenish the oxygen used by the cells, the kidneys maintain constant ion concentrations, and the gastrointestinal system provides nutrients.
A large segment of this text is concerned with the manner in which each organ or tissue contributes to homeostasis. To begin this discussion, the different functional systems of the body and their contributions to homeostasis are outlined in this chapter; then we briefly outline the basic theory of the body’s control systems that allow the functional systems to operate in support of one another.

Extracellular Fluid Transport and Mixing System—The Blood Circulatory System
Extracellular fluid is transported through all parts of the body in two stages. The first stage is movement of blood through the body in the blood vessels, and the second is movement of fluid between the blood capillaries and the intercellular spaces between the tissue cells.
Figure 1-1 shows the overall circulation of blood. All the blood in the circulation traverses the entire circulatory circuit an average of once each minute when the body is at rest and as many as six times each minute when a person is extremely active.

Figure 1-1 General organization of the circulatory system.
As blood passes through the blood capillaries, continual exchange of extracellular fluid also occurs between the plasma portion of the blood and the interstitial fluid that fills the intercellular spaces. This process is shown in Figure 1-2 . The walls of the capillaries are permeable to most molecules in the plasma of the blood, with the exception of plasma protein molecules, which are too large to readily pass through the capillaries. Therefore, large amounts of fluid and its dissolved constituents diffuse back and forth between the blood and the tissue spaces, as shown by the arrows. This process of diffusion is caused by kinetic motion of the molecules in both the plasma and the interstitial fluid. That is, the fluid and dissolved molecules are continually moving and bouncing in all directions within the plasma and the fluid in the intercellular spaces, as well as through the capillary pores. Few cells are located more than 50 micrometers from a capillary, which ensures diffusion of almost any substance from the capillary to the cell within a few seconds. Thus, the extracellular fluid everywhere in the body—both that of the plasma and that of the interstitial fluid—is continually being mixed, thereby maintaining homogeneity of the extracellular fluid throughout the body.

Figure 1-2 Diffusion of fluid and dissolved constituents through the capillary walls and through the interstitial spaces.

Origin of Nutrients in the Extracellular Fluid

Respiratory System
Figure 1-1 shows that each time the blood passes through the body, it also flows through the lungs. The blood picks up oxygen in the alveoli, thus acquiring the oxygen needed by the cells. The membrane between the alveoli and the lumen of the pulmonary capillaries, the alveolar membrane, is only 0.4 to 2.0 micrometers thick, and oxygen rapidly diffuses by molecular motion through this membrane into the blood.

Gastrointestinal Tract
A large portion of the blood pumped by the heart also passes through the walls of the gastrointestinal tract. Here different dissolved nutrients, including carbohydrates, fatty acids, and amino acids, are absorbed from the ingested food into the extracellular fluid of the blood.

Liver and Other Organs That Perform Primarily Metabolic Functions
Not all substances absorbed from the gastrointestinal tract can be used in their absorbed form by the cells. The liver changes the chemical compositions of many of these substances to more usable forms, and other tissues of the body—fat cells, gastrointestinal mucosa, kidneys, and endocrine glands—help modify the absorbed substances or store them until they are needed. The liver also eliminates certain waste products produced in the body and toxic substances that are ingested.

Musculoskeletal System
How does the musculoskeletal system contribute to homeostasis? The answer is obvious and simple: Were it not for the muscles, the body could not move to the appropriate place at the appropriate time to obtain the foods required for nutrition. The musculoskeletal system also provides motility for protection against adverse surroundings, without which the entire body, along with its homeostatic mechanisms, could be destroyed instantaneously.

Removal of Metabolic End Products

Removal of Carbon Dioxide by the Lungs
At the same time that blood picks up oxygen in the lungs, carbon dioxide is released from the blood into the lung alveoli; the respiratory movement of air into and out of the lungs carries the carbon dioxide to the atmosphere. Carbon dioxide is the most abundant of all the end products of metabolism.

Passage of the blood through the kidneys removes from the plasma most of the other substances besides carbon dioxide that are not needed by the cells. These substances include different end products of cellular metabolism, such as urea and uric acid; they also include excesses of ions and water from the food that might have accumulated in the extracellular fluid.
The kidneys perform their function by first filtering large quantities of plasma through the glomeruli into the tubules and then reabsorbing into the blood those substances needed by the body, such as glucose, amino acids, appropriate amounts of water, and many of the ions. Most of the other substances that are not needed by the body, especially the metabolic end products such as urea, are reabsorbed poorly and pass through the renal tubules into the urine.

Gastrointestinal Tract
Undigested material that enters the gastrointestinal tract and some waste products of metabolism are eliminated in the feces.

Among the functions of the liver is the detoxification or removal of many drugs and chemicals that are ingested. The liver secretes many of these wastes into the bile to be eventually eliminated in the feces.

Regulation of Body Functions

Nervous System
The nervous system is composed of three major parts: the sensory input portion, the central nervous system (or integrative portion ), and the motor output portion. Sensory receptors detect the state of the body or the state of the surroundings. For instance, receptors in the skin apprise one whenever an object touches the skin at any point. The eyes are sensory organs that give one a visual image of the surrounding area. The ears are also sensory organs. The central nervous system is composed of the brain and spinal cord. The brain can store information, generate thoughts, create ambition, and determine reactions that the body performs in response to the sensations. Appropriate signals are then transmitted through the motor output portion of the nervous system to carry out one’s desires.
An important segment of the nervous system is called the autonomic system. It operates at a subconscious level and controls many functions of the internal organs, including the level of pumping activity by the heart, movements of the gastrointestinal tract, and secretion by many of the body’s glands.

Hormone Systems
Located in the body are eight major endocrine glands that secrete chemical substances called hormones. Hormones are transported in the extracellular fluid to all parts of the body to help regulate cellular function. For instance, thyroid hormone increases the rates of most chemical reactions in all cells, thus helping to set the tempo of bodily activity. Insulin controls glucose metabolism; adrenocortical hormones control sodium ion, potassium ion, and protein metabolism; and parathyroid hormone controls bone calcium and phosphate. Thus, the hormones provide a system for regulation that complements the nervous system. The nervous system regulates many muscular and secretory activities of the body, whereas the hormonal system regulates many metabolic functions.

Protection of the Body

Immune System
The immune system consists of the white blood cells, tissue cells derived from white blood cells, the thymus, lymph nodes, and lymph vessels that protect the body from pathogens such as bacteria, viruses, parasites, and fungi. The immune system provides a mechanism for the body to (1) distinguish its own cells from foreign cells and substances and (2) destroy the invader by phagocytosis or by producing sensitized lymphocytes or specialized proteins (e.g., antibodies) that either destroy or neutralize the invader.

Integumentary System
The skin and its various appendages, including the hair, nails, glands, and other structures, cover, cushion, and protect the deeper tissues and organs of the body and generally provide a boundary between the body’s internal environment and the outside world. The integumentary system is also important for temperature regulation and excretion of wastes and it provides a sensory interface between the body and the external environment. The skin generally comprises about 12 to 15 percent of body weight.

Sometimes reproduction is not considered a homeostatic function. It does, however, help maintain homeostasis by generating new beings to take the place of those that are dying. This may sound like a permissive usage of the term homeostasis, but it illustrates that, in the final analysis, essentially all body structures are organized such that they help maintain the automaticity and continuity of life.

Control Systems of the Body
The human body has thousands of control systems. The most intricate of these are the genetic control systems that operate in all cells to help control intracellular function and extracellular functions. This subject is discussed in Chapter 3 .
Many other control systems operate within the organs to control functions of the individual parts of the organs; others operate throughout the entire body to control the interrelations between the organs. For instance, the respiratory system, operating in association with the nervous system, regulates the concentration of carbon dioxide in the extracellular fluid. The liver and pancreas regulate the concentration of glucose in the extracellular fluid, and the kidneys regulate concentrations of hydrogen, sodium, potassium, phosphate, and other ions in the extracellular fluid.

Examples of Control Mechanisms

Regulation of Oxygen and Carbon Dioxide Concentrations in the Extracellular Fluid
Because oxygen is one of the major substances required for chemical reactions in the cells, the body has a special control mechanism to maintain an almost exact and constant oxygen concentration in the extracellular fluid. This mechanism depends principally on the chemical characteristics of hemoglobin, which is present in all red blood cells. Hemoglobin combines with oxygen as the blood passes through the lungs. Then, as the blood passes through the tissue capillaries, hemoglobin, because of its own strong chemical affinity for oxygen, does not release oxygen into the tissue fluid if too much oxygen is already there. But if the oxygen concentration in the tissue fluid is too low, sufficient oxygen is released to re-establish an adequate concentration. Thus, regulation of oxygen concentration in the tissues is vested principally in the chemical characteristics of hemoglobin itself. This regulation is called the oxygen-buffering function of hemoglobin.
Carbon dioxide concentration in the extracellular fluid is regulated in a much different way. Carbon dioxide is a major end product of the oxidative reactions in cells. If all the carbon dioxide formed in the cells continued to accumulate in the tissue fluids, all energy-giving reactions of the cells would cease. Fortunately, a higher than normal carbon dioxide concentration in the blood excites the respiratory center, causing a person to breathe rapidly and deeply. This increases expiration of carbon dioxide and, therefore, removes excess carbon dioxide from the blood and tissue fluids. This process continues until the concentration returns to normal.

Regulation of Arterial Blood Pressure
Several systems contribute to the regulation of arterial blood pressure. One of these, the baroreceptor system, is a simple and excellent example of a rapidly acting control mechanism. In the walls of the bifurcation region of the carotid arteries in the neck, and also in the arch of the aorta in the thorax, are many nerve receptors called baroreceptors, which are stimulated by stretch of the arterial wall. When the arterial pressure rises too high, the baroreceptors send barrages of nerve impulses to the medulla of the brain. Here these impulses inhibit the vasomotor center, which in turn decreases the number of impulses transmitted from the vasomotor center through the sympathetic nervous system to the heart and blood vessels. Lack of these impulses causes diminished pumping activity by the heart and also dilation of the peripheral blood vessels, allowing increased blood flow through the vessels. Both of these effects decrease the arterial pressure back toward normal.
Conversely, a decrease in arterial pressure below normal relaxes the stretch receptors, allowing the vasomotor center to become more active than usual, thereby causing vasoconstriction and increased heart pumping. The decrease in arterial pressure also raises arterial pressure back toward normal.

Normal Ranges and Physical Characteristics of Important Extracellular Fluid Constituents
Table 1-1 lists some of the important constituents and physical characteristics of extracellular fluid, along with their normal values, normal ranges, and maximum limits without causing death. Note the narrowness of the normal range for each one. Values outside these ranges are usually caused by illness.

Table 1-1 Important Constituents and Physical Characteristics of Extracellular Fluid
Most important are the limits beyond which abnormalities can cause death. For example, an increase in the body temperature of only 11 °F (7 °C) above normal can lead to a vicious cycle of increasing cellular metabolism that destroys the cells. Note also the narrow range for acid-base balance in the body, with a normal pH value of 7.4 and lethal values only about 0.5 on either side of normal. Another important factor is the potassium ion concentration because whenever it decreases to less than one-third normal, a person is likely to be paralyzed as a result of the nerves’ inability to carry signals. Alternatively, if the potassium ion concentration increases to two or more times normal, the heart muscle is likely to be severely depressed. Also, when the calcium ion concentration falls below about one-half normal, a person is likely to experience tetanic contraction of muscles throughout the body because of the spontaneous generation of excess nerve impulses in the peripheral nerves. When the glucose concentration falls below one-half normal, a person frequently develops extreme mental irritability and sometimes even convulsions.
These examples should give one an appreciation for the extreme value and even the necessity of the vast numbers of control systems that keep the body operating in health; in the absence of any one of these controls, serious body malfunction or death can result.

Characteristics of Control Systems
The aforementioned examples of homeostatic control mechanisms are only a few of the many thousands in the body, all of which have certain characteristics in common as explained in this section.

Negative Feedback Nature of Most Control Systems
Most control systems of the body act by negative feedback, which can best be explained by reviewing some of the homeostatic control systems mentioned previously. In the regulation of carbon dioxide concentration, a high concentration of carbon dioxide in the extracellular fluid increases pulmonary ventilation. This, in turn, decreases the extracellular fluid carbon dioxide concentration because the lungs expire greater amounts of carbon dioxide from the body. In other words, the high concentration of carbon dioxide initiates events that decrease the concentration toward normal, which is negative to the initiating stimulus. Conversely, if the carbon dioxide concentration falls too low, this causes feedback to increase the concentration. This response is also negative to the initiating stimulus.
In the arterial pressure-regulating mechanisms, a high pressure causes a series of reactions that promote a lowered pressure, or a low pressure causes a series of reactions that promote an elevated pressure. In both instances, these effects are negative with respect to the initiating stimulus.
Therefore, in general, if some factor becomes excessive or deficient, a control system initiates negative feedback, which consists of a series of changes that return the factor toward a certain mean value, thus maintaining homeostasis.

“Gain” of a Control System
The degree of effectiveness with which a control system maintains constant conditions is determined by the gain of the negative feedback. For instance, let us assume that a large volume of blood is transfused into a person whose baroreceptor pressure control system is not functioning, and the arterial pressure rises from the normal level of 100 mm Hg up to 175 mm Hg. Then, let us assume that the same volume of blood is injected into the same person when the baroreceptor system is functioning, and this time the pressure increases only 25 mm Hg. Thus, the feedback control system has caused a “correction” of −50 mm Hg—that is, from 175 mm Hg to 125 mm Hg. There remains an increase in pressure of +25 mm Hg, called the “error,” which means that the control system is not 100 percent effective in preventing change. The gain of the system is then calculated by the following formula:

Thus, in the baroreceptor system example, the correction is −50 mm Hg and the error persisting is +25 mm Hg. Therefore, the gain of the person’s baroreceptor system for control of arterial pressure is −50 divided by +25, or −2. That is, a disturbance that increases or decreases the arterial pressure does so only one-third as much as would occur if this control system were not present.
The gains of some other physiologic control systems are much greater than that of the baroreceptor system. For instance, the gain of the system controlling internal body temperature when a person is exposed to moderately cold weather is about −33. Therefore, one can see that the temperature control system is much more effective than the baroreceptor pressure control system.

Positive Feedback Can Sometimes Cause Vicious Cycles and Death
One might ask the question, Why do most control systems of the body operate by negative feedback rather than positive feedback? If one considers the nature of positive feedback, one immediately sees that positive feedback does not lead to stability but to instability and, in some cases, can cause death.
Figure 1-3 shows an example in which death can ensue from positive feedback. This figure depicts the pumping effectiveness of the heart, showing that the heart of a healthy human being pumps about 5 liters of blood per minute. If the person is suddenly bled 2 liters, the amount of blood in the body is decreased to such a low level that not enough blood is available for the heart to pump effectively. As a result, the arterial pressure falls and the flow of blood to the heart muscle through the coronary vessels diminishes. This results in weakening of the heart, further diminished pumping, a further decrease in coronary blood flow, and still more weakness of the heart; the cycle repeats itself again and again until death occurs. Note that each cycle in the feedback results in further weakening of the heart. In other words, the initiating stimulus causes more of the same, which is positive feedback.

Figure 1-3 Recovery of heart pumping caused by negative feedback after 1 liter of blood is removed from the circulation. Death is caused by positive feedback when 2 liters of blood are removed.
Positive feedback is better known as a “vicious cycle,” but a mild degree of positive feedback can be overcome by the negative feedback control mechanisms of the body and the vicious cycle fails to develop. For instance, if the person in the aforementioned example were bled only 1 liter instead of 2 liters, the normal negative feedback mechanisms for controlling cardiac output and arterial pressure would overbalance the positive feedback and the person would recover, as shown by the dashed curve of Figure 1-3 .

Positive Feedback Can Sometimes Be Useful
In some instances, the body uses positive feedback to its advantage. Blood clotting is an example of a valuable use of positive feedback. When a blood vessel is ruptured and a clot begins to form, multiple enzymes called clotting factors are activated within the clot itself. Some of these enzymes act on other unactivated enzymes of the immediately adjacent blood, thus causing more blood clotting. This process continues until the hole in the vessel is plugged and bleeding no longer occurs. On occasion, this mechanism can get out of hand and cause the formation of unwanted clots. In fact, this is what initiates most acute heart attacks, which are caused by a clot beginning on the inside surface of an atherosclerotic plaque in a coronary artery and then growing until the artery is blocked.
Childbirth is another instance in which positive feedback plays a valuable role. When uterine contractions become strong enough for the baby’s head to begin pushing through the cervix, stretch of the cervix sends signals through the uterine muscle back to the body of the uterus, causing even more powerful contractions. Thus, the uterine contractions stretch the cervix and the cervical stretch causes stronger contractions. When this process becomes powerful enough, the baby is born. If it is not powerful enough, the contractions usually die out and a few days pass before they begin again.
Another important use of positive feedback is for the generation of nerve signals. That is, when the membrane of a nerve fiber is stimulated, this causes slight leakage of sodium ions through sodium channels in the nerve membrane to the fiber’s interior. The sodium ions entering the fiber then change the membrane potential, which in turn causes more opening of channels, more change of potential, still more opening of channels, and so forth. Thus, a slight leak becomes an explosion of sodium entering the interior of the nerve fiber, which creates the nerve action potential. This action potential in turn causes electrical current to flow along both the outside and the inside of the fiber and initiates additional action potentials. This process continues again and again until the nerve signal goes all the way to the end of the fiber.
In each case in which positive feedback is useful, the positive feedback itself is part of an overall negative feedback process. For example, in the case of blood clotting, the positive feedback clotting process is a negative feedback process for maintenance of normal blood volume. Also, the positive feedback that causes nerve signals allows the nerves to participate in thousands of negative feedback nervous control systems.

More Complex Types of Control Systems—Adaptive Control
Later in this text, when we study the nervous system, we shall see that this system contains great numbers of interconnected control mechanisms. Some are simple feedback systems similar to those already discussed. Many are not. For instance, some movements of the body occur so rapidly that there is not enough time for nerve signals to travel from the peripheral parts of the body all the way to the brain and then back to the periphery again to control the movement. Therefore, the brain uses a principle called feed-forward control to cause required muscle contractions. That is, sensory nerve signals from the moving parts apprise the brain whether the movement is performed correctly. If not, the brain corrects the feed-forward signals that it sends to the muscles the next time the movement is required. Then, if still further correction is necessary, this will be done again for subsequent movements. This is called adaptive control. Adaptive control, in a sense, is delayed negative feedback.
Thus, one can see how complex the feedback control systems of the body can be. A person’s life depends on all of them. Therefore, a major share of this text is devoted to discussing these life-giving mechanisms.

Summary—Automaticity of the Body
The purpose of this chapter has been to point out, first, the overall organization of the body and, second, the means by which the different parts of the body operate in harmony. To summarize, the body is actually a social order of about 100 trillion cells organized into different functional structures, some of which are called organs. Each functional structure contributes its share to the maintenance of homeostatic conditions in the extracellular fluid, which is called the internal environment. As long as normal conditions are maintained in this internal environment, the cells of the body continue to live and function properly. Each cell benefits from homeostasis, and in turn, each cell contributes its share toward the maintenance of homeostasis. This reciprocal interplay provides continuous automaticity of the body until one or more functional systems lose their ability to contribute their share of function. When this happens, all the cells of the body suffer. Extreme dysfunction leads to death; moderate dysfunction leads to sickness.


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CHAPTER 2 The Cell and Its Functions
Each of the 100 trillion cells in a human being is a living structure that can survive for months or many years, provided its surrounding fluids contain appropriate nutrients. To understand the function of organs and other structures of the body, it is essential that we first understand the basic organization of the cell and the functions of its component parts.

Organization of the Cell
A typical cell, as seen by the light microscope, is shown in Figure 2-1 . Its two major parts are the nucleus and the cytoplasm. The nucleus is separated from the cytoplasm by a nuclear membrane, and the cytoplasm is separated from the surrounding fluids by a cell membrane, also called the plasma membrane.

Figure 2-1 Structure of the cell as seen with the light microscope.
The different substances that make up the cell are collectively called protoplasm. Protoplasm is composed mainly of five basic substances: water, electrolytes, proteins, lipids, and carbohydrates.

The principal fluid medium of the cell is water, which is present in most cells, except for fat cells, in a concentration of 70 to 85 percent. Many cellular chemicals are dissolved in the water. Others are suspended in the water as solid particulates. Chemical reactions take place among the dissolved chemicals or at the surfaces of the suspended particles or membranes.

Important ions in the cell include potassium, magnesium, phosphate, sulfate, bicarbonate, and smaller quantities of sodium, chloride, and calcium. These are all discussed in more detail in Chapter 4 , which considers the interrelations between the intracellular and extracellular fluids.
The ions provide inorganic chemicals for cellular reactions. Also, they are necessary for operation of some of the cellular control mechanisms. For instance, ions acting at the cell membrane are required for transmission of electrochemical impulses in nerve and muscle fibers.

After water, the most abundant substances in most cells are proteins, which normally constitute 10 to 20 percent of the cell mass. These can be divided into two types: structural proteins and functional proteins.
Structural proteins are present in the cell mainly in the form of long filaments that are polymers of many individual protein molecules. A prominent use of such intracellular filaments is to form microtubules that provide the “cytoskeletons” of such cellular organelles as cilia, nerve axons, the mitotic spindles of mitosing cells, and a tangled mass of thin filamentous tubules that hold the parts of the cytoplasm and nucleoplasm together in their respective compartments. Extracellularly, fibrillar proteins are found especially in the collagen and elastin fibers of connective tissue and in blood vessel walls, tendons, ligaments, and so forth.
The functional proteins are an entirely different type of protein, usually composed of combinations of a few molecules in tubular-globular form. These proteins are mainly the enzymes of the cell and, in contrast to the fibrillar proteins, are often mobile in the cell fluid. Also, many of them are adherent to membranous structures inside the cell. The enzymes come into direct contact with other substances in the cell fluid and thereby catalyze specific intracellular chemical reactions. For instance, the chemical reactions that split glucose into its component parts and then combine these with oxygen to form carbon dioxide and water while simultaneously providing energy for cellular function are all catalyzed by a series of protein enzymes.

Lipids are several types of substances that are grouped together because of their common property of being soluble in fat solvents. Especially important lipids are phospholipids and cholesterol, which together constitute only about 2 percent of the total cell mass. The significance of phospholipids and cholesterol is that they are mainly insoluble in water and, therefore, are used to form the cell membrane and intracellular membrane barriers that separate the different cell compartments.
In addition to phospholipids and cholesterol, some cells contain large quantities of triglycerides, also called neutral fat. In the fat cells, triglycerides often account for as much as 95 percent of the cell mass. The fat stored in these cells represents the body’s main storehouse of energy-giving nutrients that can later be dissoluted and used to provide energy wherever in the body it is needed.

Carbohydrates have little structural function in the cell except as parts of glycoprotein molecules, but they play a major role in nutrition of the cell. Most human cells do not maintain large stores of carbohydrates; the amount usually averages about 1 percent of their total mass but increases to as much as 3 percent in muscle cells and, occasionally, 6 percent in liver cells. However, carbohydrate in the form of dissolved glucose is always present in the surrounding extracellular fluid so that it is readily available to the cell. Also, a small amount of carbohydrate is stored in the cells in the form of glycogen, which is an insoluble polymer of glucose that can be depolymerized and used rapidly to supply the cells’ energy needs.

Physical Structure of the Cell
The cell is not merely a bag of fluid, enzymes, and chemicals; it also contains highly organized physical structures, called intracellular organelles. The physical nature of each organelle is as important as the cell’s chemical constituents for cell function. For instance, without one of the organelles, the mitochondria, more than 95 percent of the cell’s energy release from nutrients would cease immediately. The most important organelles and other structures of the cell are shown in Figure 2-2 .

Figure 2-2 Reconstruction of a typical cell, showing the internal organelles in the cytoplasm and in the nucleus.

Membranous Structures of the Cell
Most organelles of the cell are covered by membranes composed primarily of lipids and proteins. These membranes include the cell membrane, nuclear membrane, membrane of the endoplasmic reticulum, and membranes of the mitochondria, lysosomes, and Golgi apparatus.
The lipids of the membranes provide a barrier that impedes the movement of water and water-soluble substances from one cell compartment to another because water is not soluble in lipids. However, protein molecules in the membrane often do penetrate all the way through the membrane, thus providing specialized pathways, often organized into actual pores, for passage of specific substances through the membrane. Also, many other membrane proteins are enzymes that catalyze a multitude of different chemical reactions, discussed here and in subsequent chapters.

Cell Membrane
The cell membrane (also called the plasma membrane ), which envelops the cell, is a thin, pliable, elastic structure only 7.5 to 10 nanometers thick. It is composed almost entirely of proteins and lipids. The approximate composition is proteins, 55 percent; phospholipids, 25 percent; cholesterol, 13 percent; other lipids, 4 percent; and carbohydrates, 3 percent.

Lipid Barrier of the Cell Membrane Impedes Water Penetration
Figure 2-3 shows the structure of the cell membrane. Its basic structure is a lipid bilayer, which is a thin, double-layered film of lipids—each layer only one molecule thick—that is continuous over the entire cell surface. Interspersed in this lipid film are large globular protein molecules.

Figure 2-3 Structure of the cell membrane, showing that it is composed mainly of a lipid bilayer of phospholipid molecules, but with large numbers of protein molecules protruding through the layer. Also, carbohydrate moieties are attached to the protein molecules on the outside of the membrane and to additional protein molecules on the inside.
(Redrawn from Lodish HF, Rothman JE: The assembly of cell membranes. Sci Am 240:48, 1979. Copyright George V. Kevin.)
The basic lipid bilayer is composed of phospholipid molecules. One end of each phospholipid molecule is soluble in water; that is, it is hydrophilic. The other end is soluble only in fats; that is, it is hydrophobic. The phosphate end of the phospholipid is hydrophilic, and the fatty acid portion is hydrophobic.
Because the hydrophobic portions of the phospholipid molecules are repelled by water but are mutually attracted to one another, they have a natural tendency to attach to one another in the middle of the membrane, as shown in Figure 2-3 . The hydrophilic phosphate portions then constitute the two surfaces of the complete cell membrane, in contact with intracellular water on the inside of the membrane and extracellular water on the outside surface.
The lipid layer in the middle of the membrane is impermeable to the usual water-soluble substances, such as ions, glucose, and urea. Conversely, fat-soluble substances, such as oxygen, carbon dioxide, and alcohol, can penetrate this portion of the membrane with ease.
The cholesterol molecules in the membrane are also lipid in nature because their steroid nucleus is highly fat soluble. These molecules, in a sense, are dissolved in the bilayer of the membrane. They mainly help determine the degree of permeability (or impermeability) of the bilayer to water-soluble constituents of body fluids. Cholesterol controls much of the fluidity of the membrane as well.

Integral and Peripheral Cell Membrane Proteins
Figure 2-3 also shows globular masses floating in the lipid bilayer. These are membrane proteins, most of which are glycoproteins. There are two types of cell membrane proteins: integral proteins that protrude all the way through the membrane and peripheral proteins that are attached only to one surface of the membrane and do not penetrate all the way through.
Many of the integral proteins provide structural channels (or pores ) through which water molecules and water-soluble substances, especially ions, can diffuse between the extracellular and intracellular fluids. These protein channels also have selective properties that allow preferential diffusion of some substances over others.
Other integral proteins act as carrier proteins for transporting substances that otherwise could not penetrate the lipid bilayer. Sometimes these even transport substances in the direction opposite to their electrochemical gradients for diffusion, which is called “active transport.” Still others act as enzymes.
Integral membrane proteins can also serve as receptors for water-soluble chemicals, such as peptide hormones, that do not easily penetrate the cell membrane. Interaction of cell membrane receptors with specific ligands that bind to the receptor causes conformational changes in the receptor protein. This, in turn, enzymatically activates the intracellular part of the protein or induces interactions between the receptor and proteins in the cytoplasm that act as second messengers, thereby relaying the signal from the extracellular part of the receptor to the interior of the cell. In this way, integral proteins spanning the cell membrane provide a means of conveying information about the environment to the cell interior.
Peripheral protein molecules are often attached to the integral proteins. These peripheral proteins function almost entirely as enzymes or as controllers of transport of substances through the cell membrane “pores.”

Membrane Carbohydrates—The Cell “Glycocalyx.”
Membrane carbohydrates occur almost invariably in combination with proteins or lipids in the form of glycoproteins or glycolipids. In fact, most of the integral proteins are glycoproteins, and about one tenth of the membrane lipid molecules are glycolipids. The “glyco” portions of these molecules almost invariably protrude to the outside of the cell, dangling outward from the cell surface. Many other carbohydrate compounds, called proteoglycans —which are mainly carbohydrate substances bound to small protein cores—are loosely attached to the outer surface of the cell as well. Thus, the entire outside surface of the cell often has a loose carbohydrate coat called the glycocalyx.
The carbohydrate moieties attached to the outer surface of the cell have several important functions: (1) Many of them have a negative electrical charge, which gives most cells an overall negative surface charge that repels other negative objects. (2) The glycocalyx of some cells attaches to the glycocalyx of other cells, thus attaching cells to one another. (3) Many of the carbohydrates act as receptor substances for binding hormones, such as insulin; when bound, this combination activates attached internal proteins that, in turn, activate a cascade of intracellular enzymes. (4) Some carbohydrate moieties enter into immune reactions, as discussed in Chapter 34 .

Cytoplasm and Its Organelles
The cytoplasm is filled with both minute and large dispersed particles and organelles. The clear fluid portion of the cytoplasm in which the particles are dispersed is called cytosol; this contains mainly dissolved proteins, electrolytes, and glucose.
Dispersed in the cytoplasm are neutral fat globules, glycogen granules, ribosomes, secretory vesicles, and five especially important organelles: the endoplasmic reticulum, the Golgi apparatus, mitochondria, lysosomes, and peroxisomes.

Endoplasmic Reticulum
Figure 2-2 shows a network of tubular and flat vesicular structures in the cytoplasm; this is the endoplasmic reticulum. The tubules and vesicles interconnect with one another. Also, their walls are constructed of lipid bilayer membranes that contain large amounts of proteins, similar to the cell membrane. The total surface area of this structure in some cells—the liver cells, for instance—can be as much as 30 to 40 times the cell membrane area.
The detailed structure of a small portion of endoplasmic reticulum is shown in Figure 2-4 . The space inside the tubules and vesicles is filled with endoplasmic matrix, a watery medium that is different from the fluid in the cytosol outside the endoplasmic reticulum. Electron micrographs show that the space inside the endoplasmic reticulum is connected with the space between the two membrane surfaces of the nuclear membrane.

Figure 2-4 Structure of the endoplasmic reticulum.
(Modified from DeRobertis EDP, Saez FA, DeRobertis EMF: Cell Biology, 6th ed. Philadelphia: WB Saunders, 1975.)
Substances formed in some parts of the cell enter the space of the endoplasmic reticulum and are then conducted to other parts of the cell. Also, the vast surface area of this reticulum and the multiple enzyme systems attached to its membranes provide machinery for a major share of the metabolic functions of the cell.

Ribosomes and the Granular Endoplasmic Reticulum
Attached to the outer surfaces of many parts of the endoplasmic reticulum are large numbers of minute granular particles called ribosomes. Where these are present, the reticulum is called the granular endoplasmic reticulum. The ribosomes are composed of a mixture of RNA and proteins, and they function to synthesize new protein molecules in the cell, as discussed later in this chapter and in Chapter 3 .

Agranular Endoplasmic Reticulum
Part of the endoplasmic reticulum has no attached ribosomes. This part is called the agranular, or smooth, endoplasmic reticulum. The agranular reticulum functions for the synthesis of lipid substances and for other processes of the cells promoted by intrareticular enzymes.

Golgi Apparatus
The Golgi apparatus, shown in Figure 2-5 , is closely related to the endoplasmic reticulum. It has membranes similar to those of the agranular endoplasmic reticulum. It is usually composed of four or more stacked layers of thin, flat, enclosed vesicles lying near one side of the nucleus. This apparatus is prominent in secretory cells, where it is located on the side of the cell from which the secretory substances are extruded.

Figure 2-5 A typical Golgi apparatus and its relationship to the endoplasmic reticulum (ER) and the nucleus.
The Golgi apparatus functions in association with the endoplasmic reticulum. As shown in Figure 2-5 , small “transport vesicles” (also called endoplasmic reticulum vesicles, or ER vesicles ) continually pinch off from the endoplasmic reticulum and shortly thereafter fuse with the Golgi apparatus. In this way, substances entrapped in the ER vesicles are transported from the endoplasmic reticulum to the Golgi apparatus. The transported substances are then processed in the Golgi apparatus to form lysosomes, secretory vesicles, and other cytoplasmic components that are discussed later in the chapter.

Lysosomes, shown in Figure 2-2 , are vesicular organelles that form by breaking off from the Golgi apparatus and then dispersing throughout the cytoplasm. The lysosomes provide an intracellular digestive system that allows the cell to digest (1) damaged cellular structures, (2) food particles that have been ingested by the cell, and (3) unwanted matter such as bacteria. The lysosome is quite different in different cell types, but it is usually 250 to 750 nanometers in diameter. It is surrounded by a typical lipid bilayer membrane and is filled with large numbers of small granules 5 to 8 nanometers in diameter, which are protein aggregates of as many as 40 different hydrolase (digestive) enzymes. A hydrolytic enzyme is capable of splitting an organic compound into two or more parts by combining hydrogen from a water molecule with one part of the compound and combining the hydroxyl portion of the water molecule with the other part of the compound. For instance, protein is hydrolyzed to form amino acids, glycogen is hydrolyzed to form glucose, and lipids are hydrolyzed to form fatty acids and glycerol.
Ordinarily, the membrane surrounding the lysosome prevents the enclosed hydrolytic enzymes from coming in contact with other substances in the cell and, therefore, prevents their digestive actions. However, some conditions of the cell break the membranes of some of the lysosomes, allowing release of the digestive enzymes. These enzymes then split the organic substances with which they come in contact into small, highly diffusible substances such as amino acids and glucose. Some of the specific functions of lysosomes are discussed later in the chapter.

Peroxisomes are similar physically to lysosomes, but they are different in two important ways. First, they are believed to be formed by self-replication (or perhaps by budding off from the smooth endoplasmic reticulum) rather than from the Golgi apparatus. Second, they contain oxidases rather than hydrolases. Several of the oxidases are capable of combining oxygen with hydrogen ions derived from different intracellular chemicals to form hydrogen peroxide (H 2 O 2 ). Hydrogen peroxide is a highly oxidizing substance and is used in association with catalase, another oxidase enzyme present in large quantities in peroxisomes, to oxidize many substances that might otherwise be poisonous to the cell. For instance, about half the alcohol a person drinks is detoxified by the peroxisomes of the liver cells in this manner.

Secretory Vesicles
One of the important functions of many cells is secretion of special chemical substances. Almost all such secretory substances are formed by the endoplasmic reticulum–Golgi apparatus system and are then released from the Golgi apparatus into the cytoplasm in the form of storage vesicles called secretory vesicles or secretory granules. Figure 2-6 shows typical secretory vesicles inside pancreatic acinar cells; these vesicles store protein proenzymes (enzymes that are not yet activated). The proenzymes are secreted later through the outer cell membrane into the pancreatic duct and thence into the duodenum, where they become activated and perform digestive functions on the food in the intestinal tract.

Figure 2-6 Secretory granules (secretory vesicles) in acinar cells of the pancreas.

The mitochondria, shown in Figures 2-2 and 2-7 , are called the “powerhouses” of the cell. Without them, cells would be unable to extract enough energy from the nutrients, and essentially all cellular functions would cease.

Figure 2-7 Structure of a mitochondrion.
(Modified from DeRobertis EDP, Saez FA, DeRobertis EMF: Cell Biology, 6th ed. Philadelphia: WB Saunders, 1975.)
Mitochondria are present in all areas of each cell’s cytoplasm, but the total number per cell varies from less than a hundred up to several thousand, depending on the amount of energy required by the cell. Further, the mitochondria are concentrated in those portions of the cell that are responsible for the major share of its energy metabolism. They are also variable in size and shape. Some are only a few hundred nanometers in diameter and globular in shape, whereas others are elongated—as large as 1 micrometer in diameter and 7 micrometers long; still others are branching and filamentous.
The basic structure of the mitochondrion, shown in Figure 2-7 , is composed mainly of two lipid bilayer–protein membranes: an outer membrane and an inner membrane. Many infoldings of the inner membrane form shelves onto which oxidative enzymes are attached. In addition, the inner cavity of the mitochondrion is filled with a matrix that contains large quantities of dissolved enzymes that are necessary for extracting energy from nutrients. These enzymes operate in association with the oxidative enzymes on the shelves to cause oxidation of the nutrients, thereby forming carbon dioxide and water and at the same time releasing energy. The liberated energy is used to synthesize a “high-energy” substance called adenosine triphosphate (ATP). ATP is then transported out of the mitochondrion, and it diffuses throughout the cell to release its own energy wherever it is needed for performing cellular functions. The chemical details of ATP formation by the mitochondrion are given in Chapter 67 , but some of the basic functions of ATP in the cell are introduced later in this chapter.
Mitochondria are self-replicative, which means that one mitochondrion can form a second one, a third one, and so on, whenever there is a need in the cell for increased amounts of ATP. Indeed, the mitochondria contain DNA similar to that found in the cell nucleus. In Chapter 3 we will see that DNA is the basic chemical of the nucleus that controls replication of the cell. The DNA of the mitochondrion plays a similar role, controlling replication of the mitochondrion.

Cell Cytoskeleton—Filament and Tubular Structures
The fibrillar proteins of the cell are usually organized into filaments or tubules. These originate as precursor protein molecules synthesized by ribosomes in the cytoplasm. The precursor molecules then polymerize to form filaments. As an example, large numbers of actin filaments frequently occur in the outer zone of the cytoplasm, called the ectoplasm, to form an elastic support for the cell membrane. Also, in muscle cells, actin and myosin filaments are organized into a special contractile machine that is the basis for muscle contraction, as discussed in detail in Chapter 6 .
A special type of stiff filament composed of polymerized tubulin molecules is used in all cells to construct strong tubular structures, the microtubules. Figure 2-8 shows typical microtubules that were teased from the flagellum of a sperm.

Figure 2-8 Microtubules teased from the flagellum of a sperm.
(From Wolstenholme GEW, O’Connor M, and the publisher, JA Churchill, 1967. Figure 4, page 314. Copyright the Novartis Foundation, formerly the Ciba Foundation.)
Another example of microtubules is the tubular skeletal structure in the center of each cilium that radiates upward from the cell cytoplasm to the tip of the cilium. This structure is discussed later in the chapter and is illustrated in Figure 2-17 . Also, both the centrioles and the mitotic spindle of the mitosing cell are composed of stiff microtubules.

Figure 2-17 Structure and function of the cilium.
(Modified from Satir P: Cilia. Sci Am 204:108, 1961. Copyright Donald Garber: Executor of the estate of Bunji Tagawa.)
Thus, a primary function of microtubules is to act as a cytoskeleton, providing rigid physical structures for certain parts of cells.

The nucleus is the control center of the cell. Briefly, the nucleus contains large quantities of DNA, which are the genes. The genes determine the characteristics of the cell’s proteins, including the structural proteins, as well as the intracellular enzymes that control cytoplasmic and nuclear activities.
The genes also control and promote reproduction of the cell itself. The genes first reproduce to give two identical sets of genes; then the cell splits by a special process called mitosis to form two daughter cells, each of which receives one of the two sets of DNA genes. All these activities of the nucleus are considered in detail in the next chapter.
Unfortunately, the appearance of the nucleus under the microscope does not provide many clues to the mechanisms by which the nucleus performs its control activities. Figure 2-9 shows the light microscopic appearance of the interphase nucleus (during the period between mitoses), revealing darkly staining chromatin material throughout the nucleoplasm. During mitosis, the chromatin material organizes in the form of highly structured chromosomes, which can then be easily identified using the light microscope, as illustrated in the next chapter.

Figure 2-9 Structure of the nucleus.

Nuclear Membrane
The nuclear membrane, also called the nuclear envelope, is actually two separate bilayer membranes, one inside the other. The outer membrane is continuous with the endoplasmic reticulum of the cell cytoplasm, and the space between the two nuclear membranes is also continuous with the space inside the endoplasmic reticulum, as shown in Figure 2-9 .
The nuclear membrane is penetrated by several thousand nuclear pores. Large complexes of protein molecules are attached at the edges of the pores so that the central area of each pore is only about 9 nanometers in diameter. Even this size is large enough to allow molecules up to 44,000 molecular weight to pass through with reasonable ease.

Nucleoli and Formation of Ribosomes
The nuclei of most cells contain one or more highly staining structures called nucleoli. The nucleolus, unlike most other organelles discussed here, does not have a limiting membrane. Instead, it is simply an accumulation of large amounts of RNA and proteins of the types found in ribosomes. The nucleolus becomes considerably enlarged when the cell is actively synthesizing proteins.
Formation of the nucleoli (and of the ribosomes in the cytoplasm outside the nucleus) begins in the nucleus. First, specific DNA genes in the chromosomes cause RNA to be synthesized. Some of this is stored in the nucleoli, but most of it is transported outward through the nuclear pores into cytoplasm. Here, it is used in conjunction with specific proteins to assemble “mature” ribosomes that play an essential role in forming cytoplasmic proteins, as discussed more fully in Chapter 3 .

Comparison of the Animal Cell with Precellular Forms of Life
The cell is a complicated organism that required many hundreds of millions of years to develop after the earliest form of life, an organism similar to the present-day virus, first appeared on earth. Figure 2-10 shows the relative sizes of (1) the smallest known virus, (2) a large virus, (3) a rickettsia, (4) a bacterium, and (5) a nucleated cell, demonstrating that the cell has a diameter about 1000 times that of the smallest virus and, therefore, a volume about 1 billion times that of the smallest virus. Correspondingly, the functions and anatomical organization of the cell are also far more complex than those of the virus.

Figure 2-10 Comparison of sizes of precellular organisms with that of the average cell in the human body.
The essential life-giving constituent of the small virus is a nucleic acid embedded in a coat of protein. This nucleic acid is composed of the same basic nucleic acid constituents (DNA or RNA) found in mammalian cells, and it is capable of reproducing itself under appropriate conditions. Thus, the virus propagates its lineage from generation to generation and is therefore a living structure in the same way that the cell and the human being are living structures.
As life evolved, other chemicals besides nucleic acid and simple proteins became integral parts of the organism, and specialized functions began to develop in different parts of the virus. A membrane formed around the virus, and inside the membrane, a fluid matrix appeared. Specialized chemicals then developed inside the fluid to perform special functions; many protein enzymes appeared that were capable of catalyzing chemical reactions and, therefore, determining the organism’s activities.
In still later stages of life, particularly in the rickettsial and bacterial stages, organelles developed inside the organism, representing physical structures of chemical aggregates that perform functions in a more efficient manner than can be achieved by dispersed chemicals throughout the fluid matrix.
Finally, in the nucleated cell, still more complex organelles developed, the most important of which is the nucleus itself. The nucleus distinguishes this type of cell from all lower forms of life; the nucleus provides a control center for all cellular activities, and it provides for exact reproduction of new cells generation after generation, each new cell having almost exactly the same structure as its progenitor.

Functional Systems of the Cell
In the remainder of this chapter, we discuss several representative functional systems of the cell that make it a living organism.

Ingestion by the Cell—Endocytosis
If a cell is to live and grow and reproduce, it must obtain nutrients and other substances from the surrounding fluids. Most substances pass through the cell membrane by diffusion and active transport.
Diffusion involves simple movement through the membrane caused by the random motion of the molecules of the substance; substances move either through cell membrane pores or, in the case of lipid-soluble substances, through the lipid matrix of the membrane.
Active transport involves the actual carrying of a substance through the membrane by a physical protein structure that penetrates all the way through the membrane. These active transport mechanisms are so important to cell function that they are presented in detail in Chapter 4 .
Very large particles enter the cell by a specialized function of the cell membrane called endocytosis. The principal forms of endocytosis are pinocytosis and phagocytosis. Pinocytosis means ingestion of minute particles that form vesicles of extracellular fluid and particulate constituents inside the cell cytoplasm. Phagocytosis means ingestion of large particles, such as bacteria, whole cells, or portions of degenerating tissue.

Pinocytosis occurs continually in the cell membranes of most cells, but it is especially rapid in some cells. For instance, it occurs so rapidly in macrophages that about 3 percent of the total macrophage membrane is engulfed in the form of vesicles each minute. Even so, the pinocytotic vesicles are so small—usually only 100 to 200 nanometers in diameter—that most of them can be seen only with the electron microscope.
Pinocytosis is the only means by which most large macromolecules, such as most protein molecules, can enter cells. In fact, the rate at which pinocytotic vesicles form is usually enhanced when such macromolecules attach to the cell membrane.
Figure 2-11 demonstrates the successive steps of pinocytosis, showing three molecules of protein attaching to the membrane. These molecules usually attach to specialized protein receptors on the surface of the membrane that are specific for the type of protein that is to be absorbed. The receptors generally are concentrated in small pits on the outer surface of the cell membrane, called coated pits. On the inside of the cell membrane beneath these pits is a latticework of fibrillar protein called clathrin, as well as other proteins, perhaps including contractile filaments of actin and myosin. Once the protein molecules have bound with the receptors, the surface properties of the local membrane change in such a way that the entire pit invaginates inward and the fibrillar proteins surrounding the invaginating pit cause its borders to close over the attached proteins, as well as over a small amount of extracellular fluid. Immediately thereafter, the invaginated portion of the membrane breaks away from the surface of the cell, forming a pinocytotic vesicle inside the cytoplasm of the cell.

Figure 2-11 Mechanism of pinocytosis.
What causes the cell membrane to go through the necessary contortions to form pinocytotic vesicles is still unclear. This process requires energy from within the cell; this is supplied by ATP, a high-energy substance discussed later in the chapter. Also, it requires the presence of calcium ions in the extracellular fluid, which probably react with contractile protein filaments beneath the coated pits to provide the force for pinching the vesicles away from the cell membrane.

Phagocytosis occurs in much the same way as pinocytosis, except that it involves large particles rather than molecules. Only certain cells have the capability of phagocytosis, most notably the tissue macrophages and some of the white blood cells.
Phagocytosis is initiated when a particle such as a bacterium, a dead cell, or tissue debris binds with receptors on the surface of the phagocyte. In the case of bacteria, each bacterium is usually already attached to a specific antibody, and it is the antibody that attaches to the phagocyte receptors, dragging the bacterium along with it. This intermediation of antibodies is called opsonization, which is discussed in Chapters 33 and 34 .
Phagocytosis occurs in the following steps:
1. The cell membrane receptors attach to the surface ligands of the particle.
2. The edges of the membrane around the points of attachment evaginate outward within a fraction of a second to surround the entire particle; then, progressively more and more membrane receptors attach to the particle ligands. All this occurs suddenly in a zipper-like manner to form a closed phagocytic vesicle.
3. Actin and other contractile fibrils in the cytoplasm surround the phagocytic vesicle and contract around its outer edge, pushing the vesicle to the interior.
4. The contractile proteins then pinch the stem of the vesicle so completely that the vesicle separates from the cell membrane, leaving the vesicle in the cell interior in the same way that pinocytotic vesicles are formed.

Digestion of Pinocytotic and Phagocytic Foreign Substances Inside the Cell—Function of the Lysosomes
Almost immediately after a pinocytotic or phagocytic vesicle appears inside a cell, one or more lysosomes become attached to the vesicle and empty their acid hydrolases to the inside of the vesicle, as shown in Figure 2-12 . Thus, a digestive vesicle is formed inside the cell cytoplasm in which the vesicular hydrolases begin hydrolyzing the proteins, carbohydrates, lipids, and other substances in the vesicle. The products of digestion are small molecules of amino acids, glucose, phosphates, and so forth that can diffuse through the membrane of the vesicle into the cytoplasm. What is left of the digestive vesicle, called the residual body, represents indigestible substances. In most instances, this is finally excreted through the cell membrane by a process called exocytosis, which is essentially the opposite of endocytosis.

Figure 2-12 Digestion of substances in pinocytotic or phagocytic vesicles by enzymes derived from lysosomes.
Thus, the pinocytotic and phagocytic vesicles containing lysosomes can be called the digestive organs of the cells.

Regression of Tissues and Autolysis of Cells
Tissues of the body often regress to a smaller size. For instance, this occurs in the uterus after pregnancy, in muscles during long periods of inactivity, and in mammary glands at the end of lactation. Lysosomes are responsible for much of this regression. The mechanism by which lack of activity in a tissue causes the lysosomes to increase their activity is unknown.
Another special role of the lysosomes is removal of damaged cells or damaged portions of cells from tissues. Damage to the cell—caused by heat, cold, trauma, chemicals, or any other factor—induces lysosomes to rupture. The released hydrolases immediately begin to digest the surrounding organic substances. If the damage is slight, only a portion of the cell is removed and the cell is then repaired. If the damage is severe, the entire cell is digested, a process called autolysis. In this way, the cell is completely removed and a new cell of the same type ordinarily is formed by mitotic reproduction of an adjacent cell to take the place of the old one.
The lysosomes also contain bactericidal agents that can kill phagocytized bacteria before they can cause cellular damage. These agents include (1) lysozyme, which dissolves the bacterial cell membrane; (2) lysoferrin, which binds iron and other substances before they can promote bacterial growth; and (3) acid at a pH of about 5.0, which activates the hydrolases and inactivates bacterial metabolic systems.

Synthesis and Formation of Cellular Structures by Endoplasmic Reticulum and Golgi Apparatus

Specific Functions of the Endoplasmic Reticulum
The extensiveness of the endoplasmic reticulum and the Golgi apparatus in secretory cells has already been emphasized. These structures are formed primarily of lipid bilayer membranes similar to the cell membrane, and their walls are loaded with protein enzymes that catalyze the synthesis of many substances required by the cell.
Most synthesis begins in the endoplasmic reticulum. The products formed there are then passed on to the Golgi apparatus, where they are further processed before being released into the cytoplasm. But first, let us note the specific products that are synthesized in specific portions of the endoplasmic reticulum and the Golgi apparatus.

Proteins Are Formed by the Granular Endoplasmic Reticulum
The granular portion of the endoplasmic reticulum is characterized by large numbers of ribosomes attached to the outer surfaces of the endoplasmic reticulum membrane. As discussed in Chapter 3 , protein molecules are synthesized within the structures of the ribosomes. The ribosomes extrude some of the synthesized protein molecules directly into the cytosol, but they also extrude many more through the wall of the endoplasmic reticulum to the interior of the endoplasmic vesicles and tubules, into the endoplasmic matrix.

Synthesis of Lipids by the Smooth Endoplasmic Reticulum
The endoplasmic reticulum also synthesizes lipids, especially phospholipids and cholesterol. These are rapidly incorporated into the lipid bilayer of the endoplasmic reticulum itself, thus causing the endoplasmic reticulum to grow more extensive. This occurs mainly in the smooth portion of the endoplasmic reticulum.
To keep the endoplasmic reticulum from growing beyond the needs of the cell, small vesicles called ER vesicles or transport vesicles continually break away from the smooth reticulum; most of these vesicles then migrate rapidly to the Golgi apparatus.

Other Functions of the Endoplasmic Reticulum
Other significant functions of the endoplasmic reticulum, especially the smooth reticulum, include the following:
1. It provides the enzymes that control glycogen breakdown when glycogen is to be used for energy.
2. It provides a vast number of enzymes that are capable of detoxifying substances, such as drugs, that might damage the cell. It achieves detoxification by coagulation, oxidation, hydrolysis, conjugation with glycuronic acid, and in other ways.

Specific Functions of the Golgi Apparatus

Synthetic Functions of the Golgi Apparatus
Although the major function of the Golgi apparatus is to provide additional processing of substances already formed in the endoplasmic reticulum, it also has the capability of synthesizing certain carbohydrates that cannot be formed in the endoplasmic reticulum. This is especially true for the formation of large saccharide polymers bound with small amounts of protein; important examples include hyaluronic acid and chondroitin sulfate.
A few of the many functions of hyaluronic acid and chondroitin sulfate in the body are as follows: (1) they are the major components of proteoglycans secreted in mucus and other glandular secretions; (2) they are the major components of the ground substance outside the cells in the interstitial spaces, acting as fillers between collagen fibers and cells; (3) they are principal components of the organic matrix in both cartilage and bone; and (4) they are important in many cell activities including migration and proliferation.

Processing of Endoplasmic Secretions by the Golgi Apparatus—Formation of Vesicles
Figure 2-13 summarizes the major functions of the endoplasmic reticulum and Golgi apparatus. As substances are formed in the endoplasmic reticulum, especially the proteins, they are transported through the tubules toward portions of the smooth endoplasmic reticulum that lie nearest the Golgi apparatus. At this point, small transport vesicles composed of small envelopes of smooth endoplasmic reticulum continually break away and diffuse to the deepest layer of the Golgi apparatus. Inside these vesicles are the synthesized proteins and other products from the endoplasmic reticulum.

Figure 2-13 Formation of proteins, lipids, and cellular vesicles by the endoplasmic reticulum and Golgi apparatus.
The transport vesicles instantly fuse with the Golgi apparatus and empty their contained substances into the vesicular spaces of the Golgi apparatus. Here, additional carbohydrate moieties are added to the secretions. Also, an important function of the Golgi apparatus is to compact the endoplasmic reticular secretions into highly concentrated packets. As the secretions pass toward the outermost layers of the Golgi apparatus, the compaction and processing proceed. Finally, both small and large vesicles continually break away from the Golgi apparatus, carrying with them the compacted secretory substances, and in turn, the vesicles diffuse throughout the cell.
To give an idea of the timing of these processes: When a glandular cell is bathed in radioactive amino acids, newly formed radioactive protein molecules can be detected in the granular endoplasmic reticulum within 3 to 5 minutes. Within 20 minutes, newly formed proteins are already present in the Golgi apparatus, and within 1 to 2 hours, radioactive proteins are secreted from the surface of the cell.

Types of Vesicles Formed by the Golgi Apparatus—Secretory Vesicles and Lysosomes
In a highly secretory cell, the vesicles formed by the Golgi apparatus are mainly secretory vesicles containing protein substances that are to be secreted through the surface of the cell membrane. These secretory vesicles first diffuse to the cell membrane, then fuse with it and empty their substances to the exterior by the mechanism called exocytosis. Exocytosis, in most cases, is stimulated by the entry of calcium ions into the cell; calcium ions interact with the vesicular membrane in some way that is not understood and cause its fusion with the cell membrane, followed by exocytosis—that is, opening of the membrane’s outer surface and extrusion of its contents outside the cell.
Some vesicles, however, are destined for intracellular use.

Use of Intracellular Vesicles to Replenish Cellular Membranes
Some of the intracellular vesicles formed by the Golgi apparatus fuse with the cell membrane or with the membranes of intracellular structures such as the mitochondria and even the endoplasmic reticulum. This increases the expanse of these membranes and thereby replenishes the membranes as they are used up. For instance, the cell membrane loses much of its substance every time it forms a phagocytic or pinocytotic vesicle, and the vesicular membranes of the Golgi apparatus continually replenish the cell membrane.
In summary, the membranous system of the endoplasmic reticulum and Golgi apparatus represents a highly metabolic organ capable of forming new intracellular structures, as well as secretory substances to be extruded from the cell.

Extraction of Energy from Nutrients—Function of the Mitochondria
The principal substances from which cells extract energy are foodstuffs that react chemically with oxygen—carbohydrates, fats, and proteins. In the human body, essentially all carbohydrates are converted into glucose by the digestive tract and liver before they reach the other cells of the body. Similarly, proteins are converted into amino acids and fats into fatty acids. Figure 2-14 shows oxygen and the foodstuffs—glucose, fatty acids, and amino acids—all entering the cell. Inside the cell, the foodstuffs react chemically with oxygen, under the influence of enzymes that control the reactions and channel the energy released in the proper direction. The details of all these digestive and metabolic functions are given in Chapters 62 through 72 .

Figure 2-14 Formation of adenosine triphosphate (ATP) in the cell, showing that most of the ATP is formed in the mitochondria. ADP, adenosine diphosphate.
Briefly, almost all these oxidative reactions occur inside the mitochondria and the energy that is released is used to form the high-energy compound ATP. Then, ATP, not the original foodstuffs, is used throughout the cell to energize almost all the subsequent intracellular metabolic reactions.

Functional Characteristics of ATP

ATP is a nucleotide composed of (1) the nitrogenous base adenine, (2) the pentose sugar ribose, and (3) three phosphate radicals. The last two phosphate radicals are connected with the remainder of the molecule by so-called high-energy phosphate bonds, which are represented in the formula shown by the symbol ∼. Under the physical and chemical conditions of the body, each of these high-energy bonds contains about 12,000 calories of energy per mole of ATP, which is many times greater than the energy stored in the average chemical bond, thus giving rise to the term high-energy bond. Further, the high-energy phosphate bond is very labile so that it can be split instantly on demand whenever energy is required to promote other intracellular reactions.
When ATP releases its energy, a phosphoric acid radical is split away and adenosine diphosphate (ADP) is formed. This released energy is used to energize virtually many of the cell’s other functions, such as synthesis of substances and muscular contraction.
To reconstitute the cellular ATP as it is used up, energy derived from the cellular nutrients causes ADP and phosphoric acid to recombine to form new ATP, and the entire process repeats over and over again. For these reasons, ATP has been called the energy currency of the cell because it can be spent and remade continually, having a turnover time of only a few minutes.

Chemical Processes in the Formation of ATP—Role of the Mitochondria
On entry into the cells, glucose is subjected to enzymes in the cytoplasm that convert it into pyruvic acid (a process called glycolysis ). A small amount of ADP is changed into ATP by the energy released during this conversion, but this amount accounts for less than 5 percent of the overall energy metabolism of the cell.
About 95 percent of the cell’s ATP formation occurs in the mitochondria. The pyruvic acid derived from carbohydrates, fatty acids from lipids, and amino acids from proteins is eventually converted into the compound acetyl-CoA in the matrix of the mitochondrion. This substance, in turn, is further dissoluted (for the purpose of extracting its energy) by another series of enzymes in the mitochondrion matrix, undergoing dissolution in a sequence of chemical reactions called the citric acid cycle, or Krebs cycle. These chemical reactions are so important that they are explained in detail in Chapter 67 .
In this citric acid cycle, acetyl-CoA is split into its component parts, hydrogen atoms and carbon dioxide. The carbon dioxide diffuses out of the mitochondria and eventually out of the cell; finally, it is excreted from the body through the lungs.
The hydrogen atoms, conversely, are highly reactive, and they combine instantly with oxygen that has also diffused into the mitochondria. This releases a tremendous amount of energy, which is used by the mitochondria to convert large amounts of ADP to ATP. The processes of these reactions are complex, requiring the participation of many protein enzymes that are integral parts of mitochondrial membranous shelves that protrude into the mitochondrial matrix. The initial event is removal of an electron from the hydrogen atom, thus converting it to a hydrogen ion. The terminal event is combination of hydrogen ions with oxygen to form water plus the release of tremendous amounts of energy to large globular proteins, called ATP synthetase, that protrude like knobs from the membranes of the mitochondrial shelves. Finally, the enzyme ATP synthetase uses the energy from the hydrogen ions to cause the conversion of ADP to ATP. The newly formed ATP is transported out of the mitochondria into all parts of the cell cytoplasm and nucleoplasm, where its energy is used to energize multiple cell functions.
This overall process for formation of ATP is called the chemiosmotic mechanism of ATP formation. The chemical and physical details of this mechanism are presented in Chapter 67 , and many of the detailed metabolic functions of ATP in the body are presented in Chapters 67 through 71 .

Uses of ATP for Cellular Function
Energy from ATP is used to promote three major categories of cellular functions: (1) transport of substances through multiple membranes in the cell, (2) synthesis of chemical compounds throughout the cell, and (3) mechanical work. These uses of ATP are illustrated by examples in Figure 2-15 : (1) to supply energy for the transport of sodium through the cell membrane, (2) to promote protein synthesis by the ribosomes, and (3) to supply the energy needed during muscle contraction.

Figure 2-15 Use of adenosine triphosphate (ATP) (formed in the mitochondrion) to provide energy for three major cellular functions: membrane transport, protein synthesis, and muscle contraction. ADP, adenosine diphosphate.
In addition to membrane transport of sodium, energy from ATP is required for membrane transport of potassium ions, calcium ions, magnesium ions, phosphate ions, chloride ions, urate ions, hydrogen ions, and many other ions and various organic substances. Membrane transport is so important to cell function that some cells—the renal tubular cells, for instance—use as much as 80 percent of the ATP that they form for this purpose alone.
In addition to synthesizing proteins, cells make phospholipids, cholesterol, purines, pyrimidines, and a host of other substances. Synthesis of almost any chemical compound requires energy. For instance, a single protein molecule might be composed of as many as several thousand amino acids attached to one another by peptide linkages; the formation of each of these linkages requires energy derived from the breakdown of four high-energy bonds; thus, many thousand ATP molecules must release their energy as each protein molecule is formed. Indeed, some cells use as much as 75 percent of all the ATP formed in the cell simply to synthesize new chemical compounds, especially protein molecules; this is particularly true during the growth phase of cells.
The final major use of ATP is to supply energy for special cells to perform mechanical work. We see in Chapter 6 that each contraction of a muscle fiber requires expenditure of tremendous quantities of ATP energy. Other cells perform mechanical work in other ways, especially by ciliary and ameboid motion, described later in this chapter. The source of energy for all these types of mechanical work is ATP.
In summary, ATP is always available to release its energy rapidly and almost explosively wherever in the cell it is needed. To replace the ATP used by the cell, much slower chemical reactions break down carbohydrates, fats, and proteins and use the energy derived from these to form new ATP. More than 95 percent of this ATP is formed in the mitochondria, which accounts for the mitochondria being called the “powerhouses” of the cell.

Locomotion of Cells
By far the most important type of movement that occurs in the body is that of the muscle cells in skeletal, cardiac, and smooth muscle, which constitute almost 50 percent of the entire body mass. The specialized functions of these cells are discussed in Chapters 6 through 9 . Two other types of movement— ameboid locomotion and ciliary movement —occur in other cells.

Ameboid Movement
Ameboid movement is movement of an entire cell in relation to its surroundings, such as movement of white blood cells through tissues. It receives its name from the fact that amebae move in this manner and have provided an excellent tool for studying the phenomenon.
Typically, ameboid locomotion begins with protrusion of a pseudopodium from one end of the cell. The pseudopodium projects far out, away from the cell body, and partially secures itself in a new tissue area. Then the remainder of the cell is pulled toward the pseudopodium. Figure 2-16 demonstrates this process, showing an elongated cell, the right-hand end of which is a protruding pseudopodium. The membrane of this end of the cell is continually moving forward, and the membrane at the left-hand end of the cell is continually following along as the cell moves.

Figure 2-16 Ameboid motion by a cell.

Mechanism of Ameboid Locomotion
Figure 2-16 shows the general principle of ameboid motion. Basically, it results from continual formation of new cell membrane at the leading edge of the pseudopodium and continual absorption of the membrane in mid and rear portions of the cell. Also, two other effects are essential for forward movement of the cell. The first effect is attachment of the pseudopodium to surrounding tissues so that it becomes fixed in its leading position, while the remainder of the cell body is pulled forward toward the point of attachment. This attachment is effected by receptor proteins that line the insides of exocytotic vesicles. When the vesicles become part of the pseudopodial membrane, they open so that their insides evert to the outside, and the receptors now protrude to the outside and attach to ligands in the surrounding tissues.
At the opposite end of the cell, the receptors pull away from their ligands and form new endocytotic vesicles. Then, inside the cell, these vesicles stream toward the pseudopodial end of the cell, where they are used to form still new membrane for the pseudopodium.
The second essential effect for locomotion is to provide the energy required to pull the cell body in the direction of the pseudopodium. Experiments suggest the following as an explanation: In the cytoplasm of all cells is a moderate to large amount of the protein actin. Much of the actin is in the form of single molecules that do not provide any motive power; however, these polymerize to form a filamentous network, and the network contracts when it binds with an actin-binding protein such as myosin. The whole process is energized by the high-energy compound ATP. This is what happens in the pseudopodium of a moving cell, where such a network of actin filaments forms anew inside the enlarging pseudopodium. Contraction also occurs in the ectoplasm of the cell body, where a preexisting actin network is already present beneath the cell membrane.

Types of Cells That Exhibit Ameboid Locomotion
The most common cells to exhibit ameboid locomotion in the human body are the white blood cells when they move out of the blood into the tissues to form tissue macrophages. Other types of cells can also move by ameboid locomotion under certain circumstances. For instance, fibroblasts move into a damaged area to help repair the damage and even the germinal cells of the skin, though ordinarily completely sessile cells, move toward a cut area to repair the opening. Finally, cell locomotion is especially important in development of the embryo and fetus after fertilization of an ovum. For instance, embryonic cells often must migrate long distances from their sites of origin to new areas during development of special structures.

Control of Ameboid Locomotion—Chemotaxis
The most important initiator of ameboid locomotion is the process called chemotaxis. This results from the appearance of certain chemical substances in the tissues. Any chemical substance that causes chemotaxis to occur is called a chemotactic substance. Most cells that exhibit ameboid locomotion move toward the source of a chemotactic substance—that is, from an area of lower concentration toward an area of higher concentration—which is called positive chemotaxis. Some cells move away from the source, which is called negative chemotaxis.
But how does chemotaxis control the direction of ameboid locomotion? Although the answer is not certain, it is known that the side of the cell most exposed to the chemotactic substance develops membrane changes that cause pseudopodial protrusion.

Cilia and Ciliary Movements
A second type of cellular motion, ciliary movement, is a whiplike movement of cilia on the surfaces of cells. This occurs in only two places in the human body: on the surfaces of the respiratory airways and on the inside surfaces of the uterine tubes (fallopian tubes) of the reproductive tract. In the nasal cavity and lower respiratory airways, the whiplike motion of cilia causes a layer of mucus to move at a rate of about 1 cm/min toward the pharynx, in this way continually clearing these passageways of mucus and particles that have become trapped in the mucus. In the uterine tubes, the cilia cause slow movement of fluid from the ostium of the uterine tube toward the uterus cavity; this movement of fluid transports the ovum from the ovary to the uterus.
As shown in Figure 2-17 , a cilium has the appearance of a sharp-pointed straight or curved hair that projects 2 to 4 micrometers from the surface of the cell. Many cilia often project from a single cell—for instance, as many as 200 cilia on the surface of each epithelial cell inside the respiratory passageways. The cilium is covered by an outcropping of the cell membrane, and it is supported by 11 microtubules—9 double tubules located around the periphery of the cilium and 2 single tubules down the center, as demonstrated in the cross section shown in Figure 2-17 . Each cilium is an outgrowth of a structure that lies immediately beneath the cell membrane, called the basal body of the cilium.
The flagellum of a sperm is similar to a cilium; in fact, it has much the same type of structure and same type of contractile mechanism. The flagellum, however, is much longer and moves in quasi-sinusoidal waves instead of whiplike movements.
In the inset of Figure 2-17 , movement of the cilium is shown. The cilium moves forward with a sudden, rapid whiplike stroke 10 to 20 times per second, bending sharply where it projects from the surface of the cell. Then it moves backward slowly to its initial position. The rapid forward-thrusting, whiplike movement pushes the fluid lying adjacent to the cell in the direction that the cilium moves; the slow, dragging movement in the backward direction has almost no effect on fluid movement. As a result, the fluid is continually propelled in the direction of the fast-forward stroke. Because most ciliated cells have large numbers of cilia on their surfaces and because all the cilia are oriented in the same direction, this is an effective means for moving fluids from one part of the surface to another.

Mechanism of Ciliary Movement
Although not all aspects of ciliary movement are clear, we do know the following: First, the nine double tubules and the two single tubules are all linked to one another by a complex of protein cross-linkages; this total complex of tubules and cross-linkages is called the axoneme. Second, even after removal of the membrane and destruction of other elements of the cilium besides the axoneme, the cilium can still beat under appropriate conditions. Third, there are two necessary conditions for continued beating of the axoneme after removal of the other structures of the cilium: (1) the availability of ATP and (2) appropriate ionic conditions, especially appropriate concentrations of magnesium and calcium. Fourth, during forward motion of the cilium, the double tubules on the front edge of the cilium slide outward toward the tip of the cilium, while those on the back edge remain in place. Fifth, multiple protein arms composed of the protein dynein, which has ATPase enzymatic activity, project from each double tubule toward an adjacent double tubule.
Given this basic information, it has been determined that the release of energy from ATP in contact with the ATPase dynein arms causes the heads of these arms to “crawl” rapidly along the surface of the adjacent double tubule. If the front tubules crawl outward while the back tubules remain stationary, this will cause bending.
The way in which cilia contraction is controlled is not understood. The cilia of some genetically abnormal cells do not have the two central single tubules, and these cilia fail to beat. Therefore, it is presumed that some signal, perhaps an electrochemical signal, is transmitted along these two central tubules to activate the dynein arms.


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CHAPTER 3 Genetic Control of Protein Synthesis, Cell Function, and Cell Reproduction
Virtually everyone knows that the genes, located in the nuclei of all cells of the body, control heredity from parents to children, but most people do not realize that these same genes also control day-to-day function of all the body’s cells. The genes control cell function by determining which substances are synthesized within the cell—which structures, which enzymes, which chemicals.
Figure 3-1 shows the general schema of genetic control. Each gene, which is a nucleic acid called deoxyribonucleic acid (DNA), automatically controls the formation of another nucleic acid, ribonucleic acid (RNA); this RNA then spreads throughout the cell to control the formation of a specific protein. The entire process, from transcription of the genetic code in the nucleus to translation of the RNA code and formation or proteins in the cell cytoplasm, is often referred to as gene expression.

Figure 3-1 General schema by which the genes control cell function.
Because there are approximately 30,000 different genes in each cell, it is theoretically possible to form a large number of different cellular proteins.
Some of the cellular proteins are structural proteins, which, in association with various lipids and carbohydrates, form the structures of the various intracellular organelles discussed in Chapter 2 . However, the majority of the proteins are enzymes that catalyze the different chemical reactions in the cells. For instance, enzymes promote all the oxidative reactions that supply energy to the cell, and they promote synthesis of all the cell chemicals, such as lipids, glycogen, and adenosine triphosphate (ATP).

Genes in the Cell Nucleus
In the cell nucleus, large numbers of genes are attached end on end in extremely long double-stranded helical molecules of DNA having molecular weights measured in the billions. A very short segment of such a molecule is shown in Figure 3-2 . This molecule is composed of several simple chemical compounds bound together in a regular pattern, details of which are explained in the next few paragraphs.

Figure 3-2 The helical, double-stranded structure of the gene. The outside strands are composed of phosphoric acid and the sugar deoxyribose. The internal molecules connecting the two strands of the helix are purine and pyrimidine bases; these determine the “code” of the gene.

Basic Building Blocks of DNA
Figure 3-3 shows the basic chemical compounds involved in the formation of DNA. These include (1) phosphoric acid, (2) a sugar called deoxyribose, and (3) four nitrogenous bases (two purines, adenine and guanine, and two pyrimidines, thymine and cytosine ). The phosphoric acid and deoxyribose form the two helical strands that are the backbone of the DNA molecule, and the nitrogenous bases lie between the two strands and connect them, as illustrated in Figure 3-6 .

Figure 3-3 The basic building blocks of DNA.

The first stage in the formation of DNA is to combine one molecule of phosphoric acid, one molecule of deoxyribose, and one of the four bases to form an acidic nucleotide. Four separate nucleotides are thus formed, one for each of the four bases: deoxyadenylic, deoxythymidylic, deoxyguanylic, and deoxycytidylic acids . Figure 3-4 shows the chemical structure of deoxyadenylic acid, and Figure 3-5 shows simple symbols for the four nucleotides that form DNA.

Figure 3-4 Deoxyadenylic acid, one of the nucleotides that make up DNA.

Figure 3-5 Symbols for the four nucleotides that combine to form DNA. Each nucleotide contains phosphoric acid ( P ), deoxyribose ( D ), and one of the four nucleotide bases: A, adenine; T, thymine; G, guanine; or C, cytosine.

Organization of the Nucleotides to Form Two Strands of DNA Loosely Bound to Each Other
Figure 3-6 shows the manner in which multiple numbers of nucleotides are bound together to form two strands of DNA. The two strands are, in turn, loosely bonded with each other by weak cross-linkages, illustrated in Figure 3-6 by the central dashed lines. Note that the backbone of each DNA strand is composed of alternating phosphoric acid and deoxyribose molecules. In turn, purine and pyrimidine bases are attached to the sides of the deoxyribose molecules. Then, by means of loose hydrogen bonds (dashed lines) between the purine and pyrimidine bases, the two respective DNA strands are held together. But note the following:
1. Each purine base adenine of one strand always bonds with a pyrimidine base thymine of the other strand, and
2. Each purine base guanine always bonds with a pyrimidine base cytosine.

Figure 3-6 Arrangement of deoxyribose nucleotides in a double strand of DNA.
Thus, in Figure 3-6 , the sequence of complementary pairs of bases is CG, CG, GC, TA, CG, TA, GC, AT, and AT. Because of the looseness of the hydrogen bonds, the two strands can pull apart with ease, and they do so many times during the course of their function in the cell.
To put the DNA of Figure 3-6 into its proper physical perspective, one could merely pick up the two ends and twist them into a helix. Ten pairs of nucleotides are present in each full turn of the helix in the DNA molecule, as shown in Figure 3-2 .

Genetic Code
The importance of DNA lies in its ability to control the formation of proteins in the cell. It does this by means of a genetic code. That is, when the two strands of a DNA molecule are split apart, this exposes the purine and pyrimidine bases projecting to the side of each DNA strand, as shown by the top strand in Figure 3-7 . It is these projecting bases that form the genetic code.

Figure 3-7 Combination of ribose nucleotides with a strand of DNA to form a molecule of RNA that carries the genetic code from the gene to the cytoplasm. The RNA polymerase enzyme moves along the DNA strand and builds the RNA molecule.
The genetic code consists of successive “triplets” of bases—that is, each three successive bases is a code word. The successive triplets eventually control the sequence of amino acids in a protein molecule that is to be synthesized in the cell. Note in Figure 3-6 that the top strand of DNA, reading from left to right, has the genetic code GGC, AGA, CTT, the triplets being separated from one another by the arrows. As we follow this genetic code through Figures 3-7 and 3-8 , we see that these three respective triplets are responsible for successive placement of the three amino acids, proline, serine, and glutamic acid, in a newly formed molecule of protein.

Figure 3-8 Portion of an RNA molecule, showing three RNA “codons”—CCG, UCU, and GAA—that control attachment of the three amino acids, proline, serine, and glutamic acid, respectively, to the growing RNA chain.

The DNA Code in the Cell Nucleus Is Transferred to an RNA Code in the Cell Cytoplasm—The Process of Transcription
Because the DNA is located in the nucleus of the cell, yet most of the functions of the cell are carried out in the cytoplasm, there must be some means for the DNA genes of the nucleus to control the chemical reactions of the cytoplasm. This is achieved through the intermediary of another type of nucleic acid, RNA, the formation of which is controlled by the DNA of the nucleus. Thus, as shown in Figure 3-7 , the code is transferred to the RNA; this process is called transcription. The RNA, in turn, diffuses from the nucleus through nuclear pores into the cytoplasmic compartment, where it controls protein synthesis.

Synthesis of RNA
During synthesis of RNA, the two strands of the DNA molecule separate temporarily; one of these strands is used as a template for synthesis of an RNA molecule. The code triplets in the DNA cause formation of complementary code triplets (called codons ) in the RNA; these codons, in turn, will control the sequence of amino acids in a protein to be synthesized in the cell cytoplasm.

Basic Building Blocks of RNA
The basic building blocks of RNA are almost the same as those of DNA, except for two differences. First, the sugar deoxyribose is not used in the formation of RNA. In its place is another sugar of slightly different composition, ribose, containing an extra hydroxyl ion appended to the ribose ring structure. Second, thymine is replaced by another pyrimidine, uracil.

Formation of RNA Nucleotides
The basic building blocks of RNA form RNA nucleotides, exactly as previously described for DNA synthesis. Here again, four separate nucleotides are used in the formation of RNA. These nucleotides contain the bases adenine, guanine, cytosine, and uracil. Note that these are the same bases as in DNA, except that uracil in RNA replaces thymine in DNA.

“Activation” of the RNA Nucleotides
The next step in the synthesis of RNA is “activation” of the RNA nucleotides by an enzyme, RNA polymerase. This occurs by adding to each nucleotide two extra phosphate radicals to form triphosphates (shown in Figure 3-7 by the two RNA nucleotides to the far right during RNA chain formation). These last two phosphates are combined with the nucleotide by high-energy phosphate bonds derived from ATP in the cell.
The result of this activation process is that large quantities of ATP energy are made available to each of the nucleotides, and this energy is used to promote the chemical reactions that add each new RNA nucleotide at the end of the developing RNA chain.

Assembly of the RNA Chain from Activated Nucleotides Using the DNA Strand as a Template—The Process of “Transcription”
Assembly of the RNA molecule is accomplished in the manner shown in Figure 3-7 under the influence of an enzyme, RNA polymerase. This is a large protein enzyme that has many functional properties necessary for formation of the RNA molecule. They are as follows:
1. In the DNA strand immediately ahead of the initial gene is a sequence of nucleotides called the promoter. The RNA polymerase has an appropriate complementary structure that recognizes this promoter and becomes attached to it. This is the essential step for initiating formation of the RNA molecule.
2. After the RNA polymerase attaches to the promoter, the polymerase causes unwinding of about two turns of the DNA helix and separation of the unwound portions of the two strands.
3. Then the polymerase moves along the DNA strand, temporarily unwinding and separating the two DNA strands at each stage of its movement. As it moves along, it adds at each stage a new activated RNA nucleotide to the end of the newly forming RNA chain by the following steps:
a. First, it causes a hydrogen bond to form between the end base of the DNA strand and the base of an RNA nucleotide in the nucleoplasm.
b. Then, one at a time, the RNA polymerase breaks two of the three phosphate radicals away from each of these RNA nucleotides, liberating large amounts of energy from the broken high-energy phosphate bonds; this energy is used to cause covalent linkage of the remaining phosphate on the nucleotide with the ribose on the end of the growing RNA chain.
c. When the RNA polymerase reaches the end of the DNA gene, it encounters a new sequence of DNA nucleotides called the chain-terminating sequence; this causes the polymerase and the newly formed RNA chain to break away from the DNA strand. Then the polymerase can be used again and again to form still more new RNA chains.
d. As the new RNA strand is formed, its weak hydrogen bonds with the DNA template break away, because the DNA has a high affinity for rebonding with its own complementary DNA strand. Thus, the RNA chain is forced away from the DNA and is released into the nucleoplasm.
Thus, the code that is present in the DNA strand is eventually transmitted in complementary form to the RNA chain. The ribose nucleotide bases always combine with the deoxyribose bases in the following combinations:
DNA Base   RNA Base guanine ……………… cytosine cytosine ……………… guanine adenine ……………… uracil thymine ……………… adenine

Four Different Types of RNA
Each type of RNA plays an independent and entirely different role in protein formation:
1. Messenger RNA (mRNA), which carries the genetic code to the cytoplasm for controlling the type of protein formed.
2. Transfer RNA (tRNA), which transports activated amino acids to the ribosomes to be used in assembling the protein molecule.
3. Ribosomal RNA, which, along with about 75 different proteins, forms ribosomes, the physical and chemical structures on which protein molecules are actually assembled.
4. MicroRNA (miRNA), which are single-stranded RNA molecules of 21 to 23 nucleotides that can regulate gene transcription and translation.

Messenger RNA—The Codons
mRNA molecules are long, single RNA strands that are suspended in the cytoplasm. These molecules are composed of several hundred to several thousand RNA nucleotides in unpaired strands, and they contain codons that are exactly complementary to the code triplets of the DNA genes. Figure 3-8 shows a small segment of a molecule of messenger RNA. Its codons are CCG, UCU, and GAA. These are the codons for the amino acids proline, serine, and glutamic acid. The transcription of these codons from the DNA molecule to the RNA molecule is shown in Figure 3-7 .

RNA Codons for the Different Amino Acids
Table 3-1 gives the RNA codons for the 22 common amino acids found in protein molecules. Note that most of the amino acids are represented by more than one codon; also, one codon represents the signal “start manufacturing the protein molecule,” and three codons represent “stop manufacturing the protein molecule.” In Table 3-1 , these two types of codons are designated CI for “chain-initiating” and CT for “chain-terminating.”

Table 3-1 RNA Codons for Amino Acids and for Start and Stop

Transfer RNA—The Anticodons
Another type of RNA that plays an essential role in protein synthesis is called tRNA because it transfers amino acid molecules to protein molecules as the protein is being synthesized. Each type of tRNA combines specifically with 1 of the 20 amino acids that are to be incorporated into proteins. The tRNA then acts as a carrier to transport its specific type of amino acid to the ribosomes, where protein molecules are forming. In the ribosomes, each specific type of transfer RNA recognizes a particular codon on the mRNA (described later) and thereby delivers the appropriate amino acid to the appropriate place in the chain of the newly forming protein molecule.
Transfer RNA, which contains only about 80 nucleotides, is a relatively small molecule in comparison with mRNA. It is a folded chain of nucleotides with a cloverleaf appearance similar to that shown in Figure 3-9 . At one end of the molecule is always an adenylic acid; it is to this that the transported amino acid attaches at a hydroxyl group of the ribose in the adenylic acid.

Figure 3-9 A messenger RNA strand is moving through two ribosomes. As each “codon” passes through, an amino acid is added to the growing protein chain, which is shown in the right-hand ribosome. The transfer RNA molecule transports each specific amino acid to the newly forming protein.
Because the function of tRNA is to cause attachment of a specific amino acid to a forming protein chain, it is essential that each type of tRNA also have specificity for a particular codon in the mRNA. The specific code in the tRNA that allows it to recognize a specific codon is again a triplet of nucleotide bases and is called an anticodon. This is located approximately in the middle of the tRNA molecule (at the bottom of the cloverleaf configuration shown in Figure 3-9 ). During formation of the protein molecule, the anticodon bases combine loosely by hydrogen bonding with the codon bases of the mRNA. In this way, the respective amino acids are lined up one after another along the mRNA chain, thus establishing the appropriate sequence of amino acids in the newly forming protein molecule.

Ribosomal RNA
The third type of RNA in the cell is ribosomal RNA; it constitutes about 60 percent of the ribosome. The remainder of the ribosome is protein, containing about 75 types of proteins that are both structural proteins and enzymes needed in the manufacture of protein molecules.
The ribosome is the physical structure in the cytoplasm on which protein molecules are actually synthesized. However, it always functions in association with the other two types of RNA as well: tRNA transports amino acids to the ribosome for incorporation into the developing protein molecule, whereas mRNA provides the information necessary for sequencing the amino acids in proper order for each specific type of protein to be manufactured.
Thus, the ribosome acts as a manufacturing plant in which the protein molecules are formed.

Formation of Ribosomes in the Nucleolus
The DNA genes for formation of ribosomal RNA are located in five pairs of chromosomes in the nucleus, and each of these chromosomes contains many duplicates of these particular genes because of the large amounts of ribosomal RNA required for cellular function.
As the ribosomal RNA forms, it collects in the nucleolus, a specialized structure lying adjacent to the chromosomes. When large amounts of ribosomal RNA are being synthesized, as occurs in cells that manufacture large amounts of protein, the nucleolus is a large structure, whereas in cells that synthesize little protein, the nucleolus may not even be seen. Ribosomal RNA is specially processed in the nucleolus, where it binds with “ribosomal proteins” to form granular condensation products that are primordial subunits of ribosomes. These subunits are then released from the nucleolus and transported through the large pores of the nuclear envelope to almost all parts of the cytoplasm. After the subunits enter the cytoplasm, they are assembled to form mature, functional ribosomes. Therefore, proteins are formed in the cytoplasm of the cell, but not in the cell nucleus, because the nucleus does not contain mature ribosomes.

A fourth type of RNA in the cell is miRNA. These are short (21 to 23 nucleotides) single-stranded RNA fragments that regulate gene expression ( Figure 3-10 ). The miRNAs are encoded from the transcribed DNA of genes, but they are not translated into proteins and are therefore often called noncoding RNA . The miRNAs are processed by the cell into molecules that are complementary to mRNA and act to decrease gene expression. Generation of miRNAs involves special processing of longer primary precursor RNAs called pri-miRNAs, which are the primary transcripts of the gene. The pri-miRNAs are then processed in the cell nucleus by the microprocessor complex to pre-miRNAs, which are 70 nucleotide stem-loop structures. These pre-miRNAs are then further processed in the cytoplasm by a specific dicer enzyme that helps assemble an RNA-induced silencing complex (RISC) and generates miRNAs.

Figure 3-10 Regulation of gene expression by microRNA (miRNA). Primary miRNA (pri-miRNA), the primary transcripts of a gene processed in the cell nucleus by the microprocessor complex to pre-miRNAs. These pre-miRNAs are then further processed in the cytoplasm by dicer, an enzyme that helps assemble an RNA-induced silencing complex (RISC) and generates miRNAs. The miRNAs regulate gene expression by binding to the complementary region of the RNA and repressing translation or promoting degradation of the mRNA before it can be translated by the ribosome.
The miRNAs regulate gene expression by binding to the complementary region of the RNA and promoting repression of translation or degradation of the mRNA before it can be translated by the ribosome. miRNAs are believed to play an important role in the normal regulation of cell function, and alterations in miRNA function have been associated with diseases such as cancer and heart disease.
Another type of microRNA is small interfering RNA (siRNA), also called silencing RNA or short interfering RNA. The siRNAs are short, double-stranded RNA molecules, 20 to 25 nucleotides in length, that interfere with the expression of specific genes. siRNAs generally refer to synthetic miRNAs and can be administered to silence expression of specific genes. They are designed to avoid the nuclear processing by the microprocessor complex, and after the siRNA enters the cytoplasm it activates the RISC silencing complex, blocking the translation of mRNA. Because siRNAs can be tailored for any specific sequence in the gene, they can be used to block translation of any mRNA and therefore expression by any gene for which the nucleotide sequence is known. Some researchers have proposed that siRNAs may become useful therapeutic tools to silence genes that contribute to the pathophysiology of diseases.

Formation of Proteins on the Ribosomes—The Process of “Translation”
When a molecule of messenger RNA comes in contact with a ribosome, it travels through the ribosome, beginning at a predetermined end of the RNA molecule specified by an appropriate sequence of RNA bases called the “chain-initiating” codon. Then, as shown in Figure 3-9 , while the messenger RNA travels through the ribosome, a protein molecule is formed—a process called translation. Thus, the ribosome reads the codons of the messenger RNA in much the same way that a tape is “read” as it passes through the playback head of a tape recorder. Then, when a “stop” (or “chain-terminating”) codon slips past the ribosome, the end of a protein molecule is signaled and the protein molecule is freed into the cytoplasm.

A single messenger RNA molecule can form protein molecules in several ribosomes at the same time because the initial end of the RNA strand can pass to a successive ribosome as it leaves the first, as shown at the bottom left in Figure 3-9 and in Figure 3-11 . The protein molecules are in different stages of development in each ribosome. As a result, clusters of ribosomes frequently occur, 3 to 10 ribosomes being attached to a single messenger RNA at the same time. These clusters are called polyribosomes.

Figure 3-11 Physical structure of the ribosomes, as well as their functional relation to messenger RNA, transfer RNA, and the endoplasmic reticulum during the formation of protein molecules.
(Courtesy Dr. Don W. Fawcett, Montana.)
It is especially important to note that a messenger RNA can cause the formation of a protein molecule in any ribosome; that is, there is no specificity of ribosomes for given types of protein. The ribosome is simply the physical manufacturing plant in which the chemical reactions take place.

Many Ribosomes Attach to the Endoplasmic Reticulum
In Chapter 2 , it was noted that many ribosomes become attached to the endoplasmic reticulum. This occurs because the initial ends of many forming protein molecules have amino acid sequences that immediately attach to specific receptor sites on the endoplasmic reticulum; this causes these molecules to penetrate the reticulum wall and enter the endoplasmic reticulum matrix. This gives a granular appearance to those portions of the reticulum where proteins are being formed and entering the matrix of the reticulum.
Figure 3-11 shows the functional relation of messenger RNA to the ribosomes and the manner in which the ribosomes attach to the membrane of the endoplasmic reticulum. Note the process of translation occurring in several ribosomes at the same time in response to the same strand of messenger RNA. Note also the newly forming polypeptide (protein) chains passing through the endoplasmic reticulum membrane into the endoplasmic matrix.
Yet it should be noted that except in glandular cells in which large amounts of protein-containing secretory vesicles are formed, most proteins synthesized by the ribosomes are released directly into the cytosol instead of into the endoplasmic reticulum. These proteins are enzymes and internal structural proteins of the cell.

Chemical Steps in Protein Synthesis
Some of the chemical events that occur in synthesis of a protein molecule are shown in Figure 3-12 . This figure shows representative reactions for three separate amino acids, AA 1 , AA 2 , and AA 20 . The stages of the reactions are the following: (1) Each amino acid is activated by a chemical process in which ATP combines with the amino acid to form an adenosine monophosphate complex with the amino acid, giving up two high-energy phosphate bonds in the process. (2) The activated amino acid, having an excess of energy, then combines with its specific transfer RNA to form an amino acid–tRNA complex and, at the same time, releases the adenosine monophosphate. (3) The transfer RNA carrying the amino acid complex then comes in contact with the messenger RNA molecule in the ribosome, where the anticodon of the transfer RNA attaches temporarily to its specific codon of the messenger RNA, thus lining up the amino acid in appropriate sequence to form a protein molecule. Then, under the influence of the enzyme peptidyl transferase (one of the proteins in the ribosome), peptide bonds are formed between the successive amino acids, thus adding progressively to the protein chain. These chemical events require energy from two additional high-energy phosphate bonds, making a total of four high-energy bonds used for each amino acid added to the protein chain. Thus, the synthesis of proteins is one of the most energy-consuming processes of the cell.

Figure 3-12 Chemical events in the formation of a protein molecule.

Peptide Linkage
The successive amino acids in the protein chain combine with one another according to the typical reaction:
In this chemical reaction, a hydroxyl radical (OH − ) is removed from the COOH portion of the first amino acid and a hydrogen (H + ) of the NH 2 portion of the other amino acid is removed. These combine to form water, and the two reactive sites left on the two successive amino acids bond with each other, resulting in a single molecule. This process is called peptide linkage. As each additional amino acid is added, an additional peptide linkage is formed.

Synthesis of Other Substances in the Cell
Many thousand protein enzymes formed in the manner just described control essentially all the other chemical reactions that take place in cells. These enzymes promote synthesis of lipids, glycogen, purines, pyrimidines, and hundreds of other substances. We discuss many of these synthetic processes in relation to carbohydrate, lipid, and protein metabolism in Chapters 67 through 69 . It is by means of all these substances that the many functions of the cells are performed.

Control of Gene Function and Biochemical Activity in Cells
From our discussion thus far, it is clear that the genes control both the physical and chemical functions of the cells. However, the degree of activation of respective genes must be controlled as well; otherwise, some parts of the cell might overgrow or some chemical reactions might overact until they kill the cell. Each cell has powerful internal feedback control mechanisms that keep the various functional operations of the cell in step with one another. For each gene (approximately 30,000 genes in all), there is at least one such feedback mechanism.
There are basically two methods by which the biochemical activities in the cell are controlled: (1) genetic regulation, in which the degree of activation of the genes and the formation of gene products are themselves controlled and (2) enzyme regulation, in which the activity levels of already formed enzymes in the cell are controlled.

Genetic Regulation
Genetic regulation, or regulation of gene expression, covers the entire process from transcription of the genetic code in the nucleus to the formation or proteins in the cytoplasm. Regulation of gene expression provides all living organisms the ability to respond to changes in their environment. In animals that have many different types of cells, tissues, and organs, differential regulation of gene expression also permits the many different cell types in the body to each perform their specialized functions. Although a cardiac myocyte contains the same genetic code as a renal tubular epithelia cell, many genes are expressed in cardiac cells that are not expressed in renal tubular cells. The ultimate measure of gene “expression” is whether (and how much) of the gene products (proteins) are produced because proteins carry out cell functions specified by the genes. Regulation of gene expression can occur at any point in the pathways of transcription, RNA processing, and translation.

The Promoter Controls Gene Expression
Synthesis of cellular proteins is a complex process that starts with the transcription of DNA into RNA. The transcription of DNA is controlled by regulatory elements found in the promoter of a gene ( Figure 3-13 ). In eukaryotes, which includes all mammals, the basal promoter consists of a sequence of seven bases (TATAAAA) called the TATA box, the binding site for the TATA-binding protein (TBP) and several other important transcription factors that are collectively referred to as the transcription factor IID complex. In addition to the transcription factor IID complex, this region is where transcription factor IIB binds to both the DNA and RNA polymerase 2 to facilitate transcription of the DNA into RNA. This basal promoter is found in all protein-coding genes and the polymerase must bind with this basal promoter before it can begin traveling along the DNA strand to synthesize RNA. The upstream promoter is located farther upstream from the transcription start site and contains several binding sites for positive or negative transcription factors that can effect transcription through interactions with proteins bound to the basal promoter. The structure and transcription factor binding sites in the upstream promoter vary from gene to gene to give rise to the different expression patterns of genes in different tissues.

Figure 3-13 Gene transcriptional in eukaryotic cells. A complex arrangement of multiple clustered enhancer modules interspersed with insulator elements, which can be located either upstream or downstream of a basal promoter containing TATA box (TATA), proximal promoter elements (response elements, RE), and Initiator sequences (INR).
Transcription of genes in eukaryotes is also influenced by enhancers, which are regions of DNA that can bind transcription factors. Enhancers can be located a great distance from the gene they act on or even on a different chromosome. They can also be located either upstream or downstream of the gene that they regulate. Although enhancers may be located a great distance away from their target gene, they may be relatively close when DNA is coiled in the nucleus. It is estimated that there are 110,000 gene enhancer sequences in the human genome.
In the organization of the chromosome, it is important to separate active genes that are being transcribed from genes that are repressed. This can be challenging because multiple genes may be located close together on the chromosome. This is achieved by chromosomal insulators. These insulators are gene sequences that provide a barrier so that a specific gene is isolated against transcriptional influences from surrounding genes. Insulators can vary greatly in their DNA sequence and the proteins that bind to them. One way an insulator activity can be modulated is by DNA methylation . This is the case for the mammalian insulin-like growth factor 2 (IGF-2) gene. The mother’s allele has an insulator between the enhancer and promoter of the gene that allows for the binding of a transcriptional repressor. However, the paternal DNA sequence is methylated such that the transcriptional repressor cannot bind to the insulator and the IGF-2 gene is expressed from the paternal copy of the gene.

Other Mechanisms for Control of Transcription by the Promoter
Variations in the basic mechanism for control of the promoter have been discovered with rapidity in the past 2 decades. Without giving details, let us list some of them:
1. A promoter is frequently controlled by transcription factors located elsewhere in the genome. That is, the regulatory gene causes the formation of a regulatory protein that in turn acts either as an activator or a repressor of transcription.
2. Occasionally, many different promoters are controlled at the same time by the same regulatory protein. In some instances, the same regulatory protein functions as an activator for one promoter and as a repressor for another promoter.
3. Some proteins are controlled not at the starting point of transcription on the DNA strand but farther along the strand. Sometimes the control is not even at the DNA strand itself but during the processing of the RNA molecules in the nucleus before they are released into the cytoplasm; rarely, control might occur at the level of protein formation in the cytoplasm during RNA translation by the ribosomes.
4. In nucleated cells, the nuclear DNA is packaged in specific structural units, the chromosomes. Within each chromosome, the DNA is wound around small proteins called histones, which in turn are held tightly together in a compacted state by still other proteins. As long as the DNA is in this compacted state, it cannot function to form RNA. However, multiple control mechanisms are beginning to be discovered that can cause selected areas of chromosomes to become decompacted one part at a time so that partial RNA transcription can occur. Even then, specific transcriptor factor s control the actual rate of transcription by the promoter in the chromosome. Thus, still higher orders of control are used for establishing proper cell function. In addition, signals from outside the cell, such as some of the body’s hormones, can activate specific chromosomal areas and specific transcription factors, thus controlling the chemical machinery for function of the cell.
Because there are more than 30,000 different genes in each human cell, the large number of ways in which genetic activity can be controlled is not surprising. The gene control systems are especially important for controlling intracellular concentrations of amino acids, amino acid derivatives, and intermediate substrates and products of carbohydrate, lipid, and protein metabolism.

Control of Intracellular Function by Enzyme Regulation
In addition to control of cell function by genetic regulation, some cell activities are controlled by intracellular inhibitors or activators that act directly on specific intracellular enzymes. Thus, enzyme regulation represents a second category of mechanisms by which cellular biochemical functions can be controlled.

Enzyme Inhibition
Some chemical substances formed in the cell have direct feedback effects in inhibiting the specific enzyme systems that synthesize them. Almost always the synthesized product acts on the first enzyme in a sequence, rather than on the subsequent enzymes, usually binding directly with the enzyme and causing an allosteric conformational change that inactivates it. One can readily recognize the importance of inactivating the first enzyme: this prevents buildup of intermediary products that are not used.
Enzyme inhibition is another example of negative feedback control; it is responsible for controlling intracellular concentrations of multiple amino acids, purines, pyrimidines, vitamins, and other substances.

Enzyme Activation
Enzymes that are normally inactive often can be activated when needed. An example of this occurs when most of the ATP has been depleted in a cell. In this case, a considerable amount of cyclic adenosine monophosphate (cAMP) begins to be formed as a breakdown product of the ATP; the presence of this cAMP, in turn, immediately activates the glycogen-splitting enzyme phosphorylase, liberating glucose molecules that are rapidly metabolized and their energy used for replenishment of the ATP stores. Thus, cAMP acts as an enzyme activator for the enzyme phosphorylase and thereby helps control intracellular ATP concentration.
Another interesting instance of both enzyme inhibition and enzyme activation occurs in the formation of the purines and pyrimidines. These substances are needed by the cell in approximately equal quantities for formation of DNA and RNA. When purines are formed, they inhibit the enzymes that are required for formation of additional purines. However, they activate the enzymes for formation of pyrimidines. Conversely, the pyrimidines inhibit their own enzymes but activate the purine enzymes. In this way, there is continual cross-feed between the synthesizing systems for these two substances, resulting in almost exactly equal amounts of the two substances in the cells at all times.

In summary, there are two principal methods by which cells control proper proportions and proper quantities of different cellular constituents: (1) the mechanism of genetic regulation and (2) the mechanism of enzyme regulation. The genes can be either activated or inhibited, and likewise, the enzyme systems can be either activated or inhibited. These regulatory mechanisms most often function as feedback control systems that continually monitor the cell’s biochemical composition and make corrections as needed. But on occasion, substances from without the cell (especially some of the hormones discussed throughout this text) also control the intracellular biochemical reactions by activating or inhibiting one or more of the intracellular control systems.

The DNA-Genetic System Also Controls Cell Reproduction
Cell reproduction is another example of the ubiquitous role that the DNA-genetic system plays in all life processes. The genes and their regulatory mechanisms determine the growth characteristics of the cells and also when or whether these cells will divide to form new cells. In this way, the all-important genetic system controls each stage in the development of the human being, from the single-cell fertilized ovum to the whole functioning body. Thus, if there is any central theme to life, it is the DNA-genetic system.

Life Cycle of the Cell
The life cycle of a cell is the period from cell reproduction to the next cell reproduction. When mammalian cells are not inhibited and are reproducing as rapidly as they can, this life cycle may be as little as 10 to 30 hours. It is terminated by a series of distinct physical events called mitosis that cause division of the cell into two new daughter cells. The events of mitosis are shown in Figure 3-14 and are described later. The actual stage of mitosis, however, lasts for only about 30 minutes, so more than 95 percent of the life cycle of even rapidly reproducing cells is represented by the interval between mitosis, called interphase.

Figure 3-14 Stages of cell reproduction. A, B, and C, Prophase. D, Prometaphase. E, Metaphase. F, Anaphase. G and H, Telophase.
(From Margaret C. Gladbach, Estate of Mary E. and Dan Todd, Kansas.)
Except in special conditions of rapid cellular reproduction, inhibitory factors almost always slow or stop the uninhibited life cycle of the cell. Therefore, different cells of the body actually have life cycle periods that vary from as little as 10 hours for highly stimulated bone marrow cells to an entire lifetime of the human body for most nerve cells.

Cell Reproduction Begins with Replication of DNA
As is true of almost all other important events in the cell, reproduction begins in the nucleus itself. The first step is replication (duplication) of all DNA in the chromosomes. Only after this has occurred can mitosis take place.
The DNA begins to be duplicated some 5 to 10 hours before mitosis, and this is completed in 4 to 8 hours. The net result is two exact replicas of all DNA. These replicas become the DNA in the two new daughter cells that will be formed at mitosis. After replication of the DNA, there is another period of 1 to 2 hours before mitosis begins abruptly. Even during this period, preliminary changes that will lead to the mitotic process are beginning to take place.

Chemical and Physical Events of DNA Replication
DNA is replicated in much the same way that RNA is transcribed in response to DNA, except for a few important differences:
1. Both strands of the DNA in each chromosome are replicated, not simply one of them.
2. Both entire strands of the DNA helix are replicated from end to end, rather than small portions of them, as occurs in the transcription of RNA.
3. The principal enzymes for replicating DNA are a complex of multiple enzymes called DNA polymerase, which is comparable to RNA polymerase. It attaches to and moves along the DNA template strand while another enzyme, DNA ligase, causes bonding of successive DNA nucleotides to one another, using high-energy phosphate bonds to energize these attachments.
4. Formation of each new DNA strand occurs simultaneously in hundreds of segments along each of the two strands of the helix until the entire strand is replicated. Then the ends of the subunits are joined together by the DNA ligase enzyme.
5. Each newly formed strand of DNA remains attached by loose hydrogen bonding to the original DNA strand that was used as its template. Therefore, two DNA helixes are coiled together.
6. Because the DNA helixes in each chromosome are approximately 6 centimeters in length and have millions of helix turns, it would be impossible for the two newly formed DNA helixes to uncoil from each other were it not for some special mechanism. This is achieved by enzymes that periodically cut each helix along its entire length, rotate each segment enough to cause separation, and then resplice the helix. Thus, the two new helixes become uncoiled.

DNA Repair, DNA “Proofreading,” and “Mutation.”
During the hour or so between DNA replication and the beginning of mitosis, there is a period of active repair and “proofreading” of the DNA strands. That is, wherever inappropriate DNA nucleotides have been matched up with the nucleotides of the original template strand, special enzymes cut out the defective areas and replace these with appropriate complementary nucleotides. This is achieved by the same DNA polymerases and DNA ligases that are used in replication. This repair process is referred to as DNA proofreading.
Because of repair and proofreading, the transcription process rarely makes a mistake. But when a mistake is made, this is called a mutation. The mutation causes formation of some abnormal protein in the cell rather than a needed protein, often leading to abnormal cellular function and sometimes even cell death. Yet given that there are 30,000 or more genes in the human genome and that the period from one human generation to another is about 30 years, one would expect as many as 10 or many more mutations in the passage of the genome from parent to child. As a further protection, however, each human genome is represented by two separate sets of chromosomes with almost identical genes. Therefore, one functional gene of each pair is almost always available to the child despite mutations.

Chromosomes and Their Replication
The DNA helixes of the nucleus are packaged in chromosomes. The human cell contains 46 chromosomes arranged in 23 pairs. Most of the genes in the two chromosomes of each pair are identical or almost identical to each other, so it is usually stated that the different genes also exist in pairs, although occasionally this is not the case.
In addition to DNA in the chromosome, there is a large amount of protein in the chromosome, composed mainly of many small molecules of electropositively charged histones. The histones are organized into vast numbers of small, bobbin-like cores. Small segments of each DNA helix are coiled sequentially around one core after another.
The histone cores play an important role in the regulation of DNA activity because as long as the DNA is packaged tightly, it cannot function as a template for either the formation of RNA or the replication of new DNA. Further, some of the regulatory proteins have been shown to decondense the histone packaging of the DNA and to allow small segments at a time to form RNA.
Several nonhistone proteins are also major components of chromosomes, functioning both as chromosomal structural proteins and, in connection with the genetic regulatory machinery, as activators, inhibitors, and enzymes.
Replication of the chromosomes in their entirety occurs during the next few minutes after replication of the DNA helixes has been completed; the new DNA helixes collect new protein molecules as needed. The two newly formed chromosomes remain attached to each other (until time for mitosis) at a point called the centromere located near their center. These duplicated but still attached chromosomes are called chromatids.

Cell Mitosis
The actual process by which the cell splits into two new cells is called mitosis. Once each chromosome has been replicated to form the two chromatids, in many cells, mitosis follows automatically within 1 or 2 hours.

Mitotic Apparatus: Function of the Centrioles
One of the first events of mitosis takes place in the cytoplasm, occurring during the latter part of interphase in or around the small structures called centrioles. As shown in Figure 3-14 , two pairs of centrioles lie close to each other near one pole of the nucleus. These centrioles, like the DNA and chromosomes, are also replicated during interphase, usually shortly before replication of the DNA. Each centriole is a small cylindrical body about 0.4 micrometer long and about 0.15 micrometer in diameter, consisting mainly of nine parallel tubular structures arranged in the form of a cylinder. The two centrioles of each pair lie at right angles to each other. Each pair of centrioles, along with attached pericentriolar material, is called a centrosome.
Shortly before mitosis is to take place, the two pairs of centrioles begin to move apart from each other. This is caused by polymerization of protein microtubules growing between the respective centriole pairs and actually pushing them apart. At the same time, other microtubules grow radially away from each of the centriole pairs, forming a spiny star, called the aster, in each end of the cell. Some of the spines of the aster penetrate the nuclear membrane and help separate the two sets of chromatids during mitosis. The complex of microtubules extending between the two new centriole pairs is called the spindle, and the entire set of microtubules plus the two pairs of centrioles is called the mitotic apparatus.

The first stage of mitosis, called prophase, is shown in Figure 3-14 A, B, and C . While the spindle is forming, the chromosomes of the nucleus (which in interphase consist of loosely coiled strands) become condensed into well-defined chromosomes.

During this stage (see Figure 3-14 D ), the growing microtubular spines of the aster fragment the nuclear envelope. At the same time, multiple microtubules from the aster attach to the chromatids at the centromeres, where the paired chromatids are still bound to each other; the tubules then pull one chromatid of each pair toward one cellular pole and its partner toward the opposite pole.

During metaphase (see Figure 3-14 E ), the two asters of the mitotic apparatus are pushed farther apart. This is believed to occur because the microtubular spines from the two asters, where they interdigitate with each other to form the mitotic spindle, actually push each other away. There is reason to believe that minute contractile protein molecules called “ molecular motors, ” perhaps composed of the muscle protein actin, extend between the respective spines and, using a stepping action as in muscle, actively slide the spines in a reverse direction along each other. Simultaneously, the chromatids are pulled tightly by their attached microtubules to the very center of the cell, lining up to form the equatorial plate of the mitotic spindle.

During this phase (see Figure 3-14 F ), the two chromatids of each chromosome are pulled apart at the centromere. All 46 pairs of chromatids are separated, forming two separate sets of 46 daughter chromosomes. One of these sets is pulled toward one mitotic aster and the other toward the other aster as the two respective poles of the dividing cell are pushed still farther apart.

In telophase (see Figure 3-14 G and H ), the two sets of daughter chromosomes are pushed completely apart. Then the mitotic apparatus dissolutes, and a new nuclear membrane develops around each set of chromosomes. This membrane is formed from portions of the endoplasmic reticulum that are already present in the cytoplasm. Shortly thereafter, the cell pinches in two, midway between the two nuclei. This is caused by formation of a contractile ring of microfilaments composed of actin and probably myosin (the two contractile proteins of muscle) at the juncture of the newly developing cells that pinches them off from each other.

Control of Cell Growth and Cell Reproduction
We know that certain cells grow and reproduce all the time, such as the blood-forming cells of the bone marrow, the germinal layers of the skin, and the epithelium of the gut. Many other cells, however, such as smooth muscle cells, may not reproduce for many years. A few cells, such as the neurons and most striated muscle cells, do not reproduce during the entire life of a person, except during the original period of fetal life.
In certain tissues, an insufficiency of some types of cells causes these to grow and reproduce rapidly until appropriate numbers of them are again available. For instance, in some young animals, seven eighths of the liver can be removed surgically, and the cells of the remaining one eighth will grow and divide until the liver mass returns to almost normal. The same occurs for many glandular cells and most cells of the bone marrow, subcutaneous tissue, intestinal epithelium, and almost any other tissue except highly differentiated cells such as nerve and muscle cells.
We know little about the mechanisms that maintain proper numbers of the different types of cells in the body. However, experiments have shown at least three ways in which growth can be controlled. First, growth often is controlled by growth factors that come from other parts of the body. Some of these circulate in the blood, but others originate in adjacent tissues. For instance, the epithelial cells of some glands, such as the pancreas, fail to grow without a growth factor from the sublying connective tissue of the gland. Second, most normal cells stop growing when they have run out of space for growth. This occurs when cells are grown in tissue culture; the cells grow until they contact a solid object, and then growth stops. Third, cells grown in tissue culture often stop growing when minute amounts of their own secretions are allowed to collect in the culture medium. This, too, could provide a means for negative feedback control of growth.

Regulation of Cell Size
Cell size is determined almost entirely by the amount of functioning DNA in the nucleus. If replication of the DNA does not occur, the cell grows to a certain size and thereafter remains at that size. Conversely, it is possible, by use of the chemical colchicine, to prevent formation of the mitotic spindle and therefore to prevent mitosis, even though replication of the DNA continues. In this event, the nucleus contains far greater quantities of DNA than it normally does, and the cell grows proportionately larger. It is assumed that this results simply from increased production of RNA and cell proteins, which in turn cause the cell to grow larger.

Cell Differentiation
A special characteristic of cell growth and cell division is cell differentiation, which refers to changes in physical and functional properties of cells as they proliferate in the embryo to form the different bodily structures and organs. The description of an especially interesting experiment that helps explain these processes follows.
When the nucleus from an intestinal mucosal cell of a frog is surgically implanted into a frog ovum from which the original ovum nucleus was removed, the result is often the formation of a normal frog. This demonstrates that even the intestinal mucosal cell, which is a well-differentiated cell, carries all the necessary genetic information for development of all structures required in the frog’s body.
Therefore, it has become clear that differentiation results not from loss of genes but from selective repression of different gene promoters. In fact, electron micrographs suggest that some segments of DNA helixes wound around histone cores become so condensed that they no longer uncoil to form RNA molecules. One explanation for this is as follows: It has been supposed that the cellular genome begins at a certain stage of cell differentiation to produce a regulatory protein that forever after represses a select group of genes. Therefore, the repressed genes never function again. Regardless of the mechanism, mature human cells produce a maximum of about 8000 to 10,000 proteins rather than the potential 30,000 or more if all genes were active.
Embryological experiments show that certain cells in an embryo control differentiation of adjacent cells. For instance, the primordial chorda-mesoderm is called the primary organizer of the embryo because it forms a focus around which the rest of the embryo develops. It differentiates into a mesodermal axis that contains segmentally arranged somites and, as a result of inductions in the surrounding tissues, causes formation of essentially all the organs of the body.
Another instance of induction occurs when the developing eye vesicles come in contact with the ectoderm of the head and cause the ectoderm to thicken into a lens plate that folds inward to form the lens of the eye. Therefore, a large share of the embryo develops as a result of such inductions, one part of the body affecting another part, and this part affecting still other parts.
Thus, although our understanding of cell differentiation is still hazy, we know many control mechanisms by which differentiation could occur.

Apoptosis—Programmed Cell Death
The 100 trillion cells of the body are members of a highly organized community in which the total number of cells is regulated not only by controlling the rate of cell division but also by controlling the rate of cell death. When cells are no longer needed or become a threat to the organism, they undergo a suicidal programmed cell death, or apoptosis. This process involves a specific proteolytic cascade that causes the cell to shrink and condense, to disassemble its cytoskeleton, and to alter its cell surface so that a neighboring phagocytic cell, such as a macrophage, can attach to the cell membrane and digest the cell.
In contrast to programmed death, cells that die as a result of an acute injury usually swell and burst due to loss of cell membrane integrity, a process called cell necrosis. Necrotic cells may spill their contents, causing inflammation and injury to neighboring cells. Apoptosis, however, is an orderly cell death that results in disassembly and phagocytosis of the cell before any leakage of its contents occurs, and neighboring cells usually remain healthy.
Apoptosis is initiated by activation of a family of proteases called caspases . These are enzymes that are synthesized and stored in the cell as inactive procaspases . The mechanisms of activation of caspases are complex, but once activated, the enzymes cleave and activate other procaspases, triggering a cascade that rapidly breaks down proteins within the cell. The cell thus dismantles itself, and its remains are rapidly digested by neighboring phagocytic cells.
A tremendous amount of apoptosis occurs in tissues that are being remodeled during development. Even in adult humans, billions of cells die each hour in tissues such as the intestine and bone marrow and are replaced by new cells. Programmed cell death, however, is normally balanced with the formation of new cells in healthy adults. Otherwise, the body’s tissues would shrink or grow excessively. Recent studies suggest that abnormalities of apoptosis may play a key role in neurodegenerative diseases such as Alzheimer’s disease, as well as in cancer and autoimmune disorders. Some drugs that have been used successfully for chemotherapy appear to induce apoptosis in cancer cells.

Cancer is caused in all or almost all instances by mutation or by some other abnormal activation of cellular genes that control cell growth and cell mitosis. The abnormal genes are called oncogenes. As many as 100 different oncogenes have been discovered.
Also present in all cells are antioncogenes, which suppress the activation of specific oncogenes. Therefore, loss or inactivation of antioncogenes can allow activation of oncogenes that lead to cancer.
Only a minute fraction of the cells that mutate in the body ever lead to cancer. There are several reasons for this. First, most mutated cells have less survival capability than normal cells and simply die. Second, only a few of the mutated cells that do survive become cancerous, because even most mutated cells still have normal feedback controls that prevent excessive growth.
Third, those cells that are potentially cancerous are often destroyed by the body’s immune system before they grow into a cancer. This occurs in the following way: Most mutated cells form abnormal proteins within their cell bodies because of their altered genes, and these proteins activate the body’s immune system, causing it to form antibodies or sensitized lymphocytes that react against the cancerous cells, destroying them. In support of this is the fact that in people whose immune systems have been suppressed, such as in those taking immunosuppressant drugs after kidney or heart transplantation, the probability of a cancer’s developing is multiplied as much as fivefold.
Fourth, usually several different activated oncogenes are required simultaneously to cause a cancer. For instance, one such gene might promote rapid reproduction of a cell line, but no cancer occurs because there is not a simultaneous mutant gene to form the needed blood vessels.
But what is it that causes the altered genes? Considering that many trillions of new cells are formed each year in humans, a better question might be, why is it that all of us do not develop millions or billions of mutant cancerous cells? The answer is the incredible precision with which DNA chromosomal strands are replicated in each cell before mitosis can take place, and also the proofreading process that cuts and repairs any abnormal DNA strand before the mitotic process is allowed to proceed. Yet despite all these inherited cellular precautions, probably one newly formed cell in every few million still has significant mutant characteristics.
Thus, chance alone is all that is required for mutations to take place, so we can suppose that a large number of cancers are merely the result of an unlucky occurrence.
However, the probability of mutations can be increased manyfold when a person is exposed to certain chemical, physical, or biological factors, including the following:
1. It is well known that ionizing radiation, such as x-rays, gamma rays, and particle radiation from radioactive substances, and even ultraviolet light can predispose individuals to cancer. Ions formed in tissue cells under the influence of such radiation are highly reactive and can rupture DNA strands, thus causing many mutations.
2. Chemical substances of certain types also have a high propensity for causing mutations. It was discovered long ago that various aniline dye derivatives are likely to cause cancer, so workers in chemical plants producing such substances, if unprotected, have a special predisposition to cancer. Chemical substances that can cause mutation are called carcinogens. The carcinogens that currently cause the greatest number of deaths are those in cigarette smoke. They cause about one quarter of all cancer deaths.
3. Physical irritants can also lead to cancer, such as continued abrasion of the linings of the intestinal tract by some types of food. The damage to the tissues leads to rapid mitotic replacement of the cells. The more rapid the mitosis, the greater the chance for mutation.
4. In many families, there is a strong hereditary tendency to cancer. This results from the fact that most cancers require not one mutation but two or more mutations before cancer occurs. In those families that are particularly predisposed to cancer, it is presumed that one or more cancerous genes are already mutated in the inherited genome. Therefore, far fewer additional mutations must take place in such family members before a cancer begins to grow.
5. In laboratory animals, certain types of viruses can cause some kinds of cancer, including leukemia. This usually results in one of two ways. In the case of DNA viruses, the DNA strand of the virus can insert itself directly into one of the chromosomes and thereby cause a mutation that leads to cancer. In the case of RNA viruses, some of these carry with them an enzyme called reverse transcriptase that causes DNA to be transcribed from the RNA. The transcribed DNA then inserts itself into the animal cell genome, leading to cancer.

Invasive Characteristic of the Cancer Cell
The major differences between the cancer cell and the normal cell are the following: (1) The cancer cell does not respect usual cellular growth limits; the reason for this is that these cells presumably do not require all the same growth factors that are necessary to cause growth of normal cells. (2) Cancer cells are often far less adhesive to one another than are normal cells. Therefore, they tend to wander through the tissues, enter the blood stream, and be transported all through the body, where they form nidi for numerous new cancerous growths. (3) Some cancers also produce angiogenic factors that cause many new blood vessels to grow into the cancer, thus supplying the nutrients required for cancer growth.

Why Do Cancer Cells Kill?
The answer to this question is usually simple. Cancer tissue competes with normal tissues for nutrients. Because cancer cells continue to proliferate indefinitely, their number multiplying day by day, cancer cells soon demand essentially all the nutrition available to the body or to an essential part of the body. As a result, normal tissues gradually suffer nutritive death.


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Membrane Physiology, Nerve, and Muscle
CHAPTER 4 Transport of Substances Through Cell Membranes
Figure 4-1 gives the approximate concentrations of important electrolytes and other substances in the extracellular fluid and intracellular fluid. Note that the extracellular fluid contains a large amount of sodium but only a small amount of potassium. Exactly the opposite is true of the intracellular fluid. Also, the extracellular fluid contains a large amount of chloride ions, whereas the intracellular fluid contains very little. But the concentrations of phosphates and proteins in the intracellular fluid are considerably greater than those in the extracellular fluid. These differences are extremely important to the life of the cell. The purpose of this chapter is to explain how the differences are brought about by the transport mechanisms of the cell membranes.

Figure 4-1 Chemical compositions of extracellular and intracellular fluids.

The Lipid Barrier of the Cell Membrane, and Cell Membrane Transport Proteins
The structure of the membrane covering the outside of every cell of the body is discussed in Chapter 2 and illustrated in Figures 2-3 and 4-2 . This membrane consists almost entirely of a lipid bilayer, but it also contains large numbers of protein molecules in the lipid, many of which penetrate all the way through the membrane, as shown in Figure 4-2 .

Figure 4-3 Diffusion of a fluid molecule during a thousandth of a second.

Figure 4-2 Transport pathways through the cell membrane, and the basic mechanisms of transport.
The lipid bilayer is not miscible with either the extracellular fluid or the intracellular fluid. Therefore, it constitutes a barrier against movement of water molecules and water-soluble substances between the extracellular and intracellular fluid compartments. However, as demonstrated in Figure 4-2 by the leftmost arrow, a few substances can penetrate this lipid bilayer, diffusing directly through the lipid substance itself; this is true mainly of lipid-soluble substances, as described later.
The protein molecules in the membrane have entirely different properties for transporting substances. Their molecular structures interrupt the continuity of the lipid bilayer, constituting an alternative pathway through the cell membrane. Most of these penetrating proteins, therefore, can function as transport proteins. Different proteins function differently. Some have watery spaces all the way through the molecule and allow free movement of water, as well as selected ions or molecules; these are called channel proteins. Others, called carrier proteins, bind with molecules or ions that are to be transported; conformational changes in the protein molecules then move the substances through the interstices of the protein to the other side of the membrane. Both the channel proteins and the carrier proteins are usually highly selective for the types of molecules or ions that are allowed to cross the membrane.

“Diffusion” Versus “Active Transport.”
Transport through the cell membrane, either directly through the lipid bilayer or through the proteins, occurs by one of two basic processes: diffusion or active transport.
Although there are many variations of these basic mechanisms, diffusion means random molecular movement of substances molecule by molecule, either through intermolecular spaces in the membrane or in combination with a carrier protein. The energy that causes diffusion is the energy of the normal kinetic motion of matter.
By contrast, active transport means movement of ions or other substances across the membrane in combination with a carrier protein in such a way that the carrier protein causes the substance to move against an energy gradient, such as from a low-concentration state to a high-concentration state. This movement requires an additional source of energy besides kinetic energy. Following is a more detailed explanation of the basic physics and physical chemistry of these two processes.

All molecules and ions in the body fluids, including water molecules and dissolved substances, are in constant motion, each particle moving its own separate way. Motion of these particles is what physicists call “heat”—the greater the motion, the higher the temperature—and the motion never ceases under any condition except at absolute zero temperature. When a moving molecule, A, approaches a stationary molecule, B, the electrostatic and other nuclear forces of molecule A repel molecule B, transferring some of the energy of motion of molecule A to molecule B. Consequently, molecule B gains kinetic energy of motion, while molecule A slows down, losing some of its kinetic energy. Thus, as shown in Figure 4-3 , a single molecule in a solution bounces among the other molecules first in one direction, then another, then another, and so forth, randomly bouncing thousands of times each second. This continual movement of molecules among one another in liquids or in gases is called diffusion.
Ions diffuse in the same manner as whole molecules, and even suspended colloid particles diffuse in a similar manner, except that the colloids diffuse far less rapidly than molecular substances because of their large size.

Diffusion Through the Cell Membrane
Diffusion through the cell membrane is divided into two subtypes called simple diffusion and facilitated diffusion. Simple diffusion means that kinetic movement of molecules or ions occurs through a membrane opening or through intermolecular spaces without any interaction with carrier proteins in the membrane. The rate of diffusion is determined by the amount of substance available, the velocity of kinetic motion, and the number and sizes of openings in the membrane through which the molecules or ions can move.
Facilitated diffusion requires interaction of a carrier protein. The carrier protein aids passage of the molecules or ions through the membrane by binding chemically with them and shuttling them through the membrane in this form.
Simple diffusion can occur through the cell membrane by two pathways: (1) through the interstices of the lipid bilayer if the diffusing substance is lipid soluble and (2) through watery channels that penetrate all the way through some of the large transport proteins, as shown to the left in Figure 4-2 .

Diffusion of Lipid-Soluble Substances Through the Lipid Bilayer
One of the most important factors that determines how rapidly a substance diffuses through the lipid bilayer is the lipid solubility of the substance. For instance, the lipid solubilities of oxygen, nitrogen, carbon dioxide, and alcohols are high, so all these can dissolve directly in the lipid bilayer and diffuse through the cell membrane in the same manner that diffusion of water solutes occurs in a watery solution. For obvious reasons, the rate of diffusion of each of these substances through the membrane is directly proportional to its lipid solubility. Especially large amounts of oxygen can be transported in this way; therefore, oxygen can be delivered to the interior of the cell almost as though the cell membrane did not exist.

Diffusion of Water and Other Lipid-Insoluble Molecules Through Protein Channels
Even though water is highly insoluble in the membrane lipids, it readily passes through channels in protein molecules that penetrate all the way through the membrane. The rapidity with which water molecules can move through most cell membranes is astounding. As an example, the total amount of water that diffuses in each direction through the red cell membrane during each second is about 100 times as great as the volume of the red cell itself.
Other lipid-insoluble molecules can pass through the protein pore channels in the same way as water molecules if they are water soluble and small enough. However, as they become larger, their penetration falls off rapidly. For instance, the diameter of the urea molecule is only 20 percent greater than that of water, yet its penetration through the cell membrane pores is about 1000 times less than that of water. Even so, given the astonishing rate of water penetration, this amount of urea penetration still allows rapid transport of urea through the membrane within minutes.

Diffusion Through Protein Pores and Channels—Selective Permeability and “Gating” of Channels
Computerized three-dimensional reconstructions of protein pores and channels have demonstrated tubular pathways all the way from the extracellular to the intracellular fluid. Therefore, substances can move by simple diffusion directly along these pores and channels from one side of the membrane to the other.
Pores are composed of integral cell membrane proteins that form open tubes through the membrane and are always open. However, the diameter of a pore and its electrical charges provide selectivity that permits only certain molecules to pass through. For example, protein pores, called aquaporins or water channels, permit rapid passage of water through cell membranes but exclude other molecules. At least 13 different types of aquaporins have been found in various cells of the human body. Aquaporins have a narrow pore that permits water molecules to diffuse through the membrane in single file. The pore is too narrow to permit passage of any hydrated ions. As discussed in Chapters 29 and 75 , the density of some aquaporins (e.g., aquaporin-2) in cell membranes is not static but is altered in different physiological conditions.
The protein channels are distinguished by two important characteristics: (1) They are often selectively permeable to certain substances, and (2) many of the channels can be opened or closed by gates that are regulated by electrical signals (voltage-gated channels) or chemicals that bind to the channel proteins (ligand-gated channels).

Selective Permeability of Protein Channels
Many of the protein channels are highly selective for transport of one or more specific ions or molecules. This results from the characteristics of the channel itself, such as its diameter, its shape, and the nature of the electrical charges and chemical bonds along its inside surfaces.
Potassium channels permit passage of potassium ions across the cell membrane about 1000 times more readily than they permit passage of sodium ions. This high degree of selectivity, however, cannot be explained entirely by molecular diameters of the ions since potassium ions are slightly larger than sodium ions. What is the mechanism for this remarkable ion selectivity? This question was partially answered when the structure of a bacterial potassium channel was determined by x-ray crystallography. Potassium channels were found to have a tetrameric structure consisting of four identical protein subunits surrounding a central pore ( Figure 4-4 ). At the top of the channel pore are pore loops that form a narrow selectivity filter . Lining the selectivity filter are carbonyl oxygens. When hydrated potassium ions enter the selectivity filter, they interact with the carbonyl oxygens and shed most of their bound water molecules, permitting the dehydrated potassium ions to pass through the channel. The carbonyl oxygens are too far apart, however, to enable them to interact closely with the smaller sodium ions, which are therefore effectively excluded by the selectivity filter from passing through the pore.

Figure 4-4 The structure of a potassium channel. The channel is composed of four subunits (only two are shown), each with two transmembrane helices. A narrow selectivity filter is formed from the pore loops and carbonyl oxygens line the walls of the selectivity filter, forming sites for transiently binding dehydrated potassium ions. The interaction of the potassium ions with carbonyl oxygens causes the potassium ions to shed their bound water molecules, permitting the dehydrated potassium ions to pass through the pore.
Different selectivity filters for the various ion channels are believed to determine, in large part, the specificity of the channel for cations or anions or for particular ions, such as Na + , K + , and Ca ++ , that gain access to the channel.
One of the most important of the protein channels, the sodium channel, is only 0.3 by 0.5 nanometer in diameter, but more important, the inner surfaces of this channel are lined with amino acids that are strongly negatively charged, as shown by the negative signs inside the channel proteins in the top panel of Figure 4-5 . These strong negative charges can pull small dehydrated sodium ions into these channels, actually pulling the sodium ions away from their hydrating water molecules. Once in the channel, the sodium ions diffuse in either direction according to the usual laws of diffusion. Thus, the sodium channel is specifically selective for passage of sodium ions.

Figure 4-5 Transport of sodium and potassium ions through protein channels. Also shown are conformational changes in the protein molecules to open or close “gates” guarding the channels.

Gating of Protein Channels
Gating of protein channels provides a means of controlling ion permeability of the channels. This is shown in both panels of Figure 4-5 for selective gating of sodium and potassium ions. It is believed that some of the gates are actual gatelike extensions of the transport protein molecule, which can close the opening of the channel or can be lifted away from the opening by a conformational change in the shape of the protein molecule itself.
The opening and closing of gates are controlled in two principal ways:
1. Voltage gating. In this instance, the molecular conformation of the gate or of its chemical bonds responds to the electrical potential across the cell membrane. For instance, in the top panel of Figure 4-5 , when there is a strong negative charge on the inside of the cell membrane, this presumably could cause the outside sodium gates to remain tightly closed; conversely, when the inside of the membrane loses its negative charge, these gates would open suddenly and allow tremendous quantities of sodium to pass inward through the sodium pores. This is the basic mechanism for eliciting action potentials in nerves that are responsible for nerve signals. In the bottom panel of Figure 4-5 , the potassium gates are on the intracellular ends of the potassium channels, and they open when the inside of the cell membrane becomes positively charged. The opening of these gates is partly responsible for terminating the action potential, as is discussed more fully in Chapter 5 .
2. Chemical (ligand) gating. Some protein channel gates are opened by the binding of a chemical substance (a ligand) with the protein; this causes a conformational or chemical bonding change in the protein molecule that opens or closes the gate. This is called chemical gating or ligand gating. One of the most important instances of chemical gating is the effect of acetylcholine on the so-called acetylcholine channel. Acetylcholine opens the gate of this channel, providing a negatively charged pore about 0.65 nanometer in diameter that allows uncharged molecules or positive ions smaller than this diameter to pass through. This gate is exceedingly important for the transmission of nerve signals from one nerve cell to another (see Chapter 45 ) and from nerve cells to muscle cells to cause muscle contraction (see Chapter 7 ).

Open-State Versus Closed-State of Gated Channels
Figure 4-6 A shows an especially interesting characteristic of most voltage-gated channels. This figure shows two recordings of electrical current flowing through a single sodium channel when there was an approximate 25-millivolt potential gradient across the membrane. Note that the channel conducts current either “all or none.” That is, the gate of the channel snaps open and then snaps closed, each open state lasting for only a fraction of a millisecond up to several milliseconds. This demonstrates the rapidity with which changes can occur during the opening and closing of the protein molecular gates. At one voltage potential, the channel may remain closed all the time or almost all the time, whereas at another voltage level, it may remain open either all or most of the time. At in-between voltages, as shown in the figure, the gates tend to snap open and closed intermittently, giving an average current flow somewhere between the minimum and the maximum.

Figure 4-6 A, Record of current flow through a single voltage-gated sodium channel, demonstrating the “all or none” principle for opening and closing of the channel. B, The “patch-clamp” method for recording current flow through a single protein channel. To the left, recording is performed from a “patch” of a living cell membrane. To the right, recording is from a membrane patch that has been torn away from the cell.

Patch-Clamp Method for Recording Ion Current Flow Through Single Channels
One might wonder how it is technically possible to record ion current flow through single protein channels as shown in Figure 4-6 A . This has been achieved by using the “patch-clamp” method illustrated in Figure 4-6 B . Very simply, a micropipette, having a tip diameter of only 1 or 2 micrometers, is abutted against the outside of a cell membrane. Then suction is applied inside the pipette to pull the membrane against the tip of the pipette. This creates a seal where the edges of the pipette touch the cell membrane. The result is a minute membrane “patch” at the tip of the pipette through which electrical current flow can be recorded.
Alternatively, as shown to the right in Figure 4-6 B , the small cell membrane patch at the end of the pipette can be torn away from the cell. The pipette with its sealed patch is then inserted into a free solution. This allows the concentrations of ions both inside the micropipette and in the outside solution to be altered as desired. Also, the voltage between the two sides of the membrane can be set at will—that is, “clamped” to a given voltage.
It has been possible to make such patches small enough so that only a single channel protein is found in the membrane patch being studied. By varying the concentrations of different ions, as well as the voltage across the membrane, one can determine the transport characteristics of the single channel and also its gating properties.

Facilitated Diffusion
Facilitated diffusion is also called carrier-mediated diffusion because a substance transported in this manner diffuses through the membrane using a specific carrier protein to help. That is, the carrier facilitates diffusion of the substance to the other side.
Facilitated diffusion differs from simple diffusion in the following important way: Although the rate of simple diffusion through an open channel increases proportionately with the concentration of the diffusing substance, in facilitated diffusion the rate of diffusion approaches a maximum, called V max, as the concentration of the diffusing substance increases. This difference between simple diffusion and facilitated diffusion is demonstrated in Figure 4-7 . The figure shows that as the concentration of the diffusing substance increases, the rate of simple diffusion continues to increase proportionately, but in the case of facilitated diffusion, the rate of diffusion cannot rise greater than the V max level.

Figure 4-7 Effect of concentration of a substance on rate of diffusion through a membrane by simple diffusion and facilitated diffusion. This shows that facilitated diffusion approaches a maximum rate called the V max .
What is it that limits the rate of facilitated diffusion? A probable answer is the mechanism illustrated in Figure 4-8 . This figure shows a carrier protein with a pore large enough to transport a specific molecule partway through. It also shows a binding “receptor” on the inside of the protein carrier. The molecule to be transported enters the pore and becomes bound. Then, in a fraction of a second, a conformational or chemical change occurs in the carrier protein, so the pore now opens to the opposite side of the membrane. Because the binding force of the receptor is weak, the thermal motion of the attached molecule causes it to break away and to be released on the opposite side of the membrane. The rate at which molecules can be transported by this mechanism can never be greater than the rate at which the carrier protein molecule can undergo change back and forth between its two states. Note specifically, though, that this mechanism allows the transported molecule to move—that is, to “diffuse”—in either direction through the membrane.

Figure 4-8 Postulated mechanism for facilitated diffusion.
Among the most important substances that cross cell membranes by facilitated diffusion are glucose and most of the amino acids. In the case of glucose, at least five glucose transporter molecules have been discovered in various tissues. Some of these can also transport other monosaccharides that have structures similar to that of glucose, including galactose and fructose. One of these, glucose transporter 4 (GLUT4), is activated by insulin, which can increase the rate of facilitated diffusion of glucose as much as 10-fold to 20-fold in insulin-sensitive tissues. This is the principal mechanism by which insulin controls glucose use in the body, as discussed in Chapter 78 .

Factors That Affect Net Rate of Diffusion
By now it is evident that many substances can diffuse through the cell membrane. What is usually important is the net rate of diffusion of a substance in the desired direction. This net rate is determined by several factors.

Net Diffusion Rate Is Proportional to the Concentration Difference Across a Membrane
Figure 4-9 A shows a cell membrane with a substance in high concentration on the outside and low concentration on the inside. The rate at which the substance diffuses inward is proportional to the concentration of molecules on the outside because this concentration determines how many molecules strike the outside of the membrane each second. Conversely, the rate at which molecules diffuse outward is proportional to their concentration inside the membrane. Therefore, the rate of net diffusion into the cell is proportional to the concentration on the outside minus the concentration on the inside, or:

Figure 4-9 Effect of concentration difference ( A ), electrical potential difference affecting negative ions ( B ), and pressure difference ( C ) to cause diffusion of molecules and ions through a cell membrane.

in which C o is concentration outside and C i is concentration inside.

Effect of Membrane Electrical Potential on Diffusion of Ions—The “Nernst Potential.”
If an electrical potential is applied across the membrane, as shown in Figure 4-9 B , the electrical charges of the ions cause them to move through the membrane even though no concentration difference exists to cause movement. Thus, in the left panel of Figure 4-9 B , the concentration of negative ions is the same on both sides of the membrane, but a positive charge has been applied to the right side of the membrane and a negative charge to the left, creating an electrical gradient across the membrane. The positive charge attracts the negative ions, whereas the negative charge repels them. Therefore, net diffusion occurs from left to right. After some time, large quantities of negative ions have moved to the right, creating the condition shown in the right panel of Figure 4-9 B , in which a concentration difference of the ions has developed in the direction opposite to the electrical potential difference. The concentration difference now tends to move the ions to the left, while the electrical difference tends to move them to the right. When the concentration difference rises high enough, the two effects balance each other. At normal body temperature (37 °C), the electrical difference that will balance a given concentration difference of univalent ions—such as sodium (Na + ) ions—can be determined from the following formula, called the Nernst equation:
in which EMF is the electromotive force (voltage) between side 1 and side 2 of the membrane, C 1 is the concentration on side 1, and C 2 is the concentration on side 2. This equation is extremely important in understanding the transmission of nerve impulses and is discussed in much greater detail in Chapter 5 .

Effect of a Pressure Difference Across the Membrane
At times, considerable pressure difference develops between the two sides of a diffusible membrane. This occurs, for instance, at the blood capillary membrane in all tissues of the body. The pressure is about 20 mm Hg greater inside the capillary than outside.
Pressure actually means the sum of all the forces of the different molecules striking a unit surface area at a given instant. Therefore, when the pressure is higher on one side of a membrane than on the other, this means that the sum of all the forces of the molecules striking the channels on that side of the membrane is greater than on the other side. In most instances, this is caused by greater numbers of molecules striking the membrane per second on one side than on the other side. The result is that increased amounts of energy are available to cause net movement of molecules from the high-pressure side toward the low-pressure side. This effect is demonstrated in Figure 4-9 C , which shows a piston developing high pressure on one side of a “pore,” thereby causing more molecules to strike the pore on this side and, therefore, more molecules to “diffuse” to the other side.

Osmosis Across Selectively Permeable Membranes—“Net Diffusion” of Water
By far the most abundant substance that diffuses through the cell membrane is water. Enough water ordinarily diffuses in each direction through the red cell membrane per second to equal about 100 times the volume of the cell itself. Yet normally the amount that diffuses in the two directions is balanced so precisely that zero net movement of water occurs. Therefore, the volume of the cell remains constant. However, under certain conditions, a concentration difference for water can develop across a membrane, just as concentration differences for other substances can occur. When this happens, net movement of water does occur across the cell membrane, causing the cell either to swell or shrink, depending on the direction of the water movement. This process of net movement of water caused by a concentration difference of water is called osmosis.
To give an example of osmosis, let us assume the conditions shown in Figure 4-10 , with pure water on one side of the cell membrane and a solution of sodium chloride on the other side. Water molecules pass through the cell membrane with ease, whereas sodium and chloride ions pass through only with difficulty. Therefore, sodium chloride solution is actually a mixture of permeant water molecules and nonpermeant sodium and chloride ions, and the membrane is said to be selectively permeable to water but much less so to sodium and chloride ions. Yet the presence of the sodium and chloride has displaced some of the water molecules on the side of the membrane where these ions are present and, therefore, has reduced the concentration of water molecules to less than that of pure water. As a result, in the example of Figure 4-10 , more water molecules strike the channels on the left side, where there is pure water, than on the right side, where the water concentration has been reduced. Thus, net movement of water occurs from left to right—that is, osmosis occurs from the pure water into the sodium chloride solution.

Figure 4-10 Osmosis at a cell membrane when a sodium chloride solution is placed on one side of the membrane and water is placed on the other side.

Osmotic Pressure
If in Figure 4-10 pressure were applied to the sodium chloride solution, osmosis of water into this solution would be slowed, stopped, or even reversed. The exact amount of pressure required to stop osmosis is called the osmotic pressure of the sodium chloride solution.
The principle of a pressure difference opposing osmosis is demonstrated in Figure 4-11 , which shows a selectively permeable membrane separating two columns of fluid, one containing pure water and the other containing a solution of water and any solute that will not penetrate the membrane. Osmosis of water from chamber B into chamber A causes the levels of the fluid columns to become farther and farther apart, until eventually a pressure difference develops between the two sides of the membrane great enough to oppose the osmotic effect. The pressure difference across the membrane at this point is equal to the osmotic pressure of the solution that contains the nondiffusible solute.

Figure 4-11 Demonstration of osmotic pressure caused by osmosis at a semipermeable membrane.

Importance of Number of Osmotic Particles (Molar Concentration) in Determining Osmotic Pressure
The osmotic pressure exerted by particles in a solution, whether they are molecules or ions, is determined by the number of particles per unit volume of fluid, not by the mass of the particles. The reason for this is that each particle in a solution, regardless of its mass, exerts, on average, the same amount of pressure against the membrane. That is, large particles, which have greater mass (m) than small particles, move at slower velocities (v). The small particles move at higher velocities in such a way that their average kinetic energies (k), determined by the equation
are the same for each small particle as for each large particle. Consequently, the factor that determines the osmotic pressure of a solution is the concentration of the solution in terms of number of particles (which is the same as its molar concentration if it is a nondissociated molecule), not in terms of mass of the solute.

“Osmolality”—The Osmole
To express the concentration of a solution in terms of numbers of particles, the unit called the osmole is used in place of grams.
One osmole is 1 gram molecular weight of osmotically active solute. Thus, 180 grams of glucose, which is 1 gram molecular weight of glucose, is equal to 1 osmole of glucose because glucose does not dissociate into ions. If a solute dissociates into two ions, 1 gram molecular weight of the solute will become 2 osmoles because the number of osmotically active particles is now twice as great as is the case for the nondissociated solute. Therefore, when fully dissociated, 1 gram molecular weight of sodium chloride, 58.5 grams, is equal to 2 osmoles.
Thus, a solution that has 1 osmole of solute dissolved in each kilogram of water is said to have an osmolality of 1 osmole per kilogram, and a solution that has 1/1000 osmole dissolved per kilogram has an osmolality of 1 milliosmole per kilogram. The normal osmolality of the extracellular and intracellular fluids is about 300 milliosmoles per kilogram of water.

Relation of Osmolality to Osmotic Pressure
At normal body temperature, 37 °C, a concentration of 1 osmole per liter will cause 19,300 mm Hg osmotic pressure in the solution. Likewise, 1 milliosmole per liter concentration is equivalent to 19.3 mm Hg osmotic pressure. Multiplying this value by the 300 milliosmolar concentration of the body fluids gives a total calculated osmotic pressure of the body fluids of 5790 mm Hg. The measured value for this, however, averages only about 5500 mm Hg. The reason for this difference is that many of the ions in the body fluids, such as sodium and chloride ions, are highly attracted to one another; consequently, they cannot move entirely unrestrained in the fluids and create their full osmotic pressure potential. Therefore, on average, the actual osmotic pressure of the body fluids is about 0.93 times the calculated value.

The Term “Osmolarity.”
Osmolarity is the osmolar concentration expressed as osmoles per liter of solution rather than osmoles per kilogram of water. Although, strictly speaking, it is osmoles per kilogram of water (osmolality) that determines osmotic pressure, for dilute solutions such as those in the body, the quantitative differences between osmolarity and osmolality are less than 1 percent. Because it is far more practical to measure osmolarity than osmolality, this is the usual practice in almost all physiological studies.

“Active Transport” of Substances Through Membranes
At times, a large concentration of a substance is required in the intracellular fluid even though the extracellular fluid contains only a small concentration. This is true, for instance, for potassium ions. Conversely, it is important to keep the concentrations of other ions very low inside the cell even though their concentrations in the extracellular fluid are great. This is especially true for sodium ions. Neither of these two effects could occur by simple diffusion because simple diffusion eventually equilibrates concentrations on the two sides of the membrane. Instead, some energy source must cause excess movement of potassium ions to the inside of cells and excess movement of sodium ions to the outside of cells. When a cell membrane moves molecules or ions “uphill” against a concentration gradient (or “uphill” against an electrical or pressure gradient), the process is called active transport.
Different substances that are actively transported through at least some cell membranes include sodium ions, potassium ions, calcium ions, iron ions, hydrogen ions, chloride ions, iodide ions, urate ions, several different sugars, and most of the amino acids.

Primary Active Transport and Secondary Active Transport
Active transport is divided into two types according to the source of the energy used to cause the transport: primary active transport and secondary active transport. In primary active transport, the energy is derived directly from breakdown of adenosine triphosphate (ATP) or of some other high-energy phosphate compound. In secondary active transport, the energy is derived secondarily from energy that has been stored in the form of ionic concentration differences of secondary molecular or ionic substances between the two sides of a cell membrane, created originally by primary active transport. In both instances, transport depends on carrier proteins that penetrate through the cell membrane, as is true for facilitated diffusion. However, in active transport, the carrier protein functions differently from the carrier in facilitated diffusion because it is capable of imparting energy to the transported substance to move it against the electrochemical gradient. Following are some examples of primary active transport and secondary active transport, with more detailed explanations of their principles of function.

Primary Active Transport

Sodium-Potassium Pump
Among the substances that are transported by primary active transport are sodium, potassium, calcium, hydrogen, chloride, and a few other ions.
The active transport mechanism that has been studied in greatest detail is the sodium-potassium (Na + -K + ) pump, a transport process that pumps sodium ions outward through the cell membrane of all cells and at the same time pumps potassium ions from the outside to the inside. This pump is responsible for maintaining the sodium and potassium concentration differences across the cell membrane, as well as for establishing a negative electrical voltage inside the cells. Indeed, Chapter 5 shows that this pump is also the basis of nerve function, transmitting nerve signals throughout the nervous system.
Figure 4-12 shows the basic physical components of the Na + -K + pump. The carrier protein is a complex of two separate globular proteins: a larger one called the α subunit, with a molecular weight of about 100,000, and a smaller one called the β subunit, with a molecular weight of about 55,000. Although the function of the smaller protein is not known (except that it might anchor the protein complex in the lipid membrane), the larger protein has three specific features that are important for the functioning of the pump:
1. It has three receptor sites for binding sodium ions on the portion of the protein that protrudes to the inside of the cell.
2. It has two receptor sites for potassium ions on the outside.
3. The inside portion of this protein near the sodium binding sites has ATPase activity.

Figure 4-12 Postulated mechanism of the sodium-potassium pump. ADP, adenosine diphosphate; ATP, adenosine triphosphate; Pi, phosphate ion.
When two potassium ions bind on the outside of the carrier protein and three sodium ions bind on the inside, the ATPase function of the protein becomes activated. This then cleaves one molecule of ATP, splitting it to adenosine diphosphate (ADP) and liberating a high-energy phosphate bond of energy. This liberated energy is then believed to cause a chemical and conformational change in the protein carrier molecule, extruding the three sodium ions to the outside and the two potassium ions to the inside.
As with other enzymes, the Na + -K + ATPase pump can run in reverse. If the electrochemical gradients for Na + and K + are experimentally increased enough so that the energy stored in their gradients is greater than the chemical energy of ATP hydrolysis, these ions will move down their concentration gradients and the Na + -K + pump will synthesize ATP from ADP and phosphate. The phosphorylated form of the Na + -K + pump, therefore, can either donate its phosphate to ADP to produce ATP or use the energy to change its conformation and pump Na + out of the cell and K + into the cell. The relative concentrations of ATP, ADP, and phosphate, as well as the electrochemical gradients for Na + and K + , determine the direction of the enzyme reaction. For some cells, such as electrically active nerve cells, 60 to 70 percent of the cells’ energy requirement may be devoted to pumping Na + out of the cell and K + into the cell.

The Na + -K + Pump Is Important For Controlling Cell Volume
One of the most important functions of the Na + -K + pump is to control the volume of each cell. Without function of this pump, most cells of the body would swell until they burst. The mechanism for controlling the volume is as follows: Inside the cell are large numbers of proteins and other organic molecules that cannot escape from the cell. Most of these are negatively charged and therefore attract large numbers of potassium, sodium, and other positive ions as well. All these molecules and ions then cause osmosis of water to the interior of the cell. Unless this is checked, the cell will swell indefinitely until it bursts. The normal mechanism for preventing this is the Na + -K + pump. Note again that this device pumps three Na + ions to the outside of the cell for every two K + ions pumped to the interior. Also, the membrane is far less permeable to sodium ions than to potassium ions, so once the sodium ions are on the outside, they have a strong tendency to stay there. Thus, this represents a net loss of ions out of the cell, which initiates osmosis of water out of the cell as well.
If a cell begins to swell for any reason, this automatically activates the Na + -K + pump, moving still more ions to the exterior and carrying water with them. Therefore, the Na + -K + pump performs a continual surveillance role in maintaining normal cell volume.

Electrogenic Nature of the Na + -K + Pump
The fact that the Na + -K + pump moves three Na + ions to the exterior for every two K + ions to the interior means that a net of one positive charge is moved from the interior of the cell to the exterior for each cycle of the pump. This creates positivity outside the cell but leaves a deficit of positive ions inside the cell; that is, it causes negativity on the inside. Therefore, the Na + -K + pump is said to be electrogenic because it creates an electrical potential across the cell membrane. As discussed in Chapter 5 , this electrical potential is a basic requirement in nerve and muscle fibers for transmitting nerve and muscle signals.

Primary Active Transport of Calcium Ions
Another important primary active transport mechanism is the calcium pump. Calcium ions are normally maintained at extremely low concentration in the intracellular cytosol of virtually all cells in the body, at a concentration about 10,000 times less than that in the extracellular fluid. This is achieved mainly by two primary active transport calcium pumps. One is in the cell membrane and pumps calcium to the outside of the cell. The other pumps calcium ions into one or more of the intracellular vesicular organelles of the cell, such as the sarcoplasmic reticulum of muscle cells and the mitochondria in all cells. In each of these instances, the carrier protein penetrates the membrane and functions as an enzyme ATPase, having the same capability to cleave ATP as the ATPase of the sodium carrier protein. The difference is that this protein has a highly specific binding site for calcium instead of for sodium.

Primary Active Transport of Hydrogen Ions
At two places in the body, primary active transport of hydrogen ions is important: (1) in the gastric glands of the stomach and (2) in the late distal tubules and cortical collecting ducts of the kidneys.
In the gastric glands, the deep-lying parietal cells have the most potent primary active mechanism for transporting hydrogen ions of any part of the body. This is the basis for secreting hydrochloric acid in the stomach digestive secretions. At the secretory ends of the gastric gland parietal cells, the hydrogen ion concentration is increased as much as a millionfold and then released into the stomach along with chloride ions to form hydrochloric acid.
In the renal tubules are special intercalated cells in the late distal tubules and cortical collecting ducts that also transport hydrogen ions by primary active transport. In this case, large amounts of hydrogen ions are secreted from the blood into the urine for the purpose of eliminating excess hydrogen ions from the body fluids. The hydrogen ions can be secreted into the urine against a concentration gradient of about 900-fold.

Energetics of Primary Active Transport
The amount of energy required to transport a substance actively through a membrane is determined by how much the substance is concentrated during transport. Compared with the energy required to concentrate a substance 10-fold, to concentrate it 100-fold requires twice as much energy, and to concentrate it 1000-fold requires three times as much energy. In other words, the energy required is proportional to the logarithm of the degree that the substance is concentrated, as expressed by the following formula:
Thus, in terms of calories, the amount of energy required to concentrate 1 osmole of a substance 10-fold is about 1400 calories; or to concentrate it 100-fold, 2800 calories. One can see that the energy expenditure for concentrating substances in cells or for removing substances from cells against a concentration gradient can be tremendous. Some cells, such as those lining the renal tubules and many glandular cells, expend as much as 90 percent of their energy for this purpose alone.

Secondary Active Transport—Co-Transport and Counter-Transport
When sodium ions are transported out of cells by primary active transport, a large concentration gradient of sodium ions across the cell membrane usually develops—high concentration outside the cell and low concentration inside. This gradient represents a storehouse of energy because the excess sodium outside the cell membrane is always attempting to diffuse to the interior. Under appropriate conditions, this diffusion energy of sodium can pull other substances along with the sodium through the cell membrane. This phenomenon is called co-transport; it is one form of secondary active transport.
For sodium to pull another substance along with it, a coupling mechanism is required. This is achieved by means of still another carrier protein in the cell membrane. The carrier in this instance serves as an attachment point for both the sodium ion and the substance to be co-transported. Once they both are attached, the energy gradient of the sodium ion causes both the sodium ion and the other substance to be transported together to the interior of the cell.
In counter-transport, sodium ions again attempt to diffuse to the interior of the cell because of their large concentration gradient. However, this time, the substance to be transported is on the inside of the cell and must be transported to the outside. Therefore, the sodium ion binds to the carrier protein where it projects to the exterior surface of the membrane, while the substance to be counter-transported binds to the interior projection of the carrier protein. Once both have bound, a conformational change occurs, and energy released by the sodium ion moving to the interior causes the other substance to move to the exterior.

Co-Transport of Glucose and Amino Acids Along with Sodium Ions
Glucose and many amino acids are transported into most cells against large concentration gradients; the mechanism of this is entirely by co-transport, as shown in Figure 4-13 . Note that the transport carrier protein has two binding sites on its exterior side, one for sodium and one for glucose. Also, the concentration of sodium ions is high on the outside and low inside, which provides energy for the transport. A special property of the transport protein is that a conformational change to allow sodium movement to the interior will not occur until a glucose molecule also attaches. When they both become attached, the conformational change takes place automatically, and the sodium and glucose are transported to the inside of the cell at the same time. Hence, this is a sodium-glucose co-transport mechanism. Sodium-glucose co-transporters are especially important mechanisms in transporting glucose across renal and intestinal epithelial cells, as discussed in Chapters 27 and 65 .

Figure 4-13 Postulated mechanism for sodium co-transport of glucose.
Sodium co-transport of the amino acids occurs in the same manner as for glucose, except that it uses a different set of transport proteins. Five amino acid transport proteins have been identified, each of which is responsible for transporting one subset of amino acids with specific molecular characteristics.
Sodium co-transport of glucose and amino acids occurs especially through the epithelial cells of the intestinal tract and the renal tubules of the kidneys to promote absorption of these substances into the blood, as is discussed in later chapters.
Other important co-transport mechanisms in at least some cells include co-transport of chloride ions, iodine ions, iron ions, and urate ions.

Sodium Counter-Transport of Calcium and Hydrogen Ions
Two especially important counter-transport mechanisms (transport in a direction opposite to the primary ion) are sodium-calcium counter-transport and sodium-hydrogen counter-transport ( Figure 4-14 ).

Figure 4-14 Sodium counter-transport of calcium and hydrogen ions.
Sodium-calcium counter-transport occurs through all or almost all cell membranes, with sodium ions moving to the interior and calcium ions to the exterior, both bound to the same transport protein in a counter-transport mode. This is in addition to primary active transport of calcium that occurs in some cells.
Sodium-hydrogen counter-transport occurs in several tissues. An especially important example is in the proximal tubules of the kidneys, where sodium ions move from the lumen of the tubule to the interior of the tubular cell, while hydrogen ions are counter-transported into the tubule lumen. As a mechanism for concentrating hydrogen ions, counter-transport is not nearly as powerful as the primary active transport of hydrogen ions that occurs in the more distal renal tubules, but it can transport extremely large numbers of hydrogen ions, thus making it a key to hydrogen ion control in the body fluids, as discussed in detail in Chapter 30 .

Active Transport Through Cellular Sheets
At many places in the body, substances must be transported all the way through a cellular sheet instead of simply through the cell membrane. Transport of this type occurs through the (1) intestinal epithelium, (2) epithelium of the renal tubules, (3) epithelium of all exocrine glands, (4) epithelium of the gallbladder, and (5) membrane of the choroid plexus of the brain and other membranes.
The basic mechanism for transport of a substance through a cellular sheet is (1) active transport through the cell membrane on one side of the transporting cells in the sheet, and then (2) either simple diffusion or facilitated diffusion through the membrane on the opposite side of the cell.
Figure 4-15 shows a mechanism for transport of sodium ions through the epithelial sheet of the intestines, gallbladder, and renal tubules. This figure shows that the epithelial cells are connected together tightly at the luminal pole by means of junctions called “kisses.” The brush border on the luminal surfaces of the cells is permeable to both sodium ions and water. Therefore, sodium and water diffuse readily from the lumen into the interior of the cell. Then, at the basal and lateral membranes of the cells, sodium ions are actively transported into the extracellular fluid of the surrounding connective tissue and blood vessels. This creates a high sodium ion concentration gradient across these membranes, which in turn causes osmosis of water as well. Thus, active transport of sodium ions at the basolateral sides of the epithelial cells results in transport not only of sodium ions but also of water.

Figure 4-15 Basic mechanism of active transport across a layer of cells.
These are the mechanisms by which almost all the nutrients, ions, and other substances are absorbed into the blood from the intestine; they are also the way the same substances are reabsorbed from the glomerular filtrate by the renal tubules.
Throughout this text are numerous examples of the different types of transport discussed in this chapter.


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CHAPTER 5 Membrane Potentials and Action Potentials
Electrical potentials exist across the membranes of virtually all cells of the body. In addition, some cells, such as nerve and muscle cells, are capable of generating rapidly changing electrochemical impulses at their membranes, and these impulses are used to transmit signals along the nerve or muscle membranes. In other types of cells, such as glandular cells, macrophages, and ciliated cells, local changes in membrane potentials also activate many of the cells’ functions. The present discussion is concerned with membrane potentials generated both at rest and during action by nerve and muscle cells.

Basic Physics of Membrane Potentials

Membrane Potentials Caused by Diffusion

“Diffusion Potential” Caused by an Ion Concentration Difference on the Two Sides of the Membrane
In Figure 5-1 A , the potassium concentration is great inside a nerve fiber membrane but very low outside the membrane. Let us assume that the membrane in this instance is permeable to the potassium ions but not to any other ions. Because of the large potassium concentration gradient from inside toward outside, there is a strong tendency for extra numbers of potassium ions to diffuse outward through the membrane. As they do so, they carry positive electrical charges to the outside, thus creating electropositivity outside the membrane and electronegativity inside because of negative anions that remain behind and do not diffuse outward with the potassium. Within a millisecond or so, the potential difference between the inside and outside, called the diffusion potential, becomes great enough to block further net potassium diffusion to the exterior, despite the high potassium ion concentration gradient. In the normal mammalian nerve fiber, the potential difference required is about 94 millivolts, with negativity inside the fiber membrane.

Figure 5-1 A, Establishment of a “diffusion” potential across a nerve fiber membrane, caused by diffusion of potassium ions from inside the cell to outside through a membrane that is selectively permeable only to potassium. B, Establishment of a “diffusion potential” when the nerve fiber membrane is permeable only to sodium ions. Note that the internal membrane potential is negative when potassium ions diffuse and positive when sodium ions diffuse because of opposite concentration gradients of these two ions.
Figure 5-1 B shows the same phenomenon as in Figure 5-1 A , but this time with high concentration of sodium ions outside the membrane and low sodium inside . These ions are also positively charged. This time, the membrane is highly permeable to the sodium ions but impermeable to all other ions. Diffusion of the positively charged sodium ions to the inside creates a membrane potential of opposite polarity to that in Figure 5-1 A , with negativity outside and positivity inside. Again, the membrane potential rises high enough within milliseconds to block further net diffusion of sodium ions to the inside; however, this time, in the mammalian nerve fiber, the potential is about 61 millivolts positive inside the fiber.
Thus, in both parts of Figure 5-1 , we see that a concentration difference of ions across a selectively permeable membrane can, under appropriate conditions, create a membrane potential. Later in this chapter, we show that many of the rapid changes in membrane potentials observed during nerve and muscle impulse transmission result from the occurrence of such rapidly changing diffusion potentials.

Relation of the Diffusion Potential to the Concentration Difference—The Nernst Potential
The diffusion potential level across a membrane that exactly opposes the net diffusion of a particular ion through the membrane is called the Nernst potential for that ion, a term that was introduced in Chapter 4 . The magnitude of this Nernst potential is determined by the ratio of the concentrations of that specific ion on the two sides of the membrane. The greater this ratio, the greater the tendency for the ion to diffuse in one direction, and therefore the greater the Nernst potential required to prevent additional net diffusion. The following equation, called the Nernst equation, can be used to calculate the Nernst potential for any univalent ion at normal body temperature of 98.6°F (37°C):

where EMF is electromotive force.
When using this formula, it is usually assumed that the potential in the extracellular fluid outside the membrane remains at zero potential, and the Nernst potential is the potential inside the membrane. Also, the sign of the potential is positive (+) if the ion diffusing from inside to outside is a negative ion, and it is negative (−) if the ion is positive. Thus, when the concentration of positive potassium ions on the inside is 10 times that on the outside, the log of 10 is 1, so the Nernst potential calculates to be −61 millivolts inside the membrane.

Calculation of the Diffusion Potential When the Membrane Is Permeable to Several Different Ions
When a membrane is permeable to several different ions, the diffusion potential that develops depends on three factors: (1) the polarity of the electrical charge of each ion, (2) the permeability of the membrane (P) to each ion, and (3) the concentrations (C) of the respective ions on the inside (i) and outside (o) of the membrane. Thus, the following formula, called the Goldman equation, or the Goldman-Hodgkin-Katz equation, gives the calculated membrane potential on the inside of the membrane when two univalent positive ions, sodium (Na + ) and potassium (K + ), and one univalent negative ion, chloride (Cl − ), are involved.

Let us study the importance and the meaning of this equation. First, sodium, potassium, and chloride ions are the most important ions involved in the development of membrane potentials in nerve and muscle fibers, as well as in the neuronal cells in the nervous system. The concentration gradient of each of these ions across the membrane helps determine the voltage of the membrane potential.
Second, the degree of importance of each of the ions in determining the voltage is proportional to the membrane permeability for that particular ion. That is, if the membrane has zero permeability to both potassium and chloride ions, the membrane potential becomes entirely dominated by the concentration gradient of sodium ions alone, and the resulting potential will be equal to the Nernst potential for sodium. The same holds for each of the other two ions if the membrane should become selectively permeable for either one of them alone.
Third, a positive ion concentration gradient from inside the membrane to the outside causes electronegativity inside the membrane. The reason for this is that excess positive ions diffuse to the outside when their concentration is higher inside than outside. This carries positive charges to the outside but leaves the nondiffusible negative anions on the inside, thus creating electronegativity on the inside. The opposite effect occurs when there is a gradient for a negative ion. That is, a chloride ion gradient from the outside to the inside causes negativity inside the cell because excess negatively charged chloride ions diffuse to the inside, while leaving the nondiffusible positive ions on the outside.
Fourth, as explained later, the permeability of the sodium and potassium channels undergoes rapid changes during transmission of a nerve impulse, whereas the permeability of the chloride channels does not change greatly during this process. Therefore, rapid changes in sodium and potassium permeability are primarily responsible for signal transmission in neurons, which is the subject of most of the remainder of this chapter.

Measuring the Membrane Potential
The method for measuring the membrane potential is simple in theory but often difficult in practice because of the small size of most of the fibers. Figure 5-2 shows a small pipette filled with an electrolyte solution. The pipette is impaled through the cell membrane to the interior of the fiber. Then another electrode, called the “indifferent electrode,” is placed in the extracellular fluid, and the potential difference between the inside and outside of the fiber is measured using an appropriate voltmeter. This voltmeter is a highly sophisticated electronic apparatus that is capable of measuring small voltages despite extremely high resistance to electrical flow through the tip of the micropipette, which has a lumen diameter usually less than 1 micrometer and a resistance more than a million ohms. For recording rapid changes in the membrane potential during transmission of nerve impulses, the microelectrode is connected to an oscilloscope, as explained later in the chapter.

Figure 5-2 Measurement of the membrane potential of the nerve fiber using a microelectrode.
The lower part of Figure 5-2 shows the electrical potential that is measured at each point in or near the nerve fiber membrane, beginning at the left side of the figure and passing to the right. As long as the electrode is outside the nerve membrane, the recorded potential is zero, which is the potential of the extracellular fluid. Then, as the recording electrode passes through the voltage change area at the cell membrane (called the electrical dipole layer ), the potential decreases abruptly to −90 millivolts. Moving across the center of the fiber, the potential remains at a steady −90-millivolt level but reverses back to zero the instant it passes through the membrane on the opposite side of the fiber.

Figure 5-3 Distribution of positively and negatively charged ions in the extracellular fluid surrounding a nerve fiber and in the fluid inside the fiber; note the alignment of negative charges along the inside surface of the membrane and positive charges along the outside surface. The lower panel displays the abrupt changes in membrane potential that occur at the membranes on the two sides of the fiber.
To create a negative potential inside the membrane, only enough positive ions to develop the electrical dipole layer at the membrane itself must be transported outward. All the remaining ions inside the nerve fiber can be both positive and negative, as shown in the upper panel of Figure 5-3 . Therefore, an incredibly small number of ions must be transferred through the membrane to establish the normal “resting potential” of −90 millivolts inside the nerve fiber; this means that only about 1/3,000,000 to 1/100,000,000 of the total positive charges inside the fiber must be transferred. Also, an equally small number of positive ions moving from outside to inside the fiber can reverse the potential from −90 millivolts to as much as +35 millivolts within as little as 1/10,000 of a second. Rapid shifting of ions in this manner causes the nerve signals discussed in subsequent sections of this chapter.

Resting Membrane Potential of Nerves
The resting membrane potential of large nerve fibers when not transmitting nerve signals is about −90 millivolts. That is, the potential inside the fiber is 90 millivolts more negative than the potential in the extracellular fluid on the outside of the fiber. In the next few paragraphs, the transport properties of the resting nerve membrane for sodium and potassium and the factors that determine the level of this resting potential are explained.

Active Transport of Sodium and Potassium Ions Through the Membrane—The Sodium-Potassium (Na + -K + ) Pump
First, let us recall from Chapter 4 that all cell membranes of the body have a powerful Na + -K + pump that continually transports sodium ions to the outside of the cell and potassium ions to the inside, as illustrated on the left-hand side in Figure 5-4 . Further, note that this is an electrogenic pump because more positive charges are pumped to the outside than to the inside (three Na + ions to the outside for each two K + ions to the inside), leaving a net deficit of positive ions on the inside; this causes a negative potential inside the cell membrane.

Figure 5-4 Functional characteristics of the Na + -K + pump and of the K + “leak” channels. ADP, adenosine diphosphate; ATP, adenosine triphosphate. The K + “leak” channels also leak Na + ions into the cell slightly, but are much more permeable to K + .
The Na + -K + pump also causes large concentration gradients for sodium and potassium across the resting nerve membrane. These gradients are the following:

The ratios of these two respective ions from the inside to the outside are

Leakage of Potassium Through the Nerve Membrane
The right side of Figure 5-4 shows a channel protein, sometimes called a “ tandem pore domain,” potassium channel , or potassium (K + ) “leak” channel, in the nerve membrane through which potassium can leak even in a resting cell. The basic structure of potassium channels was described in Chapter 4 ( Figure 4-4 ). These K + leak channels may also leak sodium ions slightly but are far more permeable to potassium than to sodium, normally about 100 times as permeable. As discussed later, this differential in permeability is a key factor in determining the level of the normal resting membrane potential.

Origin of the Normal Resting Membrane Potential
Figure 5-5 shows the important factors in the establishment of the normal resting membrane potential of −90 millivolts. They are as follows.

Figure 5-5 Establishment of resting membrane potentials in nerve fibers under three conditions: A, when the membrane potential is caused entirely by potassium diffusion alone; B, when the membrane potential is caused by diffusion of both sodium and potassium ions; and C, when the membrane potential is caused by diffusion of both sodium and potassium ions plus pumping of both these ions by the Na + -K + pump.

Contribution of the Potassium Diffusion Potential
In Figure 5-5 A , we make the assumption that the only movement of ions through the membrane is diffusion of potassium ions, as demonstrated by the open channels between the potassium symbols (K + ) inside and outside the membrane. Because of the high ratio of potassium ions inside to outside, 35:1, the Nernst potential corresponding to this ratio is −94 millivolts because the logarithm of 35 is 1.54, and this multiplied by −61 millivolts is −94 millivolts. Therefore, if potassium ions were the only factor causing the resting potential, the resting potential inside the fiber would be equal to −94 millivolts, as shown in the figure.

Contribution of Sodium Diffusion Through the Nerve Membrane
Figure 5-5 B shows the addition of slight permeability of the nerve membrane to sodium ions, caused by the minute diffusion of sodium ions through the K + -Na + leak channels. The ratio of sodium ions from inside to outside the membrane is 0.1, and this gives a calculated Nernst potential for the inside of the membrane of +61 millivolts. But also shown in Figure 5-5 B is the Nernst potential for potassium diffusion of −94 millivolts. How do these interact with each other, and what will be the summated potential? This can be answered by using the Goldman equation described previously. Intuitively, one can see that if the membrane is highly permeable to potassium but only slightly permeable to sodium, it is logical that the diffusion of potassium contributes far more to the membrane potential than does the diffusion of sodium. In the normal nerve fiber, the permeability of the membrane to potassium is about 100 times as great as its permeability to sodium. Using this value in the Goldman equation gives a potential inside the membrane of −86 millivolts, which is near the potassium potential shown in the figure.

Contribution of the Na + -K + Pump
In Figure 5-5 C , the Na + -K + pump is shown to provide an additional contribution to the resting potential. In this figure, there is continuous pumping of three sodium ions to the outside for each two potassium ions pumped to the inside of the membrane. The fact that more sodium ions are being pumped to the outside than potassium to the inside causes continual loss of positive charges from inside the membrane; this creates an additional degree of negativity (about −4 millivolts additional) on the inside beyond that which can be accounted for by diffusion alone. Therefore, as shown in Figure 5-5 C , the net membrane potential with all these factors operative at the same time is about −90 millivolts.
In summary, the diffusion potentials alone caused by potassium and sodium diffusion would give a membrane potential of about −86 millivolts, almost all of this being determined by potassium diffusion. Then, an additional −4 millivolts is contributed to the membrane potential by the continuously acting electrogenic Na + -K + pump, giving a net membrane potential of −90 millivolts.

Nerve Action Potential
Nerve signals are transmitted by action potentials, which are rapid changes in the membrane potential that spread rapidly along the nerve fiber membrane. Each action potential begins with a sudden change from the normal resting negative membrane potential to a positive potential and then ends with an almost equally rapid change back to the negative potential. To conduct a nerve signal, the action potential moves along the nerve fiber until it comes to the fiber’s end.
The upper panel of Figure 5-6 shows the changes that occur at the membrane during the action potential, with transfer of positive charges to the interior of the fiber at its onset and return of positive charges to the exterior at its end. The lower panel shows graphically the successive changes in membrane potential over a few 10,000ths of a second, illustrating the explosive onset of the action potential and the almost equally rapid recovery.

Figure 5-6 Typical action potential recorded by the method shown in the upper panel of the figure.
The successive stages of the action potential are as follows.

Resting Stage
This is the resting membrane potential before the action potential begins. The membrane is said to be “polarized” during this stage because of the −90 millivolts negative membrane potential that is present.

Depolarization Stage
At this time, the membrane suddenly becomes permeable to sodium ions, allowing tremendous numbers of positively charged sodium ions to diffuse to the interior of the axon. The normal “polarized” state of −90 millivolts is immediately neutralized by the inflowing positively charged sodium ions, with the potential rising rapidly in the positive direction. This is called depolarization. In large nerve fibers, the great excess of positive sodium ions moving to the inside causes the membrane potential to actually “overshoot” beyond the zero level and to become somewhat positive. In some smaller fibers, as well as in many central nervous system neurons, the potential merely approaches the zero level and does not overshoot to the positive state.

Repolarization Stage
Within a few 10,000ths of a second after the membrane becomes highly permeable to sodium ions, the sodium channels begin to close and the potassium channels open more than normal. Then, rapid diffusion of potassium ions to the exterior re-establishes the normal negative resting membrane potential. This is called repolarization of the membrane.
To explain more fully the factors that cause both depolarization and repolarization, we will describe the special characteristics of two other types of transport channels through the nerve membrane: the voltage-gated sodium and potassium channels.

Voltage-Gated Sodium and Potassium Channels
The necessary actor in causing both depolarization and repolarization of the nerve membrane during the action potential is the voltage-gated sodium channel. A voltage-gated potassium channel also plays an important role in increasing the rapidity of repolarization of the membrane. These two voltage-gated channels are in addition to the Na + -K + pump and the K + leak channels.

Voltage-Gated Sodium Channel—Activation and Inactivation of the Channel
The upper panel of Figure 5-7 shows the voltage-gated sodium channel in three separate states. This channel has two gates —one near the outside of the channel called the activation gate, and another near the inside called the inactivation gate. The upper left of the figure depicts the state of these two gates in the normal resting membrane when the membrane potential is −90 millivolts. In this state, the activation gate is closed, which prevents any entry of sodium ions to the interior of the fiber through these sodium channels.

Figure 5-7 Characteristics of the voltage-gated sodium ( top ) and potassium ( bottom ) channels, showing successive activation and inactivation of the sodium channels and delayed activation of the potassium channels when the membrane potential is changed from the normal resting negative value to a positive value.

Activation of the Sodium Channel
When the membrane potential becomes less negative than during the resting state, rising from −90 millivolts toward zero, it finally reaches a voltage—usually somewhere between −70 and −50 millivolts—that causes a sudden conformational change in the activation gate, flipping it all the way to the open position. This is called the activated state; during this state, sodium ions can pour inward through the channel, increasing the sodium permeability of the membrane as much as 500- to 5000-fold.

Inactivation of the Sodium Channel
The upper right panel of Figure 5-7 shows a third state of the sodium channel. The same increase in voltage that opens the activation gate also closes the inactivation gate. The inactivation gate, however, closes a few 10,000ths of a second after the activation gate opens. That is, the conformational change that flips the inactivation gate to the closed state is a slower process than the conformational change that opens the activation gate. Therefore, after the sodium channel has remained open for a few 10,000ths of a second, the inactivation gate closes, and sodium ions no longer can pour to the inside of the membrane. At this point, the membrane potential begins to recover back toward the resting membrane state, which is the repolarization process.
Another important characteristic of the sodium channel inactivation process is that the inactivation gate will not reopen until the membrane potential returns to or near the original resting membrane potential level. Therefore, it is usually not possible for the sodium channels to open again without first repolarizing the nerve fiber.

Voltage-Gated Potassium Channel and Its Activation
The lower panel of Figure 5-7 shows the voltage-gated potassium channel in two states: during the resting state (left) and toward the end of the action potential (right). During the resting state, the gate of the potassium channel is closed and potassium ions are prevented from passing through this channel to the exterior. When the membrane potential rises from −90 millivolts toward zero, this voltage change causes a conformational opening of the gate and allows increased potassium diffusion outward through the channel. However, because of the slight delay in opening of the potassium channels, for the most part, they open just at the same time that the sodium channels are beginning to close because of inactivation. Thus, the decrease in sodium entry to the cell and the simultaneous increase in potassium exit from the cell combine to speed the repolarization process, leading to full recovery of the resting membrane potential within another few 10,000ths of a second.

Research Method for Measuring the Effect of Voltage on Opening and Closing of the Voltage-Gated Channels—The “Voltage Clamp.”
The original research that led to quantitative understanding of the sodium and potassium channels was so ingenious that it led to Nobel Prizes for the scientists responsible, Hodgkin and Huxley. The essence of these studies is shown in Figures 5-8 and 5-9 .

Figure 5-8 “Voltage clamp” method for studying flow of ions through specific channels.

Figure 5-9 Typical changes in conductance of sodium and potassium ion channels when the membrane potential is suddenly increased from the normal resting value of −90 millivolts to a positive value of +10 millivolts for 2 milliseconds. This figure shows that the sodium channels open (activate) and then close (inactivate) before the end of the 2 milliseconds, whereas the potassium channels only open (activate), and the rate of opening is much slower than that of the sodium channels.
Figure 5-8 shows an experimental apparatus called a voltage clamp, which is used to measure flow of ions through the different channels. In using this apparatus, two electrodes are inserted into the nerve fiber. One of these is to measure the voltage of the membrane potential, and the other is to conduct electrical current into or out of the nerve fiber. This apparatus is used in the following way: The investigator decides which voltage he or she wants to establish inside the nerve fiber. The electronic portion of the apparatus is then adjusted to the desired voltage, and this automatically injects either positive or negative electricity through the current electrode at whatever rate is required to hold the voltage, as measured by the voltage electrode, at the level set by the operator. When the membrane potential is suddenly increased by this voltage clamp from −90 millivolts to zero, the voltage-gated sodium and potassium channels open and sodium and potassium ions begin to pour through the channels. To counterbalance the effect of these ion movements on the desired setting of the intracellular voltage, electrical current is injected automatically through the current electrode of the voltage clamp to maintain the intracellular voltage at the required steady zero level. To achieve this, the current injected must be equal to but of opposite polarity to the net current flow through the membrane channels. To measure how much current flow is occurring at each instant, the current electrode is connected to an oscilloscope that records the current flow, as demonstrated on the screen of the oscilloscope in Figure 5-8 . Finally, the investigator adjusts the concentrations of the ions to other than normal levels both inside and outside the nerve fiber and repeats the study. This can be done easily when using large nerve fibers removed from some invertebrates, especially the giant squid axon, which in some cases is as large as 1 millimeter in diameter. When sodium is the only permeant ion in the solutions inside and outside the squid axon, the voltage clamp measures current flow only through the sodium channels. When potassium is the only permeant ion, current flow only through the potassium channels is measured.
Another means for studying the flow of ions through an individual type of channel is to block one type of channel at a time. For instance, the sodium channels can be blocked by a toxin called tetrodotoxin by applying it to the outside of the cell membrane where the sodium activation gates are located. Conversely, tetraethylammonium ion blocks the potassium channels when it is applied to the interior of the nerve fiber.
Figure 5-9 shows typical changes in conductance of the voltage-gated sodium and potassium channels when the membrane potential is suddenly changed by use of the voltage clamp from −90 millivolts to +10 millivolts and then, 2 milliseconds later, back to −90 millivolts. Note the sudden opening of the sodium channels (the activation stage) within a small fraction of a millisecond after the membrane potential is increased to the positive value. However, during the next millisecond or so, the sodium channels automatically close (the inactivation stage).
Note the opening (activation) of the potassium channels. These open slowly and reach their full open state only after the sodium channels have almost completely closed. Further, once the potassium channels open, they remain open for the entire duration of the positive membrane potential and do not close again until after the membrane potential is decreased back to a negative value.

Summary of the Events That Cause the Action Potential
Figure 5-10 shows in summary form the sequential events that occur during and shortly after the action potential. The bottom of the figure shows the changes in membrane conductance for sodium and potassium ions. During the resting state, before the action potential begins, the conductance for potassium ions is 50 to 100 times as great as the conductance for sodium ions. This is caused by much greater leakage of potassium ions than sodium ions through the leak channels. However, at the onset of the action potential, the sodium channels instantaneously become activated and allow up to a 5000-fold increase in sodium conductance. Then the inactivation process closes the sodium channels within another fraction of a millisecond. The onset of the action potential also causes voltage gating of the potassium channels, causing them to begin opening more slowly a fraction of a millisecond after the sodium channels open. At the end of the action potential, the return of the membrane potential to the negative state causes the potassium channels to close back to their original status, but again, only after an additional millisecond or more delay.

Figure 5-10 Changes in sodium and potassium conductance during the course of the action potential. Sodium conductance increases several thousand-fold during the early stages of the action potential, whereas potassium conductance increases only about 30-fold during the latter stages of the action potential and for a short period thereafter. (These curves were constructed from theory presented in papers by Hodgkin and Huxley but transposed from squid axon to apply to the membrane potentials of large mammalian nerve fibers.)
The middle portion of Figure 5-10 shows the ratio of sodium conductance to potassium conductance at each instant during the action potential, and above this is the action potential itself. During the early portion of the action potential, the ratio of sodium to potassium conductance increases more than 1000-fold. Therefore, far more sodium ions flow to the interior of the fiber than do potassium ions to the exterior. This is what causes the membrane potential to become positive at the action potential onset. Then the sodium channels begin to close and the potassium channels begin to open, so the ratio of conductance shifts far in favor of high potassium conductance but low sodium conductance. This allows very rapid loss of potassium ions to the exterior but virtually zero flow of sodium ions to the interior. Consequently, the action potential quickly returns to its baseline level.

Roles of Other Ions During the Action Potential
Thus far, we have considered only the roles of sodium and potassium ions in the generation of the action potential. At least two other types of ions must be considered: negative anions and calcium ions.

Impermeant Negatively Charged Ions (Anions) Inside the Nerve Axon
Inside the axon are many negatively charged ions that cannot go through the membrane channels. They include the anions of protein molecules and of many organic phosphate compounds, sulfate compounds, and so forth. Because these ions cannot leave the interior of the axon, any deficit of positive ions inside the membrane leaves an excess of these impermeant negative anions. Therefore, these impermeant negative ions are responsible for the negative charge inside the fiber when there is a net deficit of positively charged potassium ions and other positive ions.

Calcium Ions
The membranes of almost all cells of the body have a calcium pump similar to the sodium pump, and calcium serves along with (or instead of) sodium in some cells to cause most of the action potential. Like the sodium pump, the calcium pump transports calcium ions from the interior to the exterior of the cell membrane (or into the endoplasmic reticulum of the cell), creating a calcium ion gradient of about 10,000-fold. This leaves an internal cell concentration of calcium ions of about 10 −7 molar, in contrast to an external concentration of about 10 −3 molar.
In addition, there are voltage-gated calcium channels . Because calcium ion concentration is more than 10,000 times greater in the extracellular than the intracellular fluid, there is a tremendous diffusion gradient for passive flow of calcium ions into the cells. These channels are slightly permeable to sodium ions and calcium ions, but their permeability to calcium is about 1000-fold greater than to sodium under normal physiological conditions. When they open in response to a stimulus that depolarizes the cell membrane, calcium ions flow to the interior of the cell.
A major function of the voltage-gated calcium ion channels is to contribute to the depolarizing phase on the action potential in some cells. The gating of calcium channels, however, is slow, requiring 10 to 20 times as long for activation as for the sodium channels. For this reason they are often called slow channels, in contrast to the sodium channels, which are called fast channels. Therefore, the opening of calcium channels provides a more sustained depolarization, whereas the sodium channels play a key role in initiating action potentials.
Calcium channels are numerous in both cardiac muscle and smooth muscle. In fact, in some types of smooth muscle, the fast sodium channels are hardly present; therefore, the action potentials are caused almost entirely by activation of slow calcium channels.

Increased Permeability of the Sodium Channels When There Is a Deficit of Calcium Ions
The concentration of calcium ions in the extracellular fluid also has a profound effect on the voltage level at which the sodium channels become activated. When there is a deficit of calcium ions, the sodium channels become activated (opened) by a small increase of the membrane potential from its normal, very negative level. Therefore, the nerve fiber becomes highly excitable, sometimes discharging repetitively without provocation rather than remaining in the resting state. In fact, the calcium ion concentration needs to fall only 50 percent below normal before spontaneous discharge occurs in some peripheral nerves, often causing muscle “tetany .” This is sometimes lethal because of tetanic contraction of the respiratory muscles.
The probable way in which calcium ions affect the sodium channels is as follows: These ions appear to bind to the exterior surfaces of the sodium channel protein molecule. The positive charges of these calcium ions in turn alter the electrical state of the sodium channel protein itself, in this way altering the voltage level required to open the sodium gate.

Initiation of the Action Potential
Up to this point, we have explained the changing sodium and potassium permeability of the membrane, as well as the development of the action potential itself, but we have not explained what initiates the action potential.

A Positive-Feedback Cycle Opens the Sodium Channels
First, as long as the membrane of the nerve fiber remains undisturbed, no action potential occurs in the normal nerve. However, if any event causes enough initial rise in the membrane potential from −90 millivolts toward the zero level, the rising voltage itself causes many voltage-gated sodium channels to begin opening. This allows rapid inflow of sodium ions, which causes a further rise in the membrane potential, thus opening still more voltage-gated sodium channels and allowing more streaming of sodium ions to the interior of the fiber. This process is a positive-feedback cycle that, once the feedback is strong enough, continues until all the voltage-gated sodium channels have become activated (opened). Then, within another fraction of a millisecond, the rising membrane potential causes closure of the sodium channels and opening of potassium channels and the action potential soon terminates.

Threshold for Initiation of the Action Potential
An action potential will not occur until the initial rise in membrane potential is great enough to create the positive feedback described in the preceding paragraph. This occurs when the number of Na + ions entering the fiber becomes greater than the number of K + ions leaving the fiber. A sudden rise in membrane potential of 15 to 30 millivolts is usually required. Therefore, a sudden increase in the membrane potential in a large nerve fiber from −90 millivolts up to about −65 millivolts usually causes the explosive development of an action potential. This level of −65 millivolts is said to be the threshold for stimulation.

Propagation of the Action Potential
In the preceding paragraphs, we discussed the action potential as it occurs at one spot on the membrane. However, an action potential elicited at any one point on an excitable membrane usually excites adjacent portions of the membrane, resulting in propagation of the action potential along the membrane. This mechanism is demonstrated in Figure 5-11 . Figure 5-11 A shows a normal resting nerve fiber, and Figure 5-11 B shows a nerve fiber that has been excited in its midportion—that is, the midportion suddenly develops increased permeability to sodium. The arrows show a “local circuit” of current flow from the depolarized areas of the membrane to the adjacent resting membrane areas. That is, positive electrical charges are carried by the inward-diffusing sodium ions through the depolarized membrane and then for several millimeters in both directions along the core of the axon. These positive charges increase the voltage for a distance of 1 to 3 millimeters inside the large myelinated fiber to above the threshold voltage value for initiating an action potential. Therefore, the sodium channels in these new areas immediately open, as shown in Figure 5-11 C and D, and the explosive action potential spreads. These newly depolarized areas produce still more local circuits of current flow farther along the membrane, causing progressively more and more depolarization. Thus, the depolarization process travels along the entire length of the fiber. This transmission of the depolarization process along a nerve or muscle fiber is called a nerve or muscle impulse.

Figure 5-11 Propagation of action potentials in both directions along a conductive fiber.

Direction of Propagation
As demonstrated in Figure 5-11 , an excitable membrane has no single direction of propagation, but the action potential travels in all directions away from the stimulus—even along all branches of a nerve fiber—until the entire membrane has become depolarized.

All-or-Nothing Principle
Once an action potential has been elicited at any point on the membrane of a normal fiber, the depolarization process travels over the entire membrane if conditions are right, or it does not travel at all if conditions are not right. This is called the all-or-nothing principle, and it applies to all normal excitable tissues. Occasionally, the action potential reaches a point on the membrane at which it does not generate sufficient voltage to stimulate the next area of the membrane. When this occurs, the spread of depolarization stops. Therefore, for continued propagation of an impulse to occur, the ratio of action potential to threshold for excitation must at all times be greater than 1. This “greater than 1” requirement is called the safety factor for propagation.

Re-establishing Sodium and Potassium Ionic Gradients After Action Potentials Are Completed—Importance of Energy Metabolism
The transmission of each action potential along a nerve fiber reduces slightly the concentration differences of sodium and potassium inside and outside the membrane because sodium ions diffuse to the inside during depolarization and potassium ions diffuse to the outside during repolarization. For a single action potential, this effect is so minute that it cannot be measured. Indeed, 100,000 to 50 million impulses can be transmitted by large nerve fibers before the concentration differences reach the point that action potential conduction ceases. Even so, with time, it becomes necessary to re-establish the sodium and potassium membrane concentration differences. This is achieved by action of the Na + -K + pump in the same way as described previously in the chapter for the original establishment of the resting potential. That is, sodium ions that have diffused to the interior of the cell during the action potentials and potassium ions that have diffused to the exterior must be returned to their original state by the Na + -K + pump. Because this pump requires energy for operation, this “recharging” of the nerve fiber is an active metabolic process, using energy derived from the adenosine triphosphate (ATP) energy system of the cell. Figure 5-12 shows that the nerve fiber produces excess heat during recharging, which is a measure of energy expenditure when the nerve impulse frequency increases.

Figure 5-12 Heat production in a nerve fiber at rest and at progressively increasing rates of stimulation.
A special feature of the Na + -K + ATPase pump is that its degree of activity is strongly stimulated when excess sodium ions accumulate inside the cell membrane. In fact, the pumping activity increases approximately in proportion to the third power of this intracellular sodium concentration. That is, as the internal sodium concentration rises from 10 to 20 mEq/L, the activity of the pump does not merely double but increases about eightfold. Therefore, it is easy to understand how the “recharging” process of the nerve fiber can be set rapidly into motion whenever the concentration differences of sodium and potassium ions across the membrane begin to “run down.”

Plateau in Some Action Potentials
In some instances, the excited membrane does not repolarize immediately after depolarization; instead, the potential remains on a plateau near the peak of the spike potential for many milliseconds, and only then does repolarization begin. Such a plateau is shown in Figure 5-13 ; one can readily see that the plateau greatly prolongs the period of depolarization. This type of action potential occurs in heart muscle fibers, where the plateau lasts for as long as 0.2 to 0.3 second and causes contraction of heart muscle to last for this same long period.

Figure 5-13 Action potential (in millivolts) from a Purkinje fiber of the heart, showing a “plateau.”
The cause of the plateau is a combination of several factors. First, in heart muscle, two types of channels enter into the depolarization process: (1) the usual voltage-activated sodium channels, called fast channels, and (2) voltage-activated calcium-sodium channels, which are slow to open and therefore are called slow channels. Opening of fast channels causes the spike portion of the action potential, whereas the prolonged opening of the slow calcium-sodium channels mainly allows calcium ions to enter the fiber, which is largely responsible for the plateau portion of the action potential as well.
A second factor that may be partly responsible for the plateau is that the voltage-gated potassium channels are slower than usual to open, often not opening much until the end of the plateau. This delays the return of the membrane potential toward its normal negative value of −80 to −90 millivolts.

Rhythmicity of Some Excitable Tissues—Repetitive Discharge
Repetitive self-induced discharges occur normally in the heart, in most smooth muscle, and in many of the neurons of the central nervous system. These rhythmical discharges cause (1) the rhythmical beat of the heart, (2) rhythmical peristalsis of the intestines, and (3) such neuronal events as the rhythmical control of breathing.
Also, almost all other excitable tissues can discharge repetitively if the threshold for stimulation of the tissue cells is reduced low enough. For instance, even large nerve fibers and skeletal muscle fibers, which normally are highly stable, discharge repetitively when they are placed in a solution that contains the drug veratrine or when the calcium ion concentration falls below a critical value, both of which increase sodium permeability of the membrane.

Re-excitation Process Necessary for Spontaneous Rhythmicity
For spontaneous rhythmicity to occur, the membrane even in its natural state must be permeable enough to sodium ions (or to calcium and sodium ions through the slow calcium-sodium channels) to allow automatic membrane depolarization. Thus, Figure 5-14 shows that the “resting” membrane potential in the rhythmical control center of the heart is only −60 to −70 millivolts. This is not enough negative voltage to keep the sodium and calcium channels totally closed. Therefore, the following sequence occurs: (1) some sodium and calcium ions flow inward; (2) this increases the membrane voltage in the positive direction, which further increases membrane permeability; (3) still more ions flow inward; and (4) the permeability increases more, and so on, until an action potential is generated. Then, at the end of the action potential, the membrane repolarizes. After another delay of milliseconds or seconds, spontaneous excitability causes depolarization again and a new action potential occurs spontaneously. This cycle continues over and over and causes self-induced rhythmical excitation of the excitable tissue.

Figure 5-14 Rhythmical action potentials (in millivolts) similar to those recorded in the rhythmical control center of the heart. Note their relationship to potassium conductance and to the state of hyperpolarization.
Why does the membrane of the heart control center not depolarize immediately after it has become repolarized, rather than delaying for nearly a second before the onset of the next action potential? The answer can be found by observing the curve labeled “potassium conductance” in Figure 5-14 . This shows that toward the end of each action potential, and continuing for a short period thereafter, the membrane becomes more permeable to potassium ions. The increased outflow of potassium ions carries tremendous numbers of positive charges to the outside of the membrane, leaving inside the fiber considerably more negativity than would otherwise occur. This continues for nearly a second after the preceding action potential is over, thus drawing the membrane potential nearer to the potassium Nernst potential. This is a state called hyperpolarization, also shown in Figure 5-14 . As long as this state exists, self-re-excitation will not occur. But the increased potassium conductance (and the state of hyperpolarization) gradually disappears, as shown after each action potential is completed in the figure, thereby allowing the membrane potential again to increase up to the threshold for excitation. Then, suddenly, a new action potential results and the process occurs again and again.

Special Characteristics of Signal Transmission in Nerve Trunks

Myelinated and Unmyelinated Nerve Fibers
Figure 5-15 shows a cross section of a typical small nerve, revealing many large nerve fibers that constitute most of the cross-sectional area. However, a more careful look reveals many more small fibers lying between the large ones. The large fibers are myelinated, and the small ones are unmyelinated. The average nerve trunk contains about twice as many unmyelinated fibers as myelinated fibers.

Figure 5-15 Cross section of a small nerve trunk containing both myelinated and unmyelinated fibers.
Figure 5-16 shows a typical myelinated fiber. The central core of the fiber is the axon, and the membrane of the axon is the membrane that actually conducts the action potential. The axon is filled in its center with axoplasm, which is a viscid intracellular fluid. Surrounding the axon is a myelin sheath that is often much thicker than the axon itself. About once every 1 to 3 millimeters along the length of the myelin sheath is a node of Ranvier.

Figure 5-16 Function of the Schwann cell to insulate nerve fibers. A, Wrapping of a Schwann cell membrane around a large axon to form the myelin sheath of the myelinated nerve fiber. B, Partial wrapping of the membrane and cytoplasm of a Schwann cell around multiple unmyelinated nerve fibers (shown in cross section).
( A, Modified from Leeson TS, Leeson R: Histology. Philadelphia: WB Saunders, 1979.)
The myelin sheath is deposited around the axon by Schwann cells in the following manner: The membrane of a Schwann cell first envelops the axon. Then the Schwann cell rotates around the axon many times, laying down multiple layers of Schwann cell membrane containing the lipid substance sphingomyelin. This substance is an excellent electrical insulator that decreases ion flow through the membrane about 5000-fold. At the juncture between each two successive Schwann cells along the axon, a small uninsulated area only 2 to 3 micro meters in length remains where ions still can flow with ease through the axon membrane between the extracellular fluid and the intracellular fluid inside the axon. This area is called the node of Ranvier.

“Saltatory” Conduction in Myelinated Fibers from Node to Node
Even though almost no ions can flow through the thick myelin sheaths of myelinated nerves, they can flow with ease through the nodes of Ranvier. Therefore, action potentials occur only at the nodes. Yet the action potentials are conducted from node to node, as shown in Figure 5-17 ; this is called saltatory conduction. That is, electrical current flows through the surrounding extracellular fluid outside the myelin sheath, as well as through the axoplasm inside the axon from node to node, exciting successive nodes one after another. Thus, the nerve impulse jumps along the fiber, which is the origin of the term “saltatory.”

Figure 5-17 Saltatory conduction along a myelinated axon. Flow of electrical current from node to node is illustrated by the arrows.
Saltatory conduction is of value for two reasons. First, by causing the depolarization process to jump long intervals along the axis of the nerve fiber, this mechanism increases the velocity of nerve transmission in myelinated fibers as much as 5- to 50-fold. Second, saltatory conduction conserves energy for the axon because only the nodes depolarize, allowing perhaps 100 times less loss of ions than would otherwise be necessary, and therefore requiring little metabolism for re-establishing the sodium and potassium concentration differences across the membrane after a series of nerve impulses.
Still another feature of saltatory conduction in large myelinated fibers is the following: The excellent insulation afforded by the myelin membrane and the 50-fold decrease in membrane capacitance allow repolarization to occur with little transfer of ions.

Velocity of Conduction in Nerve Fibers
The velocity of action potential conduction in nerve fibers varies from as little as 0.25 m/sec in small unmyelinated fibers to as great as 100 m/sec (the length of a football field in 1 second) in large myelinated fibers.

Excitation—The Process of Eliciting the Action Potential
Basically, any factor that causes sodium ions to begin to diffuse inward through the membrane in sufficient numbers can set off automatic regenerative opening of the sodium channels. This can result from mechanical disturbance of the membrane, chemical effects on the membrane, or passage of electricity through the membrane. All these are used at different points in the body to elicit nerve or muscle action potentials: mechanical pressure to excite sensory nerve endings in the skin, chemical neurotransmitters to transmit signals from one neuron to the next in the brain, and electrical current to transmit signals between successive muscle cells in the heart and intestine. For the purpose of understanding the excitation process, let us begin by discussing the principles of electrical stimulation.

Excitation of a Nerve Fiber by a Negatively Charged Metal Electrode
The usual means for exciting a nerve or muscle in the experimental laboratory is to apply electricity to the nerve or muscle surface through two small electrodes, one of which is negatively charged and the other positively charged. When this is done, the excitable membrane becomes stimulated at the negative electrode.
The cause of this effect is the following: Remember that the action potential is initiated by the opening of voltage-gated sodium channels. Further, these channels are opened by a decrease in the normal resting electrical voltage across the membrane. That is, negative current from the electrode decreases the voltage on the outside of the membrane to a negative value nearer to the voltage of the negative potential inside the fiber. This decreases the electrical voltage across the membrane and allows the sodium channels to open, resulting in an action potential. Conversely, at the positive electrode, the injection of positive charges on the outside of the nerve membrane heightens the voltage difference across the membrane rather than lessening it. This causes a state of hyperpolarization, which actually decreases the excitability of the fiber rather than causing an action potential.

Threshold for Excitation, and “Acute Local Potentials.”
A weak negative electrical stimulus may not be able to excite a fiber. However, when the voltage of the stimulus is increased, there comes a point at which excitation does take place. Figure 5-18 shows the effects of successively applied stimuli of progressing strength. A weak stimulus at point A causes the membrane potential to change from −90 to −85 millivolts, but this is not a sufficient change for the automatic regenerative processes of the action potential to develop. At point B, the stimulus is greater, but again, the intensity is still not enough. The stimulus does, however, disturb the membrane potential locally for as long as 1 millisecond or more after both of these weak stimuli. These local potential changes are called acute local potentials, and when they fail to elicit an action potential, they are called acute subthreshold potentials.

Figure 5-18 Effect of stimuli of increasing voltages to elicit an action potential. Note development of “acute subthreshold potentials” when the stimuli are below the threshold value required for eliciting an action potential.
At point C in Figure 5-18 , the stimulus is even stronger. Now the local potential has barely reached the level required to elicit an action potential, called the threshold level, but this occurs only after a short “latent period.” At point D, the stimulus is still stronger, the acute local potential is also stronger, and the action potential occurs after less of a latent period.
Thus, this figure shows that even a weak stimulus causes a local potential change at the membrane, but the intensity of the local potential must rise to a threshold level before the action potential is set off.

“Refractory Period” After an Action Potential, During Which a New Stimulus Cannot Be Elicited
A new action potential cannot occur in an excitable fiber as long as the membrane is still depolarized from the preceding action potential. The reason for this is that shortly after the action potential is initiated, the sodium channels (or calcium channels, or both) become inactivated and no amount of excitatory signal applied to these channels at this point will open the inactivation gates. The only condition that will allow them to reopen is for the membrane potential to return to or near the original resting membrane potential level. Then, within another small fraction of a second, the inactivation gates of the channels open and a new action potential can be initiated.
The period during which a second action potential cannot be elicited, even with a strong stimulus, is called the absolute refractory period. This period for large myelinated nerve fibers is about 1/2500 second. Therefore, one can readily calculate that such a fiber can transmit a maximum of about 2500 impulses per second.

Inhibition of Excitability—“Stabilizers” and Local Anesthetics
In contrast to the factors that increase nerve excitability, still others, called membrane-stabilizing factors, can decrease excitability. For instance, a high extracellular fluid calcium ion concentration decreases membrane permeability to sodium ions and simultaneously reduces excitability. Therefore, calcium ions are said to be a “stabilizer.”

Local Anesthetics
Among the most important stabilizers are the many substances used clinically as local anesthetics, including procaine and tetracaine. Most of these act directly on the activation gates of the sodium channels, making it much more difficult for these gates to open, thereby reducing membrane excitability. When excitability has been reduced so low that the ratio of action potential strength to excitability threshold (called the “safety factor”) is reduced below 1.0, nerve impulses fail to pass along the anesthetized nerves.

Recording Membrane Potentials and Action Potentials

Cathode Ray Oscilloscope
Earlier in this chapter, we noted that the membrane potential changes extremely rapidly during the course of an action potential. Indeed, most of the action potential complex of large nerve fibers takes place in less than 1/1000 second. In some figures of this chapter, an electrical meter has been shown recording these potential changes. However, it must be understood that any meter capable of recording most action potentials must be capable of responding extremely rapidly. For practical purposes, the only common type of meter that is capable of responding accurately to the rapid membrane potential changes is the cathode ray oscilloscope.
Figure 5-19 shows the basic components of a cathode ray oscilloscope. The cathode ray tube itself is composed basically of an electron gun and a fluorescent screen against which electrons are fired. Where the electrons hit the screen surface, the fluorescent material glows. If the electron beam is moved across the screen, the spot of glowing light also moves and draws a fluorescent line on the screen.

Figure 5-19 Cathode ray oscilloscope for recording transient action potentials.
In addition to the electron gun and fluorescent surface, the cathode ray tube is provided with two sets of electrically charged plates—one set positioned on the two sides of the electron beam, and the other set positioned above and below. Appropriate electronic control circuits change the voltage on these plates so that the electron beam can be bent up or down in response to electrical signals coming from recording electrodes on nerves. The beam of electrons also is swept horizontally across the screen at a constant time rate by an internal electronic circuit of the oscilloscope. This gives the record shown on the face of the cathode ray tube in the figure, giving a time base horizontally and voltage changes from the nerve electrodes shown vertically. Note at the left end of the record a small stimulus artifact caused by the electrical stimulus used to elicit the nerve action potential. Then further to the right is the recorded action potential itself.


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CHAPTER 6 Contraction of Skeletal Muscle
About 40 percent of the body is skeletal muscle, and perhaps another 10 percent is smooth and cardiac muscle. Some of the same basic principles of contraction apply to all three different types of muscle. In this chapter, function of skeletal muscle is considered mainly; the specialized functions of smooth muscle are discussed in Chapter 8 , and cardiac muscle is discussed in Chapter 9 .

Physiologic Anatomy of Skeletal Muscle

Skeletal Muscle Fiber
Figure 6-1 shows the organization of skeletal muscle, demonstrating that all skeletal muscles are composed of numerous fibers ranging from 10 to 80 micrometers in diameter. Each of these fibers is made up of successively smaller subunits, also shown in Figure 6-1 and described in subsequent paragraphs.

Figure 6-1 Organization of skeletal muscle, from the gross to the molecular level. F, G, H, and I are cross sections at the levels indicated.
In most skeletal muscles, each fiber extends the entire length of the muscle. Except for about 2 percent of the fibers, each fiber is usually innervated by only one nerve ending, located near the middle of the fiber.

The Sarcolemma Is a Thin Membrane Enclosing a Skeletal Muscle Fiber
The sarcolemma consists of a true cell membrane, called the plasma membrane, and an outer coat made up of a thin layer of polysaccharide material that contains numerous thin collagen fibrils. At each end of the muscle fiber, this surface layer of the sarcolemma fuses with a tendon fiber. The tendon fibers in turn collect into bundles to form the muscle tendons that then insert into the bones.

Myofibrils Are Composed of Actin and Myosin Filaments
Each muscle fiber contains several hundred to several thousand myofibrils, which are demonstrated by the many small open dots in the cross-sectional view of Figure 6-1 C. Each myofibril ( Figure 6-1 D and E ) is composed of about 1500 adjacent myosin filaments and 3000 actin filaments, which are large polymerized protein molecules that are responsible for the actual muscle contraction. These can be seen in longitudinal view in the electron micrograph of Figure 6-2 and are represented diagrammatically in Figure 6-1 , parts E through L. The thick filaments in the diagrams are myosin, and the thin filaments are actin.

Figure 6-2 Electron micrograph of muscle myofibrils showing the detailed organization of actin and myosin filaments. Note the mitochondria lying between the myofibrils.
(From Fawcett DW: The Cell. Philadelphia: WB Saunders, 1981.)
Note in Figure 6-1 E that the myosin and actin filaments partially interdigitate and thus cause the myofibrils to have alternate light and dark bands, as illustrated in Figure 6-2 . The light bands contain only actin filaments and are called I bands because they are isotropic to polarized light. The dark bands contain myosin filaments, as well as the ends of the actin filaments where they overlap the myosin, and are called A bands because they are anisotropic to polarized light. Note also the small projections from the sides of the myosin filaments in Figure 6-1 E and L. These are cross-bridges. It is the interaction between these cross-bridges and the actin filaments that causes contraction.
Figure 6-1 E also shows that the ends of the actin filaments are attached to a so-called Z disc. From this disc, these filaments extend in both directions to interdigitate with the myosin filaments. The Z disc, which itself is composed of filamentous proteins different from the actin and myosin filaments, passes crosswise across the myofibril and also crosswise from myofibril to myofibril, attaching the myofibrils to one another all the way across the muscle fiber. Therefore, the entire muscle fiber has light and dark bands, as do the individual myofibrils. These bands give skeletal and cardiac muscle their striated appearance.
The portion of the myofibril (or of the whole muscle fiber) that lies between two successive Z discs is called a sarcomere. When the muscle fiber is contracted, as shown at the bottom of Figure 6-5 , the length of the sarcomere is about 2 micrometers. At this length, the actin filaments completely overlap the myosin filaments, and the tips of the actin filaments are just beginning to overlap one another. As discussed later, at this length the muscle is capable of generating its greatest force of contraction.

Figure 6-5 Relaxed and contracted states of a myofibril showing ( top ) sliding of the actin filaments ( pink ) into the spaces between the myosin filaments ( red ) and ( bottom ) pulling of the Z membranes toward each other.

Titin Filamentous Molecules Keep the Myosin and Actin Filaments in Place
The side-by-side relationship between the myosin and actin filaments is difficult to maintain. This is achieved by a large number of filamentous molecules of a protein called titin ( Figure 6-3 ) . Each titin molecule has a molecular weight of about 3 million, which makes it one of the largest protein molecules in the body. Also, because it is filamentous, it is very springy. These springy titin molecules act as a framework that holds the myosin and actin filaments in place so that the contractile machinery of the sarcomere will work. One end of the titin molecule is elastic and is attached to the Z disk, acting as a spring and changing length as the sarcomere contracts and relaxes. The other part of the titin molecule tethers it to the myosin thick filament. The titin molecule itself also appears to act as a template for initial formation of portions of the contractile filaments of the sarcomere, especially the myosin filaments.

Figure 6-3 Organization of proteins in a sarcomere. Each titin molecule extends from the Z disc to the M line. Part of the titin molecule is closely associated with the myosin thick filament, whereas the rest of the molecule is springy and changes length as the sarcomere contracts and relaxes.

Sarcoplasm Is the Intracellular Fluid Between Myofibrils
The many myofibrils of each muscle fiber are suspended side by side in the muscle fiber. The spaces between the myofibrils are filled with intracellular fluid called sarcoplasm, containing large quantities of potassium, magnesium, and phosphate, plus multiple protein enzymes. Also present are tremendous numbers of mitochondria that lie parallel to the myofibrils. These supply the contracting myofibrils with large amounts of energy in the form of adenosine triphosphate (ATP) formed by the mitochondria.

Sarcoplasmic Reticulum Is a Specialized Endoplasmic Reticulum of Skeletal Muscle
Also in the sarcoplasm surrounding the myofibrils of each muscle fiber is an extensive reticulum ( Figure 6-4 ), called the sarcoplasmic reticulum. This reticulum has a special organization that is extremely important in controlling muscle contraction, as discussed in Chapter 7 . The rapidly contracting types of muscle fibers have especially extensive sarcoplasmic reticula.

Figure 6-4 Sarcoplasmic reticulum in the extracellular spaces between the myofibrils, showing a longitudinal system paralleling the myofibrils. Also shown in cross section are T tubules ( arrows ) that lead to the exterior of the fiber membrane and are important for conducting the electrical signal into the center of the muscle fiber.
(From Fawcett DW: The Cell. Philadelphia: WB Saunders, 1981.)

General Mechanism of Muscle Contraction
The initiation and execution of muscle contraction occur in the following sequential steps.
1. An action potential travels along a motor nerve to its endings on muscle fibers.
2. At each ending, the nerve secretes a small amount of the neurotransmitter substance acetylcholine.
3. The acetylcholine acts on a local area of the muscle fiber membrane to open multiple “acetylcholine-gated” cation channels through protein molecules floating in the membrane.
4. Opening of the acetylcholine-gated channels allows large quantities of sodium ions to diffuse to the interior of the muscle fiber membrane. This causes a local depolarization that in turn leads to opening of voltage-gated sodium channels. This initiates an action potential at the membrane.
5. The action potential travels along the muscle fiber membrane in the same way that action potentials travel along nerve fiber membranes.
6. The action potential depolarizes the muscle membrane, and much of the action potential electricity flows through the center of the muscle fiber. Here it causes the sarcoplasmic reticulum to release large quantities of calcium ions that have been stored within this reticulum.
7. The calcium ions initiate attractive forces between the actin and myosin filaments, causing them to slide alongside each other, which is the contractile process.
8. After a fraction of a second, the calcium ions are pumped back into the sarcoplasmic reticulum by a Ca ++ membrane pump and remain stored in the reticulum until a new muscle action potential comes along; this removal of calcium ions from the myofibrils causes the muscle contraction to cease.
We now describe the molecular machinery of the muscle contractile process.

Molecular Mechanism of Muscle Contraction

Sliding Filament Mechanism of Muscle Contraction
Figure 6-5 demonstrates the basic mechanism of muscle contraction. It shows the relaxed state of a sarcomere (top) and the contracted state (bottom). In the relaxed state, the ends of the actin filaments extending from two successive Z discs barely begin to overlap one another. Conversely, in the contracted state, these actin filaments have been pulled inward among the myosin filaments, so their ends overlap one another to their maximum extent. Also, the Z discs have been pulled by the actin filaments up to the ends of the myosin filaments. Thus, muscle contraction occurs by a sliding filament mechanism.
But what causes the actin filaments to slide inward among the myosin filaments? This is caused by forces generated by interaction of the cross-bridges from the myosin filaments with the actin filaments. Under resting conditions, these forces are inactive. But when an action potential travels along the muscle fiber, this causes the sarcoplasmic reticulum to release large quantities of calcium ions that rapidly surround the myofibrils. The calcium ions in turn activate the forces between the myosin and actin filaments, and contraction begins. But energy is needed for the contractile process to proceed. This energy comes from high-energy bonds in the ATP molecule, which is degraded to adenosine diphosphate (ADP) to liberate the energy. In the next few sections, we describe what is known about the details of these molecular processes of contraction.

Molecular Characteristics of the Contractile Filaments

Myosin Filaments Are Composed of Multiple Myosin Molecules
Each of the myosin molecules, shown in Figure 6-6 A, has a molecular weight of about 480,000. Figure 6-6 B shows the organization of many molecules to form a myosin filament, as well as interaction of this filament on one side with the ends of two actin filaments.

Figure 6-6 A, Myosin molecule. B, Combination of many myosin molecules to form a myosin filament. Also shown are thousands of myosin cross-bridges and interaction between the heads o f the cross-bridges with adjacent actin filaments.
The myosin molecule (see Figure 6-6 A ) is composed of six polypeptide chains—two heavy chains, each with a molecular weight of about 200,000, and four light chains with molecular weights of about 20,000 each. The two heavy chains wrap spirally around each other to form a double helix, which is called the tail of the myosin molecule. One end of each of these chains is folded bilaterally into a globular polypeptide structure called a myosin head. Thus, there are two free heads at one end of the double-helix myosin molecule. The four light chains are also part of the myosin head, two to each head. These light chains help control the function of the head during muscle contraction.
The myosin filament is made up of 200 or more individual myosin molecules. The central portion of one of these filaments is shown in Figure 6-6 B, displaying the tails of the myosin molecules bundled together to form the body of the filament, while many heads of the molecules hang outward to the sides of the body. Also, part of the body of each myosin molecule hangs to the side along with the head, thus providing an arm that extends the head outward from the body, as shown in the figure. The protruding arms and heads together are called cross-bridges. Each cross-bridge is flexible at two points called hinges —one where the arm leaves the body of the myosin filament, and the other where the head attaches to the arm. The hinged arms allow the heads to be either extended far outward from the body of the myosin filament or brought close to the body. The hinged heads in turn participate in the actual contraction process, as discussed in the following sections.
The total length of each myosin filament is uniform, almost exactly 1.6 micrometers. Note, however, that there are no cross-bridge heads in the center of the myosin filament for a distance of about 0.2 micrometer because the hinged arms extend away from the center.
Now, to complete the picture, the myosin filament itself is twisted so that each successive pair of cross-bridges is axially displaced from the previous pair by 120 degrees. This ensures that the cross-bridges extend in all directions around the filament.

ATPase Activity of the Myosin Head
Another feature of the myosin head that is essential for muscle contraction is that it functions as an ATPase enzyme. As explained later, this property allows the head to cleave ATP and use the energy derived from the ATP’s high-energy phosphate bond to energize the contraction process.

Actin Filaments Are Composed of Actin, Tropomyosin, and Troponin
The backbone of the actin filament is a double-stranded F-actin protein molecule, represented by the two lighter-colored strands in Figure 6-7 . The two strands are wound in a helix in the same manner as the myosin molecule.

Figure 6-7 Actin filament, composed of two helical strands of F-actin molecules and two strands of tropomyosin molecules that fit in the grooves between the actin strands. Attached to one end of each tropomyosin molecule is a troponin complex that initiates contraction.
Each strand of the double F-actin helix is composed of polymerized G-actin molecules, each having a molecular weight of about 42,000. Attached to each one of the G-actin molecules is one molecule of ADP. It is believed that these ADP molecules are the active sites on the actin filaments with which the cross-bridges of the myosin filaments interact to cause muscle contraction. The active sites on the two F-actin strands of the double helix are staggered, giving one active site on the overall actin filament about every 2.7 nanometers.
Each actin filament is about 1 micrometer long. The bases of the actin filaments are inserted strongly into the Z discs; the ends of the filaments protrude in both directions to lie in the spaces between the myosin molecules, as shown in Figure 6-5 .

Tropomyosin Molecules
The actin filament also contains another protein, tropomyosin. Each molecule of tropomyosin has a molecular weight of 70,000 and a length of 40 nanometers. These molecules are wrapped spirally around the sides of the F-actin helix. In the resting state, the tropomyosin molecules lie on top of the active sites of the actin strands so that attraction cannot occur between the actin and myosin filaments to cause contraction.

Troponin and Its Role in Muscle Contraction
Attached intermittently along the sides of the tropomyosin molecules are still other protein molecules called troponin. These are actually complexes of three loosely bound protein subunits, each of which plays a specific role in controlling muscle contraction. One of the subunits (troponin I) has a strong affinity for actin, another (troponin T) for tropomyosin, and a third (troponin C) for calcium ions. This complex is believed to attach the tropomyosin to the actin. The strong affinity of the troponin for calcium ions is believed to initiate the contraction process, as explained in the next section.

Interaction of One Myosin Filament, Two Actin Filaments, and Calcium Ions to Cause Contraction

Inhibition of the Actin Filament by the Troponin-Tropomyosin Complex; Activation by Calcium Ions
A pure actin filament without the presence of the troponin-tropomyosin complex (but in the presence of magnesium ions and ATP) binds instantly and strongly with the heads of the myosin molecules. Then, if the troponin-tropomyosin complex is added to the actin filament, the binding between myosin and actin does not take place. Therefore, it is believed that the active sites on the normal actin filament of the relaxed muscle are inhibited or physically covered by the troponin-tropomyosin complex. Consequently, the sites cannot attach to the heads of the myosin filaments to cause contraction. Before contraction can take place, the inhibitory effect of the troponin-tropomyosin complex must itself be inhibited.
This brings us to the role of the calcium ions. In the presence of large amounts of calcium ions, the inhibitory effect of the troponin-tropomyosin on the actin filaments is itself inhibited. The mechanism of this is not known, but one suggestion is the following: When calcium ions combine with troponin C, each molecule of which can bind strongly with up to four calcium ions, the troponin complex supposedly undergoes a conformational change that in some way tugs on the tropomyosin molecule and moves it deeper into the groove between the two actin strands. This “uncovers” the active sites of the actin, thus allowing these to attract the myosin cross-bridge heads and cause contraction to proceed. Although this is a hypothetical mechanism, it does emphasize that the normal relation between the troponin-tropomyosin complex and actin is altered by calcium ions, producing a new condition that leads to contraction.

Interaction Between the “Activated” Actin Filament and the Myosin Cross-Bridges—The “Walk-Along” Theory of Contraction
As soon as the actin filament becomes activated by the calcium ions, the heads of the cross-bridges from the myosin filaments become attracted to the active sites of the actin filament, and this, in some way, causes contraction to occur. Although the precise manner by which this interaction between the cross-bridges and the actin causes contraction is still partly theoretical, one hypothesis for which considerable evidence exists is the “walk-along” theory (or “ratchet” theory ) of contraction.
Figure 6-8 demonstrates this postulated walk-along mechanism for contraction. The figure shows the heads of two cross-bridges attaching to and disengaging from active sites of an actin filament. It is postulated that when a head attaches to an active site, this attachment simultaneously causes profound changes in the intramolecular forces between the head and arm of its cross-bridge. The new alignment of forces causes the head to tilt toward the arm and to drag the actin filament along with it. This tilt of the head is called the power stroke. Then, immediately after tilting, the head automatically breaks away from the active site. Next, the head returns to its extended direction. In this position, it combines with a new active site farther down along the actin filament; then the head tilts again to cause a new power stroke, and the actin filament moves another step. Thus, the heads of the cross-bridges bend back and forth and step by step walk along the actin filament, pulling the ends of two successive actin filaments toward the center of the myosin filament.
Each one of the cross-bridges is believed to operate independently of all others, each attaching and pulling in a continuous repeated cycle. Therefore, the greater the number of cross-bridges in contact with the actin filament at any given time, the greater the force of contraction.

ATP as the Source of Energy for Contraction—Chemical Events in the Motion of the Myosin Heads
When a muscle contracts, work is performed and energy is required. Large amounts of ATP are cleaved to form ADP during the contraction process; the greater the amount of work performed by the muscle, the greater the amount of ATP that is cleaved, which is called the Fenn effect. The following sequence of events is believed to be the means by which this occurs:
1. Before contraction begins, the heads of the cross-bridges bind with ATP. The ATPase activity of the myosin head immediately cleaves the ATP but leaves the cleavage products, ADP plus phosphate ion, bound to the head. In this state, the conformation of the head is such that it extends perpendicularly toward the actin filament but is not yet attached to the actin.
2. When the troponin-tropomyosin complex binds with calcium ions, active sites on the actin filament are uncovered and the myosin heads then bind with these, as shown in Figure 6-8 .
3. The bond between the head of the cross-bridge and the active site of the actin filament causes a conformational change in the head, prompting the head to tilt toward the arm of the cross-bridge. This provides the power stroke for pulling the actin filament. The energy that activates the power stroke is the energy already stored, like a “cocked” spring, by the conformational change that occurred in the head when the ATP molecule was cleaved earlier.
4. Once the head of the cross-bridge tilts, this allows release of the ADP and phosphate ion that were previously attached to the head. At the site of release of the ADP, a new molecule of ATP binds. This binding of new ATP causes detachment of the head from the actin.
5. After the head has detached from the actin, the new molecule of ATP is cleaved to begin the next cycle, leading to a new power stroke. That is, the energy again “cocks” the head back to its perpendicular condition, ready to begin the new power stroke cycle.
6. When the cocked head (with its stored energy derived from the cleaved ATP) binds with a new active site on the actin filament, it becomes uncocked and once again provides a new power stroke.

Figure 6-8 “Walk-along” mechanism for contraction of the muscle.
Thus, the process proceeds again and again until the actin filaments pull the Z membrane up against the ends of the myosin filaments or until the load on the muscle becomes too great for further pulling to occur.

The Amount of Actin and Myosin Filament Overlap Determines Tension Developed by the Contracting Muscle
Figure 6-9 shows the effect of sarcomere length and amount of myosin-actin filament overlap on the active tension developed by a contracting muscle fiber. To the right, shown in black, are different degrees of overlap of the myosin and actin filaments at different sarcomere lengths. At point D on the diagram, the actin filament has pulled all the way out to the end of the myosin filament, with no actin-myosin overlap. At this point, the tension developed by the activated muscle is zero. Then, as the sarcomere shortens and the actin filament begins to overlap the myosin filament, the tension increases progressively until the sarcomere length decreases to about 2.2 micrometers. At this point, the actin filament has already overlapped all the cross-bridges of the myosin filament but has not yet reached the center of the myosin filament. With further shortening, the sarcomere maintains full tension until point B is reached, at a sarcomere length of about 2 micrometers. At this point, the ends of the two actin filaments begin to overlap each other in addition to overlapping the myosin filaments. As the sarcomere length falls from 2 micrometers down to about 1.65 micrometers, at point A, the strength of contraction decreases rapidly. At this point, the two Z discs of the sarcomere abut the ends of the myosin filaments. Then, as contraction proceeds to still shorter sarcomere lengths, the ends of the myosin filaments are crumpled and, as shown in the figure, the strength of contraction approaches zero, but the sarcomere has now contracted to its shortest length.

Figure 6-9 Length-tension diagram for a single fully contracted sarcomere, showing maximum strength of contraction when the sarcomere is 2.0 to 2.2 micrometers in length. At the upper right are the relative positions of the actin and myosin filaments at different sarcomere lengths from point A to point D.
(Modified from Gordon AM, Huxley AF, Julian FJ: The length-tension diagram of single vertebrate striated muscle fibers. J Physiol 171:28P, 1964.)

Effect of Muscle Length on Force of Contraction in the Whole Intact Muscle
The top curve of Figure 6-10 is similar to that in Figure 6-9 , but the curve in Figure 6-10 depicts tension of the intact, whole muscle rather than of a single muscle fiber. The whole muscle has a large amount of connective tissue in it; also, the sarcomeres in different parts of the muscle do not always contract the same amount. Therefore, the curve has somewhat different dimensions from those shown for the individual muscle fiber, but it exhibits the same general form for the slope in the normal range of contraction, as noted in Figure 6-10 .

Figure 6-10 Relation of muscle length to tension in the muscle both before and during muscle contraction.
Note in Figure 6-10 that when the muscle is at its normal resting length, which is at a sarcomere length of about 2 micrometers, it contracts upon activation with the approximate maximum force of contraction. However, the increase in tension that occurs during contraction, called active tension, decreases as the muscle is stretched beyond its normal length—that is, to a sarcomere length greater than about 2.2 micrometers. This is demonstrated by the decreased length of the arrow in the figure at greater than normal muscle length.

Relation of Velocity of Contraction to Load
A skeletal muscle contracts rapidly when it contracts against no load—to a state of full contraction in about 0.1 second for the average muscle. When loads are applied, the velocity of contraction becomes progressively less as the load increases, as shown in Figure 6-11 . That is, when the load has been increased to equal the maximum force that the muscle can exert, the velocity of contraction becomes zero and no contraction results, despite activation of the muscle fiber.

Figure 6-11 Relation of load to velocity of contraction in a skeletal muscle with a cross section of 1 square centimeter and a length of 8 centimeters.
This decreasing velocity of contraction with load is caused by the fact that a load on a contracting muscle is a reverse force that opposes the contractile force caused by muscle contraction. Therefore, the net force that is available to cause velocity of shortening is correspondingly reduced.

Energetics of Muscle Contraction

Work Output During Muscle Contraction
When a muscle contracts against a load, it performs work. This means that energy is transferred from the muscle to the external load to lift an object to a greater height or to overcome resistance to movement.
In mathematical terms, work is defined by the following equation:

in which W is the work output, L is the load, and D is the distance of movement against the load. The energy required to perform the work is derived from the chemical reactions in the muscle cells during contraction, as described in the following sections.

Sources of Energy for Muscle Contraction
We have already seen that muscle contraction depends on energy supplied by ATP. Most of this energy is required to actuate the walk-along mechanism by which the cross-bridges pull the actin filaments, but small amounts are required for (1) pumping calcium ions from the sarcoplasm into the sarcoplasmic reticulum after the contraction is over and (2) pumping sodium and potassium ions through the muscle fiber membrane to maintain appropriate ionic environment for propagation of muscle fiber action potentials.
The concentration of ATP in the muscle fiber, about 4 millimolar, is sufficient to maintain full contraction for only 1 to 2 seconds at most. The ATP is split to form ADP, which transfers energy from the ATP molecule to the contracting machinery of the muscle fiber. Then, as described in Chapter 2 , the ADP is rephosphorylated to form new ATP within another fraction of a second, which allows the muscle to continue its contraction. There are several sources of the energy for this rephosphorylation.
The first source of energy that is used to reconstitute the ATP is the substance phosphocreatine, which carries a high-energy phosphate bond similar to the bonds of ATP. The high-energy phosphate bond of phosphocreatine has a slightly higher amount of free energy than that of each ATP bond, as is discussed more fully in Chapters 67 and 72 . Therefore, phosphocreatine is instantly cleaved, and its released energy causes bonding of a new phosphate ion to ADP to reconstitute the ATP. However, the total amount of phosphocreatine in the muscle fiber is also very little—only about five times as great as the ATP. Therefore, the combined energy of both the stored ATP and the phosphocreatine in the muscle is capable of causing maximal muscle contraction for only 5 to 8 seconds.
The second important source of energy, which is used to reconstitute both ATP and phosphocreatine, is “glycolysis” of glycogen previously stored in the muscle cells. Rapid enzymatic breakdown of the glycogen to pyruvic acid and lactic acid liberates energy that is used to convert ADP to ATP; the ATP can then be used directly to energize additional muscle contraction and also to re-form the stores of phosphocreatine.
The importance of this glycolysis mechanism is twofold. First, the glycolytic reactions can occur even in the absence of oxygen, so muscle contraction can be sustained for many seconds and sometimes up to more than a minute, even when oxygen delivery from the blood is not available. Second, the rate of formation of ATP by the glycolytic process is about 2.5 times as rapid as ATP formation in response to cellular foodstuffs reacting with oxygen. However, so many end products of glycolysis accumulate in the muscle cells that glycolysis also loses its capability to sustain maximum muscle contraction after about 1 minute.
The third and final source of energy is oxidative metabolism. This means combining oxygen with the end products of glycolysis and with various other cellular foodstuffs to liberate ATP. More than 95 percent of all energy used by the muscles for sustained, long-term contraction is derived from this source. The foodstuffs that are consumed are carbohydrates, fats, and protein. For extremely long-term maximal muscle activity—over a period of many hours—by far the greatest proportion of energy comes from fats, but for periods of 2 to 4 hours, as much as one half of the energy can come from stored carbohydrates.
The detailed mechanisms of these energetic processes are discussed in Chapters 67 through 72 . In addition, the importance of the different mechanisms of energy release during performance of different sports is discussed in Chapter 84 on sports physiology.

Efficiency of Muscle Contraction
The efficiency of an engine or a motor is calculated as the percentage of energy input that is converted into work instead of heat. The percentage of the input energy to muscle (the chemical energy in nutrients) that can be converted into work, even under the best conditions, is less than 25 percent, with the remainder becoming heat. The reason for this low efficiency is that about one half of the energy in foodstuffs is lost during the formation of ATP, and even then, only 40 to 45 percent of the energy in the ATP itself can later be converted into work.
Maximum efficiency can be realized only when the muscle contracts at a moderate velocity. If the muscle contracts slowly or without any movement, small amounts of maintenance heat are released during contraction, even though little or no work is performed, thereby decreasing the conversion efficiency to as little as zero. Conversely, if contraction is too rapid, large proportions of the energy are used to overcome viscous friction within the muscle itself, and this, too, reduces the efficiency of contraction. Ordinarily, maximum efficiency is developed when the velocity of contraction is about 30 percent of maximum.

Characteristics of Whole Muscle Contraction
Many features of muscle contraction can be demonstrated by eliciting single muscle twitches. This can be accomplished by instantaneous electrical excitation of the nerve to a muscle or by passing a short electrical stimulus through the muscle itself, giving rise to a single, sudden contraction lasting for a fraction of a second.

Isometric Versus Isotonic Contraction
Muscle contraction is said to be isometric when the muscle does not shorten during contraction and isotonic when it does shorten but the tension on the muscle remains constant throughout the contraction. Systems for recording the two types of muscle contraction are shown in Figure 6-12 .

Figure 6-12 Isotonic and isometric systems for recording muscle contractions.
In the isometric system, the muscle contracts against a force transducer without decreasing the muscle length, as shown on the right in Figure 6-12 . In the isotonic system, the muscle shortens against a fixed load; this is illustrated on the left in the figure, showing a muscle lifting a pan of weights. The characteristics of isotonic contraction depend on the load against which the muscle contracts, as well as the inertia of the load. However, the isometric system records strictly changes in force of muscle contraction itself. Therefore, the isometric system is most often used when comparing the functional characteristics of different muscle types.

Characteristics of Isometric Twitches Recorded from Different Muscles
The human body has many sizes of skeletal muscles—from the small stapedius muscle in the middle ear, measuring only a few millimeters long and a millimeter or so in diameter, up to the large quadriceps muscle, a half million times as large as the stapedius. Further, the fibers may be as small as 10 micrometers in diameter or as large as 80 micrometers. Finally, the energetics of muscle contraction vary considerably from one muscle to another. Therefore, it is no wonder that the mechanical characteristics of muscle contraction differ among muscles.
Figure 6-13 shows records of isometric contractions of three types of skeletal muscle: an ocular muscle, which has a duration of isometric contraction of less than 1/50 second; the gastrocnemius muscle, which has a duration of contraction of about 1/15 second; and the soleus muscle, which has a duration of contraction of about 1/5 second. It is interesting that these durations of contraction are adapted to the functions of the respective muscles. Ocular movements must be extremely rapid to maintain fixation of the eyes on specific objects to provide accuracy of vision. The gastrocnemius muscle must contract moderately rapidly to provide sufficient velocity of limb movement for running and jumping, and the soleus muscle is concerned principally with slow contraction for continual, long-term support of the body against gravity.

Figure 6-13 Duration of isometric contractions for different types of mammalian skeletal muscles, showing a latent period between the action potential (depolarization) and muscle contraction.

Fast Versus Slow Muscle Fibers
As we discuss more fully in Chapter 84 on sports physiology, every muscle of the body is composed of a mixture of so-called fast and slow muscle fibers, with still other fibers gradated between these two extremes. Muscles that react rapidly, including anterior tibialis, are composed mainly of “fast” fibers with only small numbers of the slow variety. Conversely, muscles such as soleus that respond slowly but with prolonged contraction are composed mainly of “slow” fibers. The differences between these two types of fibers are as follows.

Slow Fibers (Type 1, Red Muscle)
(1) Smaller fibers. (2) Also innervated by smaller nerve fibers. (3) More extensive blood vessel system and capillaries to supply extra amounts of oxygen. (4) Greatly increased numbers of mitochondria, also to support high levels of oxidative metabolism. (5) Fibers contain large amounts of myoglobin, an iron-containing protein similar to hemoglobin in red blood cells. Myoglobin combines with oxygen and stores it until needed; this also greatly speeds oxygen transport to the mitochondria. The myoglobin gives the slow muscle a reddish appearance and the name red muscle.

Fast Fibers (Type II, White Muscle)
(1) Large fibers for great strength of contraction. (2) Extensive sarcoplasmic reticulum for rapid release of calcium ions to initiate contraction. (3) Large amounts of glycolytic enzymes for rapid release of energy by the glycolytic process. (4) Less extensive blood supply because oxidative metabolism is of secondary importance. (5) Fewer mitochondria, also because oxidative metabolism is secondary. A deficit of red myoglobin in fast muscle gives it the name white muscle.

Mechanics of Skeletal Muscle Contraction

Motor Unit—All the Muscle Fibers Innervated by a Single Nerve Fiber
Each motoneuron that leaves the spinal cord innervates multiple muscle fibers, the number depending on the type of muscle. All the muscle fibers innervated by a single nerve fiber are called a motor unit. In general, small muscles that react rapidly and whose control must be exact have more nerve fibers for fewer muscle fibers (for instance, as few as two or three muscle fibers per motor unit in some of the laryngeal muscles). Conversely, large muscles that do not require fine control, such as the soleus muscle, may have several hundred muscle fibers in a motor unit. An average figure for all the muscles of the body is questionable, but a good guess would be about 80 to 100 muscle fibers to a motor unit.
The muscle fibers in each motor unit are not all bunched together in the muscle but overlap other motor units in microbundles of 3 to 15 fibers. This interdigitation allows the separate motor units to contract in support of one another rather than entirely as individual segments.

Muscle Contractions of Different Force—Force Summation
Summation means the adding together of individual twitch contractions to increase the intensity of overall muscle contraction. Summation occurs in two ways: (1) by increasing the number of motor units contracting simultaneously, which is called multiple fiber summation, and (2) by increasing the frequency of contraction, which is called frequency summation and can lead to tetanization.

Multiple Fiber Summation
When the central nervous system sends a weak signal to contract a muscle, the smaller motor units of the muscle may be stimulated in preference to the larger motor units. Then, as the strength of the signal increases, larger and larger motor units begin to be excited as well, with the largest motor units often having as much as 50 times the contractile force of the smallest units. This is called the size principle. It is important because it allows the gradations of muscle force during weak contraction to occur in small steps, whereas the steps become progressively greater when large amounts of force are required. The cause of this size principle is that the smaller motor units are driven by small motor nerve fibers, and the small motoneurons in the spinal cord are more excitable than the larger ones, so naturally they are excited first.
Another important feature of multiple fiber summation is that the different motor units are driven asynchronously by the spinal cord, so contraction alternates among motor units one after the other, thus providing smooth contraction even at low frequencies of nerve signals.

Frequency Summation and Tetanization
Figure 6-14 shows the principles of frequency summation and tetanization. To the left are displayed individual twitch contractions occurring one after another at low frequency of stimulation. Then, as the frequency increases, there comes a point where each new contraction occurs before the preceding one is over. As a result, the second contraction is added partially to the first, so the total strength of contraction rises progressively with increasing frequency. When the frequency reaches a critical level, the successive contractions eventually become so rapid that they fuse together and the whole muscle contraction appears to be completely smooth and continuous, as shown in the figure. This is called tetanization. At a slightly higher frequency, the strength of contraction reaches its maximum, so any additional increase in frequency beyond that point has no further effect in increasing contractile force. This occurs because enough calcium ions are maintained in the muscle sarcoplasm, even between action potentials, so that full contractile state is sustained without allowing any relaxation between the action potentials.

Figure 6-14 Frequency summation and tetanization.

Maximum Strength of Contraction
The maximum strength of tetanic contraction of a muscle operating at a normal muscle length averages between 3 and 4 kilograms per square centimeter of muscle, or 50 pounds per square inch. Because a quadriceps muscle can have up to 16 square inches of muscle belly, as much as 800 pounds of tension may be applied to the patellar tendon. Thus, one can readily understand how it is possible for muscles to pull their tendons out of their insertions in bone.

Changes in Muscle Strength at the Onset of Contraction—The Staircase Effect (Treppe)
When a muscle begins to contract after a long period of rest, its initial strength of contraction may be as little as one-half its strength 10 to 50 muscle twitches later. That is, the strength of contraction increases to a plateau, a phenomenon called the staircase effect, or treppe.
Although all the possible causes of the staircase effect are not known, it is believed to be caused primarily by increasing calcium ions in the cytosol because of the release of more and more ions from the sarcoplasmic reticulum with each successive muscle action potential and failure of the sarcoplasm to recapture the ions immediately.

Skeletal Muscle Tone
Even when muscles are at rest, a certain amount of tautness usually remains. This is called muscle tone. Because normal skeletal muscle fibers do not contract without an action potential to stimulate the fibers, skeletal muscle tone results entirely from a low rate of nerve impulses coming from the spinal cord. These, in turn, are controlled partly by signals transmitted from the brain to the appropriate spinal cord anterior motoneurons and partly by signals that originate in muscle spindles located in the muscle itself. Both of these are discussed in relation to muscle spindle and spinal cord function in Chapter 54 .

Muscle Fatigue
Prolonged and strong contraction of a muscle leads to the well-known state of muscle fatigue. Studies in athletes have shown that muscle fatigue increases in almost direct proportion to the rate of depletion of muscle glycogen. Therefore, fatigue results mainly from inability of the contractile and metabolic processes of the muscle fibers to continue supplying the same work output. However, experiments have also shown that transmission of the nerve signal through the neuromuscular junction, which is discussed in Chapter 7 , can diminish at least a small amount after intense prolonged muscle activity, thus further diminishing muscle contraction. Interruption of blood flow through a contracting muscle leads to almost complete muscle fatigue within 1 or 2 minutes because of the loss of nutrient supply, especially loss of oxygen.

Lever Systems of the Body
Muscles operate by applying tension to their points of insertion into bones, and the bones in turn form various types of lever systems. Figure 6-15 shows the lever system activated by the biceps muscle to lift the forearm. If we assume that a large biceps muscle has a cross-sectional area of 6 square inches, the maximum force of contraction would be about 300 pounds. When the forearm is at right angles with the upper arm, the tendon attachment of the biceps is about 2 inches anterior to the fulcrum at the elbow and the total length of the forearm lever is about 14 inches. Therefore, the amount of lifting power of the biceps at the hand would be only one seventh of the 300 pounds of muscle force, or about 43 pounds. When the arm is fully extended, the attachment of the biceps is much less than 2 inches anterior to the fulcrum and the force with which the hand can be brought forward is also much less than 43 pounds.

Figure 6-15 Lever system activated by the biceps muscle.
In short, an analysis of the lever systems of the body depends on knowledge of (1) the point of muscle insertion, (2) its distance from the fulcrum of the lever, (3) the length of the lever arm, and (4) the position of the lever. Many types of movement are required in the body, some of which need great strength and others of which need large distances of movement. For this reason, there are many different types of muscle; some are long and contract a long distance, and some are short but have large cross-sectional areas and can provide extreme strength of contraction over short distances. The study of different types of muscles, lever systems, and their movements is called kinesiology and is an important scientific component of human physioanatomy.

“Positioning” of a Body Part by Contraction of Agonist and Antagonist Muscles on Opposite Sides of a Joint—“Coactivation” of Antagonist Muscles
Virtually all body movements are caused by simultaneous contraction of agonist and antagonist muscles on opposite sides of joints. This is called coactivation of the agonist and antagonist muscles, and it is controlled by the motor control centers of the brain and spinal cord.
The position of each separate part of the body, such as an arm or a leg, is determined by the relative degrees of contraction of the agonist and antagonist sets of muscles. For instance, let us assume that an arm or a leg is to be placed in a midrange position. To achieve this, agonist and antagonist muscles are excited about equally. Remember that an elongated muscle contracts with more force than a shortened muscle, which was demonstrated in Figure 6-10 , showing maximum strength of contraction at full functional muscle length and almost no strength of contraction at half-normal length. Therefore, the elongated muscle on one side of a joint can contract with far greater force than the shorter muscle on the opposite side. As an arm or leg moves toward its midposition, the strength of the longer muscle decreases, whereas the strength of the shorter muscle increases until the two strengths equal each other. At this point, movement of the arm or leg stops. Thus, by varying the ratios of the degree of activation of the agonist and antagonist muscles, the nervous system directs the positioning of the arm or leg.
We learn in Chapter 54 that the motor nervous system has additional important mechanisms to compensate for different muscle loads when directing this positioning process.

Remodeling of Muscle to Match Function
All the muscles of the body are continually being remodeled to match the functions that are required of them. Their diameters are altered, their lengths are altered, their strengths are altered, their vascular supplies are altered, and even the types of muscle fibers are altered at least slightly. This remodeling process is often quite rapid, within a few weeks. Indeed, experiments in animals have shown that muscle contractile proteins in some smaller, more active muscles can be replaced in as little as 2 weeks.

Muscle Hypertrophy and Muscle Atrophy
When the total mass of a muscle increases, this is called muscle hypertrophy. When it decreases, the process is called muscle atrophy.
Virtually all muscle hypertrophy results from an increase in the number of actin and myosin filaments in each muscle fiber, causing enlargement of the individual muscle fibers; this is called simply fiber hypertrophy. Hypertrophy occurs to a much greater extent when the muscle is loaded during the contractile process. Only a few strong contractions each day are required to cause significant hypertrophy within 6 to 10 weeks.
The manner in which forceful contraction leads to hypertrophy is not known. It is known, however, that the rate of synthesis of muscle contractile proteins is far greater when hypertrophy is developing, leading also to progressively greater numbers of both actin and myosin filaments in the myofibrils, often increasing as much as 50 percent. In turn, some of the myofibrils themselves have been observed to split within hypertrophying muscle to form new myofibrils, but how important this is in usual muscle hypertrophy is still unknown.
Along with the increasing size of myofibrils, the enzyme systems that provide energy also increase. This is especially true of the enzymes for glycolysis, allowing rapid supply of energy during short-term forceful muscle contraction.
When a muscle remains unused for many weeks, the rate of degradation of the contractile proteins is more rapid than the rate of replacement. Therefore, muscle atrophy occurs. The pathway that appears to account for much of the protein degradation in a muscle undergoing atrophy is the ATP-dependent ubiquitin-proteasome pathway . Proteasomes are large protein complexes that degrade damaged or unneeded proteins by proteolysis, a chemical reaction that breaks peptide bonds. Ubiquitin is a regulatory protein that basically labels which cells will be targeted for proteasomal degradation.

Adjustment of Muscle Length
Another type of hypertrophy occurs when muscles are stretched to greater than normal length. This causes new sarcomeres to be added at the ends of the muscle fibers, where they attach to the tendons. In fact, new sarcomeres can be added as rapidly as several per minute in newly developing muscle, illustrating the rapidity of this type of hypertrophy.
Conversely, when a muscle continually remains shortened to less than its normal length, sarcomeres at the ends of the muscle fibers can actually disappear. It is by these processes that muscles are continually remodeled to have the appropriate length for proper muscle contraction.

Hyperplasia of Muscle Fibers
Under rare conditions of extreme muscle force generation, the actual number of muscle fibers has been observed to increase (but only by a few percentage points), in addition to the fiber hypertrophy process. This increase in fiber number is called fiber hyperplasia. When it does occur, the mechanism is linear splitting of previously enlarged fibers.

Effects of Muscle Denervation
When a muscle loses its nerve supply, it no longer receives the contractile signals that are required to maintain normal muscle size. Therefore, atrophy begins almost immediately. After about 2 months, degenerative changes also begin to appear in the muscle fibers themselves. If the nerve supply to the muscle grows back rapidly, full return of function can occur in as little as 3 months, but from that time onward, the capability of functional return becomes less and less, with no further return of function after 1 to 2 years.
In the final stage of denervation atrophy, most of the muscle fibers are destroyed and replaced by fibrous and fatty tissue. The fibers that do remain are composed of a long cell membrane with a lineup of muscle cell nuclei but with few or no contractile properties and little or no capability of regenerating myofibrils if a nerve does regrow.
The fibrous tissue that replaces the muscle fibers during denervation atrophy also has a tendency to continue shortening for many months, which is called contracture. Therefore, one of the most important problems in the practice of physical therapy is to keep atrophying muscles from developing debilitating and disfiguring contractures. This is achieved by daily stretching of the muscles or use of appliances that keep the muscles stretched during the atrophying process.

Recovery of Muscle Contraction in Poliomyelitis: Development of Macromotor Units
When some but not all nerve fibers to a muscle are destroyed, as commonly occurs in poliomyelitis, the remaining nerve fibers branch off to form new axons that then innervate many of the paralyzed muscle fibers. This causes large motor units called macromotor units, which can contain as many as five times the normal number of muscle fibers for each motoneuron coming from the spinal cord. This decreases the fineness of control one has over the muscles but does allow the muscles to regain varying degrees of strength.

Rigor Mortis
Several hours after death, all the muscles of the body go into a state of contracture called “rigor mortis”; that is, the muscles contract and become rigid, even without action potentials. This rigidity results from loss of all the ATP, which is required to cause separation of the cross-bridges from the actin filaments during the relaxation process. The muscles remain in rigor until the muscle proteins deteriorate about 15 to 25 hours later, which presumably results from autolysis caused by enzymes released from lysosomes. All these events occur more rapidly at higher temperatures.


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CHAPTER 7 Excitation of Skeletal Muscle
Neuromuscular Transmission and Excitation-Contraction Coupling

Transmission of Impulses from Nerve Endings to Skeletal Muscle Fibers: The Neuromuscular Junction
The skeletal muscle fibers are innervated by large, myelinated nerve fibers that originate from large motoneurons in the anterior horns of the spinal cord. As pointed out in Chapter 6 , each nerve fiber, after entering the muscle belly, normally branches and stimulates from three to several hundred skeletal muscle fibers. Each nerve ending makes a junction, called the neuromuscular junction, with the muscle fiber near its midpoint. The action potential initiated in the muscle fiber by the nerve signal travels in both directions toward the muscle fiber ends. With the exception of about 2 percent of the muscle fibers, there is only one such junction per muscle fiber.

Physiologic Anatomy of the Neuromuscular Junction—The Motor End Plate
Figure 7-1 A and B shows the neuromuscular junction from a large, myelinated nerve fiber to a skeletal muscle fiber. The nerve fiber forms a complex of branching nerve terminals that invaginate into the surface of the muscle fiber but lie outside the muscle fiber plasma membrane. The entire structure is called the motor end plate. It is covered by one or more Schwann cells that insulate it from the surrounding fluids.

Figure 7-1 Different views of the motor end plate. A, Longitudinal section through the end plate. B, Surface view of the end plate. C, Electron micrographic appearance of the contact point between a single axon terminal and the muscle fiber membrane.
(Redrawn from Fawcett DW, as modified from Couteaux R, in Bloom W, Fawcett DW: A Textbook of Histology. Philadelphia: WB Saunders, 1986.)
Figure 7-1 C shows an electron micrographic sketch of the junction between a single axon terminal and the muscle fiber membrane. The invaginated membrane is called the synaptic gutter or synaptic trough, and the space between the terminal and the fiber membrane is called the synaptic space or synaptic cleft. This space is 20 to 30 nanometers wide. At the bottom of the gutter are numerous smaller folds of the muscle membrane called subneural clefts, which greatly increase the surface area at which the synaptic transmitter can act.
In the axon terminal are many mitochondria that supply adenosine triphosphate (ATP), the energy source that is used for synthesis of an excitatory transmitter, acetylcholine. The acetylcholine in turn excites the muscle fiber membrane. Acetylcholine is synthesized in the cytoplasm of the terminal, but it is absorbed rapidly into many small synaptic vesicles, about 300,000 of which are normally in the terminals of a single end plate. In the synaptic space are large quantities of the enzyme acetylcholinesterase, which destroys acetylcholine a few milliseconds after it has been released from the synaptic vesicles.

Secretion of Acetylcholine by the Nerve Terminals
When a nerve impulse reaches the neuromuscular junction, about 125 vesicles of acetylcholine are released from the terminals into the synaptic space. Some of the details of this mechanism can be seen in Figure 7-2 , which shows an expanded view of a synaptic space with the neural membrane above and the muscle membrane and its subneural clefts below.

Figure 7-2 Release of acetylcholine from synaptic vesicles at the neural membrane of the neuromuscular junction. Note the proximity of the release sites in the neural membrane to the acetylcholine receptors in the muscle membrane, at the mouths of the subneural clefts.
On the inside surface of the neural membrane are linear dense bars, shown in cross section in Figure 7-2 . To each side of each dense bar are protein particles that penetrate the neural membrane; these are voltage-gated calcium channels. When an action potential spreads over the terminal, these channels open and allow calcium ions to diffuse from the synaptic space to the interior of the nerve terminal. The calcium ions, in turn, are believed to exert an attractive influence on the acetylcholine vesicles, drawing them to the neural membrane adjacent to the dense bars. The vesicles then fuse with the neural membrane and empty their acetylcholine into the synaptic space by the process of exocytosis.
Although some of the aforementioned details are speculative, it is known that the effective stimulus for causing acetylcholine release from the vesicles is entry of calcium ions and that acetylcholine from the vesicles is then emptied through the neural membrane adjacent to the dense bars.

Effect of Acetylcholine on the Postsynaptic Muscle Fiber Membrane to Open Ion Channels
Figure 7-2 also shows many small acetylcholine receptors in the muscle fiber membrane; these are acetylcholine-gated ion channels, and they are located almost entirely near the mouths of the subneural clefts lying immediately below the dense bar areas, where the acetylcholine is emptied into the synaptic space.
Each receptor is a protein complex that has a total molecular weight of 275,000. The complex is composed of five subunit proteins, two alpha proteins and one each of beta, delta, and gamma proteins. These protein molecules penetrate all the way through the membrane, lying side by side in a circle to form a tubular channel, illustrated in Figure 7-3 . The channel remains constricted, as shown in section A of the figure, until two acetylcholine molecules attach respectively to the two alpha subunit proteins. This causes a conformational change that opens the channel, as shown in section B of the figure.

Figure 7-3 Acetylcholine-gated channel. A, Closed state. B, After acetylcholine (Ach) has become attached and a conformational change has opened the channel, allowing sodium ions to enter the muscle fiber and excite contraction. Note the negative charges at the channel mouth that prevent passage of negative ions such as chloride ions.
The acetylcholine-gated channel has a diameter of about 0.65 nanometer, which is large enough to allow the important positive ions—sodium (Na + ), potassium (K + ), and calcium (Ca ++ )—to move easily through the opening. Conversely, negative ions, such as chloride ions, do not pass through because of strong negative charges in the mouth of the channel that repel these negative ions.
In practice, far more sodium ions flow through the acetylcholine-gated channels than any other ions, for two reasons. First, there are only two positive ions in large concentration: sodium ions in the extracellular fluid and potassium ions in the intracellular fluid. Second, the negative potential on the inside of the muscle membrane, −80 to −90 millivolts, pulls the positively charged sodium ions to the inside of the fiber, while simultaneously preventing efflux of the positively charged potassium ions when they attempt to pass outward.
As shown in Figure 7-3 B , the principal effect of opening the acetylcholine-gated channels is to allow large numbers of sodium ions to pour to the inside of the fiber, carrying with them large numbers of positive charges. This creates a local positive potential change inside the muscle fiber membrane, called the end plate potential. In turn, this end plate potential initiates an action potential that spreads along the muscle membrane and thus causes muscle contraction.

Destruction of the Released Acetylcholine by Acetylcholinesterase
The acetylcholine, once released into the synaptic space, continues to activate the acetylcholine receptors as long as the acetylcholine persists in the space. However, it is removed rapidly by two means: (1) Most of the acetylcholine is destroyed by the enzyme acetylcholinesterase, which is attached mainly to the spongy layer of fine connective tissue that fills the synaptic space between the presynaptic nerve terminal and the postsynaptic muscle membrane. (2) A small amount of acetylcholine diffuses out of the synaptic space and is then no longer available to act on the muscle fiber membrane.
The short time that the acetylcholine remains in the synaptic space—a few milliseconds at most—normally is sufficient to excite the muscle fiber. Then the rapid removal of the acetylcholine prevents continued muscle re-excitation after the muscle fiber has recovered from its initial action potential.

End Plate Potential and Excitation of the Skeletal Muscle Fiber
The sudden insurgence of sodium ions into the muscle fiber when the acetylcholine-gated channels open causes the electrical potential inside the fiber at the local area of the end plate to increase in the positive direction as much as 50 to 75 millivolts, creating a local potential called the end plate potential. Recall from Chapter 5 that a sudden increase in nerve membrane potential of more than 20 to 30 millivolts is normally sufficient to initiate more and more sodium channel opening, thus initiating an action potential at the muscle fiber membrane.
Figure 7-4 shows the principle of an end plate potential initiating the action potential. This figure shows three separate end plate potentials. End plate potentials A and C are too weak to elicit an action potential, but they do produce weak local end plate voltage changes, as recorded in the figure. By contrast, end plate potential B is much stronger and causes enough sodium channels to open so that the self-regenerative effect of more and more sodium ions flowing to the interior of the fiber initiates an action potential. The weakness of the end plate potential at point A was caused by poisoning of the muscle fiber with curare, a drug that blocks the gating action of acetylcholine on the acetylcholine channels by competing for the acetylcholine receptor sites. The weakness of the end plate potential at point C resulted from the effect of botulinum toxin, a bacterial poison that decreases the quantity of acetylcholine release by the nerve terminals.

Figure 7-4 End plate potentials (in millivolts). A, Weakened end plate potential recorded in a curarized muscle, too weak to elicit an action potential. B, Normal end plate potential eliciting a muscle action potential. C, Weakened end plate potential caused by botulinum toxin that decreases end plate release of acetylcholine, again too weak to elicit a muscle action potential.

Safety Factor for Transmission at the Neuromuscular Junction; Fatigue of the Junction
Ordinarily, each impulse that arrives at the neuromuscular junction causes about three times as much end plate potential as that required to stimulate the muscle fiber. Therefore, the normal neuromuscular junction is said to have a high safety factor. However, stimulation of the nerve fiber at rates greater than 100 times per second for several minutes often diminishes the number of acetylcholine vesicles so much that impulses fail to pass into the muscle fiber. This is called fatigue of the neuromuscular junction, and it is the same effect that causes fatigue of synapses in the central nervous system when the synapses are overexcited. Under normal functioning conditions, measurable fatigue of the neuromuscular junction occurs rarely, and even then only at the most exhausting levels of muscle activity.

Molecular Biology of Acetylcholine Formation and Release
Because the neuromuscular junction is large enough to be studied easily, it is one of the few synapses of the nervous system for which most of the details of chemical transmission have been worked out. The formation and release of acetylcholine at this junction occur in the following stages:
1. Small vesicles, about 40 nanometers in size, are formed by the Golgi apparatus in the cell body of the motoneuron in the spinal cord. These vesicles are then transported by axoplasm that “streams” through the core of the axon from the central cell body in the spinal cord all the way to the neuromuscular junction at the tips of the peripheral nerve fibers. About 300,000 of these small vesicles collect in the nerve terminals of a single skeletal muscle end plate.
2. Acetylcholine is synthesized in the cytosol of the nerve fiber terminal but is immediately transported through the membranes of the vesicles to their interior, where it is stored in highly concentrated form, about 10,000 molecules of acetylcholine in each vesicle.
3. When an action potential arrives at the nerve terminal, it opens many calcium channels in the membrane of the nerve terminal because this terminal has an abundance of voltage-gated calcium channels. As a result, the calcium ion concentration inside the terminal membrane increases about 100-fold, which in turn increases the rate of fusion of the acetylcholine vesicles with the terminal membrane about 10,000-fold. This fusion makes many of the vesicles rupture, allowing exocytosis of acetylcholine into the synaptic space. About 125 vesicles usually rupture with each action potential. Then, after a few milliseconds, the acetylcholine is split by acetylcholinesterase into acetate ion and choline and the choline is reabsorbed actively into the neural terminal to be reused to form new acetylcholine. This sequence of events occurs within a period of 5 to 10 milliseconds.
4. The number of vesicles available in the nerve ending is sufficient to allow transmission of only a few thousand nerve-to-muscle impulses. Therefore, for continued function of the neuromuscular junction, new vesicles need to be re-formed rapidly. Within a few seconds after each action potential is over, “coated pits” appear in the terminal nerve membrane, caused by contractile proteins in the nerve ending, especially the protein clathrin, which is attached to the membrane in the areas of the original vesicles. Within about 20 seconds, the proteins contract and cause the pits to break away to the interior of the membrane, thus forming new vesicles. Within another few seconds, acetylcholine is transported to the interior of these vesicles, and they are then ready for a new cycle of acetylcholine release.

Drugs That Enhance or Block Transmission at the Neuromuscular Junction

Drugs That Stimulate the Muscle Fiber by Acetylcholine-Like Action
Many compounds, including methacholine, carbachol, and nicotine, have the same effect on the muscle fiber as does acetylcholine. The difference between these drugs and acetylcholine is that the drugs are not destroyed by cholinesterase or are destroyed so slowly that their action often persists for many minutes to several hours. The drugs work by causing localized areas of depolarization of the muscle fiber membrane at the motor end plate where the acetylcholine receptors are located. Then, every time the muscle fiber recovers from a previous contraction, these depolarized areas, by virtue of leaking ions, initiate a new action potential, thereby causing a state of muscle spasm.

Drugs That Stimulate the Neuromuscular Junction by Inactivating Acetylcholinesterase
Three particularly well-known drugs, neostigmine, physostigmine, and diisopropyl fluorophosphate, inactivate the acetylcholinesterase in the synapses so that it no longer hydrolyzes acetylcholine. Therefore, with each successive nerve impulse, additional acetylcholine accumulates and stimulates the muscle fiber repetitively. This causes muscle spasm when even a few nerve impulses reach the muscle. Unfortunately, it can also cause death due to laryngeal spasm, which smothers the person.
Neostigmine and physostigmine combine with acetylcholinesterase to inactivate the acetylcholinesterase for up to several hours, after which these drugs are displaced from the acetylcholinesterase so that the esterase once again becomes active. Conversely, diisopropyl fluorophosphate, which is a powerful “nerve” gas poison, inactivates acetylcholinesterase for weeks, which makes this a particularly lethal poison.

Drugs That Block Transmission at the Neuromuscular Junction
A group of drugs known as curariform drugs can prevent passage of impulses from the nerve ending into the muscle. For instance, D -tubocurarine blocks the action of acetylcholine on the muscle fiber acetylcholine receptors, thus preventing sufficient increase in permeability of the muscle membrane channels to initiate an action potential.

Myasthenia Gravis Causes Muscle Paralysis
Myasthenia gravis, which occurs in about 1 in every 20,000 persons, causes muscle paralysis because of inability of the neuromuscular junctions to transmit enough signals from the nerve fibers to the muscle fibers. Pathologically, antibodies that attack the acetylcholine receptors have been demonstrated in the blood of most patients with myasthenia gravis. Therefore, it is believed that myasthenia gravis is an autoimmune disease in which the patients have developed antibodies that block or destroy their own acetylcholine receptors at the postsynaptic neuromuscular junction.
Regardless of the cause, the end plate potentials that occur in the muscle fibers are mostly too weak to initiate opening of the voltage-gated sodium channels so that muscle fiber depolarization does not occur. If the disease is intense enough, the patient dies of paralysis—in particular, paralysis of the respiratory muscles. The disease can usually be ameliorated for several hours by administering neostigmine or some other anticholinesterase drug, which allows larger than normal amounts of acetylcholine to accumulate in the synaptic space. Within minutes, some of these paralyzed people can begin to function almost normally, until a new dose of neostigmine is required a few hours later.

Muscle Action Potential
Almost everything discussed in Chapter 5 regarding initiation and conduction of action potentials in nerve fibers applies equally to skeletal muscle fibers, except for quantitative differences. Some of the quantitative aspects of muscle potentials are the following:
1. Resting membrane potential: about −80 to −90 millivolts in skeletal fibers—the same as in large myelinated nerve fibers.
2. Duration of action potential: 1 to 5 milliseconds in skeletal muscle—about five times as long as in large myelinated nerves.
3. Velocity of conduction: 3 to 5 m/sec—about 1/13 the velocity of conduction in the large myelinated nerve fibers that excite skeletal muscle.

Spread of the Action Potential to the Interior of the Muscle Fiber by Way of “Transverse Tubules”
The skeletal muscle fiber is so large that action potentials spreading along its surface membrane cause almost no current flow deep within the fiber. Yet to cause maximum muscle contraction, current must penetrate deeply into the muscle fiber to the vicinity of the separate myofibrils. This is achieved by transmission of action potentials along transverse tubules (T tubules) that penetrate all the way through the muscle fiber from one side of the fiber to the other, as illustrated in Figure 7-5 . The T tubule action potentials cause release of calcium ions inside the muscle fiber in the immediate vicinity of the myofibrils, and these calcium ions then cause contraction. This overall process is called excitation-contraction coupling.

Figure 7-5 Transverse (T) tubule–sarcoplasmic reticulum system. Note that the T tubules communicate with the outside of the cell membrane, and deep in the muscle fiber, each T tubule lies adjacent to the ends of longitudinal sarcoplasmic reticulum tubules that surround all sides of the actual myofibrils that contract. This illustration was drawn from frog muscle, which has one T tubule per sarcomere, located at the Z line. A similar arrangement is found in mammalian heart muscle, but mammalian skeletal muscle has two T tubules per sarcomere, located at the A-I band junctions.

Excitation-Contraction Coupling

Transverse Tubule–Sarcoplasmic Reticulum System
Figure 7-5 shows myofibrils surrounded by the T tubule–sarcoplasmic reticulum system. The T tubules are small and run transverse to the myofibrils. They begin at the cell membrane and penetrate all the way from one side of the muscle fiber to the opposite side. Not shown in the figure is the fact that these tubules branch among themselves and form entire planes of T tubules interlacing among all the separate myofibrils. Also, where the T tubules originate from the cell membrane, they are open to the exterior of the muscle fiber. Therefore, they communicate with the extracellular fluid surrounding the muscle fiber and they themselves contain extracellular fluid in their lumens. In other words, the T tubules are actually internal extensions of the cell membrane. Therefore, when an action potential spreads over a muscle fiber membrane, a potential change also spreads along the T tubules to the deep interior of the muscle fiber. The electrical currents surrounding these T tubules then elicit the muscle contraction.
Figure 7-5 also shows a sarcoplasmic reticulum, in yellow. This is composed of two major parts: (1) large chambers called terminal cisternae that abut the T tubules and (2) long longitudinal tubules that surround all surfaces of the actual contracting myofibrils.

Release of Calcium Ions by the Sarcoplasmic Reticulum
One of the special features of the sarcoplasmic reticulum is that within its vesicular tubules is an excess of calcium ions in high concentration, and many of these ions are released from each vesicle when an action potential occurs in the adjacent T tubule.
Figures 7-6 and 7-7 show that the action potential of the T tubule causes current flow into the sarcoplasmic reticular cisternae where they abut the T tubule. As the action potential reaches the T tubule, the voltage change is sensed by dihydropyridine receptors that are linked to calcium release channels, also called ryanodine receptor channels, in the adjacent sarcoplasmic reticular cisternae (see Figure 7-6 ). Activation of dihydropyridine receptors triggers the opening of the calcium release channels in the cisternae, as well as in their attached longitudinal tubules. These channels remain open for a few milliseconds, releasing calcium ions into the sarcoplasm surrounding the myofibrils and causing contraction, as discussed in Chapter 6 .

Figure 7-6 Excitation-contraction coupling in skeletal muscle. The top panel shows an action potential in the T tubule that causes a conformational change in the voltage-sensing dihydropyridine (DHP) receptors, opening the Ca ++ release channels in the terminal cisternae of the sarcoplasmic reticulum and permitting Ca ++ to rapidly diffuse into the sarcoplasm and initiate muscle contraction. During repolarization ( bottom panel ) the conformational change in the DHP receptor closes the Ca ++ release channels and Ca ++ is transported from the sarcoplasm into the sarcoplasmic reticulum by an ATP-dependent calcium pump.

Figure 7-7 Excitation-contraction coupling in the muscle, showing (1) an action potential that causes release of calcium ions from the sarcoplasmic reticulum and then (2) re-uptake of the calcium ions by a calcium pump.

Calcium Pump for Removing Calcium Ions from the Myofibrillar Fluid After Contraction Occurs
Once the calcium ions have been released from the sarcoplasmic tubules and have diffused among the myofibrils, muscle contraction continues as long as the calcium ions remain in high concentration. However, a continually active calcium pump located in the walls of the sarcoplasmic reticulum pumps calcium ions away from the myofibrils back into the sarcoplasmic tubules (see Figure 7-6 ). This pump can concentrate the calcium ions about 10,000-fold inside the tubules. In addition, inside the reticulum is a protein called calsequestrin that can bind up to 40 times more calcium.

Excitatory “Pulse” of Calcium Ions
The normal resting state concentration (<10 −7 molar) of calcium ions in the cytosol that bathes the myofibrils is too little to elicit contraction. Therefore, the troponin-tropomyosin complex keeps the actin filaments inhibited and maintains a relaxed state of the muscle.
Conversely, full excitation of the T tubule and sarcoplasmic reticulum system causes enough release of calcium ions to increase the concentration in the myofibrillar fluid to as high as 2 × 10 −4 molar concentration, a 500-fold increase, which is about 10 times the level required to cause maximum muscle contraction. Immediately thereafter, the calcium pump depletes the calcium ions again. The total duration of this calcium “pulse” in the usual skeletal muscle fiber lasts about 1/20 of a second, although it may last several times as long in some fibers and several times less in others. (In heart muscle, the calcium pulse lasts about one third of a second because of the long duration of the cardiac action potential.)
During this calcium pulse, muscle contraction occurs. If the contraction is to continue without interruption for long intervals, a series of calcium pulses must be initiated by a continuous series of repetitive action potentials, as discussed in Chapter 6 .


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CHAPTER 8 Excitation and Contraction of Smooth Muscle

Contraction of Smooth Muscle
In Chapters 6 and 7 , the discussion was concerned with skeletal muscle. We now turn to smooth muscle, which is composed of far smaller fibers—usually 1 to 5 micrometers in diameter and only 20 to 500 micrometers in length. In contrast, skeletal muscle fibers are as much as 30 times greater in diameter and hundreds of times as long. Many of the same principles of contraction apply to smooth muscle as to skeletal muscle. Most important, essentially the same attractive forces between myosin and actin filaments cause contraction in smooth muscle as in skeletal muscle, but the internal physical arrangement of smooth muscle fibers is different.

Types of Smooth Muscle
The smooth muscle of each organ is distinctive from that of most other organs in several ways: (1) physical dimensions, (2) organization into bundles or sheets, (3) response to different types of stimuli, (4) characteristics of innervation, and (5) function. Yet for the sake of simplicity, smooth muscle can generally be divided into two major types, which are shown in Figure 8-1 : multi-unit smooth muscle and unitary (or single-unit ) smooth muscle.

Figure 8-1 Multi-unit ( A ) and unitary ( B ) smooth muscle.

Multi-Unit Smooth Muscle
This type of smooth muscle is composed of discrete, separate smooth muscle fibers. Each fiber operates independently of the others and often is innervated by a single nerve ending, as occurs for skeletal muscle fibers. Further, the outer surfaces of these fibers, like those of skeletal muscle fibers, are covered by a thin layer of basement membrane–like substance, a mixture of fine collagen and glycoprotein that helps insulate the separate fibers from one another.
The most important characteristic of multi-unit smooth muscle fibers is that each fiber can contract independently of the others, and their control is exerted mainly by nerve signals. In contrast, a major share of control of unitary smooth muscle is exerted by non-nervous stimuli. Some examples of multi-unit smooth muscle are the ciliary muscle of the eye, the iris muscle of the eye, and the piloerector muscles that cause erection of the hairs when stimulated by the sympathetic nervous system.

Unitary Smooth Muscle
This type is also called syncytial smooth muscle or visceral smooth muscle. The term “unitary” is confusing because it does not mean single muscle fibers. Instead, it means a mass of hundreds to thousands of smooth muscle fibers that contract together as a single unit. The fibers usually are arranged in sheets or bundles, and their cell membranes are adherent to one another at multiple points so that force generated in one muscle fiber can be transmitted to the next. In addition, the cell membranes are joined by many gap junctions through which ions can flow freely from one muscle cell to the next so that action potentials or simple ion flow without action potentials can travel from one fiber to the next and cause the muscle fibers to contract together. This type of smooth muscle is also known as syncytial smooth muscle because of its syncytial interconnections among fibers. It is also called visceral smooth muscle because it is found in the walls of most viscera of the body, including the gastrointestinal tract, bile ducts, ureters, uterus, and many blood vessels.

Contractile Mechanism in Smooth Muscle

Chemical Basis for Smooth Muscle Contraction
Smooth muscle contains both actin and myosin filaments, having chemical characteristics similar to those of the actin and myosin filaments in skeletal muscle. It does not contain the normal troponin complex that is required in the control of skeletal muscle contraction, so the mechanism for control of contraction is different. This is discussed in detail later in this chapter.
Chemical studies have shown that actin and myosin filaments derived from smooth muscle interact with each other in much the same way that they do in skeletal muscle. Further, the contractile process is activated by calcium ions, and adenosine triphosphate (ATP) is degraded to adenosine diphosphate (ADP) to provide the energy for contraction.
There are, however, major differences between the physical organization of smooth muscle and that of skeletal muscle, as well as differences in excitation-contraction coupling, control of the contractile process by calcium ions, duration of contraction, and amount of energy required for contraction.

Physical Basis for Smooth Muscle Contraction
Smooth muscle does not have the same striated arrangement of actin and myosin filaments as is found in skeletal muscle. Instead, electron micrographic techniques suggest the physical organization exhibited in Figure 8-2 . This figure shows large numbers of actin filaments attached to so-called dense bodies. Some of these bodies are attached to the cell membrane. Others are dispersed inside the cell. Some of the membrane-dense bodies of adjacent cells are bonded together by intercellular protein bridges. It is mainly through these bonds that the force of contraction is transmitted from one cell to the next.

Figure 8-2 Physical structure of smooth muscle. The upper left-hand fiber shows actin filaments radiating from dense bodies. The lower left-hand fiber and the right-hand diagram demonstrate the relation of myosin filaments to actin filaments.
Interspersed among the actin filaments in the muscle fiber are myosin filaments. These have a diameter more than twice that of the actin filaments. In electron micrographs, one usually finds 5 to 10 times as many actin filaments as myosin filaments.
To the right in Figure 8-2 is a postulated structure of an individual contractile unit within a smooth muscle cell, showing large numbers of actin filaments radiating from two dense bodies; the ends of these filaments overlap a myosin filament located midway between the dense bodies. This contractile unit is similar to the contractile unit of skeletal muscle, but without the regularity of the skeletal muscle structure; in fact, the dense bodies of smooth muscle serve the same role as the Z discs in skeletal muscle.
There is another difference: Most of the myosin filaments have what are called “sidepolar” cross-bridges arranged so that the bridges on one side hinge in one direction and those on the other side hinge in the opposite direction. This allows the myosin to pull an actin filament in one direction on one side while simultaneously pulling another actin filament in the opposite direction on the other side. The value of this organization is that it allows smooth muscle cells to contract as much as 80 percent of their length instead of being limited to less than 30 percent, as occurs in skeletal muscle.

Comparison of Smooth Muscle Contraction and Skeletal Muscle Contraction
Although most skeletal muscles contract and relax rapidly, most smooth muscle contraction is prolonged tonic contraction, sometimes lasting hours or even days. Therefore, it is to be expected that both the physical and the chemical characteristics of smooth muscle versus skeletal muscle contraction would differ. Following are some of the differences.

Slow Cycling of the Myosin Cross-Bridges
The rapidity of cycling of the myosin cross-bridges in smooth muscle—that is, their attachment to actin, then release from the actin, and reattachment for the next cycle—is much slower than in skeletal muscle; in fact, the frequency is as little as 1/10 to 1/300 that in skeletal muscle. Yet the fraction of time that the cross-bridges remain attached to the actin filaments, which is a major factor that determines the force of contraction, is believed to be greatly increased in smooth muscle. A possible reason for the slow cycling is that the cross-bridge heads have far less ATPase activity than in skeletal muscle, so degradation of the ATP that energizes the movements of the cross-bridge heads is greatly reduced, with corresponding slowing of the rate of cycling.

Low Energy Requirement to Sustain Smooth Muscle Contraction
Only 1/10 to 1/300 as much energy is required to sustain the same tension of contraction in smooth muscle as in skeletal muscle. This, too, is believed to result from the slow attachment and detachment cycling of the cross-bridges and because only one molecule of ATP is required for each cycle, regardless of its duration.
This sparsity of energy utilization by smooth muscle is exceedingly important to the overall energy economy of the body because organs such as the intestines, urinary bladder, gallbladder, and other viscera often maintain tonic muscle contraction almost indefinitely.

Slowness of Onset of Contraction and Relaxation of the Total Smooth Muscle Tissue
A typical smooth muscle tissue begins to contract 50 to 100 milliseconds after it is excited, reaches full contraction about 0.5 second later, and then declines in contractile force in another 1 to 2 seconds, giving a total contraction time of 1 to 3 seconds. This is about 30 times as long as a single contraction of an average skeletal muscle fiber. But because there are so many types of smooth muscle, contraction of some types can be as short as 0.2 second or as long as 30 seconds.
The slow onset of contraction of smooth muscle, as well as its prolonged contraction, is caused by the slowness of attachment and detachment of the cross-bridges with the actin filaments. In addition, the initiation of contraction in response to calcium ions is much slower than in skeletal muscle, as discussed later.

Maximum Force of Contraction Is Often Greater in Smooth Muscle Than in Skeletal Muscle
Despite the relatively few myosin filaments in smooth muscle, and despite the slow cycling time of the cross-bridges, the maximum force of contraction of smooth muscle is often greater than that of skeletal muscle—as great as 4 to 6 kg/cm 2 cross-sectional area for smooth muscle, in comparison with 3 to 4 kilograms for skeletal muscle. This great force of smooth muscle contraction results from the prolonged period of attachment of the myosin cross-bridges to the actin filaments.

“Latch” Mechanism Facilitates Prolonged Holding of Contractions of Smooth Muscle
Once smooth muscle has developed full contraction, the amount of continuing excitation can usually be reduced to far less than the initial level yet the muscle maintains its full force of contraction. Further, the energy consumed to maintain contraction is often minuscule, sometimes as little as 1/300 the energy required for comparable sustained skeletal muscle contraction. This is called the “latch” mechanism.
The importance of the latch mechanism is that it can maintain prolonged tonic contraction in smooth muscle for hours with little use of energy. Little continued excitatory signal is required from nerve fibers or hormonal sources.

Stress-Relaxation of Smooth Muscle
Another important characteristic of smooth muscle, especially the visceral unitary type of smooth muscle of many hollow organs, is its ability to return to nearly its original force of contraction seconds or minutes after it has been elongated or shortened. For example, a sudden increase in fluid volume in the urinary bladder, thus stretching the smooth muscle in the bladder wall, causes an immediate large increase in pressure in the bladder. However, during the next 15 seconds to a minute or so, despite continued stretch of the bladder wall, the pressure returns almost exactly back to the original level. Then, when the volume is increased by another step, the same effect occurs again.
Conversely, when the volume is suddenly decreased, the pressure falls drastically at first but then rises in another few seconds or minutes to or near to the original level. These phenomena are called stress-relaxation and reverse stress-relaxation. Their importance is that, except for short periods of time, they allow a hollow organ to maintain about the same amount of pressure inside its lumen despite long-term, large changes in volume.

Regulation of Contraction by Calcium Ions
As is true for skeletal muscle, the initiating stimulus for most smooth muscle contraction is an increase in intracellular calcium ions. This increase can be caused in different types of smooth muscle by nerve stimulation of the smooth muscle fiber, hormonal stimulation, stretch of the fiber, or even change in the chemical environment of the fiber.
Yet smooth muscle does not contain troponin, the regulatory protein that is activated by calcium ions to cause skeletal muscle contraction. Instead, smooth muscle contraction is activated by an entirely different mechanism, as follows.

Calcium Ions Combine with Calmodulin to Cause Activation of Myosin Kinase and Phosphorylation of the Myosin Head
In place of troponin, smooth muscle cells contain a large amount of another regulatory protein called calmodulin ( Figure 8-3 ) . Although this protein is similar to troponin, it is different in the manner in which it initiates contraction. Calmodulin does this by activating the myosin cross-bridges. This activation and subsequent contraction occur in the following sequence:
1. The calcium ions bind with calmodulin.
2. The calmodulin-calcium complex then joins with and activates myosin light chain kinase, a phosphorylating enzyme.
3. One of the light chains of each myosin head, called the regulatory chain, becomes phosphorylated in response to this myosin kinase. When this chain is not phosphorylated, the attachment-detachment cycling of the myosin head with the actin filament does not occur. But when the regulatory chain is phosphorylated, the head has the capability of binding repetitively with the actin filament and proceeding through the entire cycling process of intermittent “pulls,” the same as occurs for skeletal muscle, thus causing muscle contraction.

Figure 8-3 Intracellular calcium ion (Ca ++ ) concentration increases when Ca ++ enters the cell through calcium channels in the cell membrane or the sarcoplasmic reticulum (SR). The Ca ++ binds to calmodulin to form a Ca ++ -calmodulin complex, which then activates myosin light chain kinase (MLCK). The MLCK phosphorylates the myosin light chain (MLC) leading to contraction of the smooth muscle. When Ca ++ concentration decreases, due to pumping of Ca ++ out of the cell, the process is reversed and myosin phosphatase removes the phosphate from MLC, leading to relaxation.

Myosin Phosphatase Is Important in Cessation of Contraction
When the calcium ion concentration falls below a critical level, the aforementioned processes automatically reverse, except for the phosphorylation of the myosin head. Reversal of this requires another enzyme, myosin phosphatase (see Figure 8-3 ) , located in the cytosol of the smooth muscle cell, which splits the phosphate from the regulatory light chain. Then the cycling stops and contraction ceases. The time required for relaxation of muscle contraction, therefore, is determined to a great extent by the amount of active myosin phosphatase in the cell.

Possible Mechanism for Regulation of the Latch Phenomenon
Because of the importance of the latch phenomenon in smooth muscle, and because this phenomenon allows long-term maintenance of tone in many smooth muscle organs without much expenditure of energy, many attempts have been made to explain it. Among the many mechanisms that have been postulated, one of the simplest is the following.
When the myosin kinase and myosin phosphatase enzymes are both strongly activated, the cycling frequency of the myosin heads and the velocity of contraction are great. Then, as the activation of the enzymes decreases, the cycling frequency decreases, but at the same time, the deactivation of these enzymes allows the myosin heads to remain attached to the actin filament for a longer and longer proportion of the cycling period. Therefore, the number of heads attached to the actin filament at any given time remains large. Because the number of heads attached to the actin determines the static force of contraction, tension is maintained, or “latched”; yet little energy is used by the muscle because ATP is not degraded to ADP except on the rare occasion when a head detaches.

Nervous and Hormonal Control of Smooth Muscle Contraction
Although skeletal muscle fibers are stimulated exclusively by the nervous system, smooth muscle can be stimulated to contract by multiple types of signals: by nervous signals, by hormonal stimulation, by stretch of the muscle, and in several other ways. The principal reason for the difference is that the smooth muscle membrane contains many types of receptor proteins that can initiate the contractile process. Still other receptor proteins inhibit smooth muscle contraction, which is another difference from skeletal muscle. Therefore, in this section, we discuss nervous control of smooth muscle contraction, followed by hormonal control and other means of control.

Neuromuscular Junctions of Smooth Muscle

Physiologic Anatomy of Smooth Muscle Neuromuscular Junctions
Neuromuscular junctions of the highly structured type found on skeletal muscle fibers do not occur in smooth muscle. Instead, the autonomic nerve fibers that innervate smooth muscle generally branch diffusely on top of a sheet of muscle fibers, as shown in Figure 8-4 . In most instances, these fibers do not make direct contact with the smooth muscle fiber cell membranes but instead form so-called diffuse junctions that secrete their transmitter substance into the matrix coating of the smooth muscle often a few nanometers to a few micrometers away from the muscle cells; the transmitter substance then diffuses to the cells. Furthermore, where there are many layers of muscle cells, the nerve fibers often innervate only the outer layer. Muscle excitation travels from this outer layer to the inner layers by action potential conduction in the muscle mass or by additional diffusion of the transmitter substance.

Figure 8-4 Innervation of smooth muscle.
The axons that innervate smooth muscle fibers do not have typical branching end feet of the type in the motor end plate on skeletal muscle fibers. Instead, most of the fine terminal axons have multiple varicosities distributed along their axes. At these points the Schwann cells that envelop the axons are interrupted so that transmitter substance can be secreted through the walls of the varicosities. In the varicosities are vesicles similar to those in the skeletal muscle end plate that contain transmitter substance. But in contrast to the vesicles of skeletal muscle junctions, which always contain acetylcholine, the vesicles of the autonomic nerve fiber endings contain acetylcholine in some fibers and norepinephrine in others—and occasionally other substances as well.
In a few instances, particularly in the multi-unit type of smooth muscle, the varicosities are separated from the muscle cell membrane by as little as 20 to 30 nanometers—the same width as the synaptic cleft that occurs in the skeletal muscle junction. These are called contact junctions, and they function in much the same way as the skeletal muscle neuromuscular junction; the rapidity of contraction of these smooth muscle fibers is considerably faster than that of fibers stimulated by the diffuse junctions.

Excitatory and Inhibitory Transmitter Substances Secreted at the Smooth Muscle Neuromuscular Junction
The most important transmitter substances secreted by the autonomic nerves innervating smooth muscle are acetylcholine and norepinephrine, but they are never secreted by the same nerve fibers. Acetylcholine is an excitatory transmitter substance for smooth muscle fibers in some organs but an inhibitory transmitter for smooth muscle in other organs. When acetylcholine excites a muscle fiber, norepinephrine ordinarily inhibits it. Conversely, when acetylcholine inhibits a fiber, norepinephrine usually excites it.
But why are these responses different? The answer is that both acetylcholine and norepinephrine excite or inhibit smooth muscle by first binding with a receptor protein on the surface of the muscle cell membrane. Some of the receptor proteins are excitatory receptors, whereas others are inhibitory receptors. Thus, the type of receptor determines whether the smooth muscle is inhibited or excited and also determines which of the two transmitters, acetylcholine or norepinephrine, is effective in causing the excitation or inhibition. These receptors are discussed in more detail in Chapter 60 in relation to function of the autonomic nervous system.

Membrane Potentials and Action Potentials in Smooth Muscle

Membrane Potentials in Smooth Muscle
The quantitative voltage of the membrane potential of smooth muscle depends on the momentary condition of the muscle. In the normal resting state, the intracellular potential is usually about −50 to −60 millivolts, which is about 30 millivolts less negative than in skeletal muscle.

Action Potentials in Unitary Smooth Muscle
Action potentials occur in unitary smooth muscle (such as visceral muscle) in the same way that they occur in skeletal muscle. They do not normally occur in most multi-unit types of smooth muscle, as discussed in a subsequent section.
The action potentials of visceral smooth muscle occur in one of two forms: (1) spike potentials or (2) action potentials with plateaus.

Spike Potentials
Typical spike action potentials, such as those seen in skeletal muscle, occur in most types of unitary smooth muscle. The duration of this type of action potential is 10 to 50 milliseconds, as shown in Figure 8-5 A . Such action potentials can be elicited in many ways, for example, by electrical stimulation, by the action of hormones on the smooth muscle, by the action of transmitter substances from nerve fibers, by stretch, or as a result of spontaneous generation in the muscle fiber itself, as discussed subsequently.

Figure 8-5 A, Typical smooth muscle action potential (spike potential) elicited by an external stimulus. B, Repetitive spike potentials, elicited by slow rhythmical electrical waves that occur spontaneously in the smooth muscle of the intestinal wall. C, Action potential with a plateau, recorded from a smooth muscle fiber of the uterus.

Action Potentials with Plateaus
Figure 8-5 C shows a smooth muscle action potential with a plateau. The onset of this action potential is similar to that of the typical spike potential. However, instead of rapid repolarization of the muscle fiber membrane, the repolarization is delayed for several hundred to as much as 1000 milliseconds (1 second). The importance of the plateau is that it can account for the prolonged contraction that occurs in some types of smooth muscle, such as the ureter, the uterus under some conditions, and certain types of vascular smooth muscle. (Also, this is the type of action potential seen in cardiac muscle fibers that have a prolonged period of contraction, as discussed in Chapters 9 and 10 .)

Calcium Channels Are Important in Generating the Smooth Muscle Action Potential
The smooth muscle cell membrane has far more voltage-gated calcium channels than does skeletal muscle but few voltage-gated sodium channels. Therefore, sodium participates little in the generation of the action potential in most smooth muscle. Instead, flow of calcium ions to the interior of the fiber is mainly responsible for the action potential. This occurs in the same self-regenerative way as occurs for the sodium channels in nerve fibers and in skeletal muscle fibers. However, the calcium channels open many times more slowly than do sodium channels, and they also remain open much longer. This accounts in large measure for the prolonged plateau action potentials of some smooth muscle fibers.
Another important feature of calcium ion entry into the cells during the action potential is that the calcium ions act directly on the smooth muscle contractile mechanism to cause contraction. Thus, the calcium performs two tasks at once.

Slow Wave Potentials in Unitary Smooth Muscle Can Lead to Spontaneous Generation of Action Potentials
Some smooth muscle is self-excitatory. That is, action potentials arise within the smooth muscle cells themselves without an extrinsic stimulus. This is often associated with a basic slow wave rhythm of the membrane potential. A typical slow wave in a visceral smooth muscle of the gut is shown in Figure 8-5 B . The slow wave itself is not the action potential. That is, it is not a self-regenerative process that spreads progressively over the membranes of the muscle fibers. Instead, it is a local property of the smooth muscle fibers that make up the muscle mass.
The cause of the slow wave rhythm is unknown. One suggestion is that the slow waves are caused by waxing and waning of the pumping of positive ions (presumably sodium ions) outward through the muscle fiber membrane; that is, the membrane potential becomes more negative when sodium is pumped rapidly and less negative when the sodium pump becomes less active. Another suggestion is that the conductances of the ion channels increase and decrease rhythmically.
The importance of the slow waves is that, when they are strong enough, they can initiate action potentials. The slow waves themselves cannot cause muscle contraction. However, when the peak of the negative slow wave potential inside the cell membrane rises in the positive direction from −60 to about −35 millivolts (the approximate threshold for eliciting action potentials in most visceral smooth muscle), an action potential develops and spreads over the muscle mass and contraction occurs. Figure 8-5 B demonstrates this effect, showing that at each peak of the slow wave, one or more action potentials occur. These repetitive sequences of action potentials elicit rhythmical contraction of the smooth muscle mass. Therefore, the slow waves are called pacemaker waves. In Chapter 62 , we see that this type of pacemaker activity controls the rhythmical contractions of the gut.

Excitation of Visceral Smooth Muscle by Muscle Stretch
When visceral (unitary) smooth muscle is stretched sufficiently, spontaneous action potentials are usually generated. They result from a combination of (1) the normal slow wave potentials and (2) decrease in overall negativity of the membrane potential caused by the stretch itself. This response to stretch allows the gut wall, when excessively stretched, to contract automatically and rhythmically. For instance, when the gut is overfilled by intestinal contents, local automatic contractions often set up peristaltic waves that move the contents away from the overfilled intestine, usually in the direction of the anus.

Depolarization of Multi-Unit Smooth Muscle Without Action Potentials
The smooth muscle fibers of multi-unit smooth muscle (such as the muscle of the iris of the eye or the piloerector muscle of each hair) normally contract mainly in response to nerve stimuli. The nerve endings secrete acetylcholine in the case of some multi-unit smooth muscles and norepinephrine in the case of others. In both instances, the transmitter substances cause depolarization of the smooth muscle membrane, and this in turn elicits contraction. Action potentials usually do not develop; the reason is that the fibers are too small to generate an action potential. (When action potentials are elicited in visceral unitary smooth muscle, 30 to 40 smooth muscle fibers must depolarize simultaneously before a self-propagating action potential ensues.) Yet in small smooth muscle cells, even without an action potential, the local depolarization (called the junctional potential ) caused by the nerve transmitter substance itself spreads “electrotonically” over the entire fiber and is all that is necessary to cause muscle contraction.

Effect of Local Tissue Factors and Hormones to Cause Smooth Muscle Contraction Without Action Potentials
Probably half of all smooth muscle contraction is initiated by stimulatory factors acting directly on the smooth muscle contractile machinery and without action potentials. Two types of non-nervous and nonaction potential stimulating factors often involved are (1) local tissue chemical factors and (2) various hormones.

Smooth Muscle Contraction in Response to Local Tissue Chemical Factors
In Chapter 17 , we discuss control of contraction of the arterioles, meta-arterioles, and precapillary sphincters. The smallest of these vessels have little or no nervous supply. Yet the smooth muscle is highly contractile, responding rapidly to changes in local chemical conditions in the surrounding interstitial fluid.
In the normal resting state, many of these small blood vessels remain contracted. But when extra blood flow to the tissue is necessary, multiple factors can relax the vessel wall, thus allowing for increased flow. In this way, a powerful local feedback control system controls the blood flow to the local tissue area. Some of the specific control factors are as follows:
1. Lack of oxygen in the local tissues causes smooth muscle relaxation and, therefore, vasodilatation.
2. Excess carbon dioxide causes vasodilatation.
3. Increased hydrogen ion concentration causes vasodilatation.
Adenosine, lactic acid, increased potassium ions, diminished calcium ion concentration, and increased body temperature can all cause local vasodilatation.

Effects of Hormones on Smooth Muscle Contraction
Many circulating hormones in the blood affect smooth muscle contraction to some degree, and some have profound effects. Among the more important of these are norepinephrine, epinephrine, acetylcholine, angiotensin, endothelin, vasopressin, oxytocin, serotonin, and histamine.
A hormone causes contraction of a smooth muscle when the muscle cell membrane contains hormone-gated excitatory receptors for the respective hormone. Conversely, the hormone causes inhibition if the membrane contains inhibitory receptors for the hormone rather than excitatory receptors.

Mechanisms of Smooth Muscle Excitation or Inhibition by Hormones or Local Tissue Factors
Some hormone receptors in the smooth muscle membrane open sodium or calcium ion channels and depolarize the membrane, the same as after nerve stimulation. Sometimes action potentials result, or action potentials that are already occurring may be enhanced. In other cases, depolarization occurs without action potentials and this depolarization allows calcium ion entry into the cell, which promotes the contraction.
Inhibition, in contrast, occurs when the hormone (or other tissue factor) closes the sodium and calcium channels to prevent entry of these positive ions; inhibition also occurs if the normally closed potassium channels are opened, allowing positive potassium ions to diffuse out of the cell. Both of these actions increase the degree of negativity inside the muscle cell, a state called hyperpolarization, which strongly inhibits muscle contraction.
Sometimes smooth muscle contraction or inhibition is initiated by hormones without directly causing any change in the membrane potential. In these instances, the hormone may activate a membrane receptor that does not open any ion channels but instead causes an internal change in the muscle fiber, such as release of calcium ions from the intracellular sarcoplasmic reticulum; the calcium then induces contraction. To inhibit contraction, other receptor mechanisms are known to activate the enzyme adenylate cyclase or guanylate cyclase in the cell membrane; the portions of the receptors that protrude to the interior of the cells are coupled to these enzymes, causing the formation of cyclic adenosine monophosphate (cAMP) or cyclic guanosine monophosphate (cGMP), so-called second messengers. The cAMP or cGMP has many effects, one of which is to change the degree of phosphorylation of several enzymes that indirectly inhibit contraction. The pump that moves calcium ions from the sarcoplasm into the sarcoplasmic reticulum is activated, as well as the cell membrane pump that moves calcium ions out of the cell itself; these effects reduce the calcium ion concentration in the sarcoplasm, thereby inhibiting contraction.
Smooth muscles have considerable diversity in how they initiate contraction or relaxation in response to different hormones, neurotransmitters, and other substances. In some instances, the same substance may cause either relaxation or contraction of smooth muscles in different locations. For example, norepinephrine inhibits contraction of smooth muscle in the intestine but stimulates contraction of smooth muscle in blood vessels.

Source of Calcium Ions That Cause Contraction Through the Cell Membrane and from the Sarcoplasmic Reticulum
Although the contractile process in smooth muscle, as in skeletal muscle, is activated by calcium ions, the source of the calcium ions differs. An important difference is that the sarcoplasmic reticulum, which provides virtually all the calcium ions for skeletal muscle contraction, is only slightly developed in most smooth muscle. Instead, most of the calcium ions that cause contraction enter the muscle cell from the extracellular fluid at the time of the action potential or other stimulus. That is, the concentration of calcium ions in the extracellular fluid is greater than 10 −3 molar, in comparison with less than 10 −7 molar inside the smooth muscle cell; this causes rapid diffusion of the calcium ions into the cell from the extracellular fluid when the calcium channels open. The time required for this diffusion to occur averages 200 to 300 milliseconds and is called the latent period before contraction begins. This latent period is about 50 times as great for smooth muscle as for skeletal muscle contraction.

Role of the Smooth Muscle Sarcoplasmic Reticulum
Figure 8-6 shows a few slightly developed sarcoplasmic tubules that lie near the cell membrane in some larger smooth muscle cells. Small invaginations of the cell membrane, called caveolae, abut the surfaces of these tubules. The caveolae suggest a rudimentary analog of the transverse tubule system of skeletal muscle. When an action potential is transmitted into the caveolae, this is believed to excite calcium ion release from the abutting sarcoplasmic tubules in the same way that action potentials in skeletal muscle transverse tubules cause release of calcium ions from the skeletal muscle longitudinal sarcoplasmic tubules. In general, the more extensive the sarcoplasmic reticulum in the smooth muscle fiber, the more rapidly it contracts.

Figure 8-6 Sarcoplasmic tubules in a large smooth muscle fiber showing their relation to invaginations in the cell membrane called caveolae.

Smooth Muscle Contraction Is Dependent on Extracellular Calcium Ion Concentration
Although changing the extracellular fluid calcium ion concentration from normal has little effect on the force of contraction of skeletal muscle, this is not true for most smooth muscle. When the extracellular fluid calcium ion concentration falls to about 1/3 to 1/10 normal, smooth muscle contraction usually ceases. Therefore, the force of contraction of smooth muscle is usually highly dependent on extracellular fluid calcium ion concentration.

A Calcium Pump Is Required to Cause Smooth Muscle Relaxation
To cause relaxation of smooth muscle after it has contracted, the calcium ions must be removed from the intracellular fluids. This removal is achieved by a calcium pump that pumps calcium ions out of the smooth muscle fiber back into the extracellular fluid, or into a sarcoplasmic reticulum, if it is present. This pump is slow-acting in comparison with the fast-acting sarcoplasmic reticulum pump in skeletal muscle. Therefore, a single smooth muscle contraction often lasts for seconds rather than hundredths to tenths of a second, as occurs for skeletal muscle.


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The Heart
CHAPTER 9 Cardiac Muscle; The Heart as a Pump and Function of the Heart Valves
With this chapter we begin discussion of the heart and circulatory system. The heart, shown in Figure 9-1 , is actually two separate pumps: a right heart that pumps blood through the lungs, and a left heart that pumps blood through the peripheral organs. In turn, each of these hearts is a pulsatile two-chamber pump composed of an atrium and a ventricle. Each atrium is a weak primer pump for the ventricle, helping to move blood into the ventricle. The ventricles then supply the main pumping force that propels the blood either (1) through the pulmonary circulation by the right ventricle or (2) through the peripheral circulation by the left ventricle.

Figure 9-1 Structure of the heart, and course of blood flow through the heart chambers and heart valves.
Special mechanisms in the heart cause a continuing succession of heart contractions called cardiac rhythmicity, transmitting action potentials throughout the cardiac muscle to cause the heart’s rhythmical beat. This rhythmical control system is explained in Chapter 10 . In this chapter, we explain how the heart operates as a pump, beginning with the special features of cardiac muscle itself.

Physiology of Cardiac Muscle
The heart is composed of three major types of cardiac muscle: atrial muscle, ventricular muscle, and specialized excitatory and conductive muscle fibers. The atrial and ventricular types of muscle contract in much the same way as skeletal muscle, except that the duration of contraction is much longer. The specialized excitatory and conductive fibers, however, contract only feebly because they contain few contractile fibrils; instead, they exhibit either automatic rhythmical electrical discharge in the form of action potentials or conduction of the action potentials through the heart, providing an excitatory system that controls the rhythmical beating of the heart.

Physiologic Anatomy of Cardiac Muscle
Figure 9-2 shows the histology of cardiac muscle, demonstrating cardiac muscle fibers arranged in a latticework, with the fibers dividing, recombining, and then spreading again. One also notes immediately from this figure that cardiac muscle is striated in the same manner as in skeletal muscle. Further, cardiac muscle has typical myofibrils that contain actin and myosin filaments almost identical to those found in skeletal muscle; these filaments lie side by side and slide along one another during contraction in the same manner as occurs in skeletal muscle (see Chapter 6 ). But in other ways, cardiac muscle is quite different from skeletal muscle, as we shall see.

Figure 9-2 “Syncytial,” interconnecting nature of cardiac muscle fibers.

Cardiac Muscle as a Syncytium
The dark areas crossing the cardiac muscle fibers in Figure 9-2 are called intercalated discs; they are actually cell membranes that separate individual cardiac muscle cells from one another. That is, cardiac muscle fibers are made up of many individual cells connected in series and in parallel with one another.
At each intercalated disc the cell membranes fuse with one another in such a way that they form permeable “communicating” junctions (gap junctions) that allow rapid diffusion of ions. Therefore, from a functional point of view, ions move with ease in the intracellular fluid along the longitudinal axes of the cardiac muscle fibers so that action potentials travel easily from one cardiac muscle cell to the next, past the intercalated discs. Thus, cardiac muscle is a syncytium of many heart muscle cells in which the cardiac cells are so interconnected that when one of these cells becomes excited, the action potential spreads to all of them, from cell to cell throughout the latticework interconnections.
The heart actually is composed of two syncytiums: the atrial syncytium, which constitutes the walls of the two atria, and the ventricular syncytium, which constitutes the walls of the two ventricles. The atria are separated from the ventricles by fibrous tissue that surrounds the atrioventricular (A-V) valvular openings between the atria and ventricles. Normally, potentials are not conducted from the atrial syncytium into the ventricular syncytium directly through this fibrous tissue. Instead, they are conducted only by way of a specialized conductive system called the A-V bundle, a bundle of conductive fibers several millimeters in diameter that is discussed in detail in Chapter 10 .
This division of the muscle of the heart into two functional syncytiums allows the atria to contract a short time ahead of ventricular contraction, which is important for effectiveness of heart pumping.

Action Potentials in Cardiac Muscle
The action potential recorded in a ventricular muscle fiber, shown in Figure 9-3 , averages about 105 millivolts, which means that the intracellular potential rises from a very negative value, about −85 millivolts, between beats to a slightly positive value, about +20 millivolts, during each beat. After the initial spike, the membrane remains depolarized for about 0.2 second, exhibiting a plateau as shown in the figure, followed at the end of the plateau by abrupt repolarization. The presence of this plateau in the action potential causes ventricular contraction to last as much as 15 times as long in cardiac muscle as in skeletal muscle.

Figure 9-3 Rhythmical action potentials (in millivolts) from a Purkinje fiber and from a ventricular muscle fiber, recorded by means of microelectrodes.

What Causes the Long Action Potential and the Plateau?
At this point, we address the questions: Why is the action potential of cardiac muscle so long and why does it have a plateau, whereas that of skeletal muscle does not? The basic biophysical answers to these questions were presented in Chapter 5 , but they merit summarizing here as well.
At least two major differences between the membrane properties of cardiac and skeletal muscle account for the prolonged action potential and the plateau in cardiac muscle. First, the action potential of skeletal muscle is caused almost entirely by sudden opening of large numbers of so-called fast sodium channels that allow tremendous numbers of sodium ions to enter the skeletal muscle fiber from the extracellular fluid. These channels are called “fast” channels because they remain open for only a few thousandths of a second and then abruptly close. At the end of this closure, repolarization occurs, and the action potential is over within another thousandth of a second or so.
In cardiac muscle, the action potential is caused by opening of two types of channels: (1) the same fast sodium channels as those in skeletal muscle and (2) another entirely different population of slow calcium channels, which are also called calcium-sodium channels. This second population of channels differs from the fast sodium channels in that they are slower to open and, even more important, remain open for several tenths of a second. During this time, a large quantity of both calcium and sodium ions flows through these channels to the interior of the cardiac muscle fiber, and this maintains a prolonged period of depolarization, causing the plateau in the action potential. Further, the calcium ions that enter during this plateau phase activate the muscle contractile process, while the calcium ions that cause skeletal muscle contraction are derived from the intracellular sarcoplasmic reticulum.
The second major functional difference between cardiac muscle and skeletal muscle that helps account for both the prolonged action potential and its plateau is this: Immediately after the onset of the action potential, the permeability of the cardiac muscle membrane for potassium ions decreases about fivefold, an effect that does not occur in skeletal muscle. This decreased potassium permeability may result from the excess calcium influx through the calcium channels just noted. Regardless of the cause, the decreased potassium permeability greatly decreases the outflux of positively charged potassium ions during the action potential plateau and thereby prevents early return of the action potential voltage to its resting level. When the slow calcium-sodium channels do close at the end of 0.2 to 0.3 second and the influx of calcium and sodium ions ceases, the membrane permeability for potassium ions also increases rapidly; this rapid loss of potassium from the fiber immediately returns the membrane potential to its resting level, thus ending the action potential.

Velocity of Signal Conduction in Cardiac Muscle
The velocity of conduction of the excitatory action potential signal along both atrial and ventricular muscle fibers is about 0.3 to 0.5 m/sec, or about the velocity in very large nerve fibers and about the velocity in skeletal muscle fibers. The velocity of conduction in the specialized heart conductive system—in the Purkinje fibers —is as great as 4 m/sec in most parts of the system, which allows reasonably rapid conduction of the excitatory signal to the different parts of the heart, as explained in Chapter 10 .

Refractory Period of Cardiac Muscle
Cardiac muscle, like all excitable tissue, is refractory to restimulation during the action potential. Therefore, the refractory period of the heart is the interval of time, as shown to the left in Figure 9-4 , during which a normal cardiac impulse cannot re-excite an already excited area of cardiac muscle. The normal refractory period of the ventricle is 0.25 to 0.30 second, which is about the duration of the prolonged plateau action potential. There is an additional relative refractory period of about 0.05 second during which the muscle is more difficult than normal to excite but nevertheless can be excited by a very strong excitatory signal, as demonstrated by the early “premature” contraction in the second example of Figure 9-4 . The refractory period of atrial muscle is much shorter than that for the ventricles (about 0.15 second for the atria compared with 0.25 to 0.30 second for the ventricles).

Figure 9-4 Force of ventricular heart muscle contraction, showing also duration of the refractory period and relative refractory period, plus the effect of premature contraction. Note that premature contractions do not cause wave summation, as occurs in skeletal muscle.

Excitation-Contraction Coupling—Function of Calcium Ions and the Transverse Tubules
The term “excitation-contraction coupling” refers to the mechanism by which the action potential causes the myofibrils of muscle to contract. This was discussed for skeletal muscle in Chapter 7 . Once again, there are differences in this mechanism in cardiac muscle that have important effects on the characteristics of heart muscle contraction.
As is true for skeletal muscle, when an action potential passes over the cardiac muscle membrane, the action potential spreads to the interior of the cardiac muscle fiber along the membranes of the transverse (T) tubules. The T tubule action potentials in turn act on the membranes of the longitudinal sarcoplasmic tubules to cause release of calcium ions into the muscle sarcoplasm from the sarcoplasmic reticulum. In another few thousandths of a second, these calcium ions diffuse into the myofibrils and catalyze the chemical reactions that promote sliding of the actin and myosin filaments along one another; this produces the muscle contraction.
Thus far, this mechanism of excitation-contraction coupling is the same as that for skeletal muscle, but there is a second effect that is quite different. In addition to the calcium ions that are released into the sarcoplasm from the cisternae of the sarcoplasmic reticulum, calcium ions also diffuse into the sarcoplasm from the T tubules themselves at the time of the action potential, which opens voltage-dependent calcium channels in the membrane of the T tubule ( Figure 9-5 ). Calcium entering the cell then activates calcium release channels, also called ryanodine receptor channels, in the sarcoplasmic reticulum membrane, triggering the release of calcium into the sarcoplasm. Calcium ions in the sarcoplasm then interact with troponin to initiate cross-bridge formation and contraction by the same basic mechanism as described for skeletal muscle in Chapter 6 .

Figure 9-5 Mechanisms of excitation-contraction coupling and relaxation in cardiac muscle.
Without the calcium from the T tubules, the strength of cardiac muscle contraction would be reduced considerably because the sarcoplasmic reticulum of cardiac muscle is less well developed than that of skeletal muscle and does not store enough calcium to provide full contraction. The T tubules of cardiac muscle, however, have a diameter 5 times as great as that of the skeletal muscle tubules, which means a volume 25 times as great. Also, inside the T tubules is a large quantity of mucopolysaccharides that are electronegatively charged and bind an abundant store of calcium ions, keeping these always available for diffusion to the interior of the cardiac muscle fiber when a T tubule action potential appears.
The strength of contraction of cardiac muscle depends to a great extent on the concentration of calcium ions in the extracellular fluids. In fact, a heart placed in a calcium-free solution will quickly stop beating. The reason for this is that the openings of the T tubules pass directly through the cardiac muscle cell membrane into the extracellular spaces surrounding the cells, allowing the same extracellular fluid that is in the cardiac muscle interstitium to percolate through the T tubules as well. Consequently, the quantity of calcium ions in the T tubule system (i.e., the availability of calcium ions to cause cardiac muscle contraction) depends to a great extent on the extracellular fluid calcium ion concentration.
In contrast, the strength of skeletal muscle contraction is hardly affected by moderate changes in extracellular fluid calcium concentration because skeletal muscle contraction is caused almost entirely by calcium ions released from the sarcoplasmic reticulum inside the skeletal muscle fiber.
At the end of the plateau of the cardiac action potential, the influx of calcium ions to the interior of the muscle fiber is suddenly cut off, and the calcium ions in the sarcoplasm are rapidly pumped back out of the muscle fibers into both the sarcoplasmic reticulum and the T tubule–extracellular fluid space. Transport of calcium back into the sarcoplasmic reticulum is achieved with the help of a calcium-ATPase pump (see Figure 9-5 ). Calcium ions are also removed from the cell by a sodium-calcium exchanger. The sodium that enters the cell during this exchange is then transported out of the cell by the sodium-potassium ATPase pump. As a result, the contraction ceases until a new action potential comes along.

Duration of Contraction
Cardiac muscle begins to contract a few milliseconds after the action potential begins and continues to contract until a few milliseconds after the action potential ends. Therefore, the duration of contraction of cardiac muscle is mainly a function of the duration of the action potential, including the plateau —about 0.2 second in atrial muscle and 0.3 second in ventricular muscle.

Cardiac Cycle
The cardiac events that occur from the beginning of one heartbeat to the beginning of the next are called the cardiac cycle. Each cycle is initiated by spontaneous generation of an action potential in the sinus node, as explained in Chapter 10 . This node is located in the superior lateral wall of the right atrium near the opening of the superior vena cava, and the action potential travels from here rapidly through both atria and then through the A-V bundle into the ventricles. Because of this special arrangement of the conducting system from the atria into the ventricles, there is a delay of more than 0.1 second during passage of the cardiac impulse from the atria into the ventricles. This allows the atria to contract ahead of ventricular contraction, thereby pumping blood into the ventricles before the strong ventricular contraction begins. Thus, the atria act as primer pumps for the ventricles, and the ventricles in turn provide the major source of power for moving blood through the body’s vascular system.

Diastole and Systole
The cardiac cycle consists of a period of relaxation called diastole, during which the heart fills with blood, followed by a period of contraction called systole.
The total duration of the cardiac cycle, including systole and diastole, is the reciprocal of the heart rate. For example, if heart rate is 72 beats/min, the duration of the cardiac cycle is 1/72 beats/min—about 0.0139 minutes per beat, or 0.833 second per beat.
Figure 9-6 shows the different events during the cardiac cycle for the left side of the heart. The top three curves show the pressure changes in the aorta, left ventricle, and left atrium, respectively. The fourth curve depicts the changes in left ventricular volume, the fifth the electrocardiogram, and the sixth a phonocardiogram, which is a recording of the sounds produced by the heart—mainly by the heart valves—as it pumps. It is especially important that the reader study in detail this figure and understand the causes of all the events shown.

Figure 9-6 Events of the cardiac cycle for left ventricular function, showing changes in left atrial pressure, left ventricular pressure, aortic pressure, ventricular volume, the electrocardiogram, and the phonocardiogram.

Effect of Heart Rate on Duration of Cardiac Cycle
When heart rate increases, the duration of each cardiac cycle decreases, including the contraction and relaxation phases. The duration of the action potential and the period of contraction (systole) also decrease, but not by as great a percentage as does the relaxation phase (diastole). At a normal heart rate of 72 beats/min, systole comprises about 0.4 of the entire cardiac cycle. At three times the normal heart rate, systole is about 0.65 of the entire cardiac cycle. This means that the heart beating at a very fast rate does not remain relaxed long enough to allow complete filling of the cardiac chambers before the next contraction.

Relationship of the Electrocardiogram to the Cardiac Cycle
The electrocardiogram in Figure 9-6 shows the P, Q, R, S, and T waves, which are discussed in Chapters 11 , 12 , and 13 . They are electrical voltages generated by the heart and recorded by the electrocardiograph from the surface of the body.
The P wave is caused by spread of depolarization through the atria, and this is followed by atrial contraction, which causes a slight rise in the atrial pressure curve immediately after the electrocardiographic P wave.
About 0.16 second after the onset of the P wave, the QRS waves appear as a result of electrical depolarization of the ventricles, which initiates contraction of the ventricles and causes the ventricular pressure to begin rising, as also shown in the figure. Therefore, the QRS complex begins slightly before the onset of ventricular systole.
Finally, one observes the ventricular T wave in the electrocardiogram. This represents the stage of repolarization of the ventricles when the ventricular muscle fibers begin to relax. Therefore, the T wave occurs slightly before the end of ventricular contraction.

Function of the Atria as Primer Pumps
Blood normally flows continually from the great veins into the atria; about 80 percent of the blood flows directly through the atria into the ventricles even before the atria contract. Then, atrial contraction usually causes an additional 20 percent filling of the ventricles. Therefore, the atria simply function as primer pumps that increase the ventricular pumping effectiveness as much as 20 percent. However, the heart can continue to operate under most conditions even without this extra 20 percent effectiveness because it normally has the capability of pumping 300 to 400 percent more blood than is required by the resting body. Therefore, when the atria fail to function, the difference is unlikely to be noticed unless a person exercises; then acute signs of heart failure occasionally develop, especially shortness of breath.

Pressure Changes in the Atria—a, c, and v Waves
In the atrial pressure curve of Figure 9-6 , three minor pressure elevations, called the a, c, and v atrial pressure waves, are noted.
The a wave is caused by atrial contraction. Ordinarily, the right atrial pressure increases 4 to 6 mm Hg during atrial contraction, and the left atrial pressure increases about 7 to 8 mm Hg.
The c wave occurs when the ventricles begin to contract; it is caused partly by slight backflow of blood into the atria at the onset of ventricular contraction but mainly by bulging of the A-V valves backward toward the atria because of increasing pressure in the ventricles.
The v wave occurs toward the end of ventricular contraction; it results from slow flow of blood into the atria from the veins while the A-V valves are closed during ventricular contraction. Then, when ventricular contraction is over, the A-V valves open, allowing this stored atrial blood to flow rapidly into the ventricles and causing the v wave to disappear.

Function of the Ventricles as Pumps

Filling of the Ventricles During Diastole
During ventricular systole, large amounts of blood accumulate in the right and left atria because of the closed A-V valves. Therefore, as soon as systole is over and the ventricular pressures fall again to their low diastolic values, the moderately increased pressures that have developed in the atria during ventricular systole immediately push the A-V valves open and allow blood to flow rapidly into the ventricles, as shown by the rise of the left ventricular volume curve in Figure 9-6 . This is called the period of rapid filling of the ventricles.
The period of rapid filling lasts for about the first third of diastole. During the middle third of diastole, only a small amount of blood normally flows into the ventricles; this is blood that continues to empty into the atria from the veins and passes through the atria directly into the ventricles.
During the last third of diastole, the atria contract and give an additional thrust to the inflow of blood into the ventricles; this accounts for about 20 percent of the filling of the ventricles during each heart cycle.

Emptying of the Ventricles During Systole

Period of Isovolumic (Isometric) Contraction
Immediately after ventricular contraction begins, the ventricular pressure rises abruptly, as shown in Figure 9-6 , causing the A-V valves to close. Then an additional 0.02 to 0.03 second is required for the ventricle to build up sufficient pressure to push the semilunar (aortic and pulmonary) valves open against the pressures in the aorta and pulmonary artery. Therefore, during this period, contraction is occurring in the ventricles, but there is no emptying. This is called the period of isovolumic or isometric contraction, meaning that tension is increasing in the muscle but little or no shortening of the muscle fibers is occurring.

Period of Ejection
When the left ventricular pressure rises slightly above 80 mm Hg (and the right ventricular pressure slightly above 8 mm Hg), the ventricular pressures push the semilunar valves open. Immediately, blood begins to pour out of the ventricles, with about 70 percent of the blood emptying occurring during the first third of the period of ejection and the remaining 30 percent emptying during the next two thirds. Therefore, the first third is called the period of rapid ejection, and the last two thirds, the period of slow ejection.

Period of Isovolumic (Isometric) Relaxation
At the end of systole, ventricular relaxation begins suddenly, allowing both the right and left intraventricular pressures to decrease rapidly. The elevated pressures in the distended large arteries that have just been filled with blood from the contracted ventricles immediately push blood back toward the ventricles, which snaps the aortic and pulmonary valves closed. For another 0.03 to 0.06 second, the ventricular muscle continues to relax, even though the ventricular volume does not change, giving rise to the period of isovolumic or isometric relaxation. During this period, the intraventricular pressures decrease rapidly back to their low diastolic levels. Then the A-V valves open to begin a new cycle of ventricular pumping.

End-Diastolic Volume, End-Systolic Volume, and Stroke Volume Output
During diastole, normal filling of the ventricles increases the volume of each ventricle to about 110 to 120 ml. This volume is called the end-diastolic volume. Then, as the ventricles empty during systole, the volume decreases about 70 ml, which is called the stroke volume output. The remaining volume in each ventricle, about 40 to 50 ml, is called the end-systolic volume. The fraction of the end-diastolic volume that is ejected is called the ejection fraction —usually equal to about 60 percent.
When the heart contracts strongly, the end-systolic volume can be decreased to as little as 10 to 20 ml. Conversely, when large amounts of blood flow into the ventricles during diastole, the ventricular end-diastolic volumes can become as great as 150 to 180 ml in the healthy heart. By both increasing the end-diastolic volume and decreasing the end-systolic volume, the stroke volume output can be increased to more than double normal.

Function of the Valves

Atrioventricular Valves
The A-V valves (the tricuspid and mitral valves) prevent backflow of blood from the ventricles to the atria during systole, and the semilunar valves (the aortic and pulmonary artery valves) prevent backflow from the aorta and pulmonary arteries into the ventricles during diastole. These valves, shown in Figure 9-7 for the left ventricle, close and open passively. That is, they close when a backward pressure gradient pushes blood backward, and they open when a forward pressure gradient forces blood in the forward direction. For anatomical reasons, the thin, filmy A-V valves require almost no backflow to cause closure, whereas the much heavier semilunar valves require rather rapid backflow for a few milliseconds.

Figure 9-7 Mitral and aortic valves (the left ventricular valves).

Function of the Papillary Muscles
Figure 9-7 also shows papillary muscles that attach to the vanes of the A-V valves by the chordae tendineae. The papillary muscles contract when the ventricular walls contract, but contrary to what might be expected, they do not help the valves to close. Instead, they pull the vanes of the valves inward toward the ventricles to prevent their bulging too far backward toward the atria during ventricular contraction. If a chorda tendinea becomes ruptured or if one of the papillary muscles becomes paralyzed, the valve bulges far backward during ventricular contraction, sometimes so far that it leaks severely and results in severe or even lethal cardiac incapacity.

Aortic and Pulmonary Artery Valves
The aortic and pulmonary artery semilunar valves function quite differently from the A-V valves. First, the high pressures in the arteries at the end of systole cause the semilunar valves to snap to the closed position, in contrast to the much softer closure of the A-V valves. Second, because of smaller openings, the velocity of blood ejection through the aortic and pulmonary valves is far greater than that through the much larger A-V valves. Also, because of the rapid closure and rapid ejection, the edges of the aortic and pulmonary valves are subjected to much greater mechanical abrasion than are the A-V valves. Finally, the A-V valves are supported by the chordae tendineae, which is not true for the semilunar valves. It is obvious from the anatomy of the aortic and pulmonary valves (as shown for the aortic valve at the bottom of Figure 9-7 ) that they must be constructed with an especially strong yet very pliable fibrous tissue base to withstand the extra physical stresses.

Aortic Pressure Curve
When the left ventricle contracts, the ventricular pressure increases rapidly until the aortic valve opens. Then, after the valve opens, the pressure in the ventricle rises much less rapidly, as shown in Figure 9-6 , because blood immediately flows out of the ventricle into the aorta and then into the systemic distribution arteries.
The entry of blood into the arteries causes the walls of these arteries to stretch and the pressure to increase to about 120 mm Hg.
Next, at the end of systole, after the left ventricle stops ejecting blood and the aortic valve closes, the elastic walls of the arteries maintain a high pressure in the arteries, even during diastole.
A so-called incisura occurs in the aortic pressure curve when the aortic valve closes. This is caused by a short period of backward flow of blood immediately before closure of the valve, followed by sudden cessation of the backflow.
After the aortic valve has closed, the pressure in the aorta decreases slowly throughout diastole because the blood stored in the distended elastic arteries flows continually through the peripheral vessels back to the veins. Before the ventricle contracts again, the aortic pressure usually has fallen to about 80 mm Hg (diastolic pressure), which is two thirds the maximal pressure of 120 mm Hg (systolic pressure) that occurs in the aorta during ventricular contraction.
The pressure curves in the right ventricle and pulmonary artery are similar to those in the aorta, except that the pressures are only about one sixth as great, as discussed in Chapter 14 .

Relationship of the Heart Sounds to Heart Pumping
When listening to the heart with a stethoscope, one does not hear the opening of the valves because this is a relatively slow process that normally makes no noise. However, when the valves close, the vanes of the valves and the surrounding fluids vibrate under the influence of sudden pressure changes, giving off sound that travels in all directions through the chest.
When the ventricles contract, one first hears a sound caused by closure of the A-V valves. The vibration is low in pitch and relatively long-lasting and is known as the first heart sound. When the aortic and pulmonary valves close at the end of systole, one hears a rapid snap because these valves close rapidly, and the surroundings vibrate for a short period. This sound is called the second heart sound. The precise causes of the heart sounds are discussed more fully in Chapter 23 , in relation to listening to the sounds with the stethoscope.

Work Output of the Heart
The stroke work output of the heart is the amount of energy that the heart converts to work during each heartbeat while pumping blood into the arteries. Minute work output is the total amount of energy converted to work in 1 minute; this is equal to the stroke work output times the heart rate per minute.
Work output of the heart is in two forms. First, by far the major proportion is used to move the blood from the low-pressure veins to the high-pressure arteries. This is called volume-pressure work or external work. Second, a minor proportion of the energy is used to accelerate the blood to its velocity of ejection through the aortic and pulmonary valves. This is the kinetic energy of blood flow component of the work output.
Right ventricular external work output is normally about one sixth the work output of the left ventricle because of the sixfold difference in systolic pressures that the two ventricles pump. The additional work output of each ventricle required to create kinetic energy of blood flow is proportional to the mass of blood ejected times the square of velocity of ejection.
Ordinarily, the work output of the left ventricle required to create kinetic energy of blood flow is only about 1 percent of the total work output of the ventricle and therefore is ignored in the calculation of the total stroke work output. But in certain abnormal conditions, such as aortic stenosis, in which blood flows with great velocity through the stenosed valve, more than 50 percent of the total work output may be required to create kinetic energy of blood flow.

Graphical Analysis of Ventricular Pumping
Figure 9-8 shows a diagram that is especially useful in explaining the pumping mechanics of the left ventricle. The most important components of the diagram are the two curves labeled “diastolic pressure” and “systolic pressure.” These curves are volume-pressure curves.

Figure 9-8 Relationship between left ventricular volume and intraventricular pressure during diastole and systole. Also shown by the heavy red lines is the “volume-pressure diagram,” demonstrating changes in intraventricular volume and pressure during the normal cardiac cycle. EW, net external work.
The diastolic pressure curve is determined by filling the heart with progressively greater volumes of blood and then measuring the diastolic pressure immediately before ventricular contraction occurs, which is the end-diastolic pressure of the ventricle.
The systolic pressure curve is determined by recording the systolic pressure achieved during ventricular contraction at each volume of filling.
Until the volume of the noncontracting ventricle rises above about 150 ml, the “diastolic” pressure does not increase greatly. Therefore, up to this volume, blood can flow easily into the ventricle from the atrium. Above 150 ml, the ventricular diastolic pressure increases rapidly, partly because of fibrous tissue in the heart that will stretch no more and partly because the pericardium that surrounds the heart becomes filled nearly to its limit.
During ventricular contraction, the “systolic” pressure increases even at low ventricular volumes and reaches a maximum at a ventricular volume of 150 to 170 ml. Then, as the volume increases still further, the systolic pressure actually decreases under some conditions, as demonstrated by the falling systolic pressure curve in Figure 9-8 , because at these great volumes, the actin and myosin filaments of the cardiac muscle fibers are pulled apart far enough that the strength of each cardiac fiber contraction becomes less than optimal.
Note especially in the figure that the maximum systolic pressure for the normal left ventricle is between 250 and 300 mm Hg, but this varies widely with each person’s heart strength and degree of heart stimulation by cardiac nerves. For the normal right ventricle, the maximum systolic pressure is between 60 and 80 mm Hg.

“Volume-Pressure Diagram” During the Cardiac Cycle; Cardiac Work Output
The red lines in Figure 9-8 form a loop called the volume-pressure diagram of the cardiac cycle for normal function of the left ventricle. A more detailed version of this loop is shown in Figure 9-9 . It is divided into four phases.

Figure 9-9 The “volume-pressure diagram” demonstrating changes in intraventricular volume and pressure during a single cardiac cycle ( red line ). The tan shaded area represents the net external work ( EW ) output by the left ventricle during the cardiac cycle.
Phase I: Period of filling. This phase in the volume-pressure diagram begins at a ventricular volume of about 50 ml and a diastolic pressure of 2 to 3 mm Hg. The amount of blood that remains in the ventricle after the previous heartbeat, 50 ml, is called the end-systolic volume. As venous blood flows into the ventricle from the left atrium, the ventricular volume normally increases to about 120 ml, called the end-diastolic volume, an increase of 70 ml. Therefore, the volume-pressure diagram during phase I extends along the line labeled “I,” from point A to point B, with the volume increasing to 120 ml and the diastolic pressure rising to about 5 to 7 mm Hg.
Phase II: Period of isovolumic contraction. During isovolumic contraction, the volume of the ventricle does not change because all valves are closed. However, the pressure inside the ventricle increases to equal the pressure in the aorta, at a pressure value of about 80 mm Hg, as depicted by point C.
Phase III: Period of ejection. During ejection, the systolic pressure rises even higher because of still more contraction of the ventricle. At the same time, the volume of the ventricle decreases because the aortic valve has now opened and blood flows out of the ventricle into the aorta. Therefore, the curve labeled “III,” or “period of ejection,” traces the changes in volume and systolic pressure during this period of ejection.
Phase IV: Period of isovolumic relaxation. At the end of the period of ejection (point D), the aortic valve closes, and the ventricular pressure falls back to the diastolic pressure level. The line labeled “IV” traces this decrease in intraventricular pressure without any change in volume. Thus, the ventricle returns to its starting point, with about 50 ml of blood left in the ventricle and at an atrial pressure of 2 to 3 mm Hg.
Readers well trained in the basic principles of physics will recognize that the area subtended by this functional volume-pressure diagram (the tan shaded area, labeled EW) represents the net external work output of the ventricle during its contraction cycle. In experimental studies of cardiac contraction, this diagram is used for calculating cardiac work output.
When the heart pumps large quantities of blood, the area of the work diagram becomes much larger. That is, it extends far to the right because the ventricle fills with more blood during diastole, it rises much higher because the ventricle contracts with greater pressure, and it usually extends farther to the left because the ventricle contracts to a smaller volume—especially if the ventricle is stimulated to increased activity by the sympathetic nervous system.

Concepts of Preload and Afterload
In assessing the contractile properties of muscle, it is important to specify the degree of tension on the muscle when it begins to contract, which is called the preload, and to specify the load against which the muscle exerts its contractile force, which is called the afterload.
For cardiac contraction, the preload is usually considered to be the end-diastolic pressure when the ventricle has become filled.
The afterload of the ventricle is the pressure in the aorta leading from the ventricle. In Figure 9-8 , this corresponds to the systolic pressure described by the phase III curve of the volume-pressure diagram. (Sometimes the afterload is loosely considered to be the resistance in the circulation rather than the pressure.)
The importance of the concepts of preload and afterload is that in many abnormal functional states of the heart or circulation, the pressure during filling of the ventricle (the preload), the arterial pressure against which the ventricle must contract (the afterload), or both are severely altered from normal.

Chemical Energy Required for Cardiac Contraction: Oxygen Utilization by the Heart
Heart muscle, like skeletal muscle, uses chemical energy to provide the work of contraction. Approximately 70 to 90 percent of this energy is normally derived from oxidative metabolism of fatty acids with about 10 to 30 percent coming from other nutrients, especially lactate and glucose. Therefore, the rate of oxygen consumption by the heart is an excellent measure of the chemical energy liberated while the heart performs its work. The different chemical reactions that liberate this energy are discussed in Chapters 67 and 68 .
Experimental studies have shown that oxygen consumption of the heart and the chemical energy expended during contraction are directly related to the total shaded area in Figure 9-8 . This shaded portion consists of the external work (EW) as explained earlier and an additional portion called the potential energy, labeled PE. The potential energy represents additional work that could be accomplished by contraction of the ventricle if the ventricle should empty completely all the blood in its chamber with each contraction.
Oxygen consumption has also been shown to be nearly proportional to the tension that occurs in the heart muscle during contraction multiplied by the duration of time that the contraction persists, called the tension-time index . Because tension is high when systolic pressure is high, correspondingly more oxygen is used. Also, much more chemical energy is expended even at normal systolic pressures when the ventricle is abnormally dilated because the heart muscle tension during contraction is proportional to pressure times the diameter of the ventricle. This becomes especially important in heart failure where the heart ventricle is dilated and, paradoxically, the amount of chemical energy required for a given amount of work output is greater than normal even though the heart is already failing.

Efficiency of Cardiac Contraction
During heart muscle contraction, most of the expended chemical energy is converted into heat and a much smaller portion into work output . The ratio of work output to total chemical energy expenditure is called the efficiency of cardiac contraction, or simply efficiency of the heart. Maximum efficiency of the normal heart is between 20 and 25 percent. In heart failure, this can decrease to as low as 5 to 10 percent.

Regulation of Heart Pumping
When a person is at rest, the heart pumps only 4 to 6 liters of blood each minute. During severe exercise, the heart may be required to pump four to seven times this amount. The basic means by which the volume pumped by the heart is regulated are (1) intrinsic cardiac regulation of pumping in response to changes in volume of blood flowing into the heart and (2) control of heart rate and strength of heart pumping by the autonomic nervous system.

Intrinsic Regulation of Heart Pumping—The Frank-Starling Mechanism
In Chapter 20 , we will learn that under most conditions, the amount of blood pumped by the heart each minute is normally determined almost entirely by the rate of blood flow into the heart from the veins, which is called venous return. That is, each peripheral tissue of the body controls its own local blood flow, and all the local tissue flows combine and return by way of the veins to the right atrium. The heart, in turn, automatically pumps this incoming blood into the arteries so that it can flow around the circuit again.
This intrinsic ability of the heart to adapt to increasing volumes of inflowing blood is called the Frank-Starling mechanism of the heart, in honor of Otto Frank and Ernest Starling, two great physiologists of a century ago. Basically, the Frank-Starling mechanism means that the greater the heart muscle is stretched during filling, the greater is the force of contraction and the greater the quantity of blood pumped into the aorta. Or, stated another way: Within physiologic limits, the heart pumps all the blood that returns to it by the way of the veins.

What Is the Explanation of the Frank-Starling Mechanism?
When an extra amount of blood flows into the ventricles, the cardiac muscle itself is stretched to greater length. This in turn causes the muscle to contract with increased force because the actin and myosin filaments are brought to a more nearly optimal degree of overlap for force generation. Therefore, the ventricle, because of its increased pumping, automatically pumps the extra blood into the arteries.
This ability of stretched muscle, up to an optimal length, to contract with increased work output is characteristic of all striated muscle, as explained in Chapter 6 , and is not simply a characteristic of cardiac muscle.
In addition to the important effect of lengthening the heart muscle, still another factor increases heart pumping when its volume is increased. Stretch of the right atrial wall directly increases the heart rate by 10 to 20 percent; this, too, helps increase the amount of blood pumped each minute, although its contribution is much less than that of the Frank-Starling mechanism.

Ventricular Function Curves
One of the best ways to express the functional ability of the ventricles to pump blood is by ventricular function curves, as shown in Figures 9-10 and 9-11 . Figure 9-10 shows a type of ventricular function curve called the stroke work output curve. Note that as the atrial pressure for each side of the heart increases, the stroke work output for that side increases until it reaches the limit of the ventricle’s pumping ability.

Figure 9-10 Left and right ventricular function curves recorded from dogs, depicting ventricular stroke work output as a function of left and right mean atrial pressures.
(Curves reconstructed from data in Sarnoff SJ: Myocardial contractility as described by ventricular function curves. Physiol Rev 35:107, 1955.)

Figure 9-11 Approximate normal right and left ventricular volume output curves for the normal resting human heart as extrapolated from data obtained in dogs and data from human beings.
Figure 9-11 shows another type of ventricular function curve called the ventricular volume output curve. The two curves of this figure represent function of the two ventricles of the human heart based on data extrapolated from lower animals. As the right and left atrial pressures increase, the respective ventricular volume outputs per minute also increase.
Thus, ventricular function curves are another way of expressing the Frank-Starling mechanism of the heart. That is, as the ventricles fill in response to higher atrial pressures, each ventricular volume and strength of cardiac muscle contraction increase, causing the heart to pump increased quantities of blood into the arteries.

Control of the Heart by the Sympathetic and Parasympathetic Nerves
The pumping effectiveness of the heart also is controlled by the sympathetic and parasympathetic (vagus) nerves, which abundantly supply the heart, as shown in Figure 9-12 . For given levels of atrial pressure, the amount of blood pumped each minute (cardiac output) often can be increased more than 100 percent by sympathetic stimulation. By contrast, the output can be decreased to as low as zero or almost zero by vagal (parasympathetic) stimulation.

Figure 9-12 Cardiac sympathetic and parasympathetic nerves. (The vagus nerves to the heart are parasympathetic nerves.)

Mechanisms of Excitation of the Heart by the Sympathetic Nerves
Strong sympathetic stimulation can increase the heart rate in young adult humans from the normal rate of 70 beats/min up to 180 to 200 and, rarely, even 250 beats/min. Also, sympathetic stimulation increases the force of heart contraction to as much as double normal, thereby increasing the volume of blood pumped and increasing the ejection pressure. Thus, sympathetic stimulation often can increase the maximum cardiac output as much as twofold to threefold, in addition to the increased output caused by the Frank-Starling mechanism already discussed.
Conversely, inhibition of the sympathetic nerves to the heart can decrease cardiac pumping to a moderate extent in the following way: Under normal conditions, the sympathetic nerve fibers to the heart discharge continuously at a slow rate that maintains pumping at about 30 percent above that with no sympathetic stimulation. Therefore, when the activity of the sympathetic nervous system is depressed below normal, this decreases both heart rate and strength of ventricular muscle contraction, thereby decreasing the level of cardiac pumping as much as 30 percent below normal.

Parasympathetic (Vagal) Stimulation of the Heart
Strong stimulation of the parasympathetic nerve fibers in the vagus nerves to the heart can stop the heartbeat for a few seconds, but then the heart usually “escapes” and beats at a rate of 20 to 40 beats/min as long as the parasympathetic stimulation continues. In addition, strong vagal stimulation can decrease the strength of heart muscle contraction by 20 to 30 percent.
The vagal fibers are distributed mainly to the atria and not much to the ventricles, where the power contraction of the heart occurs. This explains the effect of vagal stimulation mainly to decrease heart rate rather than to decrease greatly the strength of heart contraction. Nevertheless, the great decrease in heart rate combined with a slight decrease in heart contraction strength can decrease ventricular pumping 50 percent or more.

Effect of Sympathetic or Parasympathetic Stimulation on the Cardiac Function Curve
Figure 9-13 shows four cardiac function curves. They are similar to the ventricular function curves of Figure 9-11 . However, they represent function of the entire heart rather than of a single ventricle; they show the relation between right atrial pressure at the input of the right heart and cardiac output from the left ventricle into the aorta.

Figure 9-13 Effect on the cardiac output curve of different degrees of sympathetic or parasympathetic stimulation.
The curves of Figure 9-13 demonstrate that at any given right atrial pressure, the cardiac output increases during increased sympathetic stimulation and decreases during increased parasympathetic stimulation. These changes in output caused by autonomic nervous system stimulation result both from changes in heart rate and from changes in contractile strength of the heart because both change in response to the nerve stimulation.

Effect of Potassium and Calcium Ions on Heart Function
In the discussion of membrane potentials in Chapter 5 , it was pointed out that potassium ions have a marked effect on membrane potentials, and in Chapter 6 it was noted that calcium ions play an especially important role in activating the muscle contractile process. Therefore, it is to be expected that the concentration of each of these two ions in the extracellular fluids should also have important effects on cardiac pumping.

Effect of Potassium Ions
Excess potassium in the extracellular fluids causes the heart to become dilated and flaccid and also slows the heart rate. Large quantities also can block conduction of the cardiac impulse from the atria to the ventricles through the A-V bundle. Elevation of potassium concentration to only 8 to 12 mEq/L—two to three times the normal value—can cause such weakness of the heart and abnormal rhythm that death occurs.
These effects result partially from the fact that a high potassium concentration in the extracellular fluids decreases the resting membrane potential in the cardiac muscle fibers, as explained in Chapter 5 . That is, high extracellular fluid potassium concentration partially depolarizes the cell membrane, causing the membrane potential to be less negative. As the membrane potential decreases, the intensity of the action potential also decreases, which makes contraction of the heart progressively weaker.

Effect of Calcium Ions
An excess of calcium ions causes effects almost exactly opposite to those of potassium ions, causing the heart to go toward spastic contraction. This is caused by a direct effect of calcium ions to initiate the cardiac contractile process, as explained earlier in the chapter.
Conversely, deficiency of calcium ions causes cardiac flaccidity , similar to the effect of high potassium. Fortunately, calcium ion levels in the blood normally are regulated within a very narrow range. Therefore, cardiac effects of abnormal calcium concentrations are seldom of clinical concern.

Effect of Temperature on Heart Function
Increased body temperature, as occurs when one has fever, causes a greatly increased heart rate, sometimes to double normal. Decreased temperature causes a greatly decreased heart rate, falling to as low as a few beats per minute when a person is near death from hypothermia in the body temperature range of 60° to 70°F. These effects presumably result from the fact that heat increases the permeability of the cardiac muscle membrane to ions that control heart rate, resulting in acceleration of the self-excitation process.
Contractile strength of the heart often is enhanced temporarily by a moderate increase in temperature, as occurs during body exercise, but prolonged elevation of temperature exhausts the metabolic systems of the heart and eventually causes weakness. Therefore, optimal function of the heart depends greatly on proper control of body temperature by the temperature control mechanisms explained in Chapter 73 .

Increasing the Arterial Pressure Load (up to a Limit) Does Not Decrease the Cardiac Output
Note in Figure 9-14 that increasing the arterial pressure in the aorta does not decrease the cardiac output until the mean arterial pressure rises above about 160 mm Hg. In other words, during normal function of the heart at normal systolic arterial pressures (80 to 140 mm Hg), the cardiac output is determined almost entirely by the ease of blood flow through the body’s tissues, which in turn controls venous return of blood to the heart. This is the principal subject of Chapter 20 .

Figure 9-14 Constancy of cardiac output up to a pressure level of 160 mm Hg. Only when the arterial pressure rises above this normal limit does the increasing pressure load cause the cardiac output to fall significantly.


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CHAPTER 10 Rhythmical Excitation of the Heart
The heart is endowed with a special system for (1) generating rhythmical electrical impulses to cause rhythmical contraction of the heart muscle and (2) conducting these impulses rapidly through the heart. When this system functions normally, the atria contract about one sixth of a second ahead of ventricular contraction, which allows filling of the ventricles before they pump the blood through the lungs and peripheral circulation. Another special importance of the system is that it allows all portions of the ventricles to contract almost simultaneously, which is essential for most effective pressure generation in the ventricular chambers.
This rhythmical and conductive system of the heart is susceptible to damage by heart disease, especially by ischemia of the heart tissues resulting from poor coronary blood flow. The effect is often a bizarre heart rhythm or abnormal sequence of contraction of the heart chambers, and the pumping effectiveness of the heart often is affected severely, even to the extent of causing death.

Specialized Excitatory and Conductive System of the Heart
Figure 10-1 shows the specialized excitatory and conductive system of the heart that controls cardiac contractions. The figure shows the sinus node (also called sinoatrial or S-A node), in which the normal rhythmical impulses are generated; the internodal pathways that conduct impulses from the sinus node to the atrioventricular (A-V) node; the A-V node, in which impulses from the atria are delayed before passing into the ventricles; the A-V bundle, which conducts impulses from the atria into the ventricles; and the left and right bundle branches of Purkinje fibers, which conduct the cardiac impulses to all parts of the ventricles.

Figure 10-1 Sinus node and the Purkinje system of the heart, showing also the A-V node, atrial internodal pathways, and ventricular bundle branches.

Sinus (Sinoatrial) Node
The sinus node (also called sinoatrial node ) is a small, flattened, ellipsoid strip of specialized cardiac muscle about 3 millimeters wide, 15 millimeters long, and 1 millimeter thick. It is located in the superior posterolateral wall of the right atrium immediately below and slightly lateral to the opening of the superior vena cava. The fibers of this node have almost no contractile muscle filaments and are each only 3 to 5 micrometers in diameter, in contrast to a diameter of 10 to 15 micrometers for the surrounding atrial muscle fibers. However, the sinus nodal fibers connect directly with the atrial muscle fibers so that any action potential that begins in the sinus node spreads immediately into the atrial muscle wall.

Automatic Electrical Rhythmicity of the Sinus Fibers
Some cardiac fibers have the capability of self-excitation , a process that can cause automatic rhythmical discharge and contraction. This is especially true of the fibers of the heart’s specialized conducting system, including the fibers of the sinus node. For this reason, the sinus node ordinarily controls the rate of beat of the entire heart, as discussed in detail later in this chapter. First, let us describe this automatic rhythmicity.

Mechanism of Sinus Nodal Rhythmicity
Figure 10-2 shows action potentials recorded from inside a sinus nodal fiber for three heartbeats and, by comparison, a single ventricular muscle fiber action potential. Note that the “resting membrane potential” of the sinus nodal fiber between discharges has a negativity of about −55 to −60 millivolts, in comparison with −85 to −90 millivolts for the ventricular muscle fiber. The cause of this lesser negativity is that the cell membranes of the sinus fibers are naturally leaky to sodium and calcium ions, and positive charges of the entering sodium and calcium ions neutralize some of the intracellular negativity.

Figure 10-2 Rhythmical discharge of a sinus nodal fiber. Also, the sinus nodal action potential is compared with that of a ventricular muscle fiber.
Before attempting to explain the rhythmicity of the sinus nodal fibers, first recall from the discussions of Chapters 5 and 9 that cardiac muscle has three types of membrane ion channels that play important roles in causing the voltage changes of the action potential. They are (1) fast sodium channels , (2) slow sodium-calcium channels , and (3) potassium channels .
Opening of the fast sodium channels for a few 10,000 ths of a second is responsible for the rapid upstroke spike of the action potential observed in ventricular muscle, because of rapid influx of positive sodium ions to the interior of the fiber. Then the “plateau” of the ventricular action potential is caused primarily by slower opening of the slow sodium-calcium channels, which lasts for about 0.3 second. Finally, opening of potassium channels allows diffusion of large amounts of positive potassium ions in the outward direction through the fiber membrane and returns the membrane potential to its resting level.
But there is a difference in the function of these channels in the sinus nodal fiber because the “resting” potential is much less negative—only −55 millivolts in the nodal fiber instead of the −90 millivolts in the ventricular muscle fiber. At this level of −55 millivolts, the fast sodium channels mainly have already become “inactivated,” which means that they have become blocked. The cause of this is that any time the membrane potential remains less negative than about −55 millivolts for more than a few milliseconds, the inactivation gates on the inside of the cell membrane that close the fast sodium channels become closed and remain so. Therefore, only the slow sodium-calcium channels can open (i.e., can become “activated”) and thereby cause the action potential. As a result, the atrial nodal action potential is slower to develop than the action potential of the ventricular muscle. Also, after the action potential does occur, return of the potential to its negative state occurs slowly as well, rather than the abrupt return that occurs for the ventricular fiber.

Self-Excitation of Sinus Nodal Fibers
Because of the high sodium ion concentration in the extracellular fluid outside the nodal fiber, as well as a moderate number of already open sodium channels, positive sodium ions from outside the fibers normally tend to leak to the inside. Therefore, between heartbeats, influx of positively charged sodium ions causes a slow rise in the resting membrane potential in the positive direction. Thus, as shown in Figure 10-2 , the “resting” potential gradually rises and becomes less negative between each two heartbeats. When the potential reaches a threshold voltage of about −40 millivolts, the sodium-calcium channels become “activated,” thus causing the action potential. Therefore, basically, the inherent leakiness of the sinus nodal fibers to sodium and calcium ions causes their self-excitation.
Why does this leakiness to sodium and calcium ions not cause the sinus nodal fibers to remain depolarized all the time? The answer is that two events occur during the course of the action potential to prevent this. First, the sodium-calcium channels become inactivated (i.e., they close) within about 100 to 150 milliseconds after opening, and second, at about the same time, greatly increased numbers of potassium channels open. Therefore, influx of positive calcium and sodium ions through the sodium-calcium channels ceases, while at the same time large quantities of positive potassium ions diffuse out of the fiber. Both of these effects reduce the intracellular potential back to its negative resting level and therefore terminate the action potential. Furthermore, the potassium channels remain open for another few tenths of a second, temporarily continuing movement of positive charges out of the cell, with resultant excess negativity inside the fiber; this is called hyperpolarization. The hyperpolarization state initially carries the “resting” membrane potential down to about −55 to −60 millivolts at the termination of the action potential.
Why is this new state of hyperpolarization not maintained forever? The reason is that during the next few tenths of a second after the action potential is over, progressively more and more potassium channels close. The inward-leaking sodium and calcium ions once again overbalance the outward flux of potassium ions, and this causes the “resting” potential to drift upward once more, finally reaching the threshold level for discharge at a potential of about −40 millivolts. Then the entire process begins again: self-excitation to cause the action potential, recovery from the action potential, hyperpolarization after the action potential is over, drift of the “resting” potential to threshold, and finally re-excitation to elicit another cycle. This process continues indefinitely throughout a person’s life.

Internodal Pathways and Transmission of the Cardiac Impulse Through the Atria
The ends of the sinus nodal fibers connect directly with surrounding atrial muscle fibers. Therefore, action potentials originating in the sinus node travel outward into these atrial muscle fibers. In this way, the action potential spreads through the entire atrial muscle mass and, eventually, to the A-V node. The velocity of conduction in most atrial muscle is about 0.3 m/sec, but conduction is more rapid, about 1 m/sec, in several small bands of atrial fibers. One of these, called the anterior interatrial band, passes through the anterior walls of the atria to the left atrium. In addition, three other small bands curve through the anterior, lateral, and posterior atrial walls and terminate in the A-V node; shown in Figures 10-1 and 10-3 , these are called, respectively, the anterior, middle, and posterior internodal pathways . The cause of more rapid velocity of conduction in these bands is the presence of specialized conduction fibers. These fibers are similar to even more rapidly conducting “Purkinje fibers” of the ventricles, which are discussed as follows.

Figure 10-3 Organization of the A-V node. The numbers represent the interval of time from the origin of the impulse in the sinus node. The values have been extrapolated to human beings.

Atrioventricular Node and Delay of Impulse Conduction from the Atria to the Ventricles
The atrial conductive system is organized so that the cardiac impulse does not travel from the atria into the ventricles too rapidly; this delay allows time for the atria to empty their blood into the ventricles before ventricular contraction begins. It is primarily the A-V node and its adjacent conductive fibers that delay this transmission into the ventricles.
The A-V node is located in the posterior wall of the right atrium immediately behind the tricuspid valve, as shown in Figure 10-1 . And Figure 10-3 shows diagrammatically the different parts of this node, plus its connections with the entering atrial internodal pathway fibers and the exiting A-V bundle. The figure also shows the approximate intervals of time in fractions of a second between initial onset of the cardiac impulse in the sinus node and its subsequent appearance in the A-V nodal system. Note that the impulse, after traveling through the internodal pathways, reaches the A-V node about 0.03 second after its origin in the sinus node. Then there is a delay of another 0.09 second in the A-V node itself before the impulse enters the penetrating portion of the A-V bundle, where it passes into the ventricles. A final delay of another 0.04 second occurs mainly in this penetrating A-V bundle, which is composed of multiple small fascicles passing through the fibrous tissue separating the atria from the ventricles.
Thus, the total delay in the A-V nodal and A-V bundle system is about 0.13 second. This, in addition to the initial conduction delay of 0.03 second from the sinus node to the A-V node, makes a total delay of 0.16 second before the excitatory signal finally reaches the contracting muscle of the ventricles.

Cause of the Slow Conduction
The slow conduction in the transitional, nodal, and penetrating A-V bundle fibers is caused mainly by diminished numbers of gap junctions between successive cells in the conducting pathways, so there is great resistance to conduction of excitatory ions from one conducting fiber to the next. Therefore, it is easy to see why each succeeding cell is slow to be excited.

Rapid Transmission in the Ventricular Purkinje System
Special Purkinje fibers lead from the A-V node through the A-V bundle into the ventricles. Except for the initial portion of these fibers where they penetrate the A-V fibrous barrier, they have functional characteristics that are quite the opposite of those of the A-V nodal fibers. They are very large fibers, even larger than the normal ventricular muscle fibers, and they transmit action potentials at a velocity of 1.5 to 4.0 m/sec, a velocity about 6 times that in the usual ventricular muscle and 150 times that in some of the A-V nodal fibers. This allows almost instantaneous transmission of the cardiac impulse throughout the entire remainder of the ventricular muscle.
The rapid transmission of action potentials by Purkinje fibers is believed to be caused by a very high level of permeability of the gap junctions at the intercalated discs between the successive cells that make up the Purkinje fibers. Therefore, ions are transmitted easily from one cell to the next, thus enhancing the velocity of transmission. The Purkinje fibers also have very few myofibrils, which means that they contract little or not at all during the course of impulse transmission.

One-Way Conduction Through the A-V Bundle
A special characteristic of the A-V bundle is the inability, except in abnormal states, of action potentials to travel backward from the ventricles to the atria. This prevents re-entry of cardiac impulses by this route from the ventricles to the atria, allowing only forward conduction from the atria to the ventricles.
Furthermore, it should be recalled that everywhere, except at the A-V bundle, the atrial muscle is separated from the ventricular muscle by a continuous fibrous barrier, a portion of which is shown in Figure 10-3 . This barrier normally acts as an insulator to prevent passage of the cardiac impulse between atrial and ventricular muscle through any other route besides forward conduction through the A-V bundle itself. (In rare instances, an abnormal muscle bridge does penetrate the fibrous barrier elsewhere besides at the A-V bundle. Under such conditions, the cardiac impulse can re-enter the atria from the ventricles and cause a serious cardiac arrhythmia.)

Distribution of the Purkinje Fibers in the Ventricles—The Left and Right Bundle Branches
After penetrating the fibrous tissue between the atrial and ventricular muscle, the distal portion of the A-V bundle passes downward in the ventricular septum for 5 to 15 millimeters toward the apex of the heart, as shown in Figures 10-1 and 10-3 . Then the bundle divides into left and right bundle branches that lie beneath the endocardium on the two respective sides of the ventricular septum. Each branch spreads downward toward the apex of the ventricle, progressively dividing into smaller branches. These branches in turn course sidewise around each ventricular chamber and back toward the base of the heart. The ends of the Purkinje fibers penetrate about one third of the way into the muscle mass and finally become continuous with the cardiac muscle fibers.
From the time the cardiac impulse enters the bundle branches in the ventricular septum until it reaches the terminations of the Purkinje fibers, the total elapsed time averages only 0.03 second. Therefore, once the cardiac impulse enters the ventricular Purkinje conductive system, it spreads almost immediately to the entire ventricular muscle mass.

Transmission of the Cardiac Impulse in the Ventricular Muscle
Once the impulse reaches the ends of the Purkinje fibers, it is transmitted through the ventricular muscle mass by the ventricular muscle fibers themselves. The velocity of transmission is now only 0.3 to 0.5 m/sec, one sixth that in the Purkinje fibers.
The cardiac muscle wraps around the heart in a double spiral, with fibrous septa between the spiraling layers; therefore, the cardiac impulse does not necessarily travel directly outward toward the surface of the heart but instead angulates toward the surface along the directions of the spirals. Because of this, transmission from the endocardial surface to the epicardial surface of the ventricle requires as much as another 0.03 second, approximately equal to the time required for transmission through the entire ventricular portion of the Purkinje system. Thus, the total time for transmission of the cardiac impulse from the initial bundle branches to the last of the ventricular muscle fibers in the normal heart is about 0.06 second.

Summary of the Spread of the Cardiac Impulse Through the Heart
Figure 10-4 shows in summary form the transmission of the cardiac impulse through the human heart. The numbers on the figure represent the intervals of time, in fractions of a second, that lapse between the origin of the cardiac impulse in the sinus node and its appearance at each respective point in the heart. Note that the impulse spreads at moderate velocity through the atria but is delayed more than 0.1 second in the A-V nodal region before appearing in the ventricular septal A-V bundle. Once it has entered this bundle, it spreads very rapidly through the Purkinje fibers to the entire endocardial surfaces of the ventricles. Then the impulse once again spreads slightly less rapidly through the ventricular muscle to the epicardial surfaces.

Figure 10-4 Transmission of the cardiac impulse through the heart, showing the time of appearance (in fractions of a second after initial appearance at the sinoatrial node) in different parts of the heart.
It is important that the student learn in detail the course of the cardiac impulse through the heart and the precise times of its appearance in each separate part of the heart, because a thorough quantitative knowledge of this process is essential to the understanding of electrocardiography, which is discussed in Chapters 11 through 13 .

Control of Excitation and Conduction in the Heart

Sinus Node as the Pacemaker of the Heart
In the discussion thus far of the genesis and transmission of the cardiac impulse through the heart, we have noted that the impulse normally arises in the sinus node. In some abnormal conditions, this is not the case. Other parts of the heart can also exhibit intrinsic rhythmical excitation in the same way that the sinus nodal fibers do; this is particularly true of the A-V nodal and Purkinje fibers.
The A-V nodal fibers, when not stimulated from some outside source, discharge at an intrinsic rhythmical rate of 40 to 60 times per minute, and the Purkinje fibers discharge at a rate somewhere between 15 and 40 times per minute. These rates are in contrast to the normal rate of the sinus node of 70 to 80 times per minute.
Why then does the sinus node rather than the A-V node or the Purkinje fibers control the heart’s rhythmicity? The answer derives from the fact that the discharge rate of the sinus node is considerably faster than the natural self-excitatory discharge rate of either the A-V node or the Purkinje fibers. Each time the sinus node discharges, its impulse is conducted into both the A-V node and the Purkinje fibers, also discharging their excitable membranes. But the sinus node discharges again before either the A-V node or the Purkinje fibers can reach their own thresholds for self-excitation. Therefore, the new impulse from the sinus node discharges both the A-V node and the Purkinje fibers before self-excitation can occur in either of these.
Thus, the sinus node controls the beat of the heart because its rate of rhythmical discharge is faster than that of any other part of the heart. Therefore, the sinus node is virtually always the pacemaker of the normal heart.

Abnormal Pacemakers—“Ectopic” Pacemaker
Occasionally some other part of the heart develops a rhythmical discharge rate that is more rapid than that of the sinus node. For instance, this sometimes occurs in the A-V node or in the Purkinje fibers when one of these becomes abnormal. In either case, the pacemaker of the heart shifts from the sinus node to the A-V node or to the excited Purkinje fibers. Under rarer conditions, a place in the atrial or ventricular muscle develops excessive excitability and becomes the pacemaker.
A pacemaker elsewhere than the sinus node is called an “ ectopic ” pacemaker . An ectopic pacemaker causes an abnormal sequence of contraction of the different parts of the heart and can cause significant debility of heart pumping.
Another cause of shift of the pacemaker is blockage of transmission of the cardiac impulse from the sinus node to the other parts of the heart. The new pacemaker then occurs most frequently at the A-V node or in the penetrating portion of the A-V bundle on the way to the ventricles.
When A-V block occurs—that is, when the cardiac impulse fails to pass from the atria into the ventricles through the A-V nodal and bundle system—the atria continue to beat at the normal rate of rhythm of the sinus node, while a new pacemaker usually develops in the Purkinje system of the ventricles and drives the ventricular muscle at a new rate somewhere between 15 and 40 beats per minute. After sudden A-V bundle block, the Purkinje system does not begin to emit its intrinsic rhythmical impulses until 5 to 20 seconds later because, before the blockage, the Purkinje fibers had been “overdriven” by the rapid sinus impulses and, consequently, are in a suppressed state. During these 5 to 20 seconds, the ventricles fail to pump blood, and the person faints after the first 4 to 5 seconds because of lack of blood flow to the brain. This delayed pickup of the heartbeat is called Stokes-Adams syndrome . If the delay period is too long, it can lead to death.

Role of the Purkinje System in Causing Synchronous Contraction of the Ventricular Muscle
It is clear from our description of the Purkinje system that normally the cardiac impulse arrives at almost all portions of the ventricles within a narrow span of time, exciting the first ventricular muscle fiber only 0.03 to 0.06 second ahead of excitation of the last ventricular muscle fiber. This causes all portions of the ventricular muscle in both ventricles to begin contracting at almost the same time and then to continue contracting for about another 0.3 second.
Effective pumping by the two ventricular chambers requires this synchronous type of contraction. If the cardiac impulse should travel through the ventricles slowly, much of the ventricular mass would contract before contraction of the remainder, in which case the overall pumping effect would be greatly depressed. Indeed, in some types of cardiac debilities, several of which are discussed in Chapters 12 and 13 , slow transmission does occur, and the pumping effectiveness of the ventricles is decreased as much as 20 to 30 percent.

Control of Heart Rhythmicity and Impulse Conduction by the Cardiac Nerves: Sympathetic and Parasympathetic Nerves
The heart is supplied with both sympathetic and parasympathetic nerves, as shown in Figure 9-10 of Chapter 9 . The parasympathetic nerves (the vagi) are distributed mainly to the S-A and A-V nodes, to a lesser extent to the muscle of the two atria, and very little directly to the ventricular muscle. The sympathetic nerves, conversely, are distributed to all parts of the heart, with strong representation to the ventricular muscle, as well as to all the other areas.

Parasympathetic (Vagal) Stimulation Can Slow or Even Block Cardiac Rhythm and Conduction—“Ventricular Escape.”
Stimulation of the parasympathetic nerves to the heart (the vagi) causes the hormone acetylcholine to be released at the vagal endings. This hormone has two major effects on the heart. First, it decreases the rate of rhythm of the sinus node, and second, it decreases the excitability of the A-V junctional fibers between the atrial musculature and the A-V node, thereby slowing transmission of the cardiac impulse into the ventricles.
Weak to moderate vagal stimulation slows the rate of heart pumping, often to as little as one-half normal. And strong stimulation of the vagi can stop completely the rhythmical excitation by the sinus node or block completely transmission of the cardiac impulse from the atria into the ventricles through the A-V mode. In either case, rhythmical excitatory signals are no longer transmitted into the ventricles. The ventricles stop beating for 5 to 20 seconds, but then some small area in the Purkinje fibers, usually in the ventricular septal portion of the A-V bundle, develops a rhythm of its own and causes ventricular contraction at a rate of 15 to 40 beats per minute. This phenomenon is called ventricular escape.

Mechanism of the Vagal Effects
The acetylcholine released at the vagal nerve endings greatly increases the permeability of the fiber membranes to potassium ions, which allows rapid leakage of potassium out of the conductive fibers. This causes increased negativity inside the fibers, an effect called hyperpolarization, which makes this excitable tissue much less excitable, as explained in Chapter 5 .
In the sinus node, the state of hyperpolarization decreases the “resting” membrane potential of the sinus nodal fibers to a level considerably more negative than usual, to −65 to −75 millivolts rather than the normal level of −55 to −60 millivolts. Therefore, the initial rise of the sinus nodal membrane potential caused by inward sodium and calcium leakage requires much longer to reach the threshold potential for excitation. This greatly slows the rate of rhythmicity of these nodal fibers. If the vagal stimulation is strong enough, it is possible to stop entirely the rhythmical self-excitation of this node.
In the A-V node, a state of hyperpolarization caused by vagal stimulation makes it difficult for the small atrial fibers entering the node to generate enough electricity to excite the nodal fibers. Therefore, the safety factor for transmission of the cardiac impulse through the transitional fibers into the A-V nodal fibers decreases. A moderate decrease simply delays conduction of the impulse, but a large decrease blocks conduction entirely.

Effect of Sympathetic Stimulation on Cardiac Rhythm and Conduction
Sympathetic stimulation causes essentially the opposite effects on the heart to those caused by vagal stimulation, as follows: First, it increases the rate of sinus nodal discharge. Second, it increases the rate of conduction, as well as the level of excitability in all portions of the heart. Third, it increases greatly the force of contraction of all the cardiac musculature, both atrial and ventricular, as discussed in Chapter 9 .
In short, sympathetic stimulation increases the overall activity of the heart. Maximal stimulation can almost triple the frequency of heartbeat and can increase the strength of heart contraction as much as twofold.

Mechanism of the Sympathetic Effect
Stimulation of the sympathetic nerves releases the hormone norepinephrine at the sympathetic nerve endings. Norepinephrine in turn stimulates beta-1 adrenergic receptors , which mediate the effects on heart rate. The precise mechanism by which beta-1 adrenergic stimulation acts on cardiac muscle fibers is somewhat unclear, but the belief is that it increases the permeability of the fiber membrane to sodium and calcium ions. In the sinus node, an increase of sodium-calcium permeability causes a more positive resting potential and also causes increased rate of upward drift of the diastolic membrane potential toward the threshold level for self-excitation, thus accelerating self-excitation and, therefore, increasing the heart rate.
In the A-V node and A-V bundles, increased sodium-calcium permeability makes it easier for the action potential to excite each succeeding portion of the conducting fiber bundles, thereby decreasing the conduction time from the atria to the ventricles.
The increase in permeability to calcium ions is at least partially responsible for the increase in contractile strength of the cardiac muscle under the influence of sympathetic stimulation, because calcium ions play a powerful role in exciting the contractile process of the myofibrils.


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CHAPTER 11 The Normal Electrocardiogram
When the cardiac impulse passes through the heart, electrical current also spreads from the heart into the adjacent tissues surrounding the heart. A small portion of the current spreads all the way to the surface of the body. If electrodes are placed on the skin on opposite sides of the heart, electrical potentials generated by the current can be recorded; the recording is known as an electrocardiogram. A normal electrocardiogram for two beats of the heart is shown in Figure 11-1 .

Figure 11-1 Normal electrocardiogram.

Characteristics of the Normal Electrocardiogram
The normal electrocardiogram (see Figure 11-1 ) is composed of a P wave, a QRS complex, and a T wave. The QRS complex is often, but not always, three separate waves: the Q wave, the R wave, and the S wave.
The P wave is caused by electrical potentials generated when the atria depolarize before atrial contraction begins. The QRS complex is caused by potentials generated when the ventricles depolarize before contraction, that is, as the depolarization wave spreads through the ventricles. Therefore, both the P wave and the components of the QRS complex are depolarization waves .
The T wave is caused by potentials generated as the ventricles recover from the state of depolarization. This process normally occurs in ventricular muscle 0.25 to 0.35 second after depolarization, and the T wave is known as a repolarization wave.
Thus, the electrocardiogram is composed of both depolarization and repolarization waves. The principles of depolarization and repolarization are discussed in Chapter 5 . The distinction between depolarization waves and repolarization waves is so important in electrocardiography that further clarification is necessary.

Depolarization Waves versus Repolarization Waves
Figure 11-2 shows a single cardiac muscle fiber in four stages of depolarization and repolarization, the color red designating depolarization. During depolarization, the normal negative potential inside the fiber reverses and becomes slightly positive inside and negative outside.

Figure 11-2 Recording the depolarization wave ( A and B ) and the repolarization wave ( C and D ) from a cardiac muscle fiber.
In Figure 11-2 A , depolarization, demonstrated by red positive charges inside and red negative charges outside, is traveling from left to right. The first half of the fiber has already depolarized, while the remaining half is still polarized. Therefore, the left electrode on the outside of the fiber is in an area of negativity, and the right electrode is in an area of positivity; this causes the meter to record positively. To the right of the muscle fiber is shown a record of changes in potential between the two electrodes, as recorded by a high-speed recording meter. Note that when depolarization has reached the halfway mark in Figure 11-2 A , the record has risen to a maximum positive value.
In Figure 11-2 B , depolarization has extended over the entire muscle fiber, and the recording to the right has returned to the zero baseline because both electrodes are now in areas of equal negativity. The completed wave is a depolarization wave because it results from spread of depolarization along the muscle fiber membrane.
Figure 11-2 C shows halfway repolarization of the same muscle fiber, with positivity returning to the outside of the fiber. At this point, the left electrode is in an area of positivity, and the right electrode is in an area of negativity. This is opposite to the polarity in Figure 11-2 A . Consequently, the recording, as shown to the right, becomes negative.
In Figure 11-2 D , the muscle fiber has completely repolarized, and both electrodes are now in areas of positivity so that no potential difference is recorded between them. Thus, in the recording to the right, the potential returns once more to zero. This completed negative wave is a repolarization wave because it results from spread of repolarization along the muscle fiber membrane.

Relation of the Monophasic Action Potential of Ventricular Muscle to the QRS and T Waves in the Standard Electrocardiogram
The monophasic action potential of ventricular muscle, discussed in Chapter 10 , normally lasts between 0.25 and 0.35 second. The top part of Figure 11-3 shows a monophasic action potential recorded from a microelectrode inserted to the inside of a single ventricular muscle fiber. The upsweep of this action potential is caused by depolarization, and the return of the potential to the baseline is caused by repolarization.

Figure 11-3 Above, Monophasic action potential from a ventricular muscle fiber during normal cardiac function, showing rapid depolarization and then repolarization occurring slowly during the plateau stage but rapidly toward the end. Below, Electrocardiogram recorded simultaneously.
Note in the lower half of the figure a simultaneous recording of the electrocardiogram from this same ventricle, which shows the QRS waves appearing at the beginning of the monophasic action potential and the T wave appearing at the end. Note especially that no potential is recorded in the electrocardiogram when the ventricular muscle is either completely polarized or completely depolarized . Only when the muscle is partly polarized and partly depolarized does current flow from one part of the ventricles to another part and therefore current also flows to the surface of the body to produce the electrocardiogram.

Relationship of Atrial and Ventricular Contraction to the Waves of the Electrocardiogram
Before contraction of muscle can occur, depolarization must spread through the muscle to initiate the chemical processes of contraction. Refer again to Figure 11-1 ; the P wave occurs at the beginning of contraction of the atria, and the QRS complex of waves occurs at the beginning of contraction of the ventricles. The ventricles remain contracted until after repolarization has occurred, that is, until after the end of the T wave.
The atria repolarize about 0.15 to 0.20 second after termination of the P wave. This is also approximately when the QRS complex is being recorded in the electrocardiogram. Therefore, the atrial repolarization wave, known as the atrial T wave, is usually obscured by the much larger QRS complex. For this reason, an atrial T wave seldom is observed in the electrocardiogram.
The ventricular repolarization wave is the T wave of the normal electrocardiogram. Ordinarily, ventricular muscle begins to repolarize in some fibers about 0.20 second after the beginning of the depolarization wave (the QRS complex), but in many other fibers, it takes as long as 0.35 second. Thus, the process of ventricular repolarization extends over a long period, about 0.15 second. For this reason, the T wave in the normal electrocardiogram is a prolonged wave, but the voltage of the T wave is considerably less than the voltage of the QRS complex, partly because of its prolonged length.

Voltage and Time Calibration of the Electrocardiogram
All recordings of electrocardiograms are made with appropriate calibration lines on the recording paper. Either these calibration lines are already ruled on the paper, as is the case when a pen recorder is used, or they are recorded on the paper at the same time that the electrocardiogram is recorded, which is the case with the photographic types of electrocardiographs.
As shown in Figure 11-1 , the horizontal calibration lines are arranged so that 10 of the small line divisions upward or downward in the standard electrocardiogram represent 1 millivolt, with positivity in the upward direction and negativity in the downward direction.
The vertical lines on the electrocardiogram are time calibration lines. A typical electrocardiogram is run at a paper speed of 25 millimeters per second, although faster speeds are sometimes used. Therefore, each 25 millimeters in the horizontal direction is 1 second, and each 5-millimeter segment, indicated by the dark vertical lines, represents 0.20 second. The 0.20-second intervals are then broken into five smaller intervals by thin lines, each of which represents 0.04 second.

Normal Voltages in the Electrocardiogram
The recorded voltages of the waves in the normal electrocardiogram depend on the manner in which the electrodes are applied to the surface of the body and how close the electrodes are to the heart. When one electrode is placed directly over the ventricles and a second electrode is placed elsewhere on the body remote from the heart, the voltage of the QRS complex may be as great as 3 to 4 millivolts. Even this voltage is small in comparison with the monophasic action potential of 110 millivolts recorded directly at the heart muscle membrane. When electrocardiograms are recorded from electrodes on the two arms or on one arm and one leg, the voltage of the QRS complex usually is 1.0 to 1.5 millivolts from the top of the R wave to the bottom of the S wave; the voltage of the P wave is between 0.1 and 0.3 millivolts; and that of the T wave is between 0.2 and 0.3 millivolts.

P-Q or P-R Interval
The time between the beginning of the P wave and the beginning of the QRS complex is the interval between the beginning of electrical excitation of the atria and the beginning of excitation of the ventricles. This period is called the P-Q interval. The normal P-Q interval is about 0.16 second. (Often this interval is called the P-R interval because the Q wave is likely to be absent.)

Q-T Interval
Contraction of the ventricle lasts almost from the beginning of the Q wave (or R wave, if the Q wave is absent) to the end of the T wave. This interval is called the Q-T interval and ordinarily is about 0.35 second.

Rate of Heartbeat as Determined from the Electrocardiogram
The rate of heartbeat can be determined easily from an electrocardiogram because the heart rate is the reciprocal of the time interval between two successive heartbeats. If the interval between two beats as determined from the time calibration lines is 1 second, the heart rate is 60 beats per minute. The normal interval between two successive QRS complexes in the adult person is about 0.83 second. This is a heart rate of 60/0.83 times per minute, or 72 beats per minute.

Methods for Recording Electrocardiograms
Sometimes the electrical currents generated by the cardiac muscle during each beat of the heart change electrical potentials and polarities on the respective sides of the heart in less than 0.01 second. Therefore, it is essential that any apparatus for recording electrocardiograms be capable of responding rapidly to these changes in potentials.

Recorders for Electrocardiographs
Many modern clinical electrocardiographs use computer-based systems and electronic display, whereas others use a direct pen recorder that writes the electrocardiogram with a pen directly on a moving sheet of paper. Sometimes the pen is a thin tube connected at one end to an inkwell, and its recording end is connected to a powerful electromagnet system that is capable of moving the pen back and forth at high speed. As the paper moves forward, the pen records the electrocardiogram. The movement of the pen is controlled by appropriate electronic amplifiers connected to electrocardiographic electrodes on the patient.
Other pen recording systems use special paper that does not require ink in the recording stylus. One such paper turns black when it is exposed to heat; the stylus itself is made very hot by electrical current flowing through its tip. Another type turns black when electrical current flows from the tip of the stylus through the paper to an electrode at its back. This leaves a black line on the paper where the stylus touches.

Flow of Current Around the Heart during the Cardiac Cycle

Recording Electrical Potentials from a Partially Depolarized Mass of Syncytial Cardiac Muscle
Figure 11-4 shows a syncytial mass of cardiac muscle that has been stimulated at its centralmost point. Before stimulation, all the exteriors of the muscle cells had been positive and the interiors negative. For reasons presented in Chapter 5 in the discussion of membrane potentials, as soon as an area of cardiac syncytium becomes depolarized, negative charges leak to the outsides of the depolarized muscle fibers, making this part of the surface electronegative, as represented by the negative signs in Figure 11-4 . The remaining surface of the heart, which is still polarized, is represented by the positive signs. Therefore, a meter connected with its negative terminal on the area of depolarization and its positive terminal on one of the still-polarized areas, as shown to the right in the figure, records positively.

Figure 11-4 Instantaneous potentials develop on the surface of a cardiac muscle mass that has been depolarized in its center.
Two other electrode placements and meter readings are also demonstrated in Figure 11-4 . These should be studied carefully, and the reader should be able to explain the causes of the respective meter readings. Because the depolarization spreads in all directions through the heart, the potential differences shown in the figure persist for only a few thousandths of a second, and the actual voltage measurements can be accomplished only with a high-speed recording apparatus.

Flow of Electrical Currents in the Chest Around the Heart
Figure 11-5 shows the ventricular muscle lying within the chest. Even the lungs, although mostly filled with air, conduct electricity to a surprising extent, and fluids in other tissues surrounding the heart conduct electricity even more easily. Therefore, the heart is actually suspended in a conductive medium. When one portion of the ventricles depolarizes and therefore becomes electronegative with respect to the remainder, electrical current flows from the depolarized area to the polarized area in large circuitous routes, as noted in the figure.

Figure 11-5 Flow of current in the chest around partially depolarized ventricles.
It should be recalled from the discussion of the Purkinje system in Chapter 10 that the cardiac impulse first arrives in the ventricles in the septum and shortly thereafter spreads to the inside surfaces of the remainder of the ventricles, as shown by the red areas and the negative signs in Figure 11-5 . This provides electronegativity on the insides of the ventricles and electropositivity on the outer walls of the ventricles, with electrical current flowing through the fluids surrounding the ventricles along elliptical paths, as demonstrated by the curving arrows in the figure. If one algebraically averages all the lines of current flow (the elliptical lines), one finds that the average current flow occurs with negativity toward the base of the heart and with positivity toward the apex .
During most of the remainder of the depolarization process, current also continues to flow in this same direction, while depolarization spreads from the endocardial surface outward through the ventricular muscle mass. Then, immediately before depolarization has completed its course through the ventricles, the average direction of current flow reverses for about 0.01 second, flowing from the ventricular apex toward the base, because the last part of the heart to become depolarized is the outer walls of the ventricles near the base of the heart.
Thus, in normal heart ventricles, current flows from negative to positive primarily in the direction from the base of the heart toward the apex during almost the entire cycle of depolarization, except at the very end. And if a meter is connected to electrodes on the surface of the body as shown in Figure 11-5 , the electrode nearer the base will be negative, whereas the electrode nearer the apex will be positive, and the recording meter will show positive recording in the electrocardiogram.

Electrocardiographic Leads

Three Bipolar Limb Leads
Figure 11-6 shows electrical connections between the patient’s limbs and the electrocardiograph for recording electrocardiograms from the so-called standard bipolar limb leads. The term “bipolar” means that the electrocardiogram is recorded from two electrodes located on different sides of the heart—in this case, on the limbs. Thus, a “lead” is not a single wire connecting from the body but a combination of two wires and their electrodes to make a complete circuit between the body and the electrocardiograph. The electrocardiograph in each instance is represented by an electrical meter in the diagram, although the actual electrocardiograph is a high-speed recording meter with a moving paper.

Lead I
In recording limb lead I, the negative terminal of the electrocardiograph is connected to the right arm and the positive terminal to the left arm. Therefore, when the point where the right arm connects to the chest is electronegative with respect to the point where the left arm connects, the electrocardiograph records positively, that is, above the zero voltage line in the electrocardiogram. When the opposite is true, the electrocardiograph records below the line.

Lead II
To record limb lead II, the negative terminal of the electrocardiograph is connected to the right arm and the positive terminal to the left leg. Therefore, when the right arm is negative with respect to the left leg, the electrocardiograph records positively.

Lead III
To record limb lead III, the negative terminal of the electrocardiograph is connected to the left arm and the positive terminal to the left leg. This means that the electrocardiograph records positively when the left arm is negative with respect to the left leg.

Einthoven’s Triangle
In Figure 11-6 , the triangle, called Einthoven’s triangle, is drawn around the area of the heart. This illustrates that the two arms and the left leg form apices of a triangle surrounding the heart. The two apices at the upper part of the triangle represent the points at which the two arms connect electrically with the fluids around the heart, and the lower apex is the point at which the left leg connects with the fluids.

Figure 11-6 Conventional arrangement of electrodes for recording the standard electrocardiographic leads. Einthoven’s triangle is superimposed on the chest.

Einthoven’s Law
Einthoven’s law states that if the electrical potentials of any two of the three bipolar limb electrocardiographic leads are known at any given instant, the third one can be determined mathematically by simply summing the first two. Note, however, that the positive and negative signs of the different leads must be observed when making this summation.
For instance, let us assume that momentarily, as noted in Figure 11-6 , the right arm is −0.2 millivolts (negative) with respect to the average potential in the body, the left arm is +0.3 millivolts (positive), and the left leg is +1.0 millivolts (positive). Observing the meters in the figure, one can see that lead I records a positive potential of +0.5 millivolts because this is the difference between the −0.2 millivolts on the right arm and the +0.3 millivolts on the left arm. Similarly, lead III records a positive potential of +0.7 millivolts, and lead II records a positive potential of +1.2 millivolts because these are the instantaneous potential differences between the respective pairs of limbs.
Now, note that the sum of the voltages in leads I and III equals the voltage in lead II; that is, 0.5 plus 0.7 equals 1.2. Mathematically, this principle, called Einthoven’s law, holds true at any given instant while the three “standard” bipolar electrocardiograms are being recorded.

Normal Electrocardiograms Recorded from the Three Standard Bipolar Limb Leads
Figure 11-7 shows recordings of the electrocardiograms in leads I, II, and III. It is obvious that the electrocardiograms in these three leads are similar to one another because they all record positive P waves and positive T waves, and the major portion of the QRS complex is also positive in each electrocardiogram.

Figure 11-7 Normal electrocardiograms recorded from the three standard electrocardiographic leads.
On analysis of the three electrocardiograms, it can be shown, with careful measurements and proper observance of polarities, that at any given instant the sum of the potentials in leads I and III equals the potential in lead II, thus illustrating the validity of Einthoven’s law.
Because the recordings from all the bipolar limb leads are similar to one another, it does not matter greatly which lead is recorded when one wants to diagnose different cardiac arrhythmias, because diagnosis of arrhythmias depends mainly on the time relations between the different waves of the cardiac cycle. But when one wants to diagnose damage in the ventricular or atrial muscle or in the Purkinje conducting system, it matters greatly which leads are recorded, because abnormalities of cardiac muscle contraction or cardiac impulse conduction do change the patterns of the electrocardiograms markedly in some leads yet may not affect other leads. Electrocardiographic interpretation of these two types of conditions—cardiac myopathies and cardiac arrhythmias—is discussed separately in Chapters 12 and 13 .

Chest Leads (Precordial Leads)
Often electrocardiograms are recorded with one electrode placed on the anterior surface of the chest directly over the heart at one of the points shown in Figure 11-8 . This electrode is connected to the positive terminal of the electrocardiograph, and the negative electrode, called the indifferent electrode, is connected through equal electrical resistances to the right arm, left arm, and left leg all at the same time, as also shown in the figure. Usually six standard chest leads are recorded, one at a time, from the anterior chest wall, the chest electrode being placed sequentially at the six points shown in the diagram. The different recordings are known as leads V1, V2, V3, V4, V5, and V6.

Figure 11-8 Connections of the body with the electrocardiograph for recording chest leads. LA, left arm; RA, right arm.
Figure 11-9 illustrates the electrocardiograms of the healthy heart as recorded from these six standard chest leads. Because the heart surfaces are close to the chest wall, each chest lead records mainly the electrical potential of the cardiac musculature immediately beneath the electrode. Therefore, relatively minute abnormalities in the ventricles, particularly in the anterior ventricular wall, can cause marked changes in the electrocardiograms recorded from individual chest leads.

Figure 11-9 Normal electrocardiograms recorded from the six standard chest leads.
In leads V1 and V2, the QRS recordings of the normal heart are mainly negative because, as shown in Figure 11-8 , the chest electrode in these leads is nearer to the base of the heart than to the apex, and the base of the heart is the direction of electronegativity during most of the ventricular depolarization process. Conversely, the QRS complexes in leads V4, V5, and V6 are mainly positive because the chest electrode in these leads is nearer the heart apex, which is the direction of electropositivity during most of depolarization.

Augmented Unipolar Limb Leads
Another system of leads in wide use is the augmented unipolar limb lead . In this type of recording, two of the limbs are connected through electrical resistances to the negative terminal of the electrocardiograph, and the third limb is connected to the positive terminal. When the positive terminal is on the right arm, the lead is known as the aVR lead; when on the left arm, the aVL lead; and when on the left leg, the aVF lead.
Normal recordings of the augmented unipolar limb leads are shown in Figure 11-10 . They are all similar to the standard limb lead recordings, except that the recording from the aVR lead is inverted. (Why does this inversion occur? Study the polarity connections to the electrocardiograph to determine this.)

Figure 11-10 Normal electrocardiograms recorded from the three augmented unipolar limb leads.


See bibliography for Chapter 13 .
CHAPTER 12 Electrocardiographic Interpretation of Cardiac Muscle and Coronary Blood Flow Abnormalities
Vectorial Analysis
From the discussion in Chapter 10 of impulse transmission through the heart, it is obvious that any change in the pattern of this transmission can cause abnormal electrical potentials around the heart and, consequently, alter the shapes of the waves in the electrocardiogram. For this reason, most serious abnormalities of the heart muscle can be diagnosed by analyzing the contours of the waves in the different electrocardiographic leads.

Principles of Vectorial Analysis of Electrocardiograms

Use of Vectors to Represent Electrical Potentials
Before it is possible to understand how cardiac abnormalities affect the contours of the electrocardiogram, one must first become thoroughly familiar with the concept of vectors and vectorial analysis as applied to electrical potentials in and around the heart.
Several times in Chapter 11 it was pointed out that heart current flows in a particular direction in the heart at a given instant during the cardiac cycle. A vector is an arrow that points in the direction of the electrical potential generated by the current flow, with the arrowhead in the positive direction. Also, by convention, the length of the arrow is drawn proportional to the voltage of the potential.

“Resultant” Vector in the Heart at Any Given Instant
Figure 12-1 shows, by the shaded area and the negative signs, depolarization of the ventricular septum and parts of the apical endocardial walls of the two ventricles. At this instant of heart excitation, electrical current flows between the depolarized areas inside the heart and the nondepolarized areas on the outside of the heart, as indicated by the long elliptical arrows. Some current also flows inside the heart chambers directly from the depolarized areas toward the still polarized areas. Overall, considerably more current flows downward from the base of the ventricles toward the apex than in the upward direction. Therefore, the summated vector of the generated potential at this particular instant, called the instantaneous mean vector, is represented by the long black arrow drawn through the center of the ventricles in a direction from base toward apex. Furthermore, because the summated current is considerable in quantity, the potential is large and the vector is long.

Figure 12-1 Mean vector through the partially depolarized ventricles.

Direction of a Vector Is Denoted in Terms of Degrees
When a vector is exactly horizontal and directed toward the person’s left side, the vector is said to extend in the direction of 0 degrees, as shown in Figure 12-2 . From this zero reference point, the scale of vectors rotates clockwise: when the vector extends from above and straight downward, it has a direction of +90 degrees; when it extends from the person’s left to right, it has a direction of +180 degrees; and when it extends straight upward, it has a direction of −90 (or +270) degrees.

Figure 12-2 Vectors drawn to represent potentials for several different hearts, and the “axis” of the potential (expressed in degrees) for each heart.
In a normal heart, the average direction of the vector during spread of the depolarization wave through the ventricles, called the mean QRS vector, is about +59 degrees, which is shown by vector A drawn through the center of Figure 12-2 in the +59-degree direction. This means that during most of the depolarization wave, the apex of the heart remains positive with respect to the base of the heart, as discussed later in the chapter.

Axis for Each Standard Bipolar Lead and Each Unipolar Limb Lead
In Chapter 11 , the three standard bipolar and the three unipolar limb leads are described. Each lead is actually a pair of electrodes connected to the body on opposite sides of the heart, and the direction from negative electrode to positive electrode is called the “axis” of the lead. Lead I is recorded from two electrodes placed respectively on the two arms. Because the electrodes lie exactly in the horizontal direction, with the positive electrode to the left, the axis of lead I is 0 degrees.
In recording lead II, electrodes are placed on the right arm and left leg. The right arm connects to the torso in the upper right-hand corner and the left leg connects in the lower left-hand corner. Therefore, the direction of this lead is about +60 degrees.
By similar analysis, it can be seen that lead III has an axis of about +120 degrees; lead aVR, +210 degrees; aVF, +90 degrees; and aVL −30 degrees. The directions of the axes of all these leads are shown in Figure 12-3 , which is known as the hexagonal reference system. The polarities of the electrodes are shown by the plus and minus signs in the figure. The reader must learn these axes and their polarities, particularly for the bipolar limb leads I, II, and III, to understand the remainder of this chapter.

Figure 12-3 Axes of the three bipolar and three unipolar leads.

Vectorial Analysis of Potentials Recorded in Different Leads
Now that we have discussed, first, the conventions for representing potentials across the heart by means of vectors and, second, the axes of the leads, it is possible to use these together to determine the instantaneous potential that will be recorded in the electrocardiogram of each lead for a given vector in the heart, as follows.
Figure 12-4 shows a partially depolarized heart; vector A represents the instantaneous mean direction of current flow in the ventricles. In this instance, the direction of the vector is +55 degrees, and the voltage of the potential, represented by the length of vector A, is 2 mv. In the diagram below the heart, vector A is shown again, and a line is drawn to represent the axis of lead I in the 0-degree direction. To determine how much of the voltage in vector A will be recorded in lead I, a line perpendicular to the axis of lead I is drawn from the tip of vector A to the lead I axis, and a so-called projected vector (B) is drawn along the lead I axis. The arrow of this projected vector points toward the positive end of the lead I axis, which means that the record momentarily being recorded in the electrocardiogram of lead I is positive. And the instantaneous recorded voltage will be equal to the length of B divided by the length of A times 2 millivolts, or about 1 millivolt.

Figure 12-4 Determination of a projected vector B along the axis of lead I when vector A represents the instantaneous potential in the ventricles.
Figure 12-5 shows another example of vectorial analysis. In this example, vector A represents the electrical potential and its axis at a given instant during ventricular depolarization in a heart in which the left side of the heart depolarizes more rapidly than the right. In this instance, the instantaneous vector has a direction of 100 degrees, and its voltage is again 2 millivolts. To determine the potential actually recorded in lead I, we draw a perpendicular line from the tip of vector A to the lead I axis and find projected vector B. Vector B is very short and this time in the negative direction, indicating that at this particular instant, the recording in lead I will be negative (below the zero line in the electrocardiogram), and the voltage recorded will be slight, about −0.3 millivolts. This figure demonstrates that when the vector in the heart is in a direction almost perpendicular to the axis of the lead, the voltage recorded in the electrocardiogram of this lead is very low. Conversely, when the heart vector has almost exactly the same axis as the lead axis, essentially the entire voltage of the vector will be recorded.

Figure 12-5 Determination of the projected vector B along the axis of lead I when vector A represents the instantaneous potential in the ventricles.

Vectorial Analysis of Potentials in the Three Standard Bipolar Limb Leads
In Figure 12-6 , vector A depicts the instantaneous electrical potential of a partially depolarized heart. To determine the potential recorded at this instant in the electrocardiogram for each one of the three standard bipolar limb leads, perpendicular lines (the dashed lines) are drawn from the tip of vector A to the three lines representing the axes of the three different standard leads, as shown in the figure. The projected vector B depicts the potential recorded at that instant in lead I, projected vector C depicts the potential in lead II, and projected vector D depicts the potential in lead III. In each of these, the record in the electrocardiogram is positive—that is, above the zero line—because the projected vectors point in the positive directions along the axes of all the leads. The potential in lead I (vector B ) is about one-half that of the actual potential in the heart (vector A ); in lead II (vector C ), it is almost equal to that in the heart; and in lead III (vector D ), it is about one-third that in the heart.

Figure 12-6 Determination of projected vectors in leads I, II, and III when vector A represents the instantaneous potential in the ventricles.
An identical analysis can be used to determine potentials recorded in augmented limb leads, except that the respective axes of the augmented leads (see Figure 12-3 ) are used in place of the standard bipolar limb lead axes used for Figure 12-6 .

Vectorial Analysis of the Normal Electrocardiogram

Vectors That Occur at Successive Intervals during Depolarization of the Ventricles— the QRS Complex
When the cardiac impulse enters the ventricles through the atrioventricular bundle, the first part of the ventricles to become depolarized is the left endocardial surface of the septum. Then depolarization spreads rapidly to involve both endocardial surfaces of the septum, as demonstrated by the darker shaded portion of the ventricle in Figure 12-7 A . Next, depolarization spreads along the endocardial surfaces of the remainder of the two ventricles, as shown in Figure 12-7 B and C. Finally, it spreads through the ventricular muscle to the outside of the heart, as shown progressively in Figure 12-7 C, D, and E.

Figure 12-7 Shaded areas of the ventricles are depolarized (−); nonshaded areas are still polarized (+). The ventricular vectors and QRS complexes 0.01 second after onset of ventricular depolarization (A); 0.02 second after onset of depolarization (B); 0.035 second after onset of depolarization (C); 0.05 second after onset of depolarization (D); and after depolarization of the ventricles is complete, 0.06 second after onset (E).
At each stage in Figure 12-7 , parts A to E, the instantaneous mean electrical potential of the ventricles is represented by a red vector superimposed on the ventricle in each figure. Each of these vectors is then analyzed by the method described in the preceding section to determine the voltages that will be recorded at each instant in each of the three standard electrocardiographic leads. To the right in each figure is shown progressive development of the electrocardiographic QRS complex. Keep in mind that a positive vector in a lead will cause recording in the electrocardiogram above the zero line, whereas a negative vector will cause recording below the zero line.
Before proceeding with further consideration of vectorial analysis, it is essential that this analysis of the successive normal vectors presented in Figure 12-7 be understood. Each of these analyses should be studied in detail by the procedure given here. A short summary of this sequence follows.
In Figure 12-7 A, the ventricular muscle has just begun to be depolarized, representing an instant about 0.01 second after the onset of depolarization. At this time, the vector is short because only a small portion of the ventricles—the septum—is depolarized. Therefore, all electrocardiographic voltages are low, as recorded to the right of the ventricular muscle for each of the leads. The voltage in lead II is greater than the voltages in leads I and III because the heart vector extends mainly in the same direction as the axis of lead II.
In Figure 12-7 B, which represents about 0.02 second after onset of depolarization, the heart vector is long because much of the ventricular muscle mass has become depolarized. Therefore, the voltages in all electrocardiographic leads have increased.
In Figure 12-7 C, about 0.035 second after onset of depolarization, the heart vector is becoming shorter and the recorded electrocardiographic voltages are lower because the outside of the heart apex is now electronegative, neutralizing much of the positivity on the other epicardial surfaces of the heart. Also, the axis of the vector is beginning to shift toward the left side of the chest because the left ventricle is slightly slower to depolarize than the right. Therefore, the ratio of the voltage in lead I to that in lead III is increasing.
In Figure 12-7 D, about 0.05 second after onset of depolarization, the heart vector points toward the base of the left ventricle, and it is short because only a minute portion of the ventricular muscle is still polarized positive. Because of the direction of the vector at this time, the voltages recorded in leads II and III are both negative—that is, below the line—whereas the voltage of lead I is still positive.
In Figure 12-7 E, about 0.06 second after onset of depolarization, the entire ventricular muscle mass is depolarized so that no current flows around the heart and no electrical potential is generated. The vector becomes zero, and the voltages in all leads become zero.
Thus, the QRS complexes are completed in the three standard bipolar limb leads.
Sometimes the QRS complex has a slight negative depression at its beginning in one or more of the leads, which is not shown in Figure 12-7 ; this depression is the Q wave. When it occurs, it is caused by initial depolarization of the left side of the septum before the right side, which creates a weak vector from left to right for a fraction of a second before the usual base-to-apex vector occurs. The major positive deflection shown in Figure 12-7 is the R wave, and the final negative deflection is the S wave.

Electrocardiogram during Repolarization—the T Wave
After the ventricular muscle has become depolarized, about 0.15 second later, repolarization begins and proceeds until complete at about 0.35 second. This repolarization causes the T wave in the electrocardiogram.
Because the septum and endocardial areas of the ventricular muscle depolarize first, it seems logical that these areas should repolarize first as well. However, this is not the usual case because the septum and other endocardial areas have a longer period of contraction than most of the external surfaces of the heart. Therefore, the greatest portion of ventricular muscle mass to repolarize first is the entire outer surface of the ventricles, especially near the apex of the heart. The endocardial areas, conversely, normally repolarize last. This sequence of repolarization is postulated to be caused by the high blood pressure inside the ventricles during contraction, which greatly reduces coronary blood flow to the endocardium, thereby slowing repolarization in the endocardial areas.
Because the outer apical surfaces of the ventricles repolarize before the inner surfaces, the positive end of the overall ventricular vector during repolarization is toward the apex of the heart. As a result, the normal T wave in all three bipolar limb leads is positive, which is also the polarity of most of the normal QRS complex.
In Figure 12-8 , five stages of repolarization of the ventricles are denoted by progressive increase of the light tan areas—the repolarized areas. At each stage, the vector extends from the base of the heart toward the apex until it disappears in the last stage. At first, the vector is relatively small because the area of repolarization is small. Later, the vector becomes stronger because of greater degrees of repolarization. Finally, the vector becomes weaker again because the areas of depolarization still persisting become so slight that the total quantity of current flow decreases. These changes also demonstrate that the vector is greatest when about half the heart is in the polarized state and about half is depolarized.

Figure 12-8 Generation of the T wave during repolarization of the ventricles, showing also vectorial analysis of the first stage of repolarization. The total time from the beginning of the T wave to its end is approximately 0.15 second.
The changes in the electrocardiograms of the three standard limb leads during repolarization are noted under each of the ventricles, depicting the progressive stages of repolarization. Thus, over about 0.15 second, the period of time required for the whole process to take place, the T wave of the electrocardiogram is generated.

Depolarization of the Atria—the P Wave
Depolarization of the atria begins in the sinus node and spreads in all directions over the atria. Therefore, the point of original electronegativity in the atria is about at the point of entry of the superior vena cava where the sinus node lies, and the direction of initial depolarization is denoted by the black vector in Figure 12-9 . Furthermore, the vector remains generally in this direction throughout the process of normal atrial depolarization. Because this direction is generally in the positive directions of the axes of the three standard bipolar limb leads I, II, and III, the electrocardiograms recorded from the atria during depolarization are also usually positive in all three of these leads, as shown in Figure 12-9 . This record of atrial depolarization is known as the atrial P wave.

Figure 12-9 Depolarization of the atria and generation of the P wave, showing the maximum vector through the atria and the resultant vectors in the three standard leads. At the right are the atrial P and T waves. SA, sinoatrial node.

Repolarization of the Atria—the Atrial T Wave
Spread of depolarization through the atrial muscle is much slower than in the ventricles because the atria have no Purkinje system for fast conduction of the depolarization signal. Therefore, the musculature around the sinus node becomes depolarized a long time before the musculature in distal parts of the atria. Because of this, the area in the atria that also becomes repolarized first is the sinus nodal region, the area that had originally become depolarized first. Thus, when repolarization begins, the region around the sinus node becomes positive with respect to the rest of the atria. Therefore, the atrial repolarization vector is backward to the vector of depolarization. (Note that this is opposite to the effect that occurs in the ventricles.) Therefore, as shown to the right in Figure 12-9 , the so-called atrial T wave follows about 0.15 second after the atrial P wave, but this T wave is on the opposite side of the zero reference line from the P wave; that is, it is normally negative rather than positive in the three standard bipolar limb leads.
In the normal electrocardiogram, the atrial T wave appears at about the same time that the QRS complex of the ventricles appears. Therefore, it is almost always totally obscured by the large ventricular QRS complex, although in some very abnormal states, it does appear in the recorded electrocardiogram.

It has been noted in the discussion up to this point that the vector of current flow through the heart changes rapidly as the impulse spreads through the myocardium. It changes in two aspects: First, the vector increases and decreases in length because of increasing and decreasing voltage of the vector. Second, the vector changes direction because of changes in the average direction of the electrical potential from the heart. The so-called vectorcardiogram depicts these changes at different times during the cardiac cycle, as shown in Figure 12-10 .

Figure 12-10 QRS and T vectorcardiograms.
In the large vectorcardiogram of Figure 12-10 , point 5 is the zero reference point, and this point is the negative end of all the successive vectors. While the heart muscle is polarized between heartbeats, the positive end of the vector remains at the zero point because there is no vectorial electrical potential. However, as soon as current begins to flow through the ventricles at the beginning of ventricular depolarization, the positive end of the vector leaves the zero reference point.
When the septum first becomes depolarized, the vector extends downward toward the apex of the ventricles, but it is relatively weak, thus generating the first portion of the ventricular vectorcardiogram, as shown by the positive end of vector 1. As more of the ventricular muscle becomes depolarized, the vector becomes stronger and stronger, usually swinging slightly to one side. Thus, vector 2 of Figure 12-10 represents the state of depolarization of the ventricles about 0.02 second after vector 1. After another 0.02 second, vector 3 represents the potential, and vector 4 occurs in another 0.01 second. Finally, the ventricles become totally depolarized, and the vector becomes zero once again, as shown at point 5.
The elliptical figure generated by the positive ends of the vectors is called the QRS vectorcardiogram. Vectorcardiograms can be recorded on an oscilloscope by connecting body surface electrodes from the neck and lower abdomen to the vertical plates of the oscilloscope and connecting chest surface electrodes from each side of the heart to the horizontal plates. When the vector changes, the spot of light on the oscilloscope follows the course of the positive end of the changing vector, thus inscribing the vectorcardiogram on the oscilloscopic screen.

Mean Electrical Axis of the Ventricular QRS—and Its Significance
The vectorcardiogram during ventricular depolarization (the QRS vectorcardiogram) shown in Figure 12-10 is that of a normal heart. Note from this vectorcardiogram that the preponderant direction of the vectors of the ventricles during depolarization is mainly toward the apex of the heart. That is, during most of the cycle of ventricular depolarization, the direction of the electrical potential (negative to positive) is from the base of the ventricles toward the apex. This preponderant direction of the potential during depolarization is called the mean electrical axis of the ventricles. The mean electrical axis of the normal ventricles is 59 degrees. In many pathological conditions of the heart, this direction changes markedly, sometimes even to opposite poles of the heart.

Determining the Electrical Axis from Standard Lead Electrocardiograms
Clinically, the electrical axis of the heart is usually estimated from the standard bipolar limb lead electrocardiograms rather than from the vectorcardiogram. Figure 12-11 shows a method for doing this. After recording the standard leads, one determines the net potential and polarity of the recordings in leads I and III. In lead I of Figure 12-11 , the recording is positive, and in lead III, the recording is mainly positive but negative during part of the cycle. If any part of a recording is negative, this negative potential is subtracted from the positive part of the potential to determine the net potential for that lead, as shown by the arrow to the right of the QRS complex for lead III. Then each net potential for leads I and III is plotted on the axes of the respective leads, with the base of the potential at the point of intersection of the axes, as shown in Figure 12-11 .

Figure 12-11 Plotting the mean electrical axis of the ventricles from two electrocardiographic leads (leads I and III).
If the net potential of lead I is positive, it is plotted in a positive direction along the line depicting lead I. Conversely, if this potential is negative, it is plotted in a negative direction. Also, for lead III, the net potential is placed with its base at the point of intersection, and, if positive, it is plotted in the positive direction along the line depicting lead III. If it is negative, it is plotted in the negative direction.
To determine the vector of the total QRS ventricular mean electrical potential, one draws perpendicular lines (the dashed lines in the figure) from the apices of leads I and III, respectively. The point of intersection of these two perpendicular lines represents, by vectorial analysis, the apex of the mean QRS vector in the ventricles, and the point of intersection of the lead I and lead III axes represents the negative end of the mean vector. Therefore, the mean QRS vector is drawn between these two points. The approximate average potential generated by the ventricles during depolarization is represented by the length of this mean QRS vector, and the mean electrical axis is represented by the direction of the mean vector. Thus, the orientation of the mean electrical axis of the normal ventricles, as determined in Figure 12-11 , is 59 degrees positive (+59 degrees).

Abnormal Ventricular Conditions That Cause Axis Deviation
Although the mean electrical axis of the ventricles averages about 59 degrees, this axis can swing even in the normal heart from about 20 degrees to about 100 degrees. The causes of the normal variations are mainly anatomical differences in the Purkinje distribution system or in the musculature itself of different hearts. However, a number of abnormal conditions of the heart can cause axis deviation beyond the normal limits, as follows.

Change in the Position of the Heart in the Chest
If the heart itself is angulated to the left, the mean electrical axis of the heart also shifts to the left . Such shift occurs (1) at the end of deep expiration, (2) when a person lies down, because the abdominal contents press upward against the diaphragm, and (3) quite frequently in obese people whose diaphragms normally press upward against the heart all the time due to increased visceral adiposity.
Likewise, angulation of the heart to the right causes the mean electrical axis of the ventricles to shift to the right. This occurs (1) at the end of deep inspiration, (2) when a person stands up, and (3) normally in tall, lanky people whose hearts hang downward.

Hypertrophy of One Ventricle
When one ventricle greatly hypertrophies, the axis of the heart shifts toward the hypertrophied ventricle for two reasons. First, a far greater quantity of muscle exists on the hypertrophied side of the heart than on the other side, and this allows generation of greater electrical potential on that side. Second, more time is required for the depolarization wave to travel through the hypertrophied ventricle than through the normal ventricle. Consequently, the normal ventricle becomes depolarized considerably in advance of the hypertrophied ventricle, and this causes a strong vector from the normal side of the heart toward the hypertrophied side, which remains strongly positively charged. Thus, the axis deviates toward the hypertrophied ventricle.

Vectorial Analysis of Left Axis Deviation Resulting from Hypertrophy of the Left Ventricle
Figure 12-12 shows the three standard bipolar limb lead electrocardiograms. Vectorial analysis demonstrates left axis deviation with mean electrical axis pointing in the −15-degree direction. This is a typical electrocardiogram caused by increased muscle mass of the left ventricle. In this instance, the axis deviation was caused by hypertension (high arterial blood pressure), which caused the left ventricle to hypertrophy so that it could pump blood against elevated systemic arterial pressure. A similar picture of left axis deviation occurs when the left ventricle hypertrophies as a result of aortic valvular stenosis, aortic valvular regurgitation, or any number of congenital heart conditions in which the left ventricle enlarges while the right ventricle remains relatively normal in size.

Figure 12-12 Left axis deviation in a hypertensive heart (hypertrophic left ventricle) . Note the slightly prolonged QRS complex as well.

Vectorial Analysis of Right Axis Deviation Resulting from Hypertrophy of the Right Ventricle
The electrocardiogram of Figure 12-13 shows intense right axis deviation, to an electrical axis of 170 degrees, which is 111 degrees to the right of the normal 59-degree mean ventricular QRS axis. The right axis deviation demonstrated in this figure was caused by hypertrophy of the right ventricle as a result of congenital pulmonary valve stenosis. Right axis deviation also can occur in other congenital heart conditions that cause hypertrophy of the right ventricle, such as tetralogy of Fallot and interventricular septal defect.

Figure 12-13 High-voltage electrocardiogram in congenital pulmonary valve stenosis with right ventricular hypertrophy. Intense right axis deviation and a slightly prolonged QRS complex also are seen.

Bundle Branch Block Causes Axis Deviation
Ordinarily, the lateral walls of the two ventricles depolarize at almost the same instant because both the left and the right bundle branches of the Purkinje system transmit the cardiac impulse to the two ventricular walls at almost the same instant. As a result, the potentials generated by the two ventricles (on the two opposite sides of the heart) almost neutralize each other. But if only one of the major bundle branches is blocked, the cardiac impulse spreads through the normal ventricle long before it spreads through the other. Therefore, depolarization of the two ventricles does not occur even nearly simultaneously, and the depolarization potentials do not neutralize each other. As a result, axis deviation occurs as follows.

Vectorial Analysis of Left Axis Deviation in Left Bundle Branch Block
When the left bundle branch is blocked, cardiac depolarization spreads through the right ventricle two to three times as rapidly as through the left ventricle. Consequently, much of the left ventricle remains polarized for as long as 0.1 second after the right ventricle has become totally depolarized. Thus, the right ventricle becomes electronegative, whereas the left ventricle remains electropositive during most of the depolarization process, and a strong vector projects from the right ventricle toward the left ventricle. In other words, there is intense left axis deviation of about −50 degrees because the positive end of the vector points toward the left ventricle. This is demonstrated in Figure 12-14 , which shows typical left axis deviation resulting from left bundle branch block.

Figure 12-14 Left axis deviation caused by left bundle branch block. Note also the greatly prolonged QRS complex.
Because of slowness of impulse conduction when the Purkinje system is blocked, in addition to axis deviation, the duration of the QRS complex is greatly prolonged because of extreme slowness of depolarization in the affected side of the heart. One can see this by observing the excessive widths of the QRS waves in Figure 12-14 . This is discussed in greater detail later in the chapter. This extremely prolonged QRS complex differentiates bundle branch block from axis deviation caused by hypertrophy.

Vectorial Analysis of Right Axis Deviation in Right Bundle Branch Block
When the right bundle branch is blocked, the left ventricle depolarizes far more rapidly than the right ventricle, so the left side of the ventricles becomes electronegative as long as 0.1 second before the right. Therefore, a strong vector develops, with its negative end toward the left ventricle and its positive end toward the right ventricle. In other words, intense right axis deviation occurs. Right axis deviation caused by right bundle branch block is demonstrated, and its vector is analyzed, in Figure 12-15 , which shows an axis of about 105 degrees instead of the normal 59 degrees and a prolonged QRS complex because of slow conduction.

Figure 12-15 Right axis deviation caused by right bundle branch block. Note also the greatly prolonged QRS complex.

Conditions That Cause Abnormal Voltages of the QRS Complex

Increased Voltage in the Standard Bipolar Limb Leads
Normally, the voltages in the three standard bipolar limb leads, as measured from the peak of the R wave to the bottom of the S wave, vary between 0.5 and 2.0 millivolts, with lead III usually recording the lowest voltage and lead II the highest. However, these relations are not invariable, even for the normal heart. In general, when the sum of the voltages of all the QRS complexes of the three standard leads is greater than 4 millivolts, the patient is considered to have a high-voltage electrocardiogram.
The cause of high-voltage QRS complexes most often is increased muscle mass of the heart, which ordinarily results from hypertrophy of the muscle in response to excessive load on one part of the heart or the other. For example, the right ventricle hypertrophies when it must pump blood through a stenotic pulmonary valve, and the left ventricle hypertrophies when a person has high blood pressure. The increased quantity of muscle causes generation of increased quantities of electricity around the heart. As a result, the electrical potentials recorded in the electrocardiographic leads are considerably greater than normal, as shown in Figures 12-12 and 12-13 .

Decreased Voltage of the Electrocardiogram

Decreased Voltage Caused by Cardiac Myopathies
One of the most common causes of decreased voltage of the QRS complex is a series of old myocardial infarctions with resultant diminished muscle mass. This also causes the depolarization wave to move through the ventricles slowly and prevents major portions of the heart from becoming massively depolarized all at once. Consequently, this condition causes some prolongation of the QRS complex along with the decreased voltage. Figure 12-16 shows a typical low-voltage electrocardiogram with prolongation of the QRS complex, which is common after multiple small infarctions of the heart have caused local delays of impulse conduction and reduced voltages due to loss of muscle mass throughout the ventricles.

Figure 12-16 Low-voltage electrocardiogram following local damage throughout the ventricles caused by previous myocardial infarction .

Decreased Voltage Caused by Conditions Surrounding the Heart
One of the most important causes of decreased voltage in electrocardiographic leads is fluid in the pericardium. Because extracellular fluid conducts electrical currents with great ease, a large portion of the electricity flowing out of the heart is conducted from one part of the heart to another through the pericardial fluid. Thus, this effusion effectively “short-circuits” the electrical potentials generated by the heart, decreasing the electrocardiographic voltages that reach the outside surfaces of the body. Pleural effusion, to a lesser extent, also can “short-circuit” the electricity around the heart so that the voltages at the surface of the body and in the electrocardiograms are decreased.
Pulmonary emphysema can decrease the electrocardiographic potentials, but for a different reason than that of pericardial effusion. In pulmonary emphysema, conduction of electrical current through the lungs is depressed considerably because of excessive quantity of air in the lungs. Also, the chest cavity enlarges, and the lungs tend to envelop the heart to a greater extent than normally. Therefore, the lungs act as an insulator to prevent spread of electrical voltage from the heart to the surface of the body, and this results in decreased electrocardiographic potentials in the various leads.

Prolonged and Bizarre Patterns of the QRS Complex

Prolonged QRS Complex as a Result of Cardiac Hypertrophy or Dilatation
The QRS complex lasts as long as depolarization continues to spread through the ventricles—that is, as long as part of the ventricles is depolarized and part is still polarized. Therefore, prolonged conduction of the impulse through the ventricles always causes a prolonged QRS complex. Such prolongation often occurs when one or both ventricles are hypertrophied or dilated, owing to the longer pathway that the impulse must then travel. The normal QRS complex lasts 0.06 to 0.08 second, whereas in hypertrophy or dilatation of the left or right ventricle, the QRS complex may be prolonged to 0.09 to 0.12 second.

Prolonged QRS Complex Resulting from Purkinje System Blocks
When the Purkinje fibers are blocked, the cardiac impulse must then be conducted by the ventricular muscle instead of by way of the Purkinje system. This decreases the velocity of impulse conduction to about one third of normal. Therefore, if complete block of one of the bundle branches occurs, the duration of the QRS complex is usually increased to 0.14 second or greater.
In general, a QRS complex is considered to be abnormally long when it lasts more than 0.09 second; when it lasts more than 0.12 second, the prolongation is almost certainly caused by pathological block somewhere in the ventricular conduction system, as shown by the electrocardiograms for bundle branch block in Figures 12-14 and 12-15 .

Conditions That Cause Bizarre QRS Complexes
Bizarre patterns of the QRS complex most frequently are caused by two conditions: (1) destruction of cardiac muscle in various areas throughout the ventricular system, with replacement of this muscle by scar tissue, and (2) multiple small local blocks in the conduction of impulses at many points in the Purkinje system. As a result, cardiac impulse conduction becomes irregular, causing rapid shifts in voltages and axis deviations. This often causes double or even triple peaks in some of the electrocardiographic leads, such as those shown in Figure 12-14 .

Current of Injury
Many different cardiac abnormalities, especially those that damage the heart muscle itself, often cause part of the heart to remain partially or totally depolarized all the time. When this occurs, current flows between the pathologically depolarized and the normally polarized areas even between heartbeats. This is called a current of injury. Note especially that the injured part of the heart is negative, because this is the part that is depolarized and emits negative charges into the surrounding fluids, whereas the remainder of the heart is neutral or positive polarity.
Some abnormalities that can cause current of injury are (1) mechanical trauma, which sometimes makes the membranes remain so permeable that full repolarization cannot take place; (2) infectious processes that damage the muscle membranes; and (3) ischemia of local areas of heart muscle caused by local coronary occlusions, which is by far the most common cause of current of injury in the heart. During ischemia, not enough nutrients from the coronary blood supply are available to the heart muscle to maintain normal membrane polarization.

Effect of Current of Injury on the QRS Complex
In Figure 12-17 , a small area in the base of the left ventricle is newly infarcted (loss of coronary blood flow). Therefore, during the T-P interval—that is, when the normal ventricular muscle is totally polarized—abnormal negative current still flows from the infarcted area at the base of the left ventricle and spreads toward the rest of the ventricles.

Figure 12-17 Effect of a current of injury on the electrocardiogram.
The vector of this “current of injury,” as shown in the first heart in Figure 12-17 , is in a direction of about 125 degrees, with the base of the vector, the negative end, toward the injured muscle. As shown in the lower portions of the figure, even before the QRS complex begins, this vector causes an initial record in lead I below the zero potential line, because the projected vector of the current of injury in lead I points toward the negative end of the lead I axis. In lead II, the record is above the line because the projected vector points more toward the positive terminal of the lead. In lead III, the projected vector points in the same direction as the positive terminal of lead III so that the record is positive. Furthermore, because the vector lies almost exactly in the direction of the axis of lead III, the voltage of the current of injury in lead III is much greater than in either lead I or lead II.
As the heart then proceeds through its normal process of depolarization, the septum first becomes depolarized; then the depolarization spreads down to the apex and back toward the bases of the ventricles. The last portion of the ventricles to become totally depolarized is the base of the right ventricle, because the base of the left ventricle is already totally and permanently depolarized. By vectorial analysis, the successive stages of electrocardiogram generation by the depolarization wave traveling through the ventricles can be constructed graphically, as demonstrated in the lower part of Figure 12-17 .
When the heart becomes totally depolarized, at the end of the depolarization process (as noted by the next-to-last stage in Figure 12-17 ), all the ventricular muscle is in a negative state. Therefore, at this instant in the electrocardiogram, no current flows from the ventricles to the electrocardiographic electrodes because now both the injured heart muscle and the contracting muscle are depolarized.
Next, as repolarization takes place, all of the heart finally repolarizes, except the area of permanent depolarization in the injured base of the left ventricle. Thus, repolarization causes a return of the current of injury in each lead, as noted at the far right in Figure 12-17 .

The J Point—the Zero Reference Potential for Analyzing Current of Injury
One might think that the electrocardiograph machines for recording electrocardiograms could determine when no current is flowing around the heart. However, many stray currents exist in the body, such as currents resulting from “skin potentials” and from differences in ionic concentrations in different fluids of the body. Therefore, when two electrodes are connected between the arms or between an arm and a leg, these stray currents make it impossible to predetermine the exact zero reference level in the electrocardiogram.
For these reasons, the following procedure must be used to determine the zero potential level: First, one notes the exact point at which the wave of depolarization just completes its passage through the heart, which occurs at the end of the QRS complex. At exactly this point, all parts of the ventricles have become depolarized, including both the damaged parts and the normal parts, so no current is flowing around the heart. Even the current of injury disappears at this point. Therefore, the potential of the electrocardiogram at this instant is at zero voltage. This point is known as the “J” point in the electrocardiogram, as shown in Figure 12-18 .

Figure 12-18 J point as the zero reference potential of the electrocardiograms for leads I and III. Also, the method for plotting the axis of the injury potential is shown by the lowermost panel.
Then, for analysis of the electrical axis of the injury potential caused by a current of injury, a horizontal line is drawn in the electrocardiogram for each lead at the level of the J point. This horizontal line is then the zero potential level in the electrocardiogram from which all potentials caused by currents of injury must be measured.

Use of the J Point in Plotting Axis of Injury Potential
Figure 12-18 shows electrocardiograms (leads I and III) from an injured heart. Both records show injury potentials. In other words, the J point of each of these two electrocardiograms is not on the same line as the T-P segment. In the figure, a horizontal line has been drawn through the J point to represent the zero voltage level in each of the two recordings. The injury potential in each lead is the difference between the voltage of the electrocardiogram immediately before onset of the P wave and the zero voltage level determined from the J point. In lead I, the recorded voltage of the injury potential is above the zero potential level and is, therefore, positive. Conversely, in lead III, the injury potential is below the zero voltage level and, therefore, is negative.
At the bottom in Figure 12-18 , the respective injury potentials in leads I and III are plotted on the coordinates of these leads, and the resultant vector of the injury potential for the whole ventricular muscle mass is determined by vectorial analysis as described. In this instance, the resultant vector extends from the right side of the ventricles toward the left and slightly upward, with an axis of about −30 degrees. If one places this vector for the injury potential directly over the ventricles, the negative end of the vector points toward the permanently depolarized, “injured” area of the ventricles. In the example shown in Figure 12-18 , the injured area would be in the lateral wall of the right ventricle.
This analysis is obviously complex. However, it is essential that the student go over it again and again until he or she understands it thoroughly. No other aspect of electrocardiographic analysis is more important.

Coronary Ischemia as a Cause of Injury Potential
Insufficient blood flow to the cardiac muscle depresses the metabolism of the muscle for three reasons: (1) lack of oxygen, (2) excess accumulation of carbon dioxide, and (3) lack of sufficient food nutrients. Consequently, repolarization of the muscle membrane cannot occur in areas of severe myocardial ischemia. Often the heart muscle does not die because the blood flow is sufficient to maintain life of the muscle even though it is not sufficient to cause repolarization of the membranes. As long as this state exists, an injury potential continues to flow during the diastolic portion (the T-P portion) of each heart cycle.
Extreme ischemia of the cardiac muscle occurs after coronary occlusion, and a strong current of injury flows from the infarcted area of the ventricles during the T-P interval between heartbeats, as shown in Figures 12-19 and 12-20 . Therefore, one of the most important diagnostic features of electrocardiograms recorded after acute coronary thrombosis is the current of injury.

Figure 12-19 Current of injury in acute anterior wall infarction . Note the intense injury potential in lead V 2 .

Figure 12-20 Injury potential in acute posterior wall, apical infarction.

Acute Anterior Wall Infarction
Figure 12-19 shows the electrocardiogram in the three standard bipolar limb leads and in one chest lead (lead V 2 ) recorded from a patient with acute anterior wall cardiac infarction. The most important diagnostic feature of this electrocardiogram is the intense injury potential in chest lead V 2 . If one draws a zero horizontal potential line through the J point of this electrocardiogram, a strong negative injury potential during the T-P interval is found, which means that the chest electrode over the front of the heart is in an area of strongly negative potential. In other words, the negative end of the injury potential vector in this heart is against the anterior chest wall. This means that the current of injury is emanating from the anterior wall of the ventricles, which diagnoses this condition as anterior wall infarction .
Analyzing the injury potentials in leads I and III, one finds a negative potential in lead I and a positive potential in lead III. This means that the resultant vector of the injury potential in the heart is about +150 degrees, with the negative end pointing toward the left ventricle and the positive end pointing toward the right ventricle. Thus, in this particular electrocardiogram, the current of injury is coming mainly from the left ventricle, as well as from the anterior wall of the heart. Therefore, one would conclude that this anterior wall infarction almost certainly is caused by thrombosis of the anterior descending branch of the left coronary artery.

Posterior Wall Infarction
Figure 12-20 shows the three standard bipolar limb leads and one chest lead (lead V 2 ) from a patient with posterior wall infarction. The major diagnostic feature of this electrocardiogram is also in the chest lead. If a zero potential reference line is drawn through the J point of this lead, it is readily apparent that during the T-P interval, the potential of the current of injury is positive. This means that the positive end of the vector is in the direction of the anterior chest wall, and the negative end (injured end of the vector) points away from the chest wall. In other words, the current of injury is coming from the back of the heart opposite to the anterior chest wall, which is the reason this type of electrocardiogram is the basis for diagnosing posterior wall infarction.
If one analyzes the injury potentials from leads II and III of Figure 12-20 , it is readily apparent that the injury potential is negative in both leads. By vectorial analysis, as shown in the figure, one finds that the resultant vector of the injury potential is about −95 degrees, with the negative end pointing downward and the positive end pointing upward. Thus, because the infarct, as indicated by the chest lead, is on the posterior wall of the heart and, as indicated by the injury potentials in leads II and III, is in the apical portion of the heart, one would suspect that this infarct is near the apex on the posterior wall of the left ventricle.

Infarction in Other Parts of the Heart
By the same procedures demonstrated in the preceding discussions of anterior and posterior wall infarctions, it is possible to determine the locus of any infarcted area emitting a current of injury, regardless of which part of the heart is involved. In making such vectorial analyses, it must be remembered that the positive end of the injury potential vector points toward the normal cardiac muscle, and the negative end points toward the injured portion of the heart that is emitting the current of injury.

Recovery from Acute Coronary Thrombosis
Figure 12-21 shows a V 3 chest lead from a patient with acute posterior wall infarction, demonstrating changes in the electrocardiogram from the day of the attack to 1 week later, 3 weeks later, and finally 1 year later. From this electrocardiogram, one can see that the injury potential is strong immediately after the acute attack (T-P segment displaced positively from the S-T segment). However, after about 1 week, the injury potential has diminished considerably, and after 3 weeks, it is gone. After that, the electrocardiogram does not change greatly during the next year. This is the usual recovery pattern after acute myocardial infarction of moderate degree, showing that the new collateral coronary blood flow develops enough to re-establish appropriate nutrition to most of the infarcted area.

Figure 12-21 Recovery of the myocardium after moderate posterior wall infarction, demonstrating disappearance of the injury potential that is present on the first day after the infarction and still slightly present at 1 week.
Conversely, in some patients with myocardial infarction, the infarcted area never redevelops adequate coronary blood supply. Often, some of the heart muscle dies, but if the muscle does not die, it will continue to show an injury potential as long as the ischemia exists, particularly during bouts of exercise when the heart is overloaded.

Old Recovered Myocardial Infarction
Figure 12-22 shows leads I and III after anterior infarction and leads I and III after posterior infarction about 1 year after the acute heart attack. The records show what might be called the “ideal” configurations of the QRS complex in these types of recovered myocardial infarction. Usually a Q wave has developed at the beginning of the QRS complex in lead I in anterior infarction because of loss of muscle mass in the anterior wall of the left ventricle, but in posterior infarction, a Q wave has developed at the beginning of the QRS complex in lead III because of loss of muscle in the posterior apical part of the ventricle.

Figure 12-22 Electrocardiograms of anterior and posterior wall infarctions that occurred about 1 year previously, showing a Q wave in lead I in anterior wall infarction and a Q wave in lead III in posterior wall infarction.
These configurations are certainly not found in all cases of old cardiac infarction. Local loss of muscle and local points of cardiac signal conduction block can cause very bizarre QRS patterns (especially prominent Q waves, for instance), decreased voltage, and QRS prolongation.

Current of Injury in Angina Pectoris
“Angina pectoris” means pain from the heart felt in the pectoral regions of the upper chest. This pain usually also radiates into the left neck area and down the left arm. The pain is typically caused by moderate ischemia of the heart. Usually, no pain is felt as long as the person is quiet, but as soon as he or she overworks the heart, the pain appears.
An injury potential sometimes appears in the electrocardiogram during an attack of severe angina pectoris because the coronary insufficiency becomes great enough to prevent adequate repolarization of some areas of the heart during diastole.

Abnormalities in the T Wave
Earlier in the chapter, it was pointed out that the T wave is normally positive in all the standard bipolar limb leads and that this is caused by repolarization of the apex and outer surfaces of the ventricles ahead of the intraventricular surfaces. That is, the T wave becomes abnormal when the normal sequence of repolarization does not occur. Several factors can change this sequence of repolarization.

Effect of Slow Conduction of the Depolarization Wave on the Characteristics of the T Wave
Referring to Figure 12-14 , note that the QRS complex is considerably prolonged. The reason for this prolongation is delayed conduction in the left ventricle resulting from left bundle branch block. This causes the left ventricle to become depolarized about 0.08 second after depolarization of the right ventricle, which gives a strong mean QRS vector to the left. However, the refractory periods of the right and left ventricular muscle masses are not greatly different from each other. Therefore, the right ventricle begins to repolarize long before the left ventricle; this causes strong positivity in the right ventricle and negativity in the left ventricle at the time that the T wave is developing. In other words, the mean axis of the T wave is now deviated to the right, which is opposite the mean electrical axis of the QRS complex in the same electrocardiogram. Thus, when conduction of the depolarization impulse through the ventricles is greatly delayed, the T wave is almost always of opposite polarity to that of the QRS complex.

Shortened Depolarization in Portions of the Ventricular Muscle as a Cause of T Wave Abnormalities
If the base of the ventricles should exhibit an abnormally short period of depolarization, that is, a shortened action potential, repolarization of the ventricles would not begin at the apex as it normally does. Instead, the base of the ventricles would repolarize ahead of the apex, and the vector of repolarization would point from the apex toward the base of the heart, opposite to the standard vector of repolarization. Consequently, the T wave in all three standard leads would be negative rather than the usual positive. Thus, the simple fact that the base of the ventricles has a shortened period of depolarization is sufficient to cause marked changes in the T wave, even to the extent of changing the entire T wave polarity, as shown in Figure 12-23 .

Figure 12-23 Inverted T wave resulting from mild ischemia at the apex of the ventricles.
Mild ischemia is by far the most common cause of shortening of depolarization of cardiac muscle because this increases current flow through the potassium channels. When the ischemia occurs in only one area of the heart, the depolarization period of this area decreases out of proportion to that in other portions. As a result, definite changes in the T wave can take place. The ischemia might result from chronic, progressive coronary occlusion; acute coronary occlusion; or relative coronary insufficiency that occurs during exercise.
One means for detecting mild coronary insufficiency is to have the patient exercise and to record the electrocardiogram, noting whether changes occur in the T waves. The changes in the T waves need not be specific because any change in the T wave in any lead—inversion, for instance, or a biphasic wave—is often evidence enough that some portion of the ventricular muscle has a period of depolarization out of proportion to the rest of the heart, caused by mild to moderate coronary insufficiency.

Effect of Digitalis on the T Wave
As discussed in Chapter 22 , digitalis is a drug that can be used during coronary insufficiency to increase the strength of cardiac muscle contraction. But when overdosages of digitalis are given, depolarization duration in one part of the ventricles may be increased out of proportion to that of other parts. As a result, nonspecific changes, such as T wave inversion or biphasic T waves, may occur in one or more of the electrocardiographic leads. A biphasic T wave caused by excessive administration of digitalis is shown in Figure 12-24 . Therefore, changes in the T wave during digitalis administration are often the earliest signs of digitalis toxicity.

Figure 12-24 Biphasic T wave caused by digitalis toxicity .


See bibliography for Chapter 13 .
CHAPTER 13 Cardiac Arrhythmias and Their Electrocardiographic Interpretation
Some of the most distressing types of heart malfunction occur not as a result of abnormal heart muscle but because of abnormal rhythm of the heart. For instance, sometimes the beat of the atria is not coordinated with the beat of the ventricles, so the atria no longer function as primer pumps for the ventricles.
The purpose of this chapter is to discuss the physiology of common cardiac arrhythmias and their effects on heart pumping, as well as their diagnosis by electrocardiography. The causes of the cardiac arrhythmias are usually one or a combination of the following abnormalities in the rhythmicity-conduction system of the heart:
1. Abnormal rhythmicity of the pacemaker.
2. Shift of the pacemaker from the sinus node to another place in the heart.
3. Blocks at different points in the spread of the impulse through the heart.
4. Abnormal pathways of impulse transmission through the heart.
5. Spontaneous generation of spurious impulses in almost any part of the heart.

Abnormal Sinus Rhythms

The term “tachycardia” means fast heart rate, usually defined in an adult person as faster than 100 beats/min. An electrocardiogram recorded from a patient with tachycardia is shown in Figure 13-1 . This electrocardiogram is normal except that the heart rate, as determined from the time intervals between QRS complexes, is about 150 per minute instead of the normal 72 per minute.

Figure 13-1 Sinus tachycardia (lead I).
Some causes of tachycardia include increased body temperature, stimulation of the heart by the sympathetic nerves, or toxic conditions of the heart.
The heart rate increases about 10 beats/min for each degree of Fahrenheit (18 beats per degree Celsius) increase in body temperature, up to a body temperature of about 105 °F (40.5 °C); beyond this, the heart rate may decrease because of progressive debility of the heart muscle as a result of the fever. Fever causes tachycardia because increased temperature increases the rate of metabolism of the sinus node, which in turn directly increases its excitability and rate of rhythm.
Many factors can cause the sympathetic nervous system to excite the heart, as we discuss at multiple points in this text. For instance, when a patient loses blood and passes into a state of shock or semishock, sympathetic reflex stimulation of the heart often increases the heart rate to 150 to 180 beats/min.
Simple weakening of the myocardium usually increases the heart rate because the weakened heart does not pump blood into the arterial tree to a normal extent, and this elicits sympathetic reflexes to increase the heart rate.

The term “bradycardia” means a slow heart rate, usually defined as fewer than 60 beats/min. Bradycardia is shown by the electrocardiogram in Figure 13-2 .

Figure 13-2 Sinus bradycardia (lead III).

Bradycardia in Athletes
The athlete’s heart is larger and considerably stronger than that of a normal person, which allows the athlete’s heart to pump a large stroke volume output per beat even during periods of rest. When the athlete is at rest, excessive quantities of blood pumped into the arterial tree with each beat initiate feedback circulatory reflexes or other effects to cause bradycardia.

Vagal Stimulation as a Cause of Bradycardia
Any circulatory reflex that stimulates the vagus nerves causes release of acetylcholine at the vagal endings in the heart, thus giving a parasympathetic effect. Perhaps the most striking example of this occurs in patients with carotid sinus syndrome. In these patients, the pressure receptors (baroreceptors) in the carotid sinus region of the carotid artery walls are excessively sensitive. Therefore, even mild external pressure on the neck elicits a strong baroreceptor reflex, causing intense vagal-acetylcholine effects on the heart, including extreme bradycardia. Indeed, sometimes this reflex is so powerful that it actually stops the heart for 5 to 10 seconds.

Sinus Arrhythmia
Figure 13-3 shows a cardiotachometer recording of the heart rate, at first during normal and then (in the second half of the record) during deep respiration. A cardiotachometer is an instrument that records by the height of successive spikes the duration of the interval between the successive QRS complexes in the electrocardiogram. Note from this record that the heart rate increased and decreased no more than 5 percent during quiet respiration (left half of the record). Then, during deep respiration, the heart rate increased and decreased with each respiratory cycle by as much as 30 percent.

Figure 13-3 Sinus arrhythmia as recorded by a cardiotachometer. To the left is the record when the subject was breathing normally; to the right, when breathing deeply.
Sinus arrhythmia can result from any one of many circulatory conditions that alter the strengths of the sympathetic and parasympathetic nerve signals to the heart sinus node. In the “respiratory” type of sinus arrhythmia, as shown in Figure 13-3 , this results mainly from “spillover” of signals from the medullary respiratory center into the adjacent vasomotor center during inspiratory and expiratory cycles of respiration. The spillover signals cause alternate increase and decrease in the number of impulses transmitted through the sympathetic and vagus nerves to the heart.

Abnormal Rhythms That Result from Block of Heart Signals Within the Intracardiac Conduction Pathways

Sinoatrial Block
In rare instances, the impulse from the sinus node is blocked before it enters the atrial muscle. This phenomenon is demonstrated in Figure 13-4 , which shows sudden cessation of P waves, with resultant standstill of the atria. However, the ventricles pick up a new rhythm, the impulse usually originating spontaneously in the atrioventricular (A-V) node, so the rate of the ventricular QRS-T complex is slowed but not otherwise altered.

Figure 13-4 Sinoatrial nodal block, with A-V nodal rhythm during the block period (lead III).

Atrioventricular Block
The only means by which impulses ordinarily can pass from the atria into the ventricles is through the A-V bundle, also known as the bundle of His. Conditions that can either decrease the rate of impulse conduction in this bundle or block the impulse entirely are as follows:
1. Ischemia of the A-V node or A-V bundle fibers often delays or blocks conduction from the atria to the ventricles. Coronary insufficiency can cause ischemia of the A-V node and bundle in the same way that it can cause ischemia of the myocardium.
2. Compression of the A-V bundle by scar tissue or by calcified portions of the heart can depress or block conduction from the atria to the ventricles.
3. Inflammation of the A-V node or A-V bundle can depress conductivity from the atria to the ventricles. Inflammation results frequently from different types of myocarditis, caused, for example, by diphtheria or rheumatic fever.
4. Extreme stimulation of the heart by the vagus nerves in rare instances blocks impulse conduction through the A-V node. Such vagal excitation occasionally results from strong stimulation of the baroreceptors in people with carotid sinus syndrome, discussed earlier in relation to bradycardia.

Incomplete Atrioventricular Heart Block

Prolonged P-R (or P-Q) Interval—First-Degree Block
The usual lapse of time between beginning of the P wave and beginning of the QRS complex is about 0.16 second when the heart is beating at a normal rate. This so-called P-R interval usually decreases in length with faster heartbeat and increases with slower heartbeat. In general, when the P-R interval increases to greater than 0.20 second, the P-R interval is said to be prolonged and the patient is said to have first-degree incomplete heart block.
Figure 13-5 shows an electrocardiogram with prolonged P-R interval; the interval in this instance is about 0.30 second instead of the normal 0.20 or less. Thus, first-degree block is defined as a delay of conduction from the atria to the ventricles but not actual blockage of conduction. The P-R interval seldom increases above 0.35 to 0.45 second because, by that time, conduction through the A-V bundle is depressed so much that conduction stops entirely. One means for determining the severity of some heart diseases— acute rheumatic heart disease , for instance—is to measure the P-R interval.

Figure 13-5 Prolonged P-R interval caused by first degree A-V heart block (lead II).

Second-Degree Block
When conduction through the A-V bundle is slowed enough to increase the P-R interval to 0.25 to 0.45 second, the action potential is sometimes strong enough to pass through the bundle into the ventricles and sometimes not strong enough. In this instance, there will be an atrial P wave but no QRS-T wave, and it is said that there are “dropped beats” of the ventricles. This condition is called second-degree heart block.
Figure 13-6 shows P-R intervals of 0.30 second, as well as one dropped ventricular beat as a result of failure of conduction from the atria to the ventricles.

Figure 13-6 Second degree A-V block, showing occasional failure of the ventricles to receive the excitatory signals (lead V 3 ).
At times, every other beat of the ventricles is dropped, so a “2:1 rhythm” develops, with the atria beating twice for every single beat of the ventricles. At other times, rhythms of 3:2 or 3:1 also develop.

Complete A-V Block (Third-Degree Block)
When the condition causing poor conduction in the A-V node or A-V bundle becomes severe, complete block of the impulse from the atria into the ventricles occurs. In this instance, the ventricles spontaneously establish their own signal, usually originating in the A-V node or A-V bundle. Therefore, the P waves become dissociated from the QRS-T complexes, as shown in Figure 13-7 . Note that the rate of rhythm of the atria in this electrocardiogram is about 100 beats per minute, whereas the rate of ventricular beat is less than 40 per minute. Furthermore, there is no relation between the rhythm of the P waves and that of the QRS-T complexes because the ventricles have “escaped” from control by the atria, and they are beating at their own natural rate, controlled most often by rhythmical signals generated in the A-V node or A-V bundle.

Figure 13-7 Complete A-V block (lead II).

Stokes-Adams Syndrome—Ventricular Escape
In some patients with A-V block, the total block comes and goes; that is, impulses are conducted from the atria into the ventricles for a period of time and then suddenly impulses are not conducted. The duration of block may be a few seconds, a few minutes, a few hours, or even weeks or longer before conduction returns. This condition occurs in hearts with borderline ischemia of the conductive system.
Each time A-V conduction ceases, the ventricles often do not start their own beating until after a delay of 5 to 30 seconds. This results from the phenomenon called overdrive suppression. This means that ventricular excitability is at first in a suppressed state because the ventricles have been driven by the atria at a rate greater than their natural rate of rhythm. However, after a few seconds, some part of the Purkinje system beyond the block, usually in the distal part of the A-V node beyond the blocked point in the node, or in the A-V bundle, begins discharging rhythmically at a rate of 15 to 40 times per minute and acting as the pacemaker of the ventricles. This is called ventricular escape.
Because the brain cannot remain active for more than 4 to 7 seconds without blood supply, most patients faint a few seconds after complete block occurs because the heart does not pump any blood for 5 to 30 seconds, until the ventricles “escape.” After escape, however, the slowly beating ventricles usually pump enough blood to allow rapid recovery from the faint and then to sustain the person. These periodic fainting spells are known as the Stokes-Adams syndrome.
Occasionally the interval of ventricular standstill at the onset of complete block is so long that it becomes detrimental to the patient’s health or even causes death. Consequently, most of these patients are provided with an artificial pacemaker, a small battery-operated electrical stimulator planted beneath the skin, with electrodes usually connected to the right ventricle. The pacemaker provides continued rhythmical impulses that take control of the ventricles.

Incomplete Intraventricular Block—Electrical Alternans
Most of the same factors that can cause A-V block can also block impulse conduction in the peripheral ventricular Purkinje system. Figure 13-8 shows the condition known as electrical alternans, which results from partial intraventricular block every other heartbeat. This electrocardiogram also shows tachycardia (rapid heart rate), which is probably the reason the block has occurred, because when the rate of the heart is rapid, it may be impossible for some portions of the Purkinje system to recover from the previous refractory period quickly enough to respond during every succeeding heartbeat. Also, many conditions that depress the heart, such as ischemia, myocarditis, or digitalis toxicity, can cause incomplete intraventricular block, resulting in electrical alternans.

Figure 13-8 Partial intraventricular block—“electrical alternans” (lead III).

Premature Contractions
A premature contraction is a contraction of the heart before the time that normal contraction would have been expected. This condition is also called extrasystole, premature beat, or ectopic beat.

Causes of Premature Contractions
Most premature contractions result from ectopic foci in the heart, which emit abnormal impulses at odd times during the cardiac rhythm. Possible causes of ectopic foci are (1) local areas of ischemia; (2) small calcified plaques at different points in the heart, which press against the adjacent cardiac muscle so that some of the fibers are irritated; and (3) toxic irritation of the A-V node, Purkinje system, or myocardium caused by drugs, nicotine, or caffeine. Mechanical initiation of premature contractions is also frequent during cardiac catheterization; large numbers of premature contractions often occur when the catheter enters the right ventricle and presses against the endocardium.

Premature Atrial Contractions
Figure 13-9 shows a single premature atrial contraction. The P wave of this beat occurred too soon in the heart cycle; the P-R interval is shortened, indicating that the ectopic origin of the beat is in the atria near the A-V node. Also, the interval between the premature contraction and the next succeeding contraction is slightly prolonged, which is called a compensatory pause. One of the reasons for this is that the premature contraction originated in the atrium some distance from the sinus node, and the impulse had to travel through a considerable amount of atrial muscle before it discharged the sinus node. Consequently, the sinus node discharged late in the premature cycle, and this made the succeeding sinus node discharge also late in appearing.

Figure 13-9 Atrial premature beat (lead I).
Premature atrial contractions occur frequently in otherwise healthy people. Indeed, they often occur in athletes whose hearts are in very healthy condition. Mild toxic conditions resulting from such factors as smoking, lack of sleep, ingestion of too much coffee, alcoholism, and use of various drugs can also initiate such contractions.

Pulse Deficit
When the heart contracts ahead of schedule, the ventricles will not have filled with blood normally, and the stroke volume output during that contraction is depressed or almost absent. Therefore, the pulse wave passing to the peripheral arteries after a premature contraction may be so weak that it cannot be felt in the radial artery. Thus, a deficit in the number of radial pulses occurs when compared with the actual number of contractions of the heart.

A-V Nodal or A-V Bundle Premature Contractions
Figure 13-10 shows a premature contraction that originated in the A-V node or in the A-V bundle. The P wave is missing from the electrocardiographic record of the premature contraction. Instead, the P wave is superimposed onto the QRS-T complex because the cardiac impulse traveled backward into the atria at the same time that it traveled forward into the ventricles; this P wave slightly distorts the QRS-T complex, but the P wave itself cannot be discerned as such. In general, A-V nodal premature contractions have the same significance and causes as atrial premature contractions.

Figure 13-10 A-V nodal premature contraction (lead III).

Premature Ventricular Contractions
The electrocardiogram of Figure 13-11 shows a series of premature ventricular contractions (PVCs) alternating with normal contractions. PVCs cause specific effects in the electrocardiogram, as follows:
1. The QRS complex is usually considerably prolonged. The reason is that the impulse is conducted mainly through slowly conducting muscle of the ventricles rather than through the Purkinje system.
2. The QRS complex has a high voltage for the following reasons: when the normal impulse passes through the heart, it passes through both ventricles nearly simultaneously; consequently, in the normal heart, the depolarization waves of the two sides of the heart—mainly of opposite polarity to each other—partially neutralize each other in the electrocardiogram. When a PVC occurs, the impulse almost always travels in only one direction, so there is no such neutralization effect, and one entire side or end of the ventricles is depolarized ahead of the other; this causes large electrical potentials, as shown for the PVCs in Figure 13-11 .
3. After almost all PVCs, the T wave has an electrical potential polarity exactly opposite to that of the QRS complex because the slow conduction of the impulse through the cardiac muscle causes the muscle fibers that depolarize first also to repolarize first.

Figure 13-11 Premature ventricular contractions (PVCs) demonstrated by the large abnormal QRS-T complexes (leads II and III). Axis of the premature contractions is plotted in accordance with the principles of vectorial analysis explained in Chapter 12 ; this shows the origin of the PVC to be near the base of the ventricles.
Some PVCs are relatively benign in their effects on overall pumping by the heart; they can result from such factors as cigarettes, excessive intake of coffee, lack of sleep, various mild toxic states, and even emotional irritability. Conversely, many other PVCs result from stray impulses or re-entrant signals that originate around the borders of infarcted or ischemic areas of the heart. The presence of such PVCs is not to be taken lightly. Statistics show that people with significant numbers of PVCs have a much higher than normal chance of developing spontaneous lethal ventricular fibrillation, presumably initiated by one of the PVCs. This is especially true when the PVCs occur during the vulnerable period for causing fibrillation, just at the end of the T wave when the ventricles are coming out of refractoriness, as explained later in the chapter.

Vector Analysis of the Origin of an Ectopic Premature Ventricular Contraction
In Chapter 12 , the principles of vectorial analysis are explained. Applying these principles, one can determine from the electrocardiogram in Figure 13-11 the point of origin of the PVC as follows: Note that the potentials of the premature contractions in leads II and III are both strongly positive. Plotting these potentials on the axes of leads II and III and solving by vectorial analysis for the mean QRS vector in the heart, one finds that the vector of this premature contraction has its negative end (origin) at the base of the heart and its positive end toward the apex. Thus, the first portion of the heart to become depolarized during this premature contraction is near the base of the ventricles, which therefore is the locus of the ectopic focus.

Disorders of Cardiac Repolarization—The Long QT Syndromes
Recall that the Q wave corresponds to ventricular depolarization while the T wave corresponds to ventricular repolarization. The Q-T interval is the time from the Q point to the end of the T wave. Disorders that delay repolarization of ventricular muscle following the action potential cause prolonged ventricular action potentials and therefore excessively long Q-T intervals on the electrocardiogram, a condition called long QT syndrome (LQTS).
The major reason that the long QT syndrome is of concern is that delayed repolarization of ventricular muscle increases a person’s susceptibility to develop ventricular arrhythmias called torsades de pointes, which literally means “twisting of the points.” This type of arrhythmia has the features shown in Figure 13-12 . The shape of the QRS complex may change over time with the onset of arrhythmia usually following a premature beat, a pause, and then another beat with a long Q-T interval, which may trigger arrhythmias, tachycardia, and in some instances ventricular fibrillation.

Figure 13-12 Development of arrhythmias in long QT syndrome (LQTS). When the ventricular muscle fiber action potential is prolonged as a result of delayed repolarization, a premature depolarization ( dashed line in top left figure ) may occur before complete repolarization. Repetitive premature depolarizations ( right top figure ) may lead to multiple depolarizations under certain conditions. In torsades de pointes ( bottom figure ), premature ventricular beats lead pauses, postpause prolongation of the Q-T interval, and arrhythmias.
(Redrawn from Murray KT, Roden DM: Disorders of cardiac repolarization: the long QT syndromes. In: Crawford MG, DiMarco JP [eds]: Cardiology. London: Mosby, 2001.)
Disorders of cardiac repolarization that lead to LQTS may be inherited or acquired. The congenital forms of LQTS are rare disorders caused by mutations of sodium or potassium channel genes. At least 10 different mutations of these genes that can cause variable degrees of Q-T prolongation have been identified.
More common are the acquired forms of LQTS that are associated with plasma electrolyte disturbances, such as hypomagnesemia, hypokalemia, or hypocalcemia, or with administration of excess amounts of antiarrhythmic drugs such as quinidine or some antibiotics such as fluoroquinolones or erythromycin that prolong the Q-T interval.
Although some people with LQTS show no major symptoms (other than the prolonged Q-T interval), others exhibit fainting and ventricular arrhythmias that may be precipitated by physical exercise, intense emotions such as fright or anger, or when startled by a noise. The ventricular arrhythmias associated with LQTS can, in some cases, deteriorate into ventricular fibrillation and sudden death.
Treatment for LQTS may include magnesium sulfate for acute LQTS, and for long-term LQTS antiarrhythmia medications, such as beta-adrenergic blockers, or surgical implantation of a cardiac defibrillator are used.

Paroxysmal Tachycardia
Some abnormalities in different portions of the heart, including the atria, the Purkinje system, or the ventricles, can occasionally cause rapid rhythmical discharge of impulses that spread in all directions throughout the heart. This is believed to be caused most frequently by re-entrant circus movement feedback pathways that set up local repeated self–re-excitation. Because of the rapid rhythm in the irritable focus, this focus becomes the pacemaker of the heart.
The term “paroxysmal” means that the heart rate becomes rapid in paroxysms, with the paroxysm beginning suddenly and lasting for a few seconds, a few minutes, a few hours, or much longer. Then the paroxysm usually ends as suddenly as it began, with the pacemaker of the heart instantly shifting back to the sinus node.
Paroxysmal tachycardia often can be stopped by eliciting a vagal reflex. A type of vagal reflex sometimes elicited for this purpose is to press on the neck in the regions of the carotid sinuses, which may cause enough of a vagal reflex to stop the paroxysm. Various drugs may also be used. Two drugs frequently used are quinidine and lidocaine, either of which depresses the normal increase in sodium permeability of the cardiac muscle membrane during generation of the action potential, thereby often blocking the rhythmical discharge of the focal point that is causing the paroxysmal attack.

Atrial Paroxysmal Tachycardia
Figure 13-13 demonstrates in the middle of the record a sudden increase in the heart rate from about 95 to about 150 beats per minute. On close study of the electrocardiogram during the rapid heartbeat, an inverted P wave is seen before each QRS-T complex, and this P wave is partially superimposed onto the normal T wave of the preceding beat. This indicates that the origin of this paroxysmal tachycardia is in the atrium, but because the P wave is abnormal in shape, the origin is not near the sinus node.

Figure 13-13 Atrial paroxysmal tachycardia—onset in middle of record (lead I).

A-V Nodal Paroxysmal Tachycardia
Paroxysmal tachycardia often results from an aberrant rhythm that involves the A-V node. This usually causes almost normal QRS-T complexes but totally missing or obscured P waves.
Atrial or A-V nodal paroxysmal tachycardia, both of which are called supraventricular tachycardias, usually occurs in young, otherwise healthy people, and they generally grow out of the predisposition to tachycardia after adolescence. In general, supraventricular tachycardia frightens a person tremendously and may cause weakness during the paroxysm, but only seldom does permanent harm come from the attack.

Ventricular Paroxysmal Tachycardia
Figure 13-14 shows a typical short paroxysm of ventricular tachycardia. The electrocardiogram of ventricular paroxysmal tachycardia has the appearance of a series of ventricular premature beats occurring one after another without any normal beats interspersed.

Figure 13-14 Ventricular paroxysmal tachycardia (lead III).
Ventricular paroxysmal tachycardia is usually a serious condition for two reasons. First, this type of tachycardia usually does not occur unless considerable ischemic damage is present in the ventricles. Second, ventricular tachycardia frequently initiates the lethal condition of ventricular fibrillation because of rapid repeated stimulation of the ventricular muscle, as we discuss in the next section.
Sometimes intoxication from the heart treatment drug digitalis causes irritable foci that lead to ventricular tachycardia. Conversely, quinidine, which increases the refractory period and threshold for excitation of cardiac muscle, may be used to block irritable foci causing ventricular tachycardia.

Ventricular Fibrillation
The most serious of all cardiac arrhythmias is ventricular fibrillation, which, if not stopped within 1 to 3 minutes, is almost invariably fatal. Ventricular fibrillation results from cardiac impulses that have gone berserk within the ventricular muscle mass, stimulating first one portion of the ventricular muscle, then another portion, then another, and eventually feeding back onto itself to re-excite the same ventricular muscle over and over—never stopping. When this happens, many small portions of the ventricular muscle will be contracting at the same time, while equally as many other portions will be relaxing. Thus, there is never a coordinate contraction of all the ventricular muscle at once, which is required for a pumping cycle of the heart. Despite massive movement of stimulatory signals throughout the ventricles, the ventricular chambers neither enlarge nor contract but remain in an indeterminate stage of partial contraction, pumping either no blood or negligible amounts. Therefore, after fibrillation begins, unconsciousness occurs within 4 to 5 seconds for lack of blood flow to the brain, and irretrievable death of tissues begins to occur throughout the body within a few minutes.
Multiple factors can spark the beginning of ventricular fibrillation—a person may have a normal heartbeat one moment, but 1 second later, the ventricles are in fibrillation. Especially likely to initiate fibrillation are (1) sudden electrical shock of the heart or (2) ischemia of the heart muscle, of its specialized conducting system, or both.

Phenomenon of Re-entry—“Circus Movements” as the Basis for Ventricular Fibrillation
When the normal cardiac impulse in the normal heart has traveled through the extent of the ventricles, it has no place to go because all the ventricular muscle is refractory and cannot conduct the impulse farther. Therefore, that impulse dies, and the heart awaits a new action potential to begin in the atrial sinus node.
Under some circumstances, however, this normal sequence of events does not occur. Therefore, let us explain more fully the background conditions that can initiate re-entry and lead to “circus movements,” which in turn cause ventricular fibrillation.
Figure 13-15 shows several small cardiac muscle strips cut in the form of circles. If such a strip is stimulated at the 12 o’clock position so that the impulse travels in only one direction, the impulse spreads progressively around the circle until it returns to the 12 o’clock position. If the originally stimulated muscle fibers are still in a refractory state, the impulse then dies out because refractory muscle cannot transmit a second impulse. But there are three different conditions that can cause this impulse to continue to travel around the circle, that is, to cause “re-entry” of the impulse into muscle that has already been excited. This is called a “circus movement.”

Figure 13-15 Circus movement, showing annihilation of the impulse in the short pathway and continued propagation of the impulse in the long pathway.
First, if the pathway around the circle is too long, by the time the impulse returns to the 12 o’clock position, the originally stimulated muscle will no longer be refractory and the impulse will continue around the circle again and again.
Second, if the length of the pathway remains constant but the velocity of conduction becomes decreased enough, an increased interval of time will elapse before the impulse returns to the 12 o’clock position. By this time, the originally stimulated muscle might be out of the refractory state, and the impulse can continue around the circle again and again.
Third, the refractory period of the muscle might become greatly shortened. In this case, the impulse could also continue around and around the circle.
All these conditions occur in different pathological states of the human heart, as follows: (1) A long pathway typically occurs in dilated hearts. (2) Decreased rate of conduction frequently results from (a) blockage of the Purkinje system, (b) ischemia of the muscle, (c) high blood potassium levels, or (d) many other factors. (3) A shortened refractory period commonly occurs in response to various drugs, such as epinephrine, or after repetitive electrical stimulation. Thus, in many cardiac disturbances, re-entry can cause abnormal patterns of cardiac contraction or abnormal cardiac rhythms that ignore the pace-setting effects of the sinus node.

Chain Reaction Mechanism of Fibrillation
In ventricular fibrillation, one sees many separate and small contractile waves spreading at the same time in different directions over the cardiac muscle. The re-entrant impulses in fibrillation are not simply a single impulse moving in a circle, as shown in Figure 13-15 . Instead, they have degenerated into a series of multiple wave fronts that have the appearance of a “chain reaction.” One of the best ways to explain this process in fibrillation is to describe the initiation of fibrillation by electric shock caused by 60-cycle alternating electric current.

Fibrillation Caused by 60-Cycle Alternating Current
At a central point in the ventricles of heart A in Figure 13-16 , a 60-cycle electrical stimulus is applied through a stimulating electrode. The first cycle of the electrical stimulus causes a depolarization wave to spread in all directions, leaving all the muscle beneath the electrode in a refractory state. After about 0.25 second, part of this muscle begins to come out of the refractory state. Some portions come out of refractoriness before other portions. This state of events is depicted in heart A by many lighter patches, which represent excitable cardiac muscle, and dark patches, which represent still refractory muscle. Now, continuing 60-cycle stimuli from the electrode can cause impulses to travel only in certain directions through the heart but not in all directions. Thus, in heart A, certain impulses travel for short distances, until they reach refractory areas of the heart, and then are blocked. But other impulses pass between the refractory areas and continue to travel in the excitable areas. Then, several events transpire in rapid succession, all occurring simultaneously and eventuating in a state of fibrillation.

Figure 13-16 A, Initiation of fibrillation in a heart when patches of refractory musculature are present. B, Continued propagation of fibrillatory impulses in the fibrillating ventricle.
First, block of the impulses in some directions but successful transmission in other directions creates one of the necessary conditions for a re-entrant signal to develop—that is, transmission of some of the depolarization waves around the heart in only some directions but not other directions.
Second, the rapid stimulation of the heart causes two changes in the cardiac muscle itself, both of which predispose to circus movement: (1) The velocity of conduction through the heart muscle decreases, which allows a longer time interval for the impulses to travel around the heart. (2) The refractory period of the muscle is shortened, allowing re-entry of the impulse into previously excited heart muscle within a much shorter time than normally.
Third, one of the most important features of fibrillation is the division of impulses, as demonstrated in heart A. When a depolarization wave reaches a refractory area in the heart, it travels to both sides around the refractory area. Thus, a single impulse becomes two impulses. Then, when each of these reaches another refractory area, it, too, divides to form two more impulses. In this way, many new wave fronts are continually being formed in the heart by progressive chain reactions until, finally, there are many small depolarization waves traveling in many directions at the same time. Furthermore, this irregular pattern of impulse travel causes many circuitous routes for the impulses to travel, greatly lengthening the conductive pathway, which is one of the conditions that sustains the fibrillation. It also results in a continual irregular pattern of patchy refractory areas in the heart.
One can readily see when a vicious circle has been initiated: More and more impulses are formed; these cause more and more patches of refractory muscle, and the refractory patches cause more and more division of the impulses. Therefore, any time a single area of cardiac muscle comes out of refractoriness, an impulse is close at hand to re-enter the area.
Heart B in Figure 13-16 demonstrates the final state that develops in fibrillation. Here one can see many impulses traveling in all directions, some dividing and increasing the number of impulses, whereas others are blocked by refractory areas. In fact, a single electric shock during this vulnerable period frequently can lead to an odd pattern of impulses spreading multidirectionally around refractory areas of muscle, which will lead to fibrillation.

Electrocardiogram in Ventricular Fibrillation
In ventricular fibrillation, the electrocardiogram is bizarre ( Figure 13-17 ) and ordinarily shows no tendency toward a regular rhythm of any type. During the first few seconds of ventricular fibrillation, relatively large masses of muscle contract simultaneously, and this causes coarse, irregular waves in the electrocardiogram. After another few seconds, the coarse contractions of the ventricles disappear, and the electrocardiogram changes into a new pattern of low-voltage, very irregular waves. Thus, no repetitive electrocardiographic pattern can be ascribed to ventricular fibrillation. Instead, the ventricular muscle contracts at as many as 30 to 50 small patches of muscle at a time, and electrocardiographic potentials change constantly and spasmodically because the electrical currents in the heart flow first in one direction and then in another and seldom repeat any specific cycle.

Figure 13-17 Ventricular fibrillation (lead II).
The voltages of the waves in the electrocardiogram in ventricular fibrillation are usually about 0.5 millivolt when ventricular fibrillation first begins, but they decay rapidly so that after 20 to 30 seconds, they are usually only 0.2 to 0.3 millivolt. Minute voltages of 0.1 millivolt or less may be recorded for 10 minutes or longer after ventricular fibrillation begins. As already pointed out, because no pumping of blood occurs during ventricular fibrillation, this state is lethal unless stopped by some heroic therapy, such as immediate electroshock through the heart, as explained in the next section.

Electroshock Defibrillation of the Ventricles
Although a moderate alternating-current voltage applied directly to the ventricles almost invariably throws the ventricles into fibrillation, a strong high-voltage alternating electrical current passed through the ventricles for a fraction of a second can stop fibrillation by throwing all the ventricular muscle into refractoriness simultaneously. This is accomplished by passing intense current through large electrodes placed on two sides of the heart. The current penetrates most of the fibers of the ventricles at the same time, thus stimulating essentially all parts of the ventricles simultaneously and causing them all to become refractory. All action potentials stop, and the heart remains quiescent for 3 to 5 seconds, after which it begins to beat again, usually with the sinus node or some other part of the heart becoming the pacemaker. However, the same re-entrant focus that had originally thrown the ventricles into fibrillation often is still present, in which case fibrillation may begin again immediately.
When electrodes are applied directly to the two sides of the heart, fibrillation can usually be stopped using 110 volts of 60-cycle alternating current applied for 0.1 second or 1000 volts of direct current applied for a few thousandths of a second. When applied through two electrodes on the chest wall, as shown in Figure 13-18 , the usual procedure is to charge a large electrical capacitor up to several thousand volts and then to cause the capacitor to discharge for a few thousandths of a second through the electrodes and through the heart.

Figure 13-18 Application of electrical current to the chest to stop ventricular fibrillation.

Hand Pumping of the Heart (Cardiopulmonary Resuscitation) as an Aid to Defibrillation
Unless defibrillated within 1 minute after fibrillation begins, the heart is usually too weak to be revived by defibrillation because of the lack of nutrition from coronary blood flow. However, it is still possible to revive the heart by preliminarily pumping the heart by hand (intermittent hand squeezing) and then defibrillating the heart later. In this way, small quantities of blood are delivered into the aorta and a renewed coronary blood supply develops. Then, after a few minutes of hand pumping, electrical defibrillation often becomes possible. Indeed, fibrillating hearts have been pumped by hand for as long as 90 minutes followed by successful defibrillation.
A technique for pumping the heart without opening the chest consists of intermittent thrusts of pressure on the chest wall along with artificial respiration. This, plus defibrillation, is called cardiopulmonary resuscitation, or CPR.
Lack of blood flow to the brain for more than 5 to 8 minutes usually causes permanent mental impairment or even destruction of brain tissue. Even if the heart is revived, the person may die from the effects of brain damage or may live with permanent mental impairment.

Atrial Fibrillation
Remember that except for the conducting pathway through the A-V bundle, the atrial muscle mass is separated from the ventricular muscle mass by fibrous tissue. Therefore, ventricular fibrillation often occurs without atrial fibrillation. Likewise, fibrillation often occurs in the atria without ventricular fibrillation (shown to the right in Figure 13-20 ).
The mechanism of atrial fibrillation is identical to that of ventricular fibrillation, except that the process occurs only in the atrial muscle mass instead of the ventricular mass. A frequent cause of atrial fibrillation is atrial enlargement resulting from heart valve lesions that prevent the atria from emptying adequately into the ventricles, or from ventricular failure with excess damming of blood in the atria. The dilated atrial walls provide ideal conditions of a long conductive pathway, as well as slow conduction, both of which predispose to atrial fibrillation.

Pumping Characteristics of the Atria during Atrial Fibrillation
For the same reasons that the ventricles will not pump blood during ventricular fibrillation, neither do the atria pump blood in atrial fibrillation. Therefore, the atria become useless as primer pumps for the ventricles. Even so, blood flows passively through the atria into the ventricles, and the efficiency of ventricular pumping is decreased only 20 to 30 percent. Therefore, in contrast to the lethality of ventricular fibrillation, a person can live for months or even years with atrial fibrillation, although at reduced efficiency of overall heart pumping.

Electrocardiogram in Atrial Fibrillation
Figure 13-19 shows the electrocardiogram during atrial fibrillation. Numerous small depolarization waves spread in all directions through the atria during atrial fibrillation. Because the waves are weak and many of them are of opposite polarity at any given time, they usually almost completely electrically neutralize one another. Therefore, in the electrocardiogram, one can see either no P waves from the atria or only a fine, high-frequency, very low voltage wavy record. Conversely, the QRS-T complexes are normal unless there is some pathology of the ventricles, but their timing is irregular, as explained next.

Figure 13-19 Atrial fibrillation (lead I). The waves that can be seen are ventricular QRS and T waves.

Irregularity of Ventricular Rhythm during Atrial Fibrillation
When the atria are fibrillating, impulses arrive from the atrial muscle at the A-V node rapidly but also irregularly. Because the A-V node will not pass a second impulse for about 0.35 second after a previous one, at least 0.35 second must elapse between one ventricular contraction and the next. Then an additional but variable interval of 0 to 0.6 second occurs before one of the irregular atrial fibrillatory impulses happens to arrive at the A-V node. Thus, the interval between successive ventricular contractions varies from a minimum of about 0.35 second to a maximum of about 0.95 second, causing a very irregular heartbeat. In fact, this irregularity, demonstrated by the variable spacing of the heartbeats in the electrocardiogram of Figure 13-19 , is one of the clinical findings used to diagnose the condition. Also, because of the rapid rate of the fibrillatory impulses in the atria, the ventricle is driven at a fast heart rate, usually between 125 and 150 beats per minute.

Electroshock Treatment of Atrial Fibrillation
In the same manner that ventricular fibrillation can be converted back to a normal rhythm by electroshock, so too can atrial fibrillation be converted by electroshock. The procedure is essentially the same as for ventricular fibrillation conversion—passage of a single strong electric shock through the heart, which throws the entire heart into refractoriness for a few seconds; a normal rhythm often follows if the heart is capable of this.

Atrial Flutter
Atrial flutter is another condition caused by a circus movement in the atria. It is different from atrial fibrillation, in that the electrical signal travels as a single large wave always in one direction around and around the atrial muscle mass, as shown to the left in Figure 13-20 . Atrial flutter causes a rapid rate of contraction of the atria, usually between 200 and 350 beats per minute. However, because one side of the atria is contracting while the other side is relaxing, the amount of blood pumped by the atria is slight. Furthermore, the signals reach the A-V node too rapidly for all of them to be passed into the ventricles, because the refractory periods of the A-V node and A-V bundle are too long to pass more than a fraction of the atrial signals. Therefore, there are usually two to three beats of the atria for every single beat of the ventricles.

Figure 13-20 Pathways of impulses in atrial flutter and atrial fibrillation.
Figure 13-21 shows a typical electrocardiogram in atrial flutter. The P waves are strong because of contraction of semicoordinate masses of muscle. However, note in the record that a QRS-T complex follows an atrial P wave only once for every two to three beats of the atria, giving a 2:1 or 3:1 rhythm.

Figure 13-21 Atrial flutter—2:1 and 3:1 atrial to ventricle rhythm (lead I).

Cardiac Arrest
A final serious abnormality of the cardiac rhythmicity-conduction system is cardiac arrest. This results from cessation of all electrical control signals in the heart. That is, no spontaneous rhythm remains.
Cardiac arrest may occur during deep anesthesia, when many patients develop severe hypoxia because of inadequate respiration. The hypoxia prevents the muscle fibers and conductive fibers from maintaining normal electrolyte concentration differentials across their membranes, and their excitability may be so affected that the automatic rhythmicity disappears.
In most instances of cardiac arrest from anesthesia, prolonged cardiopulmonary resuscitation (many minutes or even hours) is quite successful in re-establishing a normal heart rhythm. In some patients, severe myocardial disease can cause permanent or semipermanent cardiac arrest, which can cause death. To treat the condition, rhythmical electrical impulses from an implanted electronic cardiac pacemaker have been used successfully to keep patients alive for months to years.


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The Circulation
CHAPTER 14 Overview of the Circulation; Biophysics of Pressure, Flow, and Resistance
The function of the circulation is to service the needs of the body tissues—to transport nutrients to the body tissues, to transport waste products away, to transport hormones from one part of the body to another, and, in general, to maintain an appropriate environment in all the tissue fluids of the body for optimal survival and function of the cells.
The rate of blood flow through many tissues is controlled mainly in response to tissue need for nutrients. In some organs, such as the kidneys, the circulation serves additional functions. Blood flow to the kidney, for example, is far in excess of its metabolic requirements and is related to its excretory function, which demands that a large volume of blood be filtered each minute.
The heart and blood vessels, in turn, are controlled to provide the necessary cardiac output and arterial pressure to cause the needed tissue blood flow. What are the mechanisms for controlling blood volume and blood flow, and how does this relate to all the other functions of the circulation? These are some of the topics and questions that we discuss in this section on the circulation.

Physical Characteristics of the Circulation
The circulation, shown in Figure 14-1 , is divided into the systemic circulation and the pulmonary circulation. Because the systemic circulation supplies blood flow to all the tissues of the body except the lungs, it is also called the greater circulation or peripheral circulation.

Figure 14-1 Distribution of blood (in percentage of total blood) in the different parts of the circulatory system.

Functional Parts of the Circulation
Before discussing the details of circulatory function, it is important to understand the role of each part of the circulation.
The function of the arteries is to transport blood under high pressure to the tissues. For this reason, the arteries have strong vascular walls, and blood flows at a high velocity in the arteries.
The arterioles are the last small branches of the arterial system; they act as control conduits through which blood is released into the capillaries. Arterioles have strong muscular walls that can close the arterioles completely or can, by relaxing, dilate the vessels severalfold, thus having the capability of vastly altering blood flow in each tissue in response to its needs.
The function of the capillaries is to exchange fluid, nutrients, electrolytes, hormones, and other substances between the blood and the interstitial fluid. To serve this role, the capillary walls are very thin and have numerous minute capillary pores permeable to water and other small molecular substances.
The venules collect blood from the capillaries and gradually coalesce into progressively larger veins.
The veins function as conduits for transport of blood from the venules back to the heart; equally important, they serve as a major reservoir of extra blood. Because the pressure in the venous system is very low, the venous walls are thin. Even so, they are muscular enough to contract or expand and thereby act as a controllable reservoir for the extra blood, either a small or a large amount, depending on the needs of the circulation.

Volumes of Blood in the Different Parts of the Circulation
Figure 14-1 gives an overview of the circulation and lists the percentage of the total blood volume in major segments of the circulation. For instance, about 84 percent of the entire blood volume of the body is in the systemic circulation and 16 percent is in the heart and lungs. Of the 84 percent in the systemic circulation, 64 percent is in the veins, 13 percent in the arteries, and 7 percent in the systemic arterioles and capillaries. The heart contains 7 percent of the blood, and the pulmonary vessels, 9 percent.
Most surprising is the low blood volume in the capillaries. It is here, however, that the most important function of the circulation occurs, diffusion of substances back and forth between the blood and the tissues. This function is discussed in detail in Chapter 16 .

Cross-Sectional Areas and Velocities of Blood Flow
If all the systemic vessels of each type were put side by side, their approximate total cross-sectional areas for the average human being would be as follows:
Vessel Cross-Sectional Area (cm 2 ) Aorta 2.5 Small arteries 20 Arterioles 40 Capillaries 2500 Venules 250 Small veins 80 Venae cavae 8
Note particularly the much larger cross-sectional areas of the veins than of the arteries, averaging about four times those of the corresponding arteries. This explains the large blood storage capacity of the venous system in comparison with the arterial system.
Because the same volume of blood flow (F) must pass through each segment of the circulation each minute, the velocity of blood flow (v) is inversely proportional to vascular cross-sectional area (A):

Thus, under resting conditions, the velocity averages about 33 cm/sec in the aorta but only 1/1000 as rapidly in the capillaries, about 0.3 mm/sec. However, because the capillaries have a typical length of only 0.3 to 1 millimeter, the blood remains in the capillaries for only 1 to 3 seconds. This short time is surprising because all diffusion of nutrient food substances and electrolytes that occurs through the capillary walls must do so in this short time.

Pressures in the Various Portions of the Circulation
Because the heart pumps blood continually into the aorta, the mean pressure in the aorta is high, averaging about 100 mm Hg. Also, because heart pumping is pulsatile, the arterial pressure alternates between a systolic pressure level of 120 mm Hg and a diastolic pressure level of 80 mm Hg, as shown on the left side of Figure 14-2 .

Figure 14-2 Normal blood pressures in the different portions of the circulatory system when a person is lying in the horizontal position.
As the blood flows through the systemic circulation, its mean pressure falls progressively to about 0 mm Hg by the time it reaches the termination of the venae cavae where they empty into the right atrium of the heart.
The pressure in the systemic capillaries varies from as high as 35 mm Hg near the arteriolar ends to as low as 10 mm Hg near the venous ends, but their average “functional” pressure in most vascular beds is about 17 mm Hg, a pressure low enough that little of the plasma leaks through the minute pores of the capillary walls, even though nutrients can diffuse easily through these same pores to the outlying tissue cells.
Note at the far right side of Figure 14-2 the respective pressures in the different parts of the pulmonary circulation. In the pulmonary arteries, the pressure is pulsatile, just as in the aorta, but the pressure is far less: pulmonary artery systolic pressure averages about 25 mm Hg and diastolic pressure 8 mm Hg, with a mean pulmonary arterial pressure of only 16 mm Hg. The mean pulmonary capillary pressure averages only 7 mm Hg. Yet the total blood flow through the lungs each minute is the same as through the systemic circulation. The low pressures of the pulmonary system are in accord with the needs of the lungs because all that is required is to expose the blood in the pulmonary capillaries to oxygen and other gases in the pulmonary alveoli.

Basic Principles of Circulatory Function
Although the details of circulatory function are complex, there are three basic principles that underlie all functions of the system.
1. The rate of blood flow to each tissue of the body is almost always precisely controlled in relation to the tissue need. When tissues are active, they need a greatly increased supply of nutrients and therefore much more blood flow than when at rest—occasionally as much as 20 to 30 times the resting level. Yet the heart normally cannot increase its cardiac output more than four to seven times greater than resting levels. Therefore, it is not possible simply to increase blood flow everywhere in the body when a particular tissue demands increased flow. Instead, the microvessels of each tissue continuously monitor tissue needs, such as the availability of oxygen and other nutrients and the accumulation of carbon dioxide and other tissue waste products, and these in turn act directly on the local blood vessels, dilating or constricting them, to control local blood flow precisely to that level required for the tissue activity. Also, nervous control of the circulation from the central nervous system and hormones provide additional help in controlling tissue blood flow.
2. The cardiac output is controlled mainly by the sum of all the local tissue flows. When blood flows through a tissue, it immediately returns by way of the veins to the heart. The heart responds automatically to this increased inflow of blood by pumping it immediately back into the arteries. Thus, the heart acts as an automaton, responding to the demands of the tissues. The heart, however, often needs help in the form of special nerve signals to make it pump the required amounts of blood flow.
3. Arterial pressure regulation is generally independent of either local blood flow control or cardiac output control. The circulatory system is provided with an extensive system for controlling the arterial blood pressure. For instance, if at any time the pressure falls significantly below the normal level of about 100 mm Hg, within seconds a barrage of nervous reflexes elicits a series of circulatory changes to raise the pressure back toward normal. The nervous signals especially (a) increase the force of heart pumping, (b) cause contraction of the large venous reservoirs to provide more blood to the heart, and (c) cause generalized constriction of most of the arterioles throughout the body so that more blood accumulates in the large arteries to increase the arterial pressure. Then, over more prolonged periods, hours and days, the kidneys play an additional major role in pressure control both by secreting pressure-controlling hormones and by regulating the blood volume.
Thus, in summary, the needs of the individual tissues are served specifically by the circulation. In the remainder of this chapter, we begin to discuss the basic details of the management of tissue blood flow and control of cardiac output and arterial pressure.

Interrelationships of Pressure, Flow, and Resistance
Blood flow through a blood vessel is determined by two factors: (1) pressure difference of the blood between the two ends of the vessel, also sometimes called “pressure gradient” along the vessel, which is the force that pushes the blood through the vessel, and (2) the impediment to blood flow through the vessel, which is called vascular resistance. Figure 14-3 demonstrates these relationships, showing a blood vessel segment located anywhere in the circulatory system.

Figure 14-3 Interrelationships of pressure, resistance, and blood flow.
P 1 represents the pressure at the origin of the vessel; at the other end, the pressure is P 2 . Resistance occurs as a result of friction between the flowing blood and the intravascular endothelium all along the inside of the vessel. The flow through the vessel can be calculated by the following formula, which is called Ohm’s law :

in which F is blood flow, P is the pressure difference (P 1 − P 2 ) between the two ends of the vessel, and R is the resistance. This formula states that the blood flow is directly proportional to the pressure difference but inversely proportional to the resistance.
Note that it is the difference in pressure between the two ends of the vessel, not the absolute pressure in the vessel, that determines rate of flow. For example, if the pressure at both ends of a vessel is 100 mm Hg and yet no difference exists between the two ends, there will be no flow despite the presence of 100 mm Hg pressure.
Ohm’s law, illustrated in Equation 1, expresses the most important of all the relations that the reader needs to understand to comprehend the hemodynamics of the circulation. Because of the extreme importance of this formula, the reader should also become familiar with its other algebraic forms:

Blood Flow
Blood flow means the quantity of blood that passes a given point in the circulation in a given period of time. Ordinarily, blood flow is expressed in milliliters per minute or liters per minute, but it can be expressed in milliliters per second or in any other units of flow and time.
The overall blood flow in the total circulation of an adult person at rest is about 5000 ml/min. This is called the cardiac output because it is the amount of blood pumped into the aorta by the heart each minute.

Methods for Measuring Blood Flow
Many mechanical and mechanoelectrical devices can be inserted in series with a blood vessel or, in some instances, applied to the outside of the vessel to measure flow. They are called flowmeters.

Electromagnetic Flowmeter
One of the most important devices for measuring blood flow without opening the vessel is the electromagnetic flowmeter, the principles of which are illustrated in Figure 14-4 . Figure 14-4 A shows the generation of electromotive force (electrical voltage) in a wire that is moved rapidly in a cross-wise direction through a magnetic field. This is the well-known principle for production of electricity by the electric generator. Figure 14-4 B shows that the same principle applies for generation of electromotive force in blood that is moving through a magnetic field. In this case, a blood vessel is placed between the poles of a strong magnet, and electrodes are placed on the two sides of the vessel perpendicular to the magnetic lines of force. When blood flows through the vessel, an electrical voltage proportional to the rate of blood flow is generated between the two electrodes, and this is recorded using an appropriate voltmeter or electronic recording apparatus. Figure 14-4 C shows an actual “probe” that is placed on a large blood vessel to record its blood flow. The probe contains both the strong magnet and the electrodes.

Figure 14-4 Flowmeter of the electromagnetic type, showing generation of an electrical voltage in a wire as it passes through an electromagnetic field ( A ); generation of an electrical voltage in electrodes on a blood vessel when the vessel is placed in a strong magnetic field and blood flows through the vessel ( B ); and a modern electromagnetic flowmeter probe for chronic implantation around blood vessels ( C ).
A special advantage of the electromagnetic flowmeter is that it can record changes in flow in less than 1/100 of a second, allowing accurate recording of pulsatile changes in flow, as well as steady flow.

Ultrasonic Doppler Flowmeter
Another type of flowmeter that can be applied to the outside of the vessel and that has many of the same advantages as the electromagnetic flowmeter is the ultrasonic Doppler flowmeter, shown in Figure 14-5 . A minute piezoelectric crystal is mounted at one end in the wall of the device. This crystal, when energized with an appropriate electronic apparatus, transmits ultrasound at a frequency of several hundred thousand cycles per second downstream along the flowing blood. A portion of the sound is reflected by the red blood cells in the flowing blood. The reflected ultrasound waves then travel backward from the blood cells toward the crystal. These reflected waves have a lower frequency than the transmitted wave because the red cells are moving away from the transmitter crystal. This is called the Doppler effect. (It is the same effect that one experiences when a train approaches and passes by while blowing its whistle. Once the whistle has passed by the person, the pitch of the sound from the whistle suddenly becomes much lower than when the train is approaching.)

Figure 14-5 Ultrasonic Doppler flowmeter.
For the flowmeter shown in Figure 14-5 , the high-frequency ultrasound wave is intermittently cut off, and the reflected wave is received back onto the crystal and amplified greatly by the electronic apparatus. Another portion of the electronic apparatus determines the frequency difference between the transmitted wave and the reflected wave, thus determining the velocity of blood flow. As long as diameter of a blood vessel does not change, changes in blood flow in the vessel are directly related to changes in flow velocity.
Like the electromagnetic flowmeter, the ultrasonic Doppler flowmeter is capable of recording rapid, pulsatile changes in flow, as well as steady flow.

Laminar Flow of Blood in Vessels
When blood flows at a steady rate through a long, smooth blood vessel, it flows in streamlines, with each layer of blood remaining the same distance from the vessel wall. Also, the central-most portion of the blood stays in the center of the vessel. This type of flow is called laminar flow or streamline flow, and it is the opposite of turbulent flow, which is blood flowing in all directions in the vessel and continually mixing within the vessel, as discussed subsequently.

Parabolic Velocity Profile during Laminar Flow
When laminar flow occurs, the velocity of flow in the center of the vessel is far greater than that toward the outer edges. This is demonstrated in Figure 14-6 . In Figure 14-6 A , a vessel contains two fluids, the one at the left colored by a dye and the one at the right a clear fluid, but there is no flow in the vessel. When the fluids are made to flow, a parabolic interface develops between them, as shown 1 second later in Figure 14-6 B ; the portion of fluid adjacent to the vessel wall has hardly moved, the portion slightly away from the wall has moved a small distance, and the portion in the center of the vessel has moved a long distance. This effect is called the “parabolic profile for velocity of blood flow.”

Figure 14-6 A , Two fluids (one dyed red, and the other clear) before flow begins; B , the same fluids 1 second after flow begins; C , turbulent flow, with elements of the fluid moving in a disorderly pattern.
The cause of the parabolic profile is the following: The fluid molecules touching the wall move slowly because of adherence to the vessel wall. The next layer of molecules slips over these, the third layer over the second, the fourth layer over the third, and so forth. Therefore, the fluid in the middle of the vessel can move rapidly because many layers of slipping molecules exist between the middle of the vessel and the vessel wall; thus, each layer toward the center flows progressively more rapidly than the outer layers.

Turbulent Flow of Blood under Some Conditions
When the rate of blood flow becomes too great, when it passes by an obstruction in a vessel, when it makes a sharp turn, or when it passes over a rough surface, the flow may then become turbulent, or disorderly, rather than streamlined (see Figure 14-6 C ). Turbulent flow means that the blood flows crosswise in the vessel and along the vessel, usually forming whorls in the blood, called eddy currents. These are similar to the whirlpools that one frequently sees in a rapidly flowing river at a point of obstruction.
When eddy currents are present, the blood flows with much greater resistance than when the flow is streamlined, because eddies add tremendously to the overall friction of flow in the vessel.
The tendency for turbulent flow increases in direct proportion to the velocity of blood flow, the diameter of the blood vessel, and the density of the blood and is inversely proportional to the viscosity of the blood, in accordance with the following equation:

where Re is Reynolds’ number and is the measure of the tendency for turbulence to occur, ν is the mean velocity of blood flow (in centimeters/second), d is the vessel diameter (in centimeters), ρ is density, and η is the viscosity (in poise). The viscosity of blood is normally about 1/30 poise, and the density is only slightly greater than 1. When Reynolds’ number rises above 200 to 400, turbulent flow will occur at some branches of vessels but will die out along the smooth portions of the vessels. However, when Reynolds’ number rises above approximately 2000, turbulence will usually occur even in a straight, smooth vessel.
Reynolds’ number for flow in the vascular system even normally rises to 200 to 400 in large arteries; as a result there is almost always some turbulence of flow at the branches of these vessels. In the proximal portions of the aorta and pulmonary artery, Reynolds’ number can rise to several thousand during the rapid phase of ejection by the ventricles; this causes considerable turbulence in the proximal aorta and pulmonary artery where many conditions are appropriate for turbulence: (1) high velocity of blood flow, (2) pulsatile nature of the flow, (3) sudden change in vessel diameter, and (4) large vessel diameter. However, in small vessels, Reynolds’ number is almost never high enough to cause turbulence.

Blood Pressure

Standard Units of Pressure
Blood pressure almost always is measured in millimeters of mercury (mm Hg) because the mercury manometer has been used as the standard reference for measuring pressure since its invention in 1846 by Poiseuille. Actually, blood pressure means the force exerted by the blood against any unit area of the vessel wall . When one says that the pressure in a vessel is 50 mm Hg, this means that the force exerted is sufficient to push a column of mercury against gravity up to a level 50 millimeters high. If the pressure is 100 mm Hg, it will push the column of mercury up to 100 millimeters.
Occasionally, pressure is measured in centimeters of water (cm H 2 O) . A pressure of 10 cm H 2 O means a pressure sufficient to raise a column of water against gravity to a height of 10 centimeters. One millimeter of mercury pressure equals 1.36 cm water pressure because the specific gravity of mercury is 13.6 times that of water, and 1 centimeter is 10 times as great as 1 millimeter.

High-Fidelity Methods for Measuring Blood Pressure
The mercury in a manometer has so much inertia that it cannot rise and fall rapidly. For this reason, the mercury manometer, although excellent for recording steady pressures, cannot respond to pressure changes that occur more rapidly than about one cycle every 2 to 3 seconds. Whenever it is desired to record rapidly changing pressures, some other type of pressure recorder is necessary. Figure 14-7 demonstrates the basic principles of three electronic pressure transducers commonly used for converting blood pressure and/or rapid changes in pressure into electrical signals and then recording the electrical signals on a high-speed electrical recorder. Each of these transducers uses a very thin, highly stretched metal membrane that forms one wall of the fluid chamber. The fluid chamber in turn is connected through a needle or catheter inserted into the blood vessel in which the pressure is to be measured. When the pressure is high, the membrane bulges slightly, and when it is low, it returns toward its resting position.

Figure 14-7 Principles of three types of electronic transducers for recording rapidly changing blood pressures (explained in the text).
In Figure 14-7 A , a simple metal plate is placed a few hundredths of a centimeter above the membrane. When the membrane bulges, the membrane comes closer to the plate, which increases the electrical capacitance between these two, and this change in capacitance can be recorded using an appropriate electronic system.
In Figure 14-7 B , a small iron slug rests on the membrane, and this can be displaced upward into a center space inside an electrical wire coil. Movement of the iron into the coil increases the inductance of the coil, and this, too, can be recorded electronically.
Finally, in Figure 14-7 C , a very thin, stretched resistance wire is connected to the membrane. When this wire is stretched greatly, its resistance increases; when it is stretched less, its resistance decreases. These changes, too, can be recorded by an electronic system.
The electrical signals from the transducer are sent to an amplifier and then to an appropriate recording device. With some of these high-fidelity types of recording systems, pressure cycles up to 500 cycles per second have been recorded accurately. In common use are recorders capable of registering pressure changes that occur as rapidly as 20 to 100 cycles per second, in the manner shown on the recording paper in Figure 14-7 C .

Resistance to Blood Flow

Units of Resistance
Resistance is the impediment to blood flow in a vessel, but it cannot be measured by any direct means. Instead, resistance must be calculated from measurements of blood flow and pressure difference between two points in the vessel. If the pressure difference between two points is 1 mm Hg and the flow is 1 ml/sec, the resistance is said to be 1 peripheral resistance unit, usually abbreviated PRU .

Expression of Resistance in CGS Units
Occasionally, a basic physical unit called the CGS (centimeters, grams, seconds) unit is used to express resistance. This unit is dyne sec/cm 5 . Resistance in these units can be calculated by the following formula:

Total Peripheral Vascular Resistance and Total Pulmonary Vascular Resistance
The rate of blood flow through the entire circulatory system is equal to the rate of blood pumping by the heart—that is, it is equal to the cardiac output. In the adult human being, this is approximately 100 ml/sec. The pressure difference from the systemic arteries to the systemic veins is about 100 mm Hg. Therefore, the resistance of the entire systemic circulation, called the total peripheral resistance, is about 100/100, or 1 peripheral resistance unit (PRU).
In conditions in which all the blood vessels throughout the body become strongly constricted, the total peripheral resistance occasionally rises to as high as 4 PRU. Conversely, when the vessels become greatly dilated, the resistance can fall to as little as 0.2 PRU.
In the pulmonary system, the mean pulmonary arterial pressure averages 16 mm Hg and the mean left atrial pressure averages 2 mm Hg, giving a net pressure difference of 14 mm. Therefore, when the cardiac output is normal at about 100 ml/sec, the total pulmonary vascular resistance calculates to be about 0.14 PRU (about one seventh that in the systemic circulation).

“Conductance” of Blood in a Vessel and Its Relation to Resistance
Conductance is a measure of the blood flow through a vessel for a given pressure difference. This is generally expressed in terms of milliliters per second per millimeter of mercury pressure, but it can also be expressed in terms of liters per second per millimeter of mercury or in any other units of blood flow and pressure.
It is evident that conductance is the exact reciprocal of resistance in accord with the following equation:

Very Slight Changes in Diameter of a Vessel Can Change Its Conductance Tremendously!
Slight changes in the diameter of a vessel cause tremendous changes in the vessel’s ability to conduct blood when the blood flow is streamlined. This is demonstrated by the experiment illustrated in Figure 14-8 A , which shows three vessels with relative diameters of 1, 2, and 4 but with the same pressure difference of 100 mm Hg between the two ends of the vessels. Although the diameters of these vessels increase only fourfold, the respective flows are 1, 16, and 256 ml/min, which is a 256-fold increase in flow. Thus, the conductance of the vessel increases in proportion to the fourth power of the diameter, in accordance with the following formula:

Figure 14-8 A , Demonstration of the effect of vessel diameter on blood flow. B , Concentric rings of blood flowing at different velocities; the farther away from the vessel wall, the faster the flow.

Poiseuille’s Law
The cause of this great increase in conductance when the diameter increases can be explained by referring to Figure 14-8 B , which shows cross sections of a large and a small vessel. The concentric rings inside the vessels indicate that the velocity of flow in each ring is different from that in the adjacent rings because of laminar flow, which was discussed earlier in the chapter. That is, the blood in the ring touching the wall of the vessel is barely flowing because of its adherence to the vascular endothelium. The next ring of blood toward the center of the vessel slips past the first ring and, therefore, flows more rapidly. The third, fourth, fifth, and sixth rings likewise flow at progressively increasing velocities. Thus, the blood that is near the wall of the vessel flows slowly, whereas that in the middle of the vessel flows much more rapidly.
In the small vessel, essentially all the blood is near the wall, so the extremely rapidly flowing central stream of blood simply does not exist. By integrating the velocities of all the concentric rings of flowing blood and multiplying them by the areas of the rings, one can derive the following formula, known as Poiseuille’s law:

in which F is the rate of blood flow, P is the pressure difference between the ends of the vessel, r is the radius of the vessel, l is length of the vessel, and η is viscosity of the blood.
Note particularly in this equation that the rate of blood flow is directly proportional to the fourth power of the radius of the vessel, which demonstrates once again that the diameter of a blood vessel (which is equal to twice the radius) plays by far the greatest role of all factors in determining the rate of blood flow through a vessel.

Importance of the Vessel Diameter “Fourth Power Law” in Determining Arteriolar Resistance
In the systemic circulation, about two thirds of the total systemic resistance to blood flow is arteriolar resistance in the small arterioles. The internal diameters of the arterioles range from as little as 4 micrometers to as great as 25 micrometers. However, their strong vascular walls allow the internal diameters to change tremendously, often as much as fourfold. From the fourth power law discussed earlier that relates blood flow to diameter of the vessel, one can see that a fourfold increase in vessel diameter can increase the flow as much as 256-fold. Thus, this fourth power law makes it possible for the arterioles, responding with only small changes in diameter to nervous signals or local tissue chemical signals, either to turn off almost completely the blood flow to the tissue or at the other extreme to cause a vast increase in flow. Indeed, ranges of blood flow of more than 100-fold in separate tissue areas have been recorded between the limits of maximum arteriolar constriction and maximum arteriolar dilatation.

Resistance to Blood Flow in Series and Parallel Vascular Circuits
Blood pumped by the heart flows from the high-pressure part of the systemic circulation (i.e., aorta) to the low-pressure side (i.e., vena cava) through many miles of blood vessels arranged in series and in parallel. The arteries, arterioles, capillaries, venules, and veins are collectively arranged in series. When blood vessels are arranged in series, flow through each blood vessel is the same and the total resistance to blood flow (R total ) is equal to the sum of the resistances of each vessel:

The total peripheral vascular resistance is therefore equal to the sum of resistances of the arteries, arterioles, capillaries, venules, and veins. In the example shown in Figure 14-9 A , the total vascular resistance is equal to the sum of R 1 and R 2 .

Figure 14-9 Vascular resistances: A, in series and B , in parallel.
Blood vessels branch extensively to form parallel circuits that supply blood to the many organs and tissues of the body. This parallel arrangement permits each tissue to regulate its own blood flow, to a great extent, independently of flow to other tissues.
For blood vessels arranged in parallel ( Figure 14-9 B ), the total resistance to blood flow is expressed as:

It is obvious that for a given pressure gradient, far greater amounts of blood will flow through this parallel system than through any of the individual blood vessels. Therefore, the total resistance is far less than the resistance of any single blood vessel. Flow through each of the parallel vessels in Figure 14-9 B is determined by the pressure gradient and its own resistance, not the resistance of the other parallel blood vessels. However, increasing the resistance of any of the blood vessels increases the total vascular resistance.
It may seem paradoxical that adding more blood vessels to a circuit reduces the total vascular resistance. Many parallel blood vessels, however, make it easier for blood to flow through the circuit because each parallel vessel provides another pathway, or conductance, for blood flow. The total conductance (C total ) for blood flow is the sum of the conductance of each parallel pathway:

For example, brain, kidney, muscle, gastrointestinal, skin, and coronary circulations are arranged in parallel, and each tissue contributes to the overall conductance of the systemic circulation. Blood flow through each tissue is a fraction of the total blood flow (cardiac output) and is determined by the resistance (the reciprocal of conductance) for blood flow in the tissue, as well as the pressure gradient. Therefore, amputation of a limb or surgical removal of a kidney also removes a parallel circuit and reduces the total vascular conductance and total blood flow (i.e., cardiac output) while increasing total peripheral vascular resistance.

Effect of Blood Hematocrit and Blood Viscosity on Vascular Resistance and Blood Flow
Note especially that another of the important factors in Poiseuille’s equation is the viscosity of the blood. The greater the viscosity, the less the flow in a vessel if all other factors are constant. Furthermore, the viscosity of normal blood is about three times as great as the viscosity of water.
But what makes the blood so viscous? It is mainly the large numbers of suspended red cells in the blood, each of which exerts frictional drag against adjacent cells and against the wall of the blood vessel.

The proportion of the blood that is red blood cells is called the hematocrit. Thus, if a person has a hematocrit of 40, this means that 40 percent of the blood volume is cells and the remainder is plasma. The hematocrit of adult men averages about 42, while that of women averages about 38. These values vary tremendously, depending on whether the person has anemia, on the degree of bodily activity, and on the altitude at which the person resides. These changes in hematocrit are discussed in relation to the red blood cells and their oxygen transport function in Chapter 32 .
Hematocrit is determined by centrifuging blood in a calibrated tube, as shown in Figure 14-10 . The calibration allows direct reading of the percentage of cells.

Figure 14-10 Hematocrits in a healthy (normal) person and in patients with anemia and polycythemia.

Effect of Hematocrit on Blood Viscosity
The viscosity of blood increases drastically as the hematocrit increases, as shown in Figure 14-11 . The viscosity of whole blood at normal hematocrit is about 3; this means that three times as much pressure is required to force whole blood as to force water through the same blood vessel. When the hematocrit rises to 60 or 70, which it often does in polycythemia, the blood viscosity can become as great as 10 times that of water, and its flow through blood vessels is greatly retarded.

Figure 14-11 Effect of hematocrit on blood viscosity. (Water viscosity = 1.)
Other factors that affect blood viscosity are the plasma protein concentration and types of proteins in the plasma, but these effects are so much less than the effect of hematocrit that they are not significant considerations in most hemodynamic studies. The viscosity of blood plasma is about 1.5 times that of water.

Effects of Pressure on Vascular Resistance and Tissue Blood Flow

“Autoregulation” Attenuates the Effect of Arterial Pressure on Tissue Blood Flow
From the discussion thus far, one might expect an increase in arterial pressure to cause a proportionate increase in blood flow through the various tissues of the body. However, the effect of arterial pressure on blood flow in many tissues is usually far less than one would expect, as shown in Figure 14-12 . The reason for this is that an increase in arterial pressure not only increases the force that pushes blood through the vessels but it also initiates compensatory increases in vascular resistance within a few seconds through activation of the local control mechanisms discussed in Chapter 17 . Conversely, with reductions in arterial pressure most vascular resistance is promptly reduced in most tissues and blood flow is maintained relatively constant. The ability of each tissue to adjust its vascular resistance and to maintain normal blood flow during changes in arterial pressure between approximately 70 and 175 mm Hg is called blood flow autoregulation.

Figure 14-12 Effect of changes in arterial pressure over a period of several minutes on blood flow in a tissue such as skeletal muscle. Note that between pressure of 70 and 175 mm Hg blood flow is “autoregulated.” The blue line shows the effect of sympathetic nerve stimulation or vasoconstriction by hormones such as norepinephrine, angiotensin II, vasopressin, or endothelin on this relationship. Reduced tissue blood flow is rarely maintained for more than a few hours due to activation of local autoregulatory mechanisms that eventually return blood flow toward normal.
Note in Figure 14-12 that changes in blood flow can be caused by strong sympathetic stimulation, which constricts the blood vessels. Likewise, hormonal vasoconstrictors, such as norepinephrine, angiotensin II, vasopressin, or endothelin, can also reduce blood flow, at least transiently.
Changes in tissue blood flow rarely last for more than a few hours even when increases in arterial pressure or increased levels of vasoconstrictors are sustained. The reason for the relative constancy of blood flow is that each tissue’s local autoregulatory mechanisms eventually override most of the effects of vasoconstrictors in order to provide a blood flow that is appropriate for the needs of the tissue.

Pressure-Flow Relationship in Passive Vascular Beds
In isolated blood vessels or in tissues that do not exhibit autoregulation, changes in arterial pressure may have important effects on blood flow. In fact, the effect of pressure on blood flow may be greater than predicted by Poiseuille’s equation, as shown by the upward curving lines in Figure 14-13 . The reason for this is that increased arterial pressure not only increases the force that pushes blood through the vessels but it also distends the elastic vessels, actually decreasing vascular resistance. Conversely, decreased arterial pressure in passive blood vessels increases resistance as the elastic vessels gradually collapse due to reduced distending pressure. When pressure falls below a critical level, called the critical closing pressure, flow ceases as the blood vessels are completely collapsed.

Figure 14-13 Effect of arterial pressure on blood flow through a passive blood vessel at different degrees of vascular tone caused by increased or decreased sympathetic stimulation of the vessel.
Sympathetic stimulation and other vasoconstrictors can alter the passive pressure-flow relationship shown in Figure 14-13 . Thus, inhibition of sympathetic activity g reatly dilates the vessels and can increase the blood flow twofold or more. Conversely, very strong sympathetic stimulation can constrict the vessels so much that blood flow occasionally decreases to as low as zero for a few seconds despite high arterial pressure.
In reality, there are few physiological conditions in which tissues display the passive pressure-flow relationship shown in Figure 14-13 . Even in tissues that do not effectively autoregulate blood flow during acute changes in arterial pressure, blood flow is regulated according to the needs of the tissue when the pressure changes are sustained, as discussed in Chapter 17 .


See bibliography for Chapter 15 .
CHAPTER 15 Vascular Distensibility and Functions of the Arterial and Venous Systems

Vascular Distensibility
A valuable characteristic of the vascular system is that all blood vessels are distensible. The distensible nature of the arteries allows them to accommodate the pulsatile output of the heart and to average out the pressure pulsations. This provides smooth, continuous flow of blood through the very small blood vessels of the tissues.
The most distensible by far of all the vessels are the veins. Even slight increases in venous pressure cause the veins to store 0.5 to 1.0 liter of extra blood. Therefore, the veins provide a reservoir function for storing large quantities of extra blood that can be called into use whenever required elsewhere in the circulation.

Units of Vascular Distensibility
Vascular distensibility normally is expressed as the fractional increase in volume for each millimeter of mercury rise in pressure, in accordance with the following formula:
That is, if 1 mm Hg causes a vessel that originally contained 10 millimeters of blood to increase its volume by 1 milliliter, the distensibility would be 0.1 per mm Hg, or 10 percent per mm Hg.

Difference in Distensibility of the Arteries and the Veins
Anatomically, the walls of the arteries are far stronger than those of the veins. Consequently, the veins, on average, are about eight times more distensible than the arteries. That is, a given increase in pressure causes about eight times as much increase in blood in a vein as in an artery of comparable size.
In the pulmonary circulation, the pulmonary vein distensibilities are similar to those of the systemic circulation. But the pulmonary arteries normally operate under pressures about one sixth of those in the systemic arterial system, and their distensibilities are correspondingly greater, about six times the distensibility of systemic arteries.

Vascular Compliance (or Vascular Capacitance)
In hemodynamic studies, it usually is much more important to know the total quantity of blood that can be stored in a given portion of the circulation for each mm Hg pressure rise than to know the distensibilities of the individual vessels. This value is called the compliance or capacitance of the respective vascular bed; that is,
Compliance and distensibility are quite different. A highly distensible vessel that has a slight volume may have far less compliance than a much less distensible vessel that has a large volume because compliance is equal to distensibility times volume.
The compliance of a systemic vein is about 24 times that of its corresponding artery because it is about 8 times as distensible and it has a volume about 3 times as great (8 × 3 = 24).

Volume-Pressure Curves of the Arterial and Venous Circulations
A convenient method for expressing the relation of pressure to volume in a vessel or in any portion of the circulation is to use the so-called volume-pressure curve. The red and blue solid curves in Figure 15-1 represent, respectively, the volume-pressure curves of the normal systemic arterial system and venous system, showing that when the arterial system of the average adult person (including all the large arteries, small arteries, and arterioles) is filled with about 700 milliliters of blood, the mean arterial pressure is 100 mm Hg, but when it is filled with only 400 milliliters of blood, the pressure falls to zero.

Figure 15-1 “Volume-pressure curves” of the systemic arterial and venous systems, showing the effects of stimulation or inhibition of the sympathetic nerves to the circulatory system.
In the entire systemic venous system, the volume normally ranges from 2000 to 3500 milliliters, and a change of several hundred millimeters in this volume is required to change the venous pressure only 3 to 5 mm Hg. This mainly explains why as much as one half liter of blood can be transfused into a healthy person in only a few minutes without greatly altering function of the circulation.

Effect of Sympathetic Stimulation or Sympathetic Inhibition on the Volume-Pressure Relations of the Arterial and Venous Systems
Also shown in Figure 15-1 are the effects of exciting or inhibiting the vascular sympathetic nerves on the volume-pressure curves. It is evident that increase in vascular smooth muscle tone caused by sympathetic stimulation increases the pressure at each volume of the arteries or veins, whereas sympathetic inhibition decreases the pressure at each volume. Control of the vessels in this manner by the sympathetics is a valuable means for diminishing the dimensions of one segment of the circulation, thus transferring blood to other segments. For instance, an increase in vascular tone throughout the systemic circulation often causes large volumes of blood to shift into the heart, which is one of the principal methods that the body uses to increase heart pumping.
Sympathetic control of vascular capacitance is also highly important during hemorrhage. Enhancement of sympathetic tone, especially to the veins, reduces the vessel sizes enough that the circulation continues to operate almost normally even when as much as 25 percent of the total blood volume has been lost.

Delayed Compliance (Stress-Relaxation) of Vessels
The term “delayed compliance” means that a vessel exposed to increased volume at first exhibits a large increase in pressure, but progressive delayed stretching of smooth muscle in the vessel wall allows the pressure to return back toward normal over a period of minutes to hours. This effect is shown in Figure 15-2 . In this figure, the pressure is recorded in a small segment of a vein that is occluded at both ends. An extra volume of blood is suddenly injected until the pressure rises from 5 to 12 mm Hg. Even though none of the blood is removed after it is injected, the pressure begins to decrease immediately and approaches about 9 mm Hg after several minutes. In other words, the volume of blood injected causes immediate elastic distention of the vein, but then the smooth muscle fibers of the vein begin to “creep” to longer lengths, and their tensions correspondingly decrease. This effect is a characteristic of all smooth muscle tissue and is called stress-relaxation, which was explained in Chapter 8 .

Figure 15-2 Effect on the intravascular pressure of injecting a volume of blood into a venous segment and later removing the excess blood, demonstrating the principle of delayed compliance.
Delayed compliance is a valuable mechanism by which the circulation can accommodate extra blood when necessary, such as after too large a transfusion. Delayed compliance in the reverse direction is one of the ways in which the circulation automatically adjusts itself over a period of minutes or hours to diminished blood volume after serious hemorrhage.

Arterial Pressure Pulsations
With each beat of the heart a new surge of blood fills the arteries. Were it not for distensibility of the arterial system, all of this new blood would have to flow through the peripheral blood vessels almost instantaneously, only during cardiac systole, and no flow would occur during diastole. However, the compliance of the arterial tree normally reduces the pressure pulsations to almost no pulsations by the time the blood reaches the capillaries; therefore, tissue blood flow is mainly continuous with very little pulsation.
A typical record of the pressure pulsations at the root of the aorta is shown in Figure 15-3 . In the healthy young adult, the pressure at the top of each pulse, called the systolic pressure, is about 120 mm Hg. At the lowest point of each pulse, called the diastolic pressure, it is about 80 mm Hg. The difference between these two pressures, about 40 mm Hg, is called the pulse pressure.

Figure 15-3 Pressure pulse contour in the ascending aorta.
Two major factors affect the pulse pressure: (1) the stroke volume output of the heart and (2) the compliance (total distensibility) of the arterial tree. A third, less important factor, is the character of ejection from the heart during systole.
In general, the greater the stroke volume output, the greater the amount of blood that must be accommodated in the arterial tree with each heartbeat, and, therefore, the greater the pressure rise and fall during systole and diastole, thus causing a greater pulse pressure. Conversely, the less the compliance of the arterial system, the greater the rise in pressure for a given stroke volume of blood pumped into the arteries. For instance, as demonstrated by the middle top curves in Figure 15-4 , the pulse pressure in old age sometimes rises to as much as twice normal, because the arteries have become hardened with arteriosclerosis and therefore are relatively noncompliant.

Figure 15-4 Aortic pressure pulse contours in arteriosclerosis, aortic stenosis, patent ductus arteriosus, and aortic regurgitation.
In effect, pulse pressure is determined approximately by the ratio of stroke volume output to compliance of the arterial tree . Any condition of the circulation that affects either of these two factors also affects the pulse pressure:

Abnormal Pressure Pulse Contours
Some conditions of the circulation also cause abnormal contours of the pressure pulse wave in addition to altering the pulse pressure. Especially distinctive among these are aortic stenosis, patent ductus arteriosus, and aortic regurgitation, each of which is shown in Figure 15-4 .
In aortic valve stenosis , the diameter of the aortic valve opening is reduced significantly, and the aortic pressure pulse is decreased significantly because of diminished blood flow outward through the stenotic valve.
In patent ductus arteriosus, one half or more of the blood pumped into the aorta by the left ventricle flows immediately backward through the wide-open ductus into the pulmonary artery and lung blood vessels, thus allowing the diastolic pressure to fall very low before the next heartbeat.
In aortic regurgitation, the aortic valve is absent or will not close completely. Therefore, after each heartbeat, the blood that has just been pumped into the aorta flows immediately backward into the left ventricle. As a result, the aortic pressure can fall all the way to zero between heartbeats. Also, there is no incisura in the aortic pulse contour because there is no aortic valve to close.

Transmission of Pressure Pulses to the Peripheral Arteries
When the heart ejects blood into the aorta during systole, at first only the proximal portion of the aorta becomes distended because the inertia of the blood prevents sudden blood movement all the way to the periphery. However, the rising pressure in the proximal aorta rapidly overcomes this inertia, and the wave front of distention spreads farther and farther along the aorta, as shown in Figure 15-5 . This is called transmission of the pressure pulse in the arteries.

Figure 15-5 Progressive stages in transmission of the pressure pulse along the aorta.
The velocity of pressure pulse transmission in the normal aorta is 3 to 5 m/sec; in the large arterial branches, 7 to 10 m/sec; and in the small arteries, 15 to 35 m/sec. In general, the greater the compliance of each vascular segment, the slower the velocity, which explains the slow transmission in the aorta and the much faster transmission in the much less compliant small distal arteries. In the aorta, the velocity of transmission of the pressure pulse is 15 or more times the velocity of blood flow because the pressure pulse is simply a moving wave of pressure that involves little forward total movement of blood volume.

Damping of the Pressure Pulses in the Smaller Arteries, Arterioles, and Capillaries
Figure 15-6 shows typical changes in the contours of the pressure pulse as the pulse travels into the peripheral vessels. Note especially in the three lower curves that the intensity of pulsation becomes progressively less in the smaller arteries, the arterioles, and, especially, the capillaries. In fact, only when the aortic pulsations are extremely large or the arterioles are greatly dilated can pulsations be observed in the capillaries.

Figure 15-6 Changes in the pulse pressure contour as the pulse wave travels toward the smaller vessels.
This progressive diminution of the pulsations in the periphery is called damping of the pressure pulses. The cause of this is twofold: (1) resistance to blood movement in the vessels and (2) compliance of the vessels. The resistance damps the pulsations because a small amount of blood must flow forward at the pulse wave front to distend the next segment of the vessel; the greater the resistance, the more difficult it is for this to occur. The compliance damps the pulsations because the more compliant a vessel, the greater the quantity of blood required at the pulse wave front to cause an increase in pressure. Therefore, the degree of damping is almost directly proportional to the product of resistance times compliance .

Clinical Methods for Measuring Systolic and Diastolic Pressures
It is not reasonable to use pressure recorders that require needle insertion into an artery for making routine arterial pressure measurements in human patients, although these are used on occasion when special studies are necessary. Instead, the clinician determines systolic and diastolic pressures by indirect means, usually by the auscultatory method .

Auscultatory Method
Figure 15-7 shows the auscultatory method for determining systolic and diastolic arterial pressures. A stethoscope is placed over the antecubital artery and a blood pressure cuff is inflated around the upper arm. As long as the cuff continues to compress the arm with too little pressure to close the brachial artery, no sounds are heard from the antecubital artery with the stethoscope. However, when the cuff pressure is great enough to close the artery during part of the arterial pressure cycle, a sound then is heard with each pulsation. These sounds are called Korotkoff sounds, named after Nikolai Korotkoff, a Russian physician who described them in 1905.

Figure 15-7 Auscultatory method for measuring systolic and diastolic arterial pressures.
The Korotkoff sounds are believed to be caused mainly by blood jetting through the partly occluded vessel and by vibrations of the vessel wall. The jet causes turbulence in the vessel beyond the cuff, and this sets up the vibrations heard through the stethoscope.
In determining blood pressure by the auscultatory method, the pressure in the cuff is first elevated well above arterial systolic pressure. As long as this cuff pressure is higher than systolic pressure, the brachial artery remains collapsed so that no blood jets into the lower artery during any part of the pressure cycle. Therefore, no Korotkoff sounds are heard in the lower artery. But then the cuff pressure gradually is reduced. Just as soon as the pressure in the cuff falls below systolic pressure (point B, Figure 15-7) , blood begins to slip through the artery beneath the cuff during the peak of systolic pressure, and one begins to hear tapping sounds from the antecubital artery in synchrony with the heartbeat. As soon as these sounds begin to be heard, the pressure level indicated by the manometer connected to the cuff is about equal to the systolic pressure.
As the pressure in the cuff is lowered still more, the Korotkoff sounds change in quality, having less of the tapping quality and more of a rhythmical and harsher quality. Then, finally, when the pressure in the cuff falls near diastolic pressure, the sounds suddenly change to a muffled quality (point C, Figure 15-7) . One notes the manometer pressure when the Korotkoff sounds change to the muffled quality and this pressure is about equal to the diastolic pressure, although it slightly overestimates the diastolic pressure determined by direct intra-arterial catheter. As the cuff pressure falls a few mm Hg further, the artery no longer closes during diastole, which means that the basic factor causing the sounds (the jetting of blood through a squeezed artery) is no longer present. Therefore, the sounds disappear entirely. Many clinicians believe that the pressure at which the Korotkoff sounds completely disappear should be used as the diastolic pressure, except in situations in which the disappearance of sounds cannot reliably be determined because sounds are audible even after complete deflation of the cuff. For example, in patients with arteriovenous fistulas for hemodialysis or with aortic insufficiency, Korotkoff sounds may be heard after complete deflation of the cuff.
The auscultatory method for determining systolic and diastolic pressures is not entirely accurate, but it usually gives values within 10 percent of those determined by direct catheter measurement from inside the arteries.

Normal Arterial Pressures as Measured by the Auscultatory Method
Figure 15-8 shows the approximate normal systolic and diastolic arterial pressures at different ages. The progressive increase in pressure with age results from the effects of aging on the blood pressure control mechanisms. We shall see in Chapter 19 that the kidneys are primarily responsible for this long-term regulation of arterial pressure; and it is well known that the kidneys exhibit definitive changes with age, especially after the age of 50 years.

Figure 15-8 Changes in systolic, diastolic, and mean arterial pressures with age. The shaded areas show the approximate normal ranges.
A slight extra increase in systolic pressure usually occurs beyond the age of 60 years. This results from decreasing distensibility, or “hardening,” of the arteries, which is often a result of atherosclerosis. The final effect is a higher systolic pressure with considerable increase in pulse pressure, as previously explained.

Mean Arterial Pressure
The mean arterial pressure is the average of the arterial pressures measured millisecond by millisecond over a period of time. It is not equal to the average of systolic and diastolic pressure because at normal heart rates, a greater fraction of the cardiac cycle is spent in diastole than is systole; thus, the arterial pressure remains nearer to diastolic pressure than to systolic pressure during the greater part of the cardiac cycle. The mean arterial pressure is therefore determined about 60 percent by the diastolic pressure and 40 percent by the systolic pressure. Note in Figure 15-8 that the mean pressure (solid green line) at all ages is nearer to the diastolic pressure than to the systolic pressure. However, at very high heart rates diastole comprises a smaller fraction of the cardiac cycle and the mean arterial pressure is more closely approximated as the average of systolic and diastolic pressures.

Veins and Their Functions
For years, the veins were considered to be nothing more than passageways for flow of blood to the heart, but it is now apparent that they perform other special functions that are necessary for operation of the circulation. Especially important, they are capable of constricting and enlarging and thereby storing either small or large quantities of blood and making this blood available when it is required by the remainder of the circulation. The peripheral veins can also propel blood forward by means of a so-called venous pump, and they even help to regulate cardiac output, an exceedingly important function that is described in detail in Chapter 20 .

Venous Pressures—Right Atrial Pressure (Central Venous Pressure) and Peripheral Venous Pressures
To understand the various functions of the veins, it is first necessary to know something about pressure in the veins and what determines the pressure.
Blood from all the systemic veins flows into the right atrium of the heart; therefore, the pressure in the right atrium is called the central venous pressure.
Right atrial pressure is regulated by a balance between (1) the ability of the heart to pump blood out of the right atrium and ventricle into the lungs and (2) the tendency for blood to flow from the peripheral veins into the right atrium. If the right heart is pumping strongly, the right atrial pressure decreases. Conversely, weakness of the heart elevates the right atrial pressure. Also, any effect that causes rapid inflow of blood into the right atrium from the peripheral veins elevates the right atrial pressure. Some of the factors that can increase this venous return (and thereby increase the right atrial pressure) are (1) increased blood volume, (2) increased large vessel tone throughout the body with resultant increased peripheral venous pressures, and (3) dilatation of the arterioles, which decreases the peripheral resistance and allows rapid flow of blood from the arteries into the veins.
The same factors that regulate right atrial pressure also contribute to regulation of cardiac output because the amount of blood pumped by the heart depends on both the ability of the heart to pump and the tendency for blood to flow into the heart from the peripheral vessels. Therefore, we will discuss regulation of right atrial pressure in much more depth in Chapter 20 in connection with regulation of cardiac output.
The normal right atrial pressure is about 0 mm Hg, which is equal to the atmospheric pressure around the body. It can increase to 20 to 30 mm Hg under very abnormal conditions, such as (1) serious heart failure or (2) after massive transfusion of blood, which greatly increases the total blood volume and causes excessive quantities of blood to attempt to flow into the heart from the peripheral vessels.
The lower limit to the right atrial pressure is usually about −3 to −5 mm Hg below atmospheric pressure. This is also the pressure in the chest cavity that surrounds the heart. The right atrial pressure approaches these low values when the heart pumps with exceptional vigor or when blood flow into the heart from the peripheral vessels is greatly depressed, such as after severe hemorrhage.

Venous Resistance and Peripheral Venous Pressure
Large veins have so little resistance to blood flow when they are distended that the resistance then is almost zero and is of almost no importance. However, as shown in Figure 15-9 , most of the large veins that enter the thorax are compressed at many points by the surrounding tissues so that blood flow is impeded at these points. For instance, the veins from the arms are compressed by their sharp angulations over the first rib. Also, the pressure in the neck veins often falls so low that the atmospheric pressure on the outside of the neck causes these veins to collapse. Finally, veins coursing through the abdomen are often compressed by different organs and by the intra-abdominal pressure, so they usually are at least partially collapsed to an ovoid or slitlike state. For these reasons, the large veins do usually offer some resistance to blood flow, and because of this, the pressure in the more peripheral small veins in a person lying down is usually +4 to +6 mm Hg greater than the right atrial pressure.

Figure 15-9 Compression points that tend to collapse the veins entering the thorax.

Effect of High Right Atrial Pressure on Peripheral Venous Pressure
When the right atrial pressure rises above its normal value of 0 mm Hg, blood begins to back up in the large veins. This enlarges the veins, and even the collapse points in the veins open up when the right atrial pressure rises above +4 to +6 mm Hg. Then, as the right atrial pressure rises still further, the additional increase causes a corresponding rise in peripheral venous pressure in the limbs and elsewhere. Because the heart must be weakened to cause a rise in right atrial pressure as high as +4 to +6 mm Hg, one often finds that the peripheral venous pressure is not noticeably elevated even in the early stages of heart failure.

Effect of Intra-abdominal Pressure on Venous Pressures of the Leg
The pressure in the abdominal cavity of a recumbent person normally averages about +6 mm Hg, but it can rise to +15 to +30 mm Hg as a result of pregnancy, large tumors, abdominal obesity, or excessive fluid (called “ascites”) in the abdominal cavity. When the intra-abdominal pressure does rise, the pressure in the veins of the legs must rise above the abdominal pressure before the abdominal veins will open and allow the blood to flow from the legs to the heart. Thus, if the intra-abdominal pressure is +20 mm Hg, the lowest possible pressure in the femoral veins is also about +20 mm Hg.

Effect of Gravitational Pressure on Venous Pressure
In any body of water that is exposed to air, the pressure at the surface of the water is equal to atmospheric pressure, but the pressure rises 1 mm Hg for each 13.6 millimeters of distance below the surface. This pressure results from the weight of the water and therefore is called gravitational pressure or hydrostatic pressure.
Gravitational pressure also occurs in the vascular system of the human being because of weight of the blood in the vessels, as shown in Figure 15-10 . When a person is standing, the pressure in the right atrium remains about 0 mm Hg because the heart pumps into the arteries any excess blood that attempts to accumulate at this point. However, in an adult who is standing absolutely still, the pressure in the veins of the feet is about +90 mm Hg simply because of the gravitational weight of the blood in the veins between the heart and the feet. The venous pressures at other levels of the body are proportionately between 0 and 90 mm Hg.

Figure 15-10 Effect of gravitational pressure on the venous pressures throughout the body in the standing person.
In the arm veins, the pressure at the level of the top rib is usually about +6 mm Hg because of compression of the subclavian vein as it passes over this rib. The gravitational pressure down the length of the arm then is determined by the distance below the level of this rib. Thus, if the gravitational difference between the level of the rib and the hand is +29 mm Hg, this gravitational pressure is added to the +6 mm Hg pressure caused by compression of the vein as it crosses the rib, making a total of +35 mm Hg pressure in the veins of the hand.
The neck veins of a person standing upright collapse almost completely all the way to the skull because of atmospheric pressure on the outside of the neck. This collapse causes the pressure in these veins to remain at zero along their entire extent. The reason for this is that any tendency for the pressure to rise above this level opens the veins and allows the pressure to fall back to zero because of flow of the blood. Conversely, any tendency for the neck vein pressure to fall below zero collapses the veins still more, which further increases their resistance and again returns the pressure back to zero.
The veins inside the skull, on the other hand, are in a noncollapsible chamber (the skull cavity) so that they cannot collapse. Consequently, negative pressure can exist in the dural sinuses of the head; in the standing position, the venous pressure in the sagittal sinus at the top of the brain is about −10 mm Hg because of the hydrostatic “suction” between the top of the skull and the base of the skull. Therefore, if the sagittal sinus is opened during surgery, air can be sucked immediately into the venous system; the air may even pass downward to cause air embolism in the heart, and death can ensue.

Effect of the Gravitational Factor on Arterial and Other Pressures
The gravitational factor also affects pressures in the peripheral arteries and capillaries, in addition to its effects in the veins. For instance, a standing person who has a mean arterial pressure of 100 mm Hg at the level of the heart has an arterial pressure in the feet of about 190 mm Hg. Therefore, when one states that the arterial pressure is 100 mm Hg, this generally means that this is the pressure at the gravitational level of the heart but not necessarily elsewhere in the arterial vessels.

Venous Valves and the “Venous Pump”: Their Effects on Venous Pressure
Were it not for valves in the veins, the gravitational pressure effect would cause the venous pressure in the feet always to be about +90 mm Hg in a standing adult. However, every time one moves the legs, one tightens the muscles and compresses the veins in or adjacent to the muscles, and this squeezes the blood out of the veins. But the valves in the veins, shown in Figure 15-11 , are arranged so that the direction of venous blood flow can be only toward the heart. Consequently, every time a person moves the legs or even tenses the leg muscles, a certain amount of venous blood is propelled toward the heart. This pumping system is known as the “venous pump” or “muscle pump,” and it is efficient enough that under ordinary circumstances, the venous pressure in the feet of a walking adult remains less than +20 mm Hg.

Figure 15-11 Venous valves of the leg.
If a person stands perfectly still, the venous pump does not work, and the venous pressures in the lower legs increase to the full gravitational value of 90 mm Hg in about 30 seconds. The pressures in the capillaries also increase greatly, causing fluid to leak from the circulatory system into the tissue spaces. As a result, the legs swell and the blood volume diminishes. Indeed, 10 to 20 percent of the blood volume can be lost from the circulatory system within the 15 to 30 minutes of standing absolutely still, as often occurs when a soldier is made to stand at rigid attention.

Venous Valve Incompetence Causes “Varicose” Veins
The valves of the venous system frequently become “incompetent” or sometimes even are destroyed. This is especially true when the veins have been overstretched by excess venous pressure lasting weeks or months, as occurs in pregnancy or when one stands most of the time. Stretching the veins increases their cross-sectional areas, but the leaflets of the valves do not increase in size. Therefore, the leaflets of the valves no longer close completely. When this develops, the pressure in the veins of the legs increases greatly because of failure of the venous pump; this further increases the sizes of the veins and finally destroys the function of the valves entirely. Thus, the person develops “varicose veins,” which are characterized by large, bulbous protrusions of the veins beneath the skin of the entire leg, particularly the lower leg.
Whenever people with varicose veins stand for more than a few minutes, the venous and capillary pressures become very high and leakage of fluid from the capillaries causes constant edema in the legs. The edema in turn prevents adequate diffusion of nutritional materials from the capillaries to the muscle and skin cells, so the muscles become painful and weak and the skin frequently becomes gangrenous and ulcerates. The best treatment for such a condition is continual elevation of the legs to a level at least as high as the heart. Tight binders on the legs also can be of considerable assistance in preventing the edema and its sequelae.

Clinical Estimation of Venous Pressure
The venous pressure often can be estimated by simply observing the degree of distention of the peripheral veins—especially of the neck veins. For instance, in the sitting position, the neck veins are never distended in the normal quietly resting person. However, when the right atrial pressure becomes increased to as much as +10 mm Hg, the lower veins of the neck begin to protrude; and at +15 mm Hg atrial pressure essentially all the veins in the neck become distended.

Direct Measurement of Venous Pressure and Right Atrial Pressure
Venous pressure can also be measured with ease by inserting a needle directly into a vein and connecting it to a pressure recorder. The only means by which right atrial pressure can be measured accurately is by inserting a catheter through the peripheral veins and into the right atrium. Pressures measured through such central venous catheters are used almost routinely in some types of hospitalized cardiac patients to provide constant assessment of heart pumping ability.

Pressure Reference Level for Measuring Venous and Other Circulatory Pressures
In discussions up to this point, we often have spoken of right atrial pressure as being 0 mm Hg and arterial pressure as being 100 mm Hg, but we have not stated the gravitational level in the circulatory system to which this pressure is referred. There is one point in the circulatory system at which gravitational pressure factors caused by changes in body position of a healthy person usually do not affect the pressure measurement by more than 1 to 2 mm Hg. This is at or near the level of the tricuspid valve, as shown by the crossed axes in Figure 15-12 . Therefore, all circulatory pressure measurements discussed in this text are referred to this level, which is called the reference level for pressure measurement.

Figure 15-12 Reference point for circulatory pressure measurement (located near the tricuspid valve).
The reason for lack of gravitational effects at the tricuspid valve is that the heart automatically prevents significant gravitational changes in pressure at this point in the following way:
If the pressure at the tricuspid valve rises slightly above normal, the right ventricle fills to a greater extent than usual, causing the heart to pump blood more rapidly and therefore to decrease the pressure at the tricuspid valve back toward the normal mean value. Conversely, if the pressure falls, the right ventricle fails to fill adequately, its pumping decreases, and blood dams up in the venous system until the pressure at the tricuspid level again rises to the normal value. In other words, the heart acts as a feedback regulator of pressure at the tricuspid valve.
When a person is lying on his or her back, the tricuspid valve is located at almost exactly 60 percent of the chest thickness in front of the back. This is the zero pressure reference level for a person lying down .

Blood Reservoir Function of the Veins
As pointed out in Chapter 14 , more than 60 percent of all the blood in the circulatory system is usually in the veins. For this reason and also because the veins are so compliant, it is said that the venous system serves as a blood reservoir for the circulation.
When blood is lost from the body and the arterial pressure begins to fall, nervous signals are elicited from the carotid sinuses and other pressure-sensitive areas of the circulation, as discussed in Chapter 18 . These in turn elicit nerve signals from the brain and spinal cord mainly through sympathetic nerves to the veins, causing them to constrict. This takes up much of the slack in the circulatory system caused by the lost blood. Indeed, even after as much as 20 percent of the total blood volume has been lost, the circulatory system often functions almost normally because of this variable reservoir function of the veins.

Specific Blood Reservoirs
Certain portions of the circulatory system are so extensive and/or so compliant that they are called “specific blood reservoirs.” These include (1) the spleen, which sometimes can decrease in size sufficiently to release as much as 100 milliliters of blood into other areas of the circulation; (2) the liver, the sinuses of which can release several hundred milliliters of blood into the remainder of the circulation; (3) the large abdominal veins, which can contribute as much as 300 milliliters; and (4) the venous plexus beneath the skin, which also can contribute several hundred milliliters. The heart and the lungs, although not parts of the systemic venous reservoir system, must also be considered blood reservoirs. The heart, for instance, shrinks during sympathetic stimulation and in this way can contribute some 50 to 100 milliliters of blood; the lungs can contribute another 100 to 200 milliliters when the pulmonary pressures decrease to low values.

The Spleen as a Reservoir for Storing Red Blood Cells
Figure 15-13 shows that the spleen has two separate areas for storing blood: the venous sinuses and the pulp. The sinuses can swell the same as any other part of the venous system and store whole blood.

Figure 15-13 Functional structures of the spleen.
(Courtesy Dr. Don W. Fawcett, Montana.)
In the splenic pulp, the capillaries are so permeable that whole blood, including the red blood cells, oozes through the capillary walls into a trabecular mesh, forming the red pulp. The red cells are trapped by the trabeculae, while the plasma flows on into the venous sinuses and then into the general circulation. As a consequence, the red pulp of the spleen is a special reservoir that contains large quantities of concentrated red blood cells. These can then be expelled into the general circulation whenever the sympathetic nervous system becomes excited and causes the spleen and its vessels to contract. As much as 50 milliliters of concentrated red blood cells can be released into the circulation, raising the hematocrit 1 to 2 percent.
In other areas of the splenic pulp are islands of white blood cells, which collectively are called the white pulp. Here lymphoid cells are manufactured similar to those manufactured in the lymph nodes. They are part of the body’s immune system, described in Chapter 34 .

Blood-Cleansing Function of the Spleen—Removal of Old Cells
Blood cells passing through the splenic pulp before entering the sinuses undergo thorough squeezing. Therefore, it is to be expected that fragile red blood cells would not withstand the trauma. For this reason, many of the red blood cells destroyed in the body have their final demise in the spleen. After the cells rupture, the released hemoglobin and the cell stroma are digested by the reticuloendothelial cells of the spleen, and the products of digestion are mainly reused by the body as nutrients, often for making new blood cells.

Reticuloendothelial Cells of the Spleen
The pulp of the spleen contains many large phagocytic reticuloendothelial cells, and the venous sinuses are lined with similar cells. These cells function as part of a cleansing system for the blood, acting in concert with a similar system of reticuloendothelial cells in the venous sinuses of the liver. When the blood is invaded by infectious agents, the reticuloendothelial cells of the spleen rapidly remove debris, bacteria, parasites, and so forth. Also, in many chronic infectious processes, the spleen enlarges in the same manner that lymph nodes enlarge and then performs its cleansing function even more avidly.


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CHAPTER 16 The Microcirculation and Lymphatic System
Capillary Fluid Exchange, Interstitial Fluid, and Lymph Flow
The most purposeful function of the circulation occurs in the microcirculation: This is transport of nutrients to the tissues and removal of cell excreta . The small arterioles control blood flow to each tissue, and local conditions in the tissues in turn control the diameters of the arterioles. Thus, each tissue, in most instances, controls its own blood flow in relation to its individual needs, a subject that is discussed in Chapter 17 .
The walls of the capillaries are extremely thin, constructed of single-layer, highly permeable endothelial cells. Therefore, water, cell nutrients, and cell excreta can all interchange quickly and easily between the tissues and the circulating blood.
The peripheral circulation of the whole body has about 10 billion capillaries with a total surface area estimated to be 500 to 700 square meters (about one-eighth the surface area of a football field). Indeed, it is rare that any single functional cell of the body is more than 20 to 30 micrometers away from a capillary.

Structure of the Microcirculation and Capillary System
The microcirculation of each organ is organized specifically to serve that organ’s needs. In general, each nutrient artery entering an organ branches six to eight times before the arteries become small enough to be called arterioles, which generally have internal diameters of only 10 to 15 micrometers. Then the arterioles themselves branch two to five times, reaching diameters of 5 to 9 micrometers at their ends where they supply blood to the capillaries.
The arterioles are highly muscular, and their diameters can change manyfold. The metarterioles (the terminal arterioles) do not have a continuous muscular coat, but smooth muscle fibers encircle the vessel at intermittent points, as shown in Figure 16-1 by the black dots on the sides of the metarteriole.

Figure 16-1 Structure of the mesenteric capillary bed.
(Redrawn from Zweifach BW: Factors Regulating Blood Pressure. New York: Josiah Macy, Jr., Foundation, 1950.)
At the point where each true capillary originates from a metarteriole, a smooth muscle fiber usually encircles the capillary. This is called the precapillary sphincter. This sphincter can open and close the entrance to the capillary.
The venules are larger than the arterioles and have a much weaker muscular coat. Yet the pressure in the venules is much less than that in the arterioles, so the venules can still contract considerably despite the weak muscle.
This typical arrangement of the capillary bed is not found in all parts of the body, although a similar arrangement may serve the same purposes. Most important, the metarterioles and the precapillary sphincters are in close contact with the tissues they serve. Therefore, the local conditions of the tissues—the concentrations of nutrients, end products of metabolism, hydrogen ions, and so forth—can cause direct effects on the vessels to control local blood flow in each small tissue area.

Structure of the Capillary Wall
Figure 16-2 shows the ultramicroscopic structure of typical endothelial cells in the capillary wall as found in most organs of the body, especially in muscles and connective tissue. Note that the wall is composed of a unicellular layer of endothelial cells and is surrounded by a thin basement membrane on the outside of the capillary. The total thickness of the capillary wall is only about 0.5 micrometer. The internal diameter of the capillary is 4 to 9 micrometers, barely large enough for red blood cells and other blood cells to squeeze through.

Figure 16-2 Structure of the capillary wall. Note especially the intercellular cleft at the junction between adjacent endothelial cells; it is believed that most water-soluble substances diffuse through the capillary membrane along the clefts. Small membrane invaginations, called caveolae, are believed to play a role in transporting macromolecules across the cell membrane. Caveolae contain caveolins, proteins which interact with cholesterol and polymerize to form the caveolae.

“Pores” in the Capillary Membrane
Figure 16-2 shows two small passageways connecting the interior of the capillary with the exterior. One of these is an intercellular cleft, which is the thin-slit, curving channel that lies at the bottom of the figure between adjacent endothelial cells. Each cleft is interrupted periodically by short ridges of protein attachments that hold the endothelial cells together, but between these ridges fluid can percolate freely through the cleft. The cleft normally has a uniform spacing with a width of about 6 to 7 nanometers (60 to 70 angstroms), slightly smaller than the diameter of an albumin protein molecule.
Because the intercellular clefts are located only at the edges of the endothelial cells, they usually represent no more than 1/1000 of the total surface area of the capillary wall. Nevertheless, the rate of thermal motion of water molecules, as well as most water-soluble ions and small solutes, is so rapid that all of these diffuse with ease between the interior and exterior of the capillaries through these “slit-pores,” the intercellular clefts.
Present in the endothelial cells are many minute plasmalemmal vesicles, also called caveolae (small caves). These form from oligomers of proteins called caveolins that are associated with molecules of cholesterol and sphingolipids . Although the precise functions of caveolae are still unclear, they are believed to play a role in endocytosis (the process by which the cell engulfs material from outside the cell) and transcytosis of macromolecules across endothelial cells. The caveolae at the surface of the cell appear to imbibe small packets of plasma or extracellular fluid that contain plasma proteins. These vesicles can then move slowly through the endothelial cell. Some of these vesicles may coalesce to form vesicular channels all the way through the endothelial cell, which is demonstrated in Figure 16-2.

Special Types of “Pores” Occur in the Capillaries of Certain Organs
The “pores” in the capillaries of some organs have special characteristics to meet the peculiar needs of the organs. Some of these characteristics are as follows:
1. In the brain, the junctions between the capillary endothelial cells are mainly “tight” junctions that allow only extremely small molecules such as water, oxygen, and carbon dioxide to pass into or out of the brain tissues.
2. In the liver, the opposite is true. The clefts between the capillary endothelial cells are wide open so that almost all dissolved substances of the plasma, including the plasma proteins, can pass from the blood into the liver tissues.
3. The pores of the gastrointestinal capillary membranes are midway between those of the muscles and those of the liver.
4. In the glomerular capillaries of the kidney, numerous small oval windows called fenestrae penetrate all the way through the middle of the endothelial cells so that tremendous amounts of very small molecular and ionic substances (but not the large molecules of the plasma proteins) can filter through the glomeruli without having to pass through the clefts between the endothelial cells.

Flow of Blood in the Capillaries—Vasomotion
Blood usually does not flow continuously through the capillaries. Instead, it flows intermittently, turning on and off every few seconds or minutes. The cause of this intermittency is the phenomenon called vasomotion, which means intermittent contraction of the metarterioles and precapillary sphincters (and sometimes even the very small arterioles as well).

Regulation of Vasomotion
The most important factor found thus far to affect the degree of opening and closing of the metarterioles and precapillary sphincters is the concentration of oxygen in the tissues. When the rate of oxygen usage by the tissue is great so that tissue oxygen concentration decreases below normal, the intermittent periods of capillary blood flow occur more often, and the duration of each period of flow lasts longer, thereby allowing the capillary blood to carry increased quantities of oxygen (as well as other nutrients) to the tissues. This effect, along with multiple other factors that control local tissue blood flow, is discussed in Chapter 17 .

Average Function of the Capillary System
Despite the fact that blood flow through each capillary is intermittent, so many capillaries are present in the tissues that their overall function becomes averaged. That is, there is an average rate of blood flow through each tissue capillary bed, an average capillary pressure within the capillaries, and an average rate of transfer of substances between the blood of the capillaries and the surrounding interstitial fluid. In the remainder of this chapter, we are concerned with these averages, although one must remember that the average functions are, in reality, the functions of literally billions of individual capillaries, each operating intermittently in response to local conditions in the tissues.

Exchange of Water, Nutrients, and Other Substances Between the Blood and Interstitial Fluid

Diffusion Through the Capillary Membrane
By far the most important means by which substances are transferred between the plasma and the interstitial fluid is diffusion . Figure 16-3 demonstrates this process, showing that as the blood flows along the lumen of the capillary, tremendous numbers of water molecules and dissolved particles diffuse back and forth through the capillary wall, providing continual mixing between the interstitial fluid and the plasma. Diffusion results from thermal motion of the water molecules and dissolved substances in the fluid, the different molecules and ions moving first in one direction and then another, bouncing randomly in every direction.

Figure 16-3 Diffusion of fluid molecules and dissolved substances between the capillary and interstitial fluid spaces.

Lipid-Soluble Substances Can Diffuse Directly Through the Cell Membranes of the Capillary Endothelium
If a substance is lipid soluble, it can diffuse directly through the cell membranes of the capillary without having to go through the pores. Such substances include oxygen and carbon dioxide. Because these substances can permeate all areas of the capillary membrane, their rates of transport through the capillary membrane are many times faster than the rates for lipid-insoluble substances, such as sodium ions and glucose that can go only through the pores.

Water-Soluble, Non-Lipid-Soluble Substances Diffuse Through Intercellular “Pores” in the Capillary Membrane
Many substances needed by the tissues are soluble in water but cannot pass through the lipid membranes of the endothelial cells; such substances include water molecules themselves, sodium ions, chloride ions, and glucose. Despite the fact that not more than 1/1000 of the surface area of the capillaries is represented by the intercellular clefts between the endothelial cells, the velocity of thermal molecular motion in the clefts is so great that even this small area is sufficient to allow tremendous diffusion of water and water-soluble substances through these cleft-pores. To give one an idea of the rapidity with which these substances diffuse, the rate at which water molecules diffuse through the capillary membrane is about 80 times as great as the rate at which plasma itself flows linearly along the capillary . That is, the water of the plasma is exchanged with the water of the interstitial fluid 80 times before the plasma can flow the entire distance through the capillary.

Effect of Molecular Size on Passage Through the Pores
The width of the capillary intercellular cleft-pores, 6 to 7 nanometers, is about 20 times the diameter of the water molecule, which is the smallest molecule that normally passes through the capillary pores. Conversely, the diameters of plasma protein molecules are slightly greater than the width of the pores. Other substances, such as sodium ions, chloride ions, glucose, and urea, have intermediate diameters. Therefore, the permeability of the capillary pores for different substances varies according to their molecular diameters.
Table 16-1 gives the relative permeabilities of the capillary pores in skeletal muscle for substances commonly encountered, demonstrating, for instance, that the permeability for glucose molecules is 0.6 times that for water molecules, whereas the permeability for albumin molecules is very, very slight, only 1/1000 that for water molecules.
Table 16-1 Relative Permeability of Skeletal Muscle Capillary Pores to Different-Sized Molecules Substance Molecular Weight Permeability Water 18 1.00 NaCl 58.5 0.96 Urea 60 0.8 Glucose 180 0.6 Sucrose 342 0.4 Inulin 5,000 0.2 Myoglobin 17,600 0.03 Hemoglobin 68,000 0.01 Albumin 69,000 0.001
Data from Pappenheimer JR: Passage of molecules through capillary walls. Physiol Rev 33:387, 1953.
A word of caution must be issued at this point. The capillaries in various tissues have extreme differences in their permeabilities. For instance, the membranes of the liver capillary sinusoids are so permeable that even plasma proteins pass freely through these walls, almost as easily as water and other substances. Also, the permeability of the renal glomerular membrane for water and electrolytes is about 500 times the permeability of the muscle capillaries, but this is not true for the plasma proteins; for these, the capillary permeabilities are very slight, as in other tissues and organs. When we study these different organs later in this text, it should become clear why some tissues—the liver, for instance—require greater degrees of capillary permeability than others to transfer tremendous amounts of nutrients between the blood and liver parenchymal cells, and the kidneys to allow filtration of large quantities of fluid for formation of urine.

Effect of Concentration Difference on Net Rate of Diffusion Through the Capillary Membrane
The “net” rate of diffusion of a substance through any membrane is proportional to the concentration difference of the substance between the two sides of the membrane. That is, the greater the difference between the concentrations of any given substance on the two sides of the capillary membrane, the greater the net movement of the substance in one direction through the membrane. For instance, the concentration of oxygen in capillary blood is normally greater than in the interstitial fluid. Therefore, large quantities of oxygen normally move from the blood toward the tissues. Conversely, the concentration of carbon dioxide is greater in the tissues than in the blood, which causes excess carbon dioxide to move into the blood and to be carried away from the tissues.
The rates of diffusion through the capillary membranes of most nutritionally important substances are so great that only slight concentration differences suffice to cause more than adequate transport between the plasma and interstitial fluid. For instance, the concentration of oxygen in the interstitial fluid immediately outside the capillary is no more than a few percent less than its concentration in the plasma of the blood, yet this slight difference causes enough oxygen to move from the blood into the interstitial spaces to provide all the oxygen required for tissue metabolism, often as much as several liters of oxygen per minute during very active states of the body.

Interstitium and Interstitial Fluid
About one sixth of the total volume of the body consists of spaces between cells, which collectively are called the interstitium. The fluid in these spaces is the interstitial fluid.
The structure of the interstitium is shown in Figure 16-4 . It contains two major types of solid structures: (1) collagen fiber bundles and (2) proteoglycan filaments. The collagen fiber bundles extend long distances in the interstitium. They are extremely strong and therefore provide most of the tensional strength of the tissues. The proteoglycan filaments, however, are extremely thin coiled or twisted molecules composed of about 98 percent hyaluronic acid and 2 percent protein. These molecules are so thin that they cannot be seen with a light microscope and are difficult to demonstrate even with the electron microscope. Nevertheless, they form a mat of very fine reticular filaments aptly described as a “brush pile.”

Figure 16-4 Structure of the interstitium. Proteoglycan filaments are everywhere in the spaces between the collagen fiber bundles. Free fluid vesicles and small amounts of free fluid in the form of rivulets occasionally also occur.

“Gel” in the Interstitium
The fluid in the interstitium is derived by filtration and diffusion from the capillaries. It contains almost the same constituents as plasma except for much lower concentrations of proteins because proteins do not easily pass outward through the pores of the capillaries. The interstitial fluid is entrapped mainly in the minute spaces among the proteoglycan filaments. This combination of proteoglycan filaments and fluid entrapped within them has the characteristics of a gel and therefore is called tissue gel.
Because of the large number of proteoglycan filaments, it is difficult for fluid to flow easily through the tissue gel. Instead, fluid mainly diffuses through the gel; that is, it moves molecule by molecule from one place to another by kinetic, thermal motion rather than by large numbers of molecules moving together.
Diffusion through the gel occurs about 95 to 99 percent as rapidly as it does through free fluid. For the short distances between the capillaries and the tissue cells, this diffusion allows rapid transport through the interstitium not only of water molecules but also of electrolytes, small molecular weight nutrients, cellular excreta, oxygen, carbon dioxide, and so forth.

“Free” Fluid in the Interstitium
Although almost all the fluid in the interstitium normally is entrapped within the tissue gel, occasionally small rivulets of “free” fluid and small free fluid vesicles are also present, which means fluid that is free of the proteoglycan molecules and therefore can flow freely. When a dye is injected into the circulating blood, it often can be seen to flow through the interstitium in the small rivulets, usually coursing along the surfaces of collagen fibers or surfaces of cells.
The amount of “free” fluid present in normal tissues is slight, usually less than 1 percent. Conversely, when the tissues develop edema, these small pockets and rivulets of free fluid expand tremendously until one half or more of the edema fluid becomes freely flowing fluid independent of the proteoglycan filaments.

Fluid Filtration Across Capillaries Is Determined by Hydrostatic and Colloid Osmotic Pressures, as Well as Capillary Filtration Coefficient
The hydrostatic pressure in the capillaries tends to force fluid and its dissolved substances through the capillary pores into the interstitial spaces. Conversely, osmotic pressure caused by the plasma proteins (called colloid osmotic pressure ) tends to cause fluid movement by osmosis from the interstitial spaces into the blood. This osmotic pressure exerted by the plasma proteins normally prevents significant loss of fluid volume from the blood into the interstitial spaces.
Also important is the lymphatic system, which returns to the circulation the small amounts of excess protein and fluid that leak from the blood into the interstitial spaces. In the remainder of this chapter, we discuss the mechanisms that control capillary filtration and lymph flow function together to regulate the respective volumes of the plasma and the interstitial fluid.

Hydrostatic and Colloid Osmotic Forces Determine Fluid Movement Through the Capillary Membrane
Figure 16-5 shows the four primary forces that determine whether fluid will move out of the blood into the interstitial fluid or in the opposite direction. These forces, called “Starling forces” in honor of the physiologist who first demonstrated their importance, are:
1. The capillary pressure (Pc), which tends to force fluid outward through the capillary membrane.
2. The interstitial fluid pressure (Pif), which tends to force fluid inward through the capillary membrane when Pif is positive but outward when Pif is negative.
3. The capillary plasma colloid osmotic pressure (Πp), which tends to cause osmosis of fluid inward through the capillary membrane.
4. The interstitial fluid colloid osmotic pressure (Πif), which tends to cause osmosis of fluid outward through the capillary membrane.

Figure 16-5 Fluid pressure and colloid osmotic pressure forces operate at the capillary membrane, tending to move fluid either outward or inward through the membrane pores.
If the sum of these forces—the net filtration pressure —is positive, there will be a net fluid filtration across the capillaries. If the sum of the Starling forces is negative, there will be a net fluid absorption from the interstitial spaces into the capillaries. The net filtration pressure (NFP) is calculated as:

As discussed later, the NFP is slightly positive under normal conditions, resulting in a net filtration of fluid across the capillaries into the interstitial space in most organs. The rate of fluid filtration in a tissue is also determined by the number and size of the pores in each capillary, as well as the number of capillaries in which blood is flowing. These factors are usually expressed together as the capillary filtration coefficient (K f ). The K f is therefore a measure of the capacity of the capillary membranes to filter water for a given NFP and is usually expressed as ml/min per mm Hg net filtration pressure.
The rate of capillary fluid filtration is therefore determined as:

In the following sections we discuss each of the forces that determine the rate of capillary fluid filtration.

Capillary Hydrostatic Pressure
Various methods have been used to estimate the capillary hydrostatic pressure: (1) direct micropipette cannulation of the capillaries, which has given an average mean capillary pressure of about 25 mm Hg in some tissues such as the skeletal muscle and the gut, and (2) indirect functional measurement of the capillary pressure, which has given a capillary pressure averaging about 17 mm Hg in these tissues.

Micropipette Method for Measuring Capillary Pressure
To measure pressure in a capillary by cannulation, a microscopic glass pipette is thrust directly into the capillary, and the pressure is measured by an appropriate micromanometer system. Using this method, capillary pressures have been measured in capillaries of exposed tissues of animals and in large capillary loops of the eponychium at the base of the fingernail in humans. These measurements have given pressures of 30 to 40 mm Hg in the arterial ends of the capillaries, 10 to 15 mm Hg in the venous ends, and about 25 mm Hg in the middle.
In some capillaries, such as the glomerular capillaries of the kidneys, the pressures measured by the micropipette method are much higher, averaging about 60 mm Hg. The peritubular capillaries of the kidneys, in contrast, have hydrostatic pressure that average only about 13 mm Hg. Thus, the capillary hydrostatic pressures in different tissues are highly variable, depending on the particular tissue and the physiological condition.

Isogravimetric Method for Indirectly Measuring “Functional” Capillary Pressure
Figure 16-6 demonstrates an isogravimetric method for indirectly estimating capillary pressure. This figure shows a section of gut held up by one arm of a gravimetric balance. Blood is perfused through the blood vessels of the gut wall. When the arterial pressure is decreased, the resulting decrease in capillary pressure allows the osmotic pressure of the plasma proteins to cause absorption of fluid out of the gut wall and makes the weight of the gut decrease. This immediately causes displacement of the balance arm. To prevent this weight decrease, the venous pressure is increased an amount sufficient to overcome the effect of decreasing the arterial pressure. In other words, the capillary pressure is kept constant while simultaneously (1) decreasing the arterial pressure and (2) increasing the venous pressure.

Figure 16-6 Isogravimetric method for measuring capillary pressure.
In the graph in the lower part of the figure, the changes in arterial and venous pressures that exactly nullify all weight changes are shown. The arterial and venous lines meet each other at a value of 17 mm Hg. Therefore, the capillary pressure must have remained at this same level of 17 mm Hg throughout these maneuvers; otherwise, either filtration or absorption of fluid through the capillary walls would have occurred. Thus, in a roundabout way, the “functional” capillary pressure in this tissue is measured to be about 17 mm Hg.
It is clear that the isogravimetric method, which determines the capillary pressure that exactly balances all the forces tending to move fluid into or out of the capillaries, gives a lower value compared with the capillary pressure measured directly with a micropipette. A major reason for this is that capillary fluid filtration is not exactly balanced with fluid reabsorption in most tissues. The fluid that is filtered in excess of what is reabsorbed is carried away by lymph vessels in most tissues. In the glomerular capillaries of the kidneys, a very large amount of fluid, approximately 125 ml/min, is continuously filtered.

Interstitial Fluid Hydrostatic Pressure
There are several methods for measuring interstitial fluid hydrostatic pressure and each of these gives slightly different values, depending on the method used and the tissue in which the pressure is measured. In loose subcutaneous tissue, interstitial fluid pressure measured by the different methods is usually a few millimeters of mercury less than atmospheric pressure; that is, the values are called negative interstitial fluid pressure. In other tissues that are surrounded by capsules, such as the kidneys, the interstitial pressure is generally positive (greater than atmospheric pressure). The methods most widely used have been (1) direct cannulation of the tissues with a micropipette, (2) measurement of the pressure from implanted perforated capsules, and (3) measurement of the pressure from a cotton wick inserted into the tissue.

Measurement of Interstitial Fluid Pressure Using the Micropipette
The same type of micropipette used for measuring capillary pressure can also be used in some tissues for measuring interstitial fluid pressure. The tip of the micropipette is about 1 micrometer in diameter, but even this is 20 or more times larger than the sizes of the spaces between the proteoglycan filaments of the interstitium. Therefore, the pressure that is measured is probably the pressure in a free fluid pocket.
The first pressures measured using the micropipette method ranged from −1 to +2 mm Hg but were usually slightly positive. With experience and improved equipment for making such measurements, more recent pressures have averaged about −2 mm Hg, giving average pressure values in loose tissues, such as skin, that are slightly less than atmospheric pressure.

Measurement of Interstitial Free Fluid Pressure in Implanted Perforated Hollow Capsules
Interstitial free fluid pressure measured by this method when using 2-centimeter diameter capsules in normal loose subcutaneous tissue averages about −6 mm Hg, but with smaller capsules, the values are not greatly different from the −2 mm Hg measured by the micropipette.

Interstitial Fluid Pressures in Tightly Encased Tissues
Some tissues of the body are surrounded by tight encasements, such as the cranial vault around the brain, the strong fibrous capsule around the kidney, the fibrous sheaths around the muscles, and the sclera around the eye. In most of these, regardless of the method used for measurement, the interstitial fluid pressures are positive. However, these interstitial fluid pressures almost invariably are still less than the pressures exerted on the outsides of the tissues by their encasements. For instance, the cerebrospinal fluid pressure surrounding the brain of an animal lying on its side averages about +10 mm Hg, whereas the brain interstitial fluid pressure averages about +4 to +6 mm Hg. In the kidneys, the capsular pressure surrounding the kidney averages about +13 mm Hg, whereas the reported renal interstitial fluid pressures have averaged about +6 mm Hg. Thus, if one remembers that the pressure exerted on the skin is atmospheric pressure, which is considered to be zero pressure, one might formulate a general rule that the normal interstitial fluid pressure is usually several millimeters of mercury negative with respect to the pressure that surrounds each tissue.

Is the True Interstitial Fluid Pressure in Loose Subcutaneous Tissue Subatmospheric?
The concept that the interstitial fluid pressure is subatmospheric in some tissues of the body began with clinical observations that could not be explained by the previously held concept that interstitial fluid pressure was always positive. Some of the pertinent observations are the following:
1. When a skin graft is placed on a concave surface of the body, such as in an eye socket after removal of the eye, before the skin becomes attached to the sublying socket, fluid tends to collect underneath the graft. Also, the skin attempts to shorten, with the result that it tends to pull it away from the concavity. Nevertheless, some negative force underneath the skin causes absorption of the fluid and usually literally pulls the skin back into the concavity.
2. Less than 1 mm Hg of positive pressure is required to inject large volumes of fluid into loose subcutaneous tissues, such as beneath the lower eyelid, in the axillary space, and in the scrotum. Amounts of fluid calculated to be more than 100 times the amount of fluid normally in the interstitial space, when injected into these areas, cause no more than about 2 mm Hg of positive pressure. The importance of these observations is that they show that such tissues do not have strong fibers that can prevent the accumulation of fluid. Therefore, some other mechanism, such as a low compliance system, must be available to prevent such fluid accumulation.
3. In most natural cavities of the body where there is free fluid in dynamic equilibrium with the surrounding interstitial fluids, the pressures that have been measured have been negative. Some of these are the following:
Intrapleural space: −8 mm Hg
Joint synovial spaces: −4 to −6 mm Hg
Epidural space: −4 to −6 mm Hg
4. The implanted capsule for measuring the interstitial fluid pressure can be used to record dynamic changes in this pressure. The changes are approximately those that one would calculate to occur (1) when the arterial pressure is increased or decreased, (2) when fluid is injected into the surrounding tissue space, or (3) when a highly concentrated colloid osmotic agent is injected into the blood to absorb fluid from the tissue spaces. It is not likely that these dynamic changes could be recorded this accurately unless the capsule pressure closely approximated the true interstitial pressure.

Summary—An Average Value for Negative Interstitial Fluid Pressure in Loose Subcutaneous Tissue
Although the aforementioned different methods give slightly different values for interstitial fluid pressure, there currently is a general belief among most physiologists that the true interstitial fluid pressure in loose subcutaneous tissue is slightly less subatmospheric, averaging about −3 mm Hg.

Pumping by the Lymphatic System Is the Basic Cause of the Negative Interstitial Fluid Pressure
The lymphatic system is discussed later in the chapter, but we need to understand here the basic role that this system plays in determining interstitial fluid pressure. The lymphatic system is a “scavenger” system that removes excess fluid, excess protein molecules, debris, and other matter from the tissue spaces. Normally, when fluid enters the terminal lymphatic capillaries, the lymph vessel walls automatically contract for a few seconds and pump the fluid into the blood circulation. This overall process creates the slight negative pressure that has been measured for fluid in the interstitial spaces.

Plasma Colloid Osmotic Pressure

Proteins in the Plasma Cause Colloid Osmotic Pressure
In the basic discussion of osmotic pressure in Chapter 4 , it was pointed out that only those molecules or ions that fail to pass through the pores of a semipermeable membrane exert osmotic pressure. Because the proteins are the only dissolved constituents in the plasma and interstitial fluids that do not readily pass through the capillary pores, it is the proteins of the plasma and interstitial fluids that are responsible for the osmotic pressures on the two sides of the capillary membrane. To distinguish this osmotic pressure from that which occurs at the cell membrane, it is called either colloid osmotic pressure or oncotic pressure. The term “colloid” osmotic pressure is derived from the fact that a protein solution resembles a colloidal solution despite the fact that it is actually a true molecular solution.

Normal Values for Plasma Colloid Osmotic Pressure
The colloid osmotic pressure of normal human plasma averages about 28 mm Hg; 19 mm of this is caused by molecular effects of the dissolved protein and 9 mm by the Donnan effect— that is, extra osmotic pressure caused by sodium, potassium, and the other cations held in the plasma by the proteins .

Effect of the Different Plasma Proteins on Colloid Osmotic Pressure
The plasma proteins are a mixture that contains albumin, with an average molecular weight of 69,000; globulins, 140,000; and fibrinogen, 400,000. Thus, 1 gram of globulin contains only half as many molecules as 1 gram of albumin, and 1 gram of fibrinogen contains only one sixth as many molecules as 1 gram of albumin. It should be recalled from the discussion of osmotic pressure in Chapter 4 that osmotic pressure is determined by the number of molecules dissolved in a fluid rather than by the mass of these molecules. Therefore, when corrected for number of molecules rather than mass, the following chart gives both the relative mass concentrations (g/dl) of the different types of proteins in normal plasma and their respective contributions to the total plasma colloid osmotic pressure (Πp).
  g/dl Πp (mm Hg) Albumin 4.5 21.8 Globulins 2.5 6.0 Fibrinogen 0.3 0.2 Total 7.3 28.0
Thus, about 80 percent of the total colloid osmotic pressure of the plasma results from the albumin fraction, 20 percent from the globulins, and almost none from the fibrinogen. Therefore, from the point of view of capillary and tissue fluid dynamics, it is mainly albumin that is important.

Interstitial Fluid Colloid Osmotic Pressure
Although the size of the usual capillary pore is smaller than the molecular sizes of the plasma proteins, this is not true of all the pores. Therefore, small amounts of plasma proteins do leak through the pores into the interstitial spaces through pores and by transcytosis in small vesicles.
The total quantity of protein in the entire 12 liters of interstitial fluid of the body is slightly greater than the total quantity of protein in the plasma itself, but because this volume is four times the volume of plasma, the average protein concentration of the interstitial fluid is usually only 40 percent of that in plasma, or about 3 g/dl. Quantitatively, one finds that the average interstitial fluid colloid osmotic pressure for this concentration of proteins is about 8 mm Hg.

Exchange of Fluid Volume Through the Capillary Membrane
Now that the different factors affecting fluid movement through the capillary membrane have been discussed, it is possible to put all these together to see how the capillary system maintains normal fluid volume distribution between the plasma and the interstitial fluid.
The average capillary pressure at the arterial ends of the capillaries is 15 to 25 mmHg greater than at the venous ends. Because of this difference, fluid “filters” out of the capillaries at their arterial ends, but at their venous ends fluid is reabsorbed back into the capillaries. Thus, a small amount of fluid actually “flows” through the tissues from the arterial ends of the capillaries to the venous ends. The dynamics of this flow are as follows.

Analysis of the Forces Causing Filtration at the Arterial End of the Capillary
The approximate average forces operative at the arterial end of the capillary that cause movement through the capillary membrane are shown as follows:
  mm Hg Forces tending to move fluid outward: Capillary pressure (arterial end of capillary) 30 Negative interstitial free fluid pressure 3 Interstitial fluid colloid osmotic pressure 8 TOTAL OUTWARD FORCE 41 Forces tending to move fluid inward: Plasma colloid osmotic pressure 28 TOTAL INWARD FORCE 28 Summation of forces: Outward 41 Inward 28 NET OUTWARD FORCE (AT ARTERIAL END) 13
Thus, the summation of forces at the arterial end of the capillary shows a net filtration pressure of 13 mm Hg, tending to move fluid outward through the capillary pores.
This 13 mm Hg filtration pressure causes, on average, about 1/200 of the plasma in the flowing blood to filter out of the arterial ends of the capillaries into the interstitial spaces each time the blood passes through the capillaries.

Analysis of Reabsorption at the Venous End of the Capillary
The low blood pressure at the venous end of the capillary changes the balance of forces in favor of absorption as follows:
  mm Hg Forces tending to move fluid inward: Plasma colloid osmotic pressure 28 TOTAL INWARD FORCE 28 Forces tending to move fluid outward: Capillary pressure (venous end of capillary) 10 Negative interstitial free fluid pressure 3 Interstitial fluid colloid osmotic pressure 8 TOTAL OUTWARD FORCE 21 Summation of forces: Inward 28 Outward 21 NET INWARD FORCE 7
Thus, the force that causes fluid to move into the capillary, 28 mm Hg, is greater than that opposing reabsorption, 21 mm Hg. The difference, 7 mm Hg, is the net reabsorption pressure at the venous ends of the capillaries. This reabsorption pressure is considerably less than the filtration pressure at the capillary arterial ends, but remember that the venous capillaries are more numerous and more permeable than the arterial capillaries, so that less reabsorption pressure is required to cause inward movement of fluid.
The reabsorption pressure causes about nine tenths of the fluid that has filtered out of the arterial ends of the capillaries to be reabsorbed at the venous ends. The remaining one tenth flows into the lymph vessels and returns to the circulating blood.

Starling Equilibrium for Capillary Exchange
Ernest H. Starling pointed out more than a century ago that under normal conditions, a state of near-equilibrium exists in most capillaries. That is, the amount of fluid filtering outward from the arterial ends of capillaries equals almost exactly the fluid returned to the circulation by absorption. The slight disequilibrium that does occur accounts for the fluid that is eventually returned to the circulation by way of the lymphatics.
The following chart shows the principles of the Starling equilibrium. For this chart, the pressures in the arterial and venous capillaries are averaged to calculate mean functional capillary pressure for the entire length of the capillary. This calculates to be 17.3 mm Hg.
  mm Hg Mean forces tending to move fluid outward: Mean capillary pressure 17.3 Negative interstitial free fluid pressure 3.0 Interstitial fluid colloid osmotic pressure 8.0 TOTAL OUTWARD FORCE 28.3 Mean force tending to move fluid inward: Plasma colloid osmotic pressure 28.0 TOTAL INWARD FORCE 28.0 Summation of mean forces: Outward 28.3 Inward 28.0 NET OUTWARD FORCE 0.3
Thus, for the total capillary circulation, we find a near-equilibrium between the total outward forces, 28.3 mm Hg, and the total inward force, 28.0 mm Hg. This slight imbalance of forces, 0.3 mm Hg, causes slightly more filtration of fluid into the interstitial spaces than reabsorption. This slight excess of filtration is called net filtration, and it is the fluid that must be returned to the circulation through the lymphatics. The normal rate of net filtration in the entire body, not including the kidneys, is only about 2 ml/min.

Filtration Coefficient
In the previous example, an average net imbalance of forces at the capillary membranes of 0.3 mm Hg causes net fluid filtration in the entire body of 2 ml/min. Expressing this for each millimeter of mercury imbalance, one finds a net filtration rate of 6.67 ml/min of fluid per mm Hg for the entire body. This is called the whole body capillary filtration coefficient.
The filtration coefficient can also be expressed for separate parts of the body in terms of rate of filtration per minute per mm Hg per 100 grams of tissue. On this basis, the filtration coefficient of the average tissue is about 0.01 ml/min/mm Hg/100 g of tissue. But, because of extreme differences in permeabilities of the capillary systems in different tissues, this coefficient varies more than 100-fold among the different tissues. It is very small in brain and muscle, moderately large in subcutaneous tissue, large in the intestine, and extreme in the liver and glomerulus of the kidney where the pores are either numerous or wide open. By the same token, the permeation of proteins through the capillary membranes varies greatly as well. The concentration of protein in the interstitial fluid of muscles is about 1.5 g/dl; in subcutaneous tissue, 2 g/dl; in intestine, 4 g/dl; and in liver, 6 g/dl.

Effect of Abnormal Imbalance of Forces at the Capillary Membrane
If the mean capillary pressure rises above 17 mm Hg, the net force tending to cause filtration of fluid into the tissue spaces rises. Thus, a 20 mm Hg rise in mean capillary pressure causes an increase in net filtration pressure from 0.3 mm Hg to 20.3 mm Hg, which results in 68 times as much net filtration of fluid into the interstitial spaces as normally occurs. To prevent accumulation of excess fluid in these spaces would require 68 times the normal flow of fluid into the lymphatic system, an amount that is 2 to 5 times too much for the lymphatics to carry away. As a result, fluid will begin to accumulate in the interstitial spaces and edema will result.
Conversely, if the capillary pressure falls very low, net reabsorption of fluid into the capillaries will occur instead of net filtration and the blood volume will increase at the expense of the interstitial fluid volume. These effects of imbalance at the capillary membrane in relation to the development of different kinds of edema are discussed in Chapter 25 .

Lymphatic System
The lymphatic system represents an accessory route through which fluid can flow from the interstitial spaces into the blood. Most important, the lymphatics can carry proteins and large particulate matter away from the tissue spaces, neither of which can be removed by absorption directly into the blood capillaries. This return of proteins to the blood from the interstitial spaces is an essential function without which we would die within about 24 hours.

Lymph Channels of the Body
Almost all tissues of the body have special lymph channels that drain excess fluid directly from the interstitial spaces. The exceptions include the superficial portions of the skin, the central nervous system, the endomysium of muscles, and the bones. But, even these tissues have minute interstitial channels called prelymphatics through which interstitial fluid can flow; this fluid eventually empties either into lymphatic vessels or, in the case of the brain, into the cerebrospinal fluid and then directly back into the blood.
Essentially all the lymph vessels from the lower part of the body eventually empty into the thoracic duct, which in turn empties into the blood venous system at the juncture of the left internal jugular vein and left subclavian vein, as shown in Figure 16-7 .

Figure 16-7 Lymphatic system.
Lymph from the left side of the head, the left arm, and parts of the chest region also enters the thoracic duct before it empties into the veins.
Lymph from the right side of the neck and head, the right arm, and parts of the right thorax enters the right lymph duct (much smaller than the thoracic duct), which empties into the blood venous system at the juncture of the right subclavian vein and internal jugular vein.

Terminal Lymphatic Capillaries and Their Permeability
Most of the fluid filtering from the arterial ends of blood capillaries flows among the cells and finally is reabsorbed back into the venous ends of the blood capillaries; but on the average, about one tenth of the fluid instead enters the lymphatic capillaries and returns to the blood through the lymphatic system rather than through the venous capillaries. The total quantity of all this lymph is normally only 2 to 3 liters each day.
The fluid that returns to the circulation by way of the lymphatics is extremely important because substances of high molecular weight, such as proteins, cannot be absorbed from the tissues in any other way, although they can enter the lymphatic capillaries almost unimpeded. The reason for this is a special structure of the lymphatic capillaries, demonstrated in Figure 16-8 . This figure shows the endothelial cells of the lymphatic capillary attached by anchoring filaments to the surrounding connective tissue. At the junctions of adjacent endothelial cells, the edge of one endothelial cell overlaps the edge of the adjacent cell in such a way that the overlapping edge is free to flap inward, thus forming a minute valve that opens to the interior of the lymphatic capillary. Interstitial fluid, along with its suspended particles, can push the valve open and flow directly into the lymphatic capillary. But this fluid has difficulty leaving the capillary once it has entered because any backflow closes the flap valve. Thus, the lymphatics have valves at the very tips of the terminal lymphatic capillaries, as well as valves along their larger vessels up to the point where they empty into the blood circulation.

Figure 16-8 Special structure of the lymphatic capillaries that permits passage of substances of high molecular weight into the lymph.

Formation of Lymph
Lymph is derived from interstitial fluid that flows into the lymphatics. Therefore, lymph as it first enters the terminal lymphatics has almost the same composition as the interstitial fluid.
The protein concentration in the interstitial fluid of most tissues averages about 2 g/dl, and the protein concentration of lymph flowing from these tissues is near this value. In the liver, lymph formed has a protein concentration as high as 6 g/dl, and lymph formed in the intestines has a protein concentration as high as 3 to 4 g/dl. Because about two thirds of all lymph normally is derived from the liver and intestines, the thoracic duct lymph, which is a mixture of lymph from all areas of the body, usually has a protein concentration of 3 to 5 g/dl.
The lymphatic system is also one of the major routes for absorption of nutrients from the gastrointestinal tract, especially for absorption of virtually all fats in food, as discussed in Chapter 65 . Indeed, after a fatty meal, thoracic duct lymph sometimes contains as much as 1 to 2 percent fat.
Finally, even large particles, such as bacteria, can push their way between the endothelial cells of the lymphatic capillaries and in this way enter the lymph. As the lymph passes through the lymph nodes, these particles are almost entirely removed and destroyed, as discussed in Chapter 33 .

Rate of Lymph Flow
About 100 milliliters per hour of lymph flows through the thoracic duct of a resting human, and approximately another 20 milliliters flows into the circulation each hour through other channels, making a total estimated lymph flow of about 120 ml/hr or 2 to 3 liters per day.

Effect of Interstitial Fluid Pressure on Lymph Flow
Figure 16-9 shows the effect of different levels of interstitial fluid pressure on lymph flow as measured in dog legs. Note that normal lymph flow is very little at interstitial fluid pressures more negative than the normal value of −6 mm Hg. Then, as the pressure rises to 0 mm Hg (atmospheric pressure), flow increases more than 20-fold. Therefore, any factor that increases interstitial fluid pressure also increases lymph flow if the lymph vessels are functioning normally. Such factors include the following:
⧫ Elevated capillary hydrostatic pressure
⧫ Decreased plasma colloid osmotic pressure
⧫ Increased interstitial fluid colloid osmotic pressure
⧫ Increased permeability of the capillaries

Figure 16-9 Relation between interstitial fluid pressure and lymph flow in the leg of a dog. Note that lymph flow reaches a maximum when the interstitial pressure, P T , rises slightly above atmospheric pressure (0 mm Hg).
(Courtesy Drs. Harry Gibson and Aubrey Taylor.)
All of these cause a balance of fluid exchange at the blood capillary membrane to favor fluid movement into the interstitium, thus increasing interstitial fluid volume, interstitial fluid pressure, and lymph flow all at the same time.
However, note in Figure 16-9 that when the interstitial fluid pressure becomes 1 or 2 mm Hg greater than atmospheric pressure (>0 mm Hg), lymph flow fails to rise any further at still higher pressures. This results from the fact that the increasing tissue pressure not only increases entry of fluid into the lymphatic capillaries but also compresses the outside surfaces of the larger lymphatics, thus impeding lymph flow. At the higher pressures, these two factors balance each other almost exactly, so lymph flow reaches what is called the “maximum lymph flow rate.” This is illustrated by the upper level plateau in Figure 16-9 .

Lymphatic Pump Increases Lymph Flow
Valves exist in all lymph channels; typical valves are shown in Figure 16-10 in collecting lymphatics into which the lymphatic capillaries empty.

Figure 16-10 Structure of lymphatic capillaries and a collecting lymphatic, showing also the lymphatic valves.
Motion pictures of exposed lymph vessels in animals and in human beings show that when a collecting lymphatic or larger lymph vessel becomes stretched with fluid, the smooth muscle in the wall of the vessel automatically contracts. Furthermore, each segment of the lymph vessel between successive valves functions as a separate automatic pump. That is, even slight filling of a segment causes it to contract and the fluid is pumped through the next valve into the next lymphatic segment. This fills the subsequent segment, and a few seconds later it, too, contracts, the process continuing all along the lymph vessel until the fluid is finally emptied into the blood circulation. In a very large lymph vessel such as the thoracic duct, this lymphatic pump can generate pressures as great as 50 to 100 mm Hg.

Pumping Caused by External Intermittent Compression of the Lymphatics
In addition to the pumping caused by intrinsic intermittent contraction of the lymph vessel walls, any external factor that intermittently compresses the lymph vessel also can cause pumping. In order of their importance, such factors are as follows:
⧫ Contraction of surrounding skeletal muscles
⧫ Movement of the parts of the body
⧫ Pulsations of arteries adjacent to the lymphatics
⧫ Compression of the tissues by objects outside the body
The lymphatic pump becomes very active during exercise, often increasing lymph flow 10- to 30-fold. Conversely, during periods of rest, lymph flow is sluggish, almost zero.

Lymphatic Capillary Pump
The terminal lymphatic capillary is also capable of pumping lymph, in addition to the pumping by the larger lymph vessels. As explained earlier in the chapter, the walls of the lymphatic capillaries are tightly adherent to the surrounding tissue cells by means of their anchoring filaments. Therefore, each time excess fluid enters the tissue and causes the tissue to swell, the anchoring filaments pull on the wall of the lymphatic capillary and fluid flows into the terminal lymphatic capillary through the junctions between the endothelial cells. Then, when the tissue is compressed, the pressure inside the capillary increases and causes the overlapping edges of the endothelial cells to close like valves. Therefore, the pressure pushes the lymph forward into the collecting lymphatic instead of backward through the cell junctions.
The lymphatic capillary endothelial cells also contain a few contractile actomyosin filaments. In some animal tissues (e.g., the bat’s wing) these filaments have been observed to cause rhythmical contraction of the lymphatic capillaries in the same way that many of the small blood and larger lymphatic vessels also contract rhythmically. Therefore, it is probable that at least part of lymph pumping results from lymph capillary endothelial cell contraction in addition to contraction of the larger muscular lymphatics.

Summary of Factors That Determine Lymph Flow
From the previous discussion, one can see that the two primary factors that determine lymph flow are (1) the interstitial fluid pressure and (2) the activity of the lymphatic pump. Therefore, one can state that, roughly, the rate of lymph flow is determined by the product of interstitial fluid pressure times the activity of the lymphatic pump .

Role of the Lymphatic System in Controlling Interstitial Fluid Protein Concentration, Interstitial Fluid Volume, and Interstitial Fluid Pressure
It is already clear that the lymphatic system functions as an “overflow mechanism” to return to the circulation excess proteins and excess fluid volume from the tissue spaces. Therefore, the lymphatic system also plays a central role in controlling (1) the concentration of proteins in the interstitial fluids, (2) the volume of interstitial fluid, and (3) the interstitial fluid pressure. Let us explain how these factors interact.
First, remember that small amounts of proteins leak continuously out of the blood capillaries into the interstitium. Only minute amounts, if any, of the leaked proteins return to the circulation by way of the venous ends of the blood capillaries. Therefore, these proteins tend to accumulate in the interstitial fluid, and this in turn increases the colloid osmotic pressure of the interstitial fluids.
Second, the increasing colloid osmotic pressure in the interstitial fluid shifts the balance of forces at the blood capillary membranes in favor of fluid filtration into the interstitium. Therefore, in effect, fluid is translocated osmotically outward through the capillary wall by the proteins and into the interstitium, thus increasing both interstitial fluid volume and interstitial fluid pressure.
Third, the increasing interstitial fluid pressure greatly increases the rate of lymph flow, as explained previously. This in turn carries away the excess interstitial fluid volume and excess protein that has accumulated in the spaces.
Thus, once the interstitial fluid protein concentration reaches a certain level and causes a comparable increase in interstitial fluid volume and interstitial fluid pressure, the return of protein and fluid by way of the lymphatic system becomes great enough to balance exactly the rate of leakage of these into the interstitium from the blood capillaries. Therefore, the quantitative values of all these factors reach a steady state; they will remain balanced at these steady state levels until something changes the rate of leakage of proteins and fluid from the blood capillaries.

Significance of Negative Interstitial Fluid Pressure as a Means for Holding the Body Tissues Together
Traditionally, it has been assumed that the different tissues of the body are held together entirely by connective tissue fibers. However, at many places in the body, connective tissue fibers are very weak or even absent. This occurs particularly at points where tissues slide over one another, such as the skin sliding over the back of the hand or over the face. Yet even at these places, the tissues are held together by the negative interstitial fluid pressure, which is actually a partial vacuum. When the tissues lose their negative pressure, fluid accumulates in the spaces and the condition known as edema occurs. This is discussed in Chapter 25 .


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CHAPTER 17 Local and Humoral Control of Tissue Blood Flow

Local Control of Blood Flow in Response to Tissue Needs
One of the most fundamental principles of circulatory function is the ability of each tissue to control its own local blood flow in proportion to its metabolic needs.
What are some of the specific needs of the tissues for blood flow? The answer to this is manyfold, including the following:
1. Delivery of oxygen to the tissues.
2. Delivery of other nutrients, such as glucose, amino acids, and fatty acids.
3. Removal of carbon dioxide from the tissues.
4. Removal of hydrogen ions from the tissues.
5. Maintenance of proper concentrations of other ions in the tissues.
6. Transport of various hormones and other substances to the different tissues.
Certain organs have special requirements. For instance, blood flow to the skin determines heat loss from the body and in this way helps to control body temperature. Also, delivery of adequate quantities of blood plasma to the kidneys allows the kidneys to excrete the waste products of the body and to regulate body fluid volumes and electrolytes.
We shall see that these factors exert extreme degrees of local blood flow control and that different tissues place different levels of importance on these factors in controlling blood flow.

Variations in Blood Flow in Different Tissues and Organs
Note in Table 17-1 the very large blood flows in some organs—for example, several hundred ml/min per 100 g of thyroid or adrenal gland tissue and a total blood flow of 1350 ml/min in the liver, which is 95 ml/min/100 g of liver tissue.

Table 17-1 Blood Flow to Different Organs and Tissues Under Basal Conditions
Also note the extremely large blood flow through the kidneys—1100 ml/min. This extreme amount of flow is required for the kidneys to perform their function of cleansing the blood of waste products.
Conversely, most surprising is the low blood flow to all the inactive muscles of the body, only a total of 750 ml/min, even though the muscles constitute between 30 and 40 percent of the total body mass. In the resting state, the metabolic activity of the muscles is very low, and so also is the blood flow, only 4 ml/min/100 g. Yet, during heavy exercise, muscle metabolic activity can increase more than 60-fold and the blood flow as much as 20-fold, increasing to as high as 16,000 ml/min in the body’s total muscle vascular bed (or 80 ml/min/100 g of muscle).

Importance of Blood Flow Control by the Local Tissues
One might ask the simple question: Why not simply allow a very large blood flow all the time through every tissue of the body, always enough to supply the tissue’s needs whether the activity of the tissue is little or great? The answer is equally simple: To do this would require many times more blood flow than the heart can pump.
Experiments have shown that the blood flow to each tissue usually is regulated at the minimal level that will supply the tissue’s requirements—no more, no less. For instance, in tissues for which the most important requirement is delivery of oxygen, the blood flow is always controlled at a level only slightly more than required to maintain full tissue oxygenation but no more than this. By controlling local blood flow in such an exact way, the tissues almost never suffer from oxygen nutritional deficiency and the workload on the heart is kept at a minimum.

Mechanisms of Blood Flow Control
Local blood flow control can be divided into two phases: (1) acute control and (2) long-term control.
Acute control is achieved by rapid changes in local vasodilation or vasoconstriction of the arterioles, metarterioles, and precapillary sphincters, occurring within seconds to minutes to provide very rapid maintenance of appropriate local tissue blood flow.
Long-term control, however, means slow, controlled changes in flow over a period of days, weeks, or even months. In general, these long-term changes provide even better control of the flow in proportion to the needs of the tissues. These changes come about as a result of an increase or decrease in the physical sizes and numbers of actual blood vessels supplying the tissues.

Acute Control of Local Blood Flow

Effect of Tissue Metabolism on Local Blood Flow
Figure 17-1 shows the approximate acute effect on blood flow of increasing the rate of metabolism in a local tissue, such as in a skeletal muscle. Note that an increase in metabolism up to eight times normal increases the blood flow acutely about fourfold.

Figure 17-1 Effect of increasing rate of metabolism on tissue blood flow.

Acute Local Blood Flow Regulation When Oxygen Availability Changes
One of the most necessary of the metabolic nutrients is oxygen. Whenever the availability of oxygen to the tissues decreases, such as (1) at high altitude at the top of a high mountain, (2) in pneumonia, (3) in carbon monoxide poisoning (which poisons the ability of hemoglobin to transport oxygen), or (4) in cyanide poisoning (which poisons the ability of the tissues to use oxygen), the blood flow through the tissues increases markedly. Figure 17-2 shows that as the arterial oxygen saturation decreases to about 25 percent of normal, the blood flow through an isolated leg increases about threefold; that is, the blood flow increases almost enough, but not quite enough, to make up for the decreased amount of oxygen in the blood, thus almost maintaining a relatively constant supply of oxygen to the tissues.

Figure 17-2 Effect of decreasing arterial oxygen saturation on blood flow through an isolated dog leg.
Total cyanide poisoning of oxygen usage by a local tissue area can cause local blood flow to increase as much as sevenfold, thus demonstrating the extreme effect of oxygen deficiency to increase blood flow.
There are two basic theories for the regulation of local blood flow when either the rate of tissue metabolism changes or the availability of oxygen changes. They are (1) the vasodilator theory and (2) the oxygen lack theory.

Vasodilator Theory for Acute Local Blood Flow Regulation—Possible Special Role of Adenosine
According to this theory, the greater the rate of metabolism or the less the availability of oxygen or some other nutrients to a tissue, the greater the rate of formation of vasodilator substances in the tissue cells . The vasodilator substances then are believed to diffuse through the tissues to the precapillary sphincters, metarterioles, and arterioles to cause dilation. Some of the different vasodilator substances that have been suggested are adenosine, carbon dioxide, adenosine phosphate compounds, histamine, potassium ions, and hydrogen ions.
Vasodilator substances may be released from the tissue in response to oxygen deficiency. For instance, experiments have shown that decreased availability of oxygen can cause both adenosine and lactic acid (containing hydrogen ions) to be released into the spaces between the tissue cells; these substances then cause intense acute vasodilation and therefore are responsible, or partially responsible, for the local blood flow regulation. Vasodilator substances, such as carbon dioxide, lactic acid, and potassium ions, tend to increase in the tissues when blood flow is reduced and cell metabolism continues at the same rate, or when cell metabolism is suddenly increased. As the concentration of vasodilator metabolites increases, this causes vasodilation of the arterioles, increasing the tissue blood flow and returning the tissue concentration of the metabolites toward normal.
Many physiologists believe that adenosine is an important local vasodilator for controlling local blood flow. For example, minute quantities of adenosine are released from heart muscle cells when coronary blood flow becomes too little, and this causes enough local vasodilation in the heart to return coronary blood flow back to normal. Also, whenever the heart becomes more active than normal and the heart’s metabolism increases an extra amount, this, too, causes increased utilization of oxygen, followed by (1) decreased oxygen concentration in the heart muscle cells with (2) consequent degradation of adenosine triphosphate (ATP), which (3) increases the release of adenosine. It is believed that much of this adenosine leaks out of the heart muscle cells to cause coronary vasodilation, providing increased coronary blood flow to supply the increased nutrient demands of the active heart.
Although research evidence is less clear, many physiologists also have suggested that the same adenosine mechanism is an important controller of blood flow in skeletal muscle and many other tissues, as well as in the heart. It has been difficult, however, to prove that sufficient quantities of any single vasodilator substance, including adenosine, are indeed formed in the tissues to cause all the measured increase in blood flow. It is likely that a combination of several different vasodilators released by the tissues contributes to blood flow regulation.

Oxygen Lack Theory for Local Blood Flow Control
Although the vasodilator theory is widely accepted, several critical facts have made other physiologists favor still another theory, which can be called either the oxygen lack theory or, more accurately, the nutrient lack theory (because other nutrients besides oxygen are involved). Oxygen (and other nutrients as well) is required as one of the metabolic nutrients to cause vascular muscle contraction. Therefore, in the absence of adequate oxygen, it is reasonable to believe that the blood vessels simply would relax and therefore naturally dilate. Also, increased utilization of oxygen in the tissues as a result of increased metabolism theoretically could decrease the availability of oxygen to the smooth muscle fibers in the local blood vessels, and this, too, would cause local vasodilation.
A mechanism by which the oxygen lack theory could operate is shown in Figure 17-3 . This figure shows a tissue unit, consisting of a metarteriole with a single sidearm capillary and its surrounding tissue. At the origin of the capillary is a precapillary sphincter, and around the metarteriole are several other smooth muscle fibers. Observing such a tissue under a microscope—for example, in a bat’s wing—one sees that the precapillary sphincters are normally either completely open or completely closed. The number of precapillary sphincters that are open at any given time is roughly proportional to the requirements of the tissue for nutrition. The precapillary sphincters and metarterioles open and close cyclically several times per minute, with the duration of the open phases being proportional to the metabolic needs of the tissues for oxygen. The cyclical opening and closing is called vasomotion.

Figure 17-3 Diagram of a tissue unit area for explanation of acute local feedback control of blood flow, showing a metarteriole passing through the tissue and a sidearm capillary with its precapillary sphincter for controlling capillary blood flow.
Let us explain how oxygen concentration in the local tissue could regulate blood flow through the area. Because smooth muscle requires oxygen to remain contracted, one might assume that the strength of contraction of the sphincters would increase with an increase in oxygen concentration. Consequently, when the oxygen concentration in the tissue rises above a certain level, the precapillary and metarteriole sphincters presumably would close until the tissue cells consume the excess oxygen. But when the excess oxygen is gone and the oxygen concentration falls low enough, the sphincters would open once more to begin the cycle again.
Thus, on the basis of available data, either a vasodilator substance theory or an oxygen lack theory could explain acute local blood flow regulation in response to the metabolic needs of the tissues. Probably the truth lies in a combination of the two mechanisms.

Possible Role of Other Nutrients Besides Oxygen in Control of Local Blood Flow
Under special conditions, it has been shown that lack of glucose in the perfusing blood can cause local tissue vasodilation. Also, it is possible that this same effect occurs when other nutrients, such as amino acids or fatty acids, are deficient, although this has not been studied adequately. In addition, vasodilation occurs in the vitamin deficiency disease beriberi, in which the patient has deficiencies of the vitamin B substances thiamine, niacin, and riboflavin. In this disease, the peripheral vascular blood flow almost everywhere in the body often increases twofold to threefold. Because all these vitamins are necessary for oxygen-induced phosphorylation, which is required to produce ATP in the tissue cells, one can well understand how deficiency of these vitamins might lead to diminished smooth muscle contractile ability and therefore also local vasodilation.

Special Examples of Acute “Metabolic” Control of Local Blood Flow
The mechanisms that we have described thus far for local blood flow control are called “metabolic mechanisms” because all of them function in response to the metabolic needs of the tissues. Two additional special examples of metabolic control of local blood flow are reactive hyperemia and active hyperemia.

Reactive Hyperemia
When the blood supply to a tissue is blocked for a few seconds to as long as an hour or more and then is unb