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Guyton and Hall Textbook of Medical Physiology
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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
T w e l f T h e d i T i o n
1600 John F. Kennedy Blvd. Ste 1800 Philadelphia, PA 19103-2899
TEXTBOOK OF MEDICAL PHYSIOLOGY 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: [email protected]. You may also complete your request on-line via the Elsevier website at http://www.elsevier.com/permissions.
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 612–dc22 2009035327
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
Notice 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
To My Family
For their abundant support, for their patience and understanding, and for their love
To 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
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vii
Preface
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 physi- ology 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 possi- ble to explain many physiology principles in the termi- nology 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 compen- dium 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 physiol- ogy needed to begin a career in the health care profes- sions, 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 orga- nization 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 stim- ulates 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 differ- ent 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 isola- tion 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 bod- ies are endowed with a vast network of feedback con- trols 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 distur- bance 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 mech- anisms 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, physi- ologists, 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 infor- mation, 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 func- tion 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 sin- cere thanks.
Preface
viii
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 ref- erences have been chosen primarily for their presentation of physiologic principles, for the quality of their own refer- ences, and for their easy accessibility. The selected biblio- graphy 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 http://www. ncbi.nlm.nih.gov/sites/entrez/. 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 physi- ologic 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, physi- ologic 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 activi- ties of the department can be found at the web site: http:// physiology.umc.edu/. 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.
John E. Hall
ix
Contents
UNIT I
Introduction to Physiology: The Cell and General Physiology
CHAPTER 1
Functional Organization of the Human Body and Control of the “Internal Environment” 3
Cells as the Living Units of the Body 3
Extracellular Fluid—The “Internal Environment” 3
“Homeostatic” Mechanisms of the Major Functional Systems 4
Control Systems of the Body 6
Summary—Automaticity of the Body 9
CHAPTER 2
The Cell and Its Functions 11
Organization of the Cell 11
Physical Structure of the Cell 12
Comparison of the Animal Cell with Precellular Forms of Life 17
Functional Systems of the Cell 18
Locomotion of Cells 23
CHAPTER 3
Genetic Control of Protein Synthesis, Cell Function, and Cell Reproduction 27
Genes in the Cell Nucleus 27
The DNA Code in the Cell Nucleus Is Transferred to an RNA Code in the Cell Cytoplasm—The Process of Transcription 30
Synthesis of Other Substances in the Cell 35
Control of Gene Function and Biochemical Activity in Cells 35
The DNA-Genetic System Also Controls Cell Reproduction 37
Cell Differentiation 39
Apoptosis—Programmed Cell Death 40
Cancer 40
UNIT II
Membrane Physiology, Nerve, and Muscle
CHAPTER 4
Transport of Substances Through Cell Membranes 45
The Lipid Barrier of the Cell Membrane, and Cell Membrane Transport Proteins 45
Diffusion 46
“Active Transport” of Substances Through Membranes 52
CHAPTER 5 Membrane Potentials and Action Potentials 57
Basic Physics of Membrane Potentials 57
Measuring the Membrane Potential 58
Resting Membrane Potential of Nerves 59
Nerve Action Potential 60
Roles of Other Ions During the Action Potential 64
Propagation of the Action Potential 64
Re-establishing Sodium and Potassium Ionic Gradients After Action Potentials Are Completed—Importance of Energy Metabolism 65
Plateau in Some Action Potentials 66
Rhythmicity of Some Excitable Tissues— Repetitive Discharge 66
Special Characteristics of Signal Transmission in Nerve Trunks 67
Excitation—The Process of Eliciting the Action Potential 68
Recording Membrane Potentials and Action Potentials 69
Contents
x
CHAPTER 6
Contraction of Skeletal Muscle 71
Physiologic Anatomy of Skeletal Muscle 71
General Mechanism of Muscle Contraction 73
Molecular Mechanism of Muscle Contraction 74
Energetics of Muscle Contraction 78
Characteristics of Whole Muscle Contraction 79
CHAPTER 7
Excitation of Skeletal Muscle: Neuromuscular Transmission and Excitation-Contraction Coupling 83
Transmission of Impulses from Nerve Endings to Skeletal Muscle Fibers: The Neuromuscular Junction 83
Molecular Biology of Acetylcholine Formation and Release 86
Drugs That Enhance or Block Transmission at the Neuromuscular Junction 86
Myasthenia Gravis Causes Muscle Paralysis 86
Muscle Action Potential 87
Excitation-Contraction Coupling 88
CHAPTER 8
Excitation and Contraction of Smooth Muscle 91
Contraction of Smooth Muscle 91
Nervous and Hormonal Control of Smooth Muscle Contraction 94
UNIT III
The Heart
CHAPTER 9
Cardiac Muscle; The Heart as a Pump and Function of the Heart Valves 101
Physiology of Cardiac Muscle 101
Cardiac Cycle 104
Relationship of the Heart Sounds to Heart Pumping 107
Work Output of the Heart 107
Chemical Energy Required for Cardiac Contraction: Oxygen Utilization by the Heart 109
Regulation of Heart Pumping 110
CHAPTER 10
Rhythmical Excitation of the Heart 115
Specialized Excitatory and Conductive System of the Heart 115
Control of Excitation and Conduction in the Heart 118
CHAPTER 11
The Normal Electrocardiogram 121
Characteristics of the Normal Electrocardiogram 121
Methods for Recording Electrocardiograms 123
Flow of Current Around the Heart during the Cardiac Cycle 123
Electrocardiographic Leads 124
CHAPTER 12
Electrocardiographic Interpretation of Cardiac Muscle and Coronary Blood Flow Abnormalities: Vectorial Analysis 129
Principles of Vectorial Analysis of Electrocardiograms 129
Vectorial Analysis of the Normal Electrocardiogram 131
Mean Electrical Axis of the Ventricular QRS—and Its Significance 134
Conditions That Cause Abnormal Voltages of the QRS Complex 137
Prolonged and Bizarre Patterns of the QRS Complex 137
Current of Injury 138
Abnormalities in the T Wave 141
CHAPTER 13
Cardiac Arrhythmias and Their Electrocardiographic Interpretation 143
Abnormal Sinus Rhythms 143
Abnormal Rhythms That Result from Block of Heart Signals Within the Intracardiac Conduction Pathways 144
Premature Contractions 146
Paroxysmal Tachycardia 148
Ventricular Fibrillation 149
Atrial Fibrillation 151
Atrial Flutter 152
Cardiac Arrest 153
UNIT IV
The Circulation
CHAPTER 14
Overview of the Circulation; Biophysics of Pressure, Flow, and Resistance 157
Physical Characteristics of the Circulation 157
Basic Principles of Circulatory Function 158
Interrelationships of Pressure, Flow, and Resistance 159
Contents
xi
CHAPTER 15
Vascular Distensibility and Functions of the Arterial and Venous Systems 167
Vascular Distensibility 167
Arterial Pressure Pulsations 168
Veins and Their Functions 171
CHAPTER 16
The Microcirculation and Lymphatic System: Capillary Fluid Exchange, Interstitial Fluid, and Lymph Flow 177
Structure of the Microcirculation and Capillary System 177
Flow of Blood in the Capillaries— Vasomotion 178
Exchange of Water, Nutrients, and Other Substances Between the Blood and Interstitial Fluid 179
Interstitium and Interstitial Fluid 180
Fluid Filtration Across Capillaries Is Determined by Hydrostatic and Colloid Osmotic Pressures, as Well as Capillary Filtration Coefficient 181
Lymphatic System 186
CHAPTER 17
Local and Humoral Control of Tissue Blood Flow 191
Local Control of Blood Flow in Response to Tissue Needs 191
Mechanisms of Blood Flow Control 191
Humoral Control of the Circulation 199
CHAPTER 18
Nervous Regulation of the Circulation, and Rapid Control of Arterial Pressure 201
Nervous Regulation of the Circulation 201
Role of the Nervous System in Rapid Control of Arterial Pressure 204
Special Features of Nervous Control of Arterial Pressure 209
CHAPTER 19
Role of the Kidneys in Long-Term Control of Arterial Pressure and in Hypertension: The Integrated System for Arterial Pressure Regulation 213
Renal–Body Fluid System for Arterial Pressure Control 213
The Renin-Angiotensin System: Its Role in Arterial Pressure Control 220
Summary of the Integrated, Multifaceted System for Arterial Pressure Regulation 226
CHAPTER 20
Cardiac Output, Venous Return, and Their Regulation 229
Normal Values for Cardiac Output at Rest and During Activity 229
Control of Cardiac Output by Venous Return—Role of the Frank-Starling Mechanism of the Heart 229
Pathologically High or Low Cardiac Outputs 232
Methods for Measuring Cardiac Output 240
CHAPTER 21
Muscle Blood Flow and Cardiac Output During Exercise; the Coronary Circulation and Ischemic Heart Disease 243
Blood Flow Regulation in Skeletal Muscle at Rest and During Exercise 243
Coronary Circulation 246
CHAPTER 22
Cardiac Failure 255
Circulatory Dynamics in Cardiac Failure 255
Unilateral Left Heart Failure 259
Low-Output Cardiac Failure— Cardiogenic Shock 259
Edema in Patients with Cardiac Failure 259
Cardiac Reserve 261
CHAPTER 23
Heart Valves and Heart Sounds; Valvular and Congenital Heart Defects 265
Heart Sounds 265
Abnormal Circulatory Dynamics in Valvular Heart Disease 268
Abnormal Circulatory Dynamics in Congenital Heart Defects 269
Use of Extracorporeal Circulation During Cardiac Surgery 271
Hypertrophy of the Heart in Valvular and Congenital Heart Disease 272
CHAPTER 24
Circulatory Shock and Its Treatment 273
Physiologic Causes of Shock 273
Shock Caused by Hypovolemia— Hemorrhagic Shock 274
Neurogenic Shock—Increased Vascular Capacity 279
Anaphylactic Shock and Histamine Shock 280
Septic Shock 280
Contents
xii
Physiology of Treatment in Shock 280
Circulatory Arrest 281
UNIT V
The Body Fluids and Kidneys
CHAPTER 25
The Body Fluid Compartments: Extracellular and Intracellular Fluids; Edema 285
Fluid Intake and Output Are Balanced During Steady-State Conditions 285
Body Fluid Compartments 286
Extracellular Fluid Compartment 287
Blood Volume 287
Constituents of Extracellular and Intracellular Fluids 287
Measurement of Fluid Volumes in the Different Body Fluid Compartments—the Indicator- Dilution Principle 287
Determination of Volumes of Specific Body Fluid Compartments 289
Regulation of Fluid Exchange and Osmotic Equilibrium Between Intracellular and Extracellular Fluid 290
Basic Principles of Osmosis and Osmotic Pressure 290
Osmotic Equilibrium Is Maintained Between Intracellular and Extracellular Fluids 291
Volume and Osmolality of Extracellular and Intracellular Fluids in Abnormal States 292
Glucose and Other Solutions Administered for Nutritive Purposes 294
Clinical Abnormalities of Fluid Volume Regulation: Hyponatremia and Hypernatremia 294
Edema: Excess Fluid in the Tissues 296
Fluids in the “Potential Spaces” of the Body 300
CHAPTER 26
Urine Formation by the Kidneys: I. Glomerular Filtration, Renal Blood Flow, and Their Control 303
Multiple Functions of the Kidneys 303
Physiologic Anatomy of the Kidneys 304
Micturition 307
Physiologic Anatomy of the Bladder 307
Transport of Urine from the Kidney Through the Ureters and into the Bladder 308
Filling of the Bladder and Bladder Wall Tone; the Cystometrogram 309
Micturition Reflex 309
Abnormalities of Micturition 310
Urine Formation Results from Glomerular Filtration, Tubular Reabsorption, and Tubular Secretion 310
Glomerular Filtration—The First Step in Urine Formation 312
Determinants of the GFR 314
Renal Blood Flow 316
Physiologic Control of Glomerular Filtration and Renal Blood Flow 317
Autoregulation of GFR and Renal Blood Flow 319
CHAPTER 27
Urine Formation by the Kidneys: II. Tubular Reabsorption and Secretion 323
Renal Tubular Reabsorption and Secretion 323
Tubular Reabsorption Includes Passive and Active Mechanisms 323
Reabsorption and Secretion Along Different Parts of the Nephron 329
Regulation of Tubular Reabsorption 334
Use of Clearance Methods to Quantify Kidney Function 340
CHAPTER 28
Urine Concentration and Dilution; Regulation of Extracellular Fluid Osmolarity and Sodium Concentration 345
Kidneys Excrete Excess Water by Forming Dilute Urine 345
Kidneys Conserve Water by Excreting Concentrated Urine 346
Quantifying Renal Urine Concentration and Dilution: “Free Water” and Osmolar Clearances 354
Disorders of Urinary Concentrating Ability 354
Control of Extracellular Fluid Osmolarity and Sodium Concentration 355
Osmoreceptor-ADH Feedback System 355
Importance of Thirst in Controlling Extracellular Fluid Osmolarity and Sodium Concentration 357
Salt-Appetite Mechanism for Controlling Extracellular Fluid Sodium Concentration and Volume 360
CHAPTER 29
Renal Regulation of Potassium, Calcium, Phosphate, and Magnesium; Integration of Renal Mechanisms for Control of Blood Volume and Extracellular Fluid Volume 361
Regulation of Extracellular Fluid Potassium Concentration and Potassium Excretion 361
Contents
xiii
Control of Renal Calcium Excretion and Extracellular Calcium Ion Concentration 367
Control of Renal Magnesium Excretion and Extracellular Magnesium Ion Concentration 369
Integration of Renal Mechanisms for Control of Extracellular Fluid 370
Importance of Pressure Natriuresis and Pressure Diuresis in Maintaining Body Sodium and Fluid Balance 371
Distribution of Extracellular Fluid Between the Interstitial Spaces and Vascular System 373
Nervous and Hormonal Factors Increase the Effectiveness of Renal–Body Fluid Feedback Control 373
Integrated Responses to Changes in Sodium Intake 376
Conditions That Cause Large Increases in Blood Volume and Extracellular Fluid Volume 376
Conditions That Cause Large Increases in Extracellular Fluid Volume but with Normal Blood Volume 377
CHAPTER 30
Acid-Base Regulation 379
H+ Concentration Is Precisely Regulated 379
Acids and Bases—Their Definitions and Meanings 379
Defending Against Changes in H+ Concentration: Buffers, Lungs, and Kidneys 380
Buffering of H+ in the Body Fluids 380
Bicarbonate Buffer System 381
Phosphate Buffer System 383
Proteins Are Important Intracellular Buffers 383
Respiratory Regulation of Acid-Base Balance 384
Renal Control of Acid-Base Balance 385
Secretion of H+ and Reabsorption of HCO 3 −
by the Renal Tubules 386
Combination of Excess H+ with Phosphate and Ammonia Buffers in the Tubule Generates “New” HCO
3 − 388
Quantifying Renal Acid-Base Excretion 389
Renal Correction of Acidosis—Increased Excretion of H+ and Addition of HCO
3 − to
the Extracellular Fluid 391
Renal Correction of Alkalosis—Decreased Tubular Secretion of H+ and Increased Excretion of HCO
3 − 391
Clinical Causes of Acid-Base Disorders 392 Treatment of Acidosis or Alkalosis 393 Clinical Measurements and Analysis of Acid-Base Disorders 393
CHAPTER 31
Diuretics, Kidney Diseases 397 Diuretics and Their Mechanisms of Action 397
Kidney Diseases 399 Acute Renal Failure 399 Chronic Renal Failure: An Irreversible Decrease in the Number of Functional Nephrons 401 Specific Tubular Disorders 408 Treatment of Renal Failure by Transplantation or by Dialysis with an Artificial Kidney 409
UNIT VI
Blood Cells, Immunity, and Blood Coagulation
CHAPTER 32
Red Blood Cells, Anemia, and Polycythemia 413 Red Blood Cells (Erythrocytes) 413 Anemias 420 Polycythemia 421
CHAPTER 33
Resistance of the Body to Infection: I. Leukocytes, Granulocytes, the Monocyte- Macrophage System, and Inflammation 423 Leukocytes (White Blood Cells) 423 Neutrophils and Macrophages Defend Against Infections 425 Monocyte-Macrophage Cell System (Reticuloendothelial System) 426 Inflammation: Role of Neutrophils and Macrophages 428 Eosinophils 430 Basophils 431 Leukopenia 431 Leukemias 431
CHAPTER 34
Resistance of the Body to Infection: II. Immunity and Allergy Innate Immunity 433
Acquired (Adaptive) Immunity 433
Allergy and Hypersensitivity 443
CHAPTER 35
Blood Types; Transfusion; Tissue and Organ Transplantation 445 Antigenicity Causes Immune Reactions of Blood 445 O-A-B Blood Types 445 Rh Blood Types 447 Transplantation of Tissues and Organs 449
Contents
xiv
CHAPTER 36
Hemostasis and Blood Coagulation 451 Events in Hemostasis 451 Vascular Constriction 451
Mechanism of Blood Coagulation 453
Conditions That Cause Excessive Bleeding in Humans 457
Thromboembolic Conditions in the Human Being 459
Anticoagulants for Clinical Use 459
Blood Coagulation Tests 460
UNIT VII
Respiration
CHAPTER 37
Pulmonary Ventilation 465
Mechanics of Pulmonary Ventilation 465
Pulmonary Volumes and Capacities 469
Minute Respiratory Volume Equals Respiratory Rate Times Tidal Volume 471
Alveolar Ventilation 471
Functions of the Respiratory Passageways 472
CHAPTER 38
Pulmonary Circulation, Pulmonary Edema, Pleural Fluid 477
Physiologic Anatomy of the Pulmonary Circulatory System 477
Pressures in the Pulmonary System 477
Blood Volume of the Lungs 478
Blood Flow Through the Lungs and Its Distribution 479
Effect of Hydrostatic Pressure Gradients in the Lungs on Regional Pulmonary Blood Flow 479
Pulmonary Capillary Dynamics 481
Fluid in the Pleural Cavity 483
CHAPTER 39
Physical Principles of Gas Exchange; Diffusion of Oxygen and Carbon Dioxide Through the Respiratory Membrane 485
Physics of Gas Diffusion and Gas Partial Pressures 485
Compositions of Alveolar Air and Atmospheric Air Are Different 487
Diffusion of Gases Through the Respiratory Membrane 489
Effect of the Ventilation-Perfusion Ratio on Alveolar Gas Concentration 492
CHAPTER 40
Transport of Oxygen and Carbon Dioxide in Blood and Tissue Fluids 495
Transport of Oxygen from the Lungs to the Body Tissues 495
Transport of Carbon Dioxide in the Blood 502
Respiratory Exchange Ratio 504
CHAPTER 41
Regulation of Respiration 505
Respiratory Center 505
Chemical Control of Respiration 507
Peripheral Chemoreceptor System for Control of Respiratory Activity—Role of Oxygen in Respiratory Control 508
Regulation of Respiration During Exercise 510
Other Factors That Affect Respiration 512
CHAPTER 42
Respiratory Insufficiency—Pathophysiology, Diagnosis, Oxygen Therapy 515
Useful Methods for Studying Respiratory Abnormalities 515
Pathophysiology of Specific Pulmonary Abnormalities 517
Hypoxia and Oxygen Therapy 520
Hypercapnia—Excess Carbon Dioxide in the Body Fluids 522 Artificial Respiration 522
UNIT VIII
Aviation, Space, and Deep-Sea Diving Physiology
CHAPTER 43
Aviation, High-Altitude, and Space Physiology 527
Effects of Low Oxygen Pressure on the Body 527
Effects of Acceleratory Forces on the Body in Aviation and Space Physiology 531
“Artificial Climate” in the Sealed Spacecraft 533
Weightlessness in Space 533
CHAPTER 44
Physiology of Deep-Sea Diving and Other Hyperbaric Conditions 535
Effect of High Partial Pressures of Individual Gases on the Body 535
Scuba (Self-Contained Underwater Breathing Apparatus) Diving 539
Special Physiologic Problems in Submarines 540
Hyperbaric Oxygen Therapy 540
Contents
xv
UNIT IX
The Nervous System: A. General Principles and Sensory Physiology
CHAPTER 45
Organization of the Nervous System, Basic Functions of Synapses, and Neurotransmitters 543
General Design of the Nervous System 543
Major Levels of Central Nervous System Function 545
Comparison of the Nervous System with a Computer 546
Central Nervous System Synapses 546
Some Special Characteristics of Synaptic Transmission 557
CHAPTER 46
Sensory Receptors, Neuronal Circuits for Processing Information 559
Types of Sensory Receptors and the Stimuli They Detect 559
Transduction of Sensory Stimuli into Nerve Impulses 560
Nerve Fibers That Transmit Different Types of Signals and Their Physiologic Classification 563
Transmission of Signals of Different Intensity in Nerve Tracts—Spatial and Temporal Summation 564
Transmission and Processing of Signals in Neuronal Pools 564
Instability and Stability of Neuronal Circuits 569
CHAPTER 47
Somatic Sensations: I. General Organization, the Tactile and Position Senses 571
Classification of Somatic Senses 571
Detection and Transmission of Tactile Sensations 571
Sensory Pathways for Transmitting Somatic Signals into the Central Nervous System 573
Transmission in the Dorsal Column–Medial Lemniscal System 573
Transmission of Less Critical Sensory Signals in the Anterolateral Pathway 580
Some Special Aspects of Somatosensory Function 581
CHAPTER 48
Somatic Sensations: II. Pain, Headache, and Thermal Sensations 583
Types of Pain and Their Qualities—Fast Pain and Slow Pain 583
Pain Receptors and Their Stimulation 583
Dual Pathways for Transmission of Pain Signals into the Central Nervous System 584
Pain Suppression (“Analgesia”) System in the Brain and Spinal Cord 586
Referred Pain 588
Visceral Pain 588
Some Clinical Abnormalities of Pain and Other Somatic Sensations 590
Headache 590
Thermal Sensations 592
UNIT X
The Nervous System: B. The Special Senses
CHAPTER 49
The Eye: I. Optics of Vision 597
Physical Principles of Optics 597
Optics of the Eye 600
Ophthalmoscope 605
Fluid System of the Eye—Intraocular Fluid 606
CHAPTER 50
The Eye: II. Receptor and Neural Function of the Retina 609
Anatomy and Function of the Structural Elements of the Retina 609
Photochemistry of Vision 611
Color Vision 615
Neural Function of the Retina 616
CHAPTER 51
The Eye: III. Central Neurophysiology of Vision 623
Visual Pathways 623
Organization and Function of the Visual Cortex 624
Neuronal Patterns of Stimulation During Analysis of the Visual Image 626
Fields of Vision; Perimetry 627
Eye Movements and Their Control 627
Autonomic Control of Accommodation and Pupillary Aperture 631
CHAPTER 52
The Sense of Hearing 633
Tympanic Membrane and the Ossicular System 633
Cochlea 634
Central Auditory Mechanisms 639
Hearing Abnormalities 642
Contents
xvi
CHAPTER 53
The Chemical Senses—Taste and Smell 645
Sense of Taste 645
Sense of Smell 648
UNIT XI
The Nervous System: C. Motor and Integrative Neurophysiology
CHAPTER 54
Motor Functions of the Spinal Cord; the Cord Reflexes 655
Organization of the Spinal Cord for Motor Functions 655
Muscle Sensory Receptors—Muscle Spindles and Golgi Tendon Organs—And Their Roles in Muscle Control 657
Flexor Reflex and the Withdrawal Reflexes 661
Crossed Extensor Reflex 663
Reciprocal Inhibition and Reciprocal Innervation 663
Reflexes of Posture and Locomotion 663
Scratch Reflex 664
Spinal Cord Reflexes That Cause Muscle Spasm 664
Autonomic Reflexes in the Spinal Cord 665
Spinal Cord Transection and Spinal Shock 665
CHAPTER 55
Cortical and Brain Stem Control of Motor Function 667
Motor Cortex and Corticospinal Tract 667
Role of the Brain Stem in Controlling Motor Function 673
Vestibular Sensations and Maintenance of Equilibrium 674
Functions of Brain Stem Nuclei in Controlling Subconscious, Stereotyped Movements 678
CHAPTER 56
Contributions of the Cerebellum and Basal Ganglia to Overall Motor Control 681
Cerebellum and Its Motor Functions 681
Basal Ganglia—Their Motor Functions 689
Integration of the Many Parts of the Total Motor Control System 694
CHAPTER 57
Cerebral Cortex, Intellectual Functions of the Brain, Learning, and Memory 697
Physiologic Anatomy of the Cerebral Cortex 697
Functions of Specific Cortical Areas 698
Function of the Brain in Communication— Language Input and Language Output 703
Function of the Corpus Callosum and Anterior Commissure to Transfer Thoughts, Memories, Training, and Other Information Between the Two Cerebral Hemispheres 704
Thoughts, Consciousness, and Memory 705
CHAPTER 58
Behavioral and Motivational Mechanisms of the Brain—The Limbic System and the Hypothalamus 711
Activating-Driving Systems of the Brain 711
Limbic System 714
Functional Anatomy of the Limbic System; Key Position of the Hypothalamus 714
Hypothalamus, a Major Control Headquarters for the Limbic System 715
Specific Functions of Other Parts of the Limbic System 718
CHAPTER 59
States of Brain Activity—Sleep, Brain Waves, Epilepsy, Psychoses 721
Sleep 721
Epilepsy 725
Psychotic Behavior and Dementia—Roles of Specific Neurotransmitter Systems 726
Schizophrenia—Possible Exaggerated Function of Part of the Dopamine System 727
CHAPTER 60
The Autonomic Nervous System and the Adrenal Medulla 729
General Organization of the Autonomic Nervous System 729
Basic Characteristics of Sympathetic and Parasympathetic Function 731
Autonomic Reflexes 738
Stimulation of Discrete Organs in Some Instances and Mass Stimulation in Other Instances by the Sympathetic and Parasympathetic Systems 738
Pharmacology of the Autonomic Nervous System 739
CHAPTER 61
Cerebral Blood Flow, Cerebrospinal Fluid, and Brain Metabolism 743
Cerebral Blood Flow 743
Cerebrospinal Fluid System 746
Brain Metabolism 749
Contents
xvii
UNIT XII
Gastrointestinal Physiology
CHAPTER 62
General Principles of Gastrointestinal Function—Motility, Nervous Control, and Blood Circulation 753
General Principles of Gastrointestinal Motility 753
Neural Control of Gastrointestinal Function— Enteric Nervous System 755
Functional Types of Movements in the Gastrointestinal Tract 759
Gastrointestinal Blood Flow—“Splanchnic Circulation” 759
CHAPTER 63
Propulsion and Mixing of Food in the Alimentary Tract 763
Ingestion of Food 763
Motor Functions of the Stomach 765
Movements of the Small Intestine 768
Movements of the Colon 770
Other Autonomic Reflexes That Affect Bowel Activity 772
CHAPTER 64
Secretory Functions of the Alimentary Tract 773
General Principles of Alimentary Tract Secretion 773
Secretion of Saliva 775
Esophageal Secretion 776
Gastric Secretion 777
Pancreatic Secretion 780
Secretion of Bile by the Liver; Functions of the Biliary Tree 783
Secretions of the Small Intestine 786
Secretion of Mucus by the Large Intestine 787
CHAPTER 65
Digestion and Absorption in the Gastrointestinal Tract 789
Digestion of the Various Foods by Hydrolysis 789
Basic Principles of Gastrointestinal Absorption 793
Absorption in the Small Intestine 794
Absorption in the Large Intestine: Formation of Feces 797
CHAPTER 66
Physiology of Gastrointestinal Disorders 799
Disorders of Swallowing and of the Esophagus 799
Disorders of the Stomach 799
Disorders of the Small Intestine 801
Disorders of the Large Intestine 802
General Disorders of the Gastrointestinal Tract 803
UNIT XIII
Metabolism and Temperature Regulation
CHAPTER 67
Metabolism of Carbohydrates, and Formation of Adenosine Triphosphate 809
Central Role of Glucose in Carbohydrate Metabolism 810
Transport of Glucose Through the Cell Membrane 810
Glycogen Is Stored in Liver and Muscle 811
Release of Energy from Glucose by the Glycolytic Pathway 812
Release of Energy from Glucose by the Pentose Phosphate Pathway 816
Formation of Carbohydrates from Proteins and Fats—“Gluconeogenesis” 817
Blood Glucose 817
CHAPTER 68
Lipid Metabolism 819
Transport of Lipids in the Body Fluids 819
Fat Deposits 821
Use of Triglycerides for Energy: Formation of Adenosine Triphosphate 822
Regulation of Energy Release from Triglycerides 825
Phospholipids and Cholesterol 826
Atherosclerosis 827
CHAPTER 69
Protein Metabolism 831
Basic Properties 831
Transport and Storage of Amino Acids 831
Functional Roles of the Plasma Proteins 833
Hormonal Regulation of Protein Metabolism 835
CHAPTER 70
The Liver as an Organ 837
Physiologic Anatomy of the Liver 837
Hepatic Vascular and Lymph Systems 837
Metabolic Functions of the Liver 839
Measurement of Bilirubin in the Bile as a Clinical Diagnostic Tool 840
Contents
xviii
CHAPTER 71
Dietary Balances; Regulation of Feeding; Obesity and Starvation; Vitamins and Minerals 843
Energy Intake and Output Are Balanced Under Steady-State Conditions 843
Dietary Balances 843
Regulation of Food Intake and Energy Storage 845
Obesity 850
Inanition, Anorexia, and Cachexia 851
Starvation 852
Vitamins 852
Mineral Metabolism 855
CHAPTER 72
Energetics and Metabolic Rate 859
Adenosine Triphosphate (ATP) Functions as an “Energy Currency” in Metabolism 859
Control of Energy Release in the Cell 861
Metabolic Rate 862
Energy Metabolism—Factors That Influence Energy Output 863
CHAPTER 73
Body Temperature Regulation, and Fever 867
Normal Body Temperatures 867
Body Temperature Is Controlled by Balancing Heat Production and Heat Loss 867
Regulation of Body Temperature— Role of the Hypothalamus 871
Abnormalities of Body Temperature Regulation 875
UNIT XIV
Endocrinology and Reproduction
CHAPTER 74
Introduction to Endocrinology 881
Coordination of Body Functions by Chemical Messengers 881
Chemical Structure and Synthesis of Hormones 881
Hormone Secretion, Transport, and Clearance from the Blood 884
Mechanisms of Action of Hormones 886
Measurement of Hormone Concentrations in the Blood 891
CHAPTER 75
Pituitary Hormones and Their Control by the Hypothalamus 895
Pituitary Gland and Its Relation to the Hypothalamus 895
Hypothalamus Controls Pituitary Secretion 897
Physiological Functions of Growth Hormone 898
Posterior Pituitary Gland and Its Relation to the Hypothalamus 904
CHAPTER 76
Thyroid Metabolic Hormones 907
Synthesis and Secretion of the Thyroid Metabolic Hormones 907
Physiological Functions of the Thyroid Hormones 910
Regulation of Thyroid Hormone Secretion 914
Diseases of the Thyroid 916
CHAPTER 77
Adrenocortical Hormones 921
Synthesis and Secretion of Adrenocortical Hormones 921
Functions of the Mineralocorticoids— Aldosterone 924
Functions of the Glucocorticoids 928
Adrenal Androgens 934
Abnormalities of Adrenocortical Secretion 934
CHAPTER 78
Insulin, Glucagon, and Diabetes Mellitus 939
Insulin and Its Metabolic Effects 939
Glucagon and Its Functions 947
Somatostatin Inhibits Glucagon and Insulin Secretion 949
Summary of Blood Glucose Regulation 949
Diabetes Mellitus 950
CHAPTER 79
Parathyroid Hormone, Calcitonin, Calcium and Phosphate Metabolism, Vitamin D, Bone, and Teeth 955
Overview of Calcium and Phosphate Regulation in the Extracellular Fluid and Plasma 955
Bone and Its Relation to Extracellular Calcium and Phosphate 957
Vitamin D 960
Parathyroid Hormone 962
Calcitonin 966
Summary of Control of Calcium Ion Concentration 966
Contents
xix
Pathophysiology of Parathyroid Hormone, Vitamin D, and Bone Disease 967
Physiology of the Teeth 969
CHAPTER 80
Reproductive and Hormonal Functions of the Male (and Function of the Pineal Gland) 973
Physiologic Anatomy of the Male Sexual Organs 973
Spermatogenesis 973
Male Sexual Act 978
Testosterone and Other Male Sex Hormones 979
Abnormalities of Male Sexual Function 984
Erectile Dysfunction in the Male 985
Pineal Gland—Its Function in Controlling Seasonal Fertility in Some Animals 986
CHAPTER 81
Female Physiology Before Pregnancy and Female Hormones 987
Physiologic Anatomy of the Female Sexual Organs 987
Female Hormonal System 987
Monthly Ovarian Cycle; Function of the Gonadotropic Hormones 988
Functions of the Ovarian Hormones— Estradiol and Progesterone 991
Regulation of the Female Monthly Rhythm—Interplay Between the Ovarian and Hypothalamic-Pituitary Hormones 996
Abnormalities of Secretion by the Ovaries 999
Female Sexual Act 1000
Female Fertility 1000
CHAPTER 82
Pregnancy and Lactation 1003
Maturation and Fertilization of the Ovum 1003
Early Nutrition of the Embryo 1005
Function of the Placenta 1005
Hormonal Factors in Pregnancy 1007
Response of the Mother’s Body to Pregnancy 1009
Parturition 1011
Lactation 1014
CHAPTER 83
Fetal and Neonatal Physiology 1019
Growth and Functional Development of the Fetus 1019
Development of the Organ Systems 1019
Adjustments of the Infant to Extrauterine Life 1021
Special Functional Problems in the Neonate 1023
Special Problems of Prematurity 1026
Growth and Development of the Child 1027
UNIT XV
Sports Physiology
CHAPTER 84
Sports Physiology 1031
Muscles in Exercise 1031
Respiration in Exercise 1036
Cardiovascular System in Exercise 1038
Body Heat in Exercise 1039
Body Fluids and Salt in Exercise 1040
Drugs and Athletes 1040
Body Fitness Prolongs Life 1041
Index 1043
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itI Introduction to Physiology: The Cell
and General Physiology 1. Functional Organization of the Human
Body and Control of the “Internal Environment”
2. The Cell and Its Functions
3. Genetic Control of Protein Synthesis, Cell Function, and Cell Reproduction
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Functional Organization of the Human Body and Control of the “Internal Environment”
chapter 1
The goal of physiology is to explain the physical and chemical factors that are responsible for the origin, development, and progres- sion 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 mech- anisms of the human body that make it a living being. The very fact that we remain alive is the result of com- plex 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, feel- ing, 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 inter- cellular 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 mark- edly from one another, all of them have certain basic char- acteristics 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 surround- ing fluids.
Almost all cells also have the ability to reproduce addi- tional 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 intracellu- lar 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 inté- rieur, 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 concentra- tions of oxygen, glucose, different ions, amino acids, fatty substances, and other constituents are available in this internal environment.
Differences Between Extracellular and Intra cellular 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
Unit I Introduction to Physiology: The Cell and General Physiology
4
being transported from the cells to the lungs to be excreted, plus other cellular waste products that are being trans- ported 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 dif- ferences between the extracellular and intracellular fluids. These transport processes are discussed in Chapter 4.
“Homeostatic” Mechanisms of the Major Functional Systems
Homeostasis
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 con- stant conditions. For instance, the lungs provide oxygen to the extracellular fluid to replenish the oxygen used by the cells, the kidneys maintain constant ion concentra- tions, and the gastrointestinal system provides nutrients.
A large segment of this text is concerned with the man- ner in which each organ or tissue contributes to homeo- stasis. To begin this discussion, the different functional systems of the body and their contributions to homeosta- sis 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 circu- latory 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.
As blood passes through the blood capillaries, con- tinual 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 diffu- sion 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 bounc- ing in all directions within the plasma and the fluid in the intercellular spaces, as well as through the capillary pores.
Lungs
Right heart pump
Left heart pump
Gut
Kidneys
ExcretionRegulation of
electrolytes
Venous end
Arterial end
Capillaries
Nutrition and excretion
O2 CO2
Figure 11 General organization of the circulatory system.
Venule
Arteriole
Figure 12 Diffusion of fluid and dissolved constituents through the capillary walls and through the interstitial spaces.
Chapter 1 Functional Organization of the Human Body and Control of the “Internal Environment”
5
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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 continu- ally being mixed, thereby maintaining homogeneity of the extracellular fluid throughout the body.
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 molec- ular 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 compo- sitions 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 musculo- skeletal 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 appropri- ate time to obtain the foods required for nutrition. The musculoskeletal system also provides motility for pro- tection 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 car- ries the carbon dioxide to the atmosphere. Carbon dioxide is the most abundant of all the end products of metabolism.
Kidneys. 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 cel- lular 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 sub- stances 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.
Liver. Among the functions of the liver is the detoxi- fication 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 out- put 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 informa- tion, generate thoughts, create ambition, and determine reactions that the body performs in response to the sen- sations. 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, includ- ing 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 extra- cellular fluid to all parts of the body to help regulate cel- lular function. For instance, thyroid hormone increases the rates of most chemical reactions in all cells, thus help- ing 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 phos- phate. Thus, the hormones provide a system for regula- tion that complements the nervous system. The nervous
Unit I Introduction to Physiology: The Cell and General Physiology
6
system regulates many muscular and secretory activi- ties 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 mech- anism 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 bound- ary between the body’s internal environment and the out- side 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.
Reproduction
Sometimes reproduction is not considered a homeo- static function. It does, however, help maintain homeo- stasis 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 con- tinuity 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 func- tion 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 respi- ratory 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 spe- cial control mechanism to maintain an almost exact and constant oxygen concentration in the extracellu- lar fluid. This mechanism depends principally on the chemical characteristics of hemoglobin, which is pres- ent 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, hemo- globin, 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 concen- tration. Thus, regulation of oxygen concentration in the tissues is vested principally in the chemical character- istics 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 nor- mal 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 concen- tration returns to normal.
Regulation of Arterial Blood Pressure. Several sys- tems contribute to the regulation of arterial blood pres- sure. One of these, the baroreceptor system, is a simple and excellent example of a rapidly acting control mecha- nism. 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 barorecep- tors, which are stimulated by stretch of the arterial wall. When the arterial pressure rises too high, the barore- ceptors 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 sym- pathetic nervous system to the heart and blood vessels. Lack of these impulses causes diminished pumping activ- ity by the heart and also dilation of the peripheral blood vessels, allowing increased blood flow through the ves- sels. Both of these effects decrease the arterial pressure back toward normal.
Conversely, a decrease in arterial pressure below nor- mal relaxes the stretch receptors, allowing the vasomotor center to become more active than usual, thereby caus- ing vasoconstriction and increased heart pumping. The decrease in arterial pressure also raises arterial pressure back toward normal.
Chapter 1 Functional Organization of the Human Body and Control of the “Internal Environment”
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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 nor- mal range for each one. Values outside these ranges are usually caused by illness.
Most important are the limits beyond which abnormal- ities 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 impor- tant 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 spontane- ous 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 num- bers 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 feed- back, 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 diox- ide from the body. In other words, the high concentra- tion of carbon dioxide initiates events that decrease the concentration toward normal, which is negative to the initiating stimulus. Conversely, if the carbon dioxide con- centration 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 exces- sive or deficient, a control system initiates negative feed- back, 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 effective- ness with which a control system maintains constant con- ditions 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 pres- sure 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 barore- ceptor system is functioning, and this time the pressure increases only 25 mm Hg. Thus, the feedback control sys- tem has caused a “correction” of −50 mm Hg—that is, from
Normal Value Normal Range Approximate ShortTerm Nonlethal Limit
Unit
Oxygen 40 35-45 10-1000 mm Hg
Carbon dioxide 40 35-45 5-80 mm Hg
Sodium ion 142 138-146 115-175 mmol/L
Potassium ion 4.2 3.8-5.0 1.5-9.0 mmol/L
Calcium ion 1.2 1.0-1.4 0.5-2.0 mmol/L
Chloride ion 108 103-112 70-130 mmol/L
Bicarbonate ion 28 24-32 8-45 mmol/L
Glucose 85 75-95 20-1500 mg/dl
Body temperature 98.4 (37.0) 98-98.8 (37.0) 65-110 (18.3-43.3) °F (°C)
Acid-base 7.4 7.3-7.5 6.9-8.0 pH
Table 11 Important Constituents and Physical Characteristics of Extracellular Fluid
Unit I Introduction to Physiology: The Cell and General Physiology
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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 pre- venting change. The gain of the system is then calculated by the following formula:
Gain = Correction
Error
Thus, in the baroreceptor system example, the correc- tion 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 moder- ately cold weather is about −33. Therefore, one can see that the temperature control system is much more effec- tive than the baroreceptor pressure control system.
Positive Feedback Can Sometimes Cause Vicious Cycles and Death
One might ask the question, Why do most control sys- tems 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 pump- ing 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 effec- tively. 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, fur- ther 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.
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 imme- diately adjacent blood, thus causing more blood clot- ting. 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 feed- back plays a valuable role. When uterine contractions become strong enough for the baby’s head to begin push- ing 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 uter- ine 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 enter- ing 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 enter- ing the interior of the nerve fiber, which creates the nerve action potential. This action potential in turn causes elec- trical current to flow along both the outside and the inside
1
Hours
Death
Bled 2 liters
Return to normal
Bled 1 liter
P u
m p
in g
e ff
ec ti
ve n
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o f
h ea
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it er
s p
u m
p ed
p er
m in
u te
)
2 3 0
1
2
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5
Figure 13 Recovery of heart pumping caused by negative feed- back after 1 liter of blood is removed from the circulation. Death is caused by positive feedback when 2 liters of blood are removed.
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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 feed- back process. For example, in the case of blood clotting, the positive feedback clotting process is a negative feed- back 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 inter- connected control mechanisms. Some are simple feed- back 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 con- trol the movement. Therefore, the brain uses a principle called feed-forward control to cause required muscle con- tractions. That is, sensory nerve signals from the moving parts apprise the brain whether the movement is per- formed correctly. If not, the brain corrects the feed-for- ward 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 move- ments. 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 har- mony. To summarize, the body is actually a social order of about 100 trillion cells organized into different func- tional structures, some of which are called organs. Each
functional structure contributes its share to the mainte- nance of homeostatic conditions in the extracellular fluid, which is called the internal environment. As long as nor- mal conditions are maintained in this internal environ- ment, 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 continu- ous automaticity of the body until one or more functional systems lose their ability to contribute their share of func- tion. When this happens, all the cells of the body suffer. Extreme dysfunction leads to death; moderate dysfunc- tion leads to sickness.
Bibliography
Adolph EF: Physiological adaptations: hypertrophies and superfunctions, Am Sci 60:608, 1972.
Bernard C: Lectures on the Phenomena of Life Common to Animals and Plants, Springfield, IL, 1974, Charles C Thomas.
Cannon WB: The Wisdom of the Body, New York, 1932, WW Norton. Chien S: Mechanotransduction and endothelial cell homeostasis: the
wisdom of the cell, Am J Physiol Heart Circ Physiol 292:H1209, 2007. Csete ME, Doyle JC: Reverse engineering of biological complexity, Science
295:1664, 2002. Danzler WH, editor: Handbook of Physiology, Sec 13: Comparative
Physiology, Bethesda, 1997, American Physiological Society. DiBona GF: Physiology in perspective: the wisdom of the body. Neural
control of the kidney, Am J Physiol Regul Integr Comp Physiol 289:R633, 2005.
Dickinson MH, Farley CT, Full RJ, et al: How animals move: an integrative view, Science 288:100, 2000.
Garland T Jr, Carter PA: Evolutionary physiology, Annu Rev Physiol 56:579, 1994.
Gao Q, Horvath TL: Neuronal control of energy homeostasis, FEBS Lett 582:132, 2008.
Guyton AC: Arterial Pressure and Hypertension, Philadelphia, 1980, WB Saunders.
Guyton AC, Jones CE, Coleman TG: Cardiac Output and Its Regulation, Philadelphia, 1973, WB Saunders.
Guyton AC, Taylor AE, Granger HJ: Dynamics and Control of the Body Fluids, Philadelphia, 1975, WB Saunders.
Herman MA, Kahn BB: Glucose transport and sensing in the maintenance of glucose homeostasis and metabolic harmony, J Clin Invest 116:1767, 2006.
Krahe R, Gabbiani F: Burst firing in sensory systems, Nat Rev Neurosci 5:13, 2004.
Orgel LE: The origin of life on the earth, Sci Am 271:76, 1994. Quarles LD: Endocrine functions of bone in mineral metabolism regulation,
J Clin Invest 118:3820, 2008. Smith HW: From Fish to Philosopher, New York, 1961, Doubleday. Tjian R: Molecular machines that control genes, Sci Am 272:54, 1995.
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The Cell and Its Functions
chapter 2
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.
The different substances that make up the cell are collectively called protoplasm. Protoplasm is composed mainly of five basic substances: water, electrolytes, pro- teins, lipids, and carbohydrates.
Water. The principal fluid medium of the cell is water, which is present in most cells, except for fat cells, in a con- centration 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.
Ions. Important ions in the cell include potassium, mag- nesium, phosphate, sulfate, bicarbonate, and smaller quanti- ties of sodium, chloride, and calcium. These are all discussed in more detail in Chapter 4, which considers the interrela- tions between the intracellular and extracellular fluids.
The ions provide inorganic chemicals for cellular reac- tions. Also, they are necessary for operation of some of the cellular control mechanisms. For instance, ions act- ing at the cell membrane are required for transmission of electrochemical impulses in nerve and muscle fibers.
Proteins. 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 fil- aments is to form microtubules that provide the “cytoskel- etons” 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 struc- tures inside the cell. The enzymes come into direct con- tact 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.
Nucleoplasm
Cytoplasm
Nucleus
Nucleolus
Cell membrane
Nuclear membrane
Figure 2-1 Structure of the cell as seen with the light microscope.
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Nucleolus
Cell membrane
Lysosome
Secretory granule
Mitochondrion
Centrioles
Microtubules
Nuclear membrane
Granular endoplasmic
reticulum
Smooth (agranular)
endoplasmic reticulum
Ribosomes
Glycogen
Golgi apparatus
Microfilaments
Chromosomes and DNA
Figure 2-2 Reconstruction of a typical cell, showing the internal organelles in the cytoplasm and in the nucleus.
Lipids. 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 consti- tute only about 2 percent of the total cell mass. The sig- nificance 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. Carbohydrates have little structural function in the cell except as parts of glycoprotein mol- ecules, but they play a major role in nutrition of the cell. Most human cells do not maintain large stores of carbo- hydrates; 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 gly- cogen, 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 chemi- cals; it also contains highly organized physical structures, called intracellular organelles. The physical nature of each organelle is as important as the cell’s chemical constitu- ents 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 immedi- ately. The most important organelles and other structures of the cell are shown in Figure 2-2.
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Membranous Structures of the Cell
Most organelles of the cell are covered by membranes composed primarily of lipids and proteins. These mem- branes 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 sub- stances 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 mem- brane, 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 compo- sition is proteins, 55 percent; phospholipids, 25 percent; cholesterol, 13 percent; other lipids, 4 percent; and carbo- hydrates, 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.
The basic lipid bilayer is composed of phospholipid molecules. One end of each phospholipid molecule is sol- uble in water; that is, it is hydrophilic. The other end is soluble only in fats; that is, it is hydrophobic. The phos- phate 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 con- stitute the two surfaces of the complete cell membrane, in contact with intracellular water on the inside of the mem- brane 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 sub- stances, such as oxygen, carbon dioxide, and alcohol, can penetrate this portion of the membrane with ease.
Integral protein
Extracellular fluid
Intracellular fluid
Cytoplasm
Lipid bilayer
Carbohydrate
Integral protein
Peripheral protein
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 out- side of the membrane and to additional protein molecules on the inside. (Redrawn from Lodish HF, Rothman JE: The assembly of cell mem- branes. Sci Am 240:48, 1979. Copyright George V. Kevin.)
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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 chan- nels (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 prefer- ential diffusion of some substances over others.
Other integral proteins act as carrier proteins for trans- porting substances that otherwise could not penetrate the lipid bilayer. Sometimes these even transport substances in the direction opposite to their electrochemical gradi- ents 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 mem- brane 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 glyco- proteins 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 out- side of the cell, dangling outward from the cell surface. Many other carbohydrate compounds, called proteogly- cans—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 sur- face 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 inter- nal proteins that, in turn, activate a cascade of intracel- lular 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 dis- persed 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 reticu- lum, the Golgi apparatus, mitochondria, lysosomes, and peroxisomes.
Endoplasmic Reticulum
Figure 2-2 shows a network of tubular and flat vesic- ular structures in the cytoplasm; this is the endoplas- mic 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 sur- face 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 endoplas- mic 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.
Substances formed in some parts of the cell enter the space of the endoplasmic reticulum and are then con- ducted 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 endo- plasmic 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.
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Agranular Endoplasmic Reticulum. Part of the endo- plasmic 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 pro- moted 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.
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 sub- stances are then processed in the Golgi apparatus to form lysosomes, secretory vesicles, and other cytoplasmic com- ponents that are discussed later in the chapter.
Lysosomes
Lysosomes, shown in Figure 2-2, are vesicular organ- elles that form by breaking off from the Golgi appara- tus 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 diame- ter, which are protein aggregates of as many as 40 differ- ent hydrolase (digestive) enzymes. A hydrolytic enzyme is capable of splitting an organic compound into two or more parts by combining hydrogen from a water mol- ecule 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 hydro- lyzed 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
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 dif- ferent intracellular chemicals to form hydrogen peroxide (H2O2). Hydrogen peroxide is a highly oxidizing substance and is used in association with catalase, another oxidase enzyme present in large quantities in peroxisomes, to oxi- dize many substances that might otherwise be poisonous
Matrix
Agranular endoplasmic reticulum
Granular endoplasmic
reticulum
Figure 2-4 Structure of the endoplasmic reticulum. (Modified from DeRobertis EDP, Saez FA, DeRobertis EMF: Cell Biology, 6th ed. Philadelphia: WB Saunders, 1975.)
Golgi apparatus
Endoplasmic reticulum
ER vesicles
Golgi vesicles
Figure 2-5 A typical Golgi apparatus and its relationship to the endoplasmic reticulum (ER) and the nucleus.
Unit I Introduction to Physiology: The Cell and General Physiology
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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 stor- age vesicles called secretory vesicles or secretory granules. Figure 2-6 shows typical secretory vesicles inside pancre- atic 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.
Mitochondria
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.
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 mito- chondria are concentrated in those portions of the cell that are responsible for the major share of its energy metabo- lism. They are also variable in size and shape. Some are only a few hundred nanometers in diameter and globu- lar in shape, whereas others are elongated—as large as 1 micrometer in diameter and 7 micrometers long; still oth- ers 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 ade- nosine 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 perform- ing cellular functions. The chemical details of ATP forma- tion by the mitochondrion are given in Chapter 67, but some of the basic functions of ATP in the cell are intro- duced 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 mito- chondrion 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 fila- ments. 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 fil- aments 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 poly- merized 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 fla- gellum of a sperm.
Secretory granules
Figure 2-6 Secretory granules (secretory vesicles) in acinar cells of the pancreas.
Outer membrane
Inner membrane
Oxidative phosphorylation
enzymesOuter chamber
MatrixCrests
Figure 2-7 Structure of a mitochondrion. (Modified from DeRobertis EDP, Saez FA, DeRobertis EMF: Cell Biology, 6th ed. Philadelphia: WB Saunders, 1975.)
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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 struc- ture is discussed later in the chapter and is illustrated in Figure 2-17. Also, both the centrioles and the mitotic spin- dle of the mitosing cell are composed of stiff microtubules.
Thus, a primary function of microtubules is to act as a cytoskeleton, providing rigid physical structures for cer- tain parts of cells.
Nucleus
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 mecha- nisms 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 micro- scope, as illustrated in the next chapter.
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 con- tinuous with the space inside the endoplasmic reticulum, as shown in Figure 2-9.
The nuclear membrane is penetrated by several thou- sand 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 stain- ing structures called nucleoli. The nucleolus, unlike most other organelles discussed here, does not have a limit- ing 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, dem- onstrating that the cell has a diameter about 1000 times that of the smallest virus and, therefore, a volume about
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.)
Endoplasmic reticulum
Nucleoplasm
Cytoplasm
Nuclear envelope- outer and inner membranes
Pores
Nucleolus
Chromatin material (DNA)
Figure 2-9 Structure of the nucleus.
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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.
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 genera- tion 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 spe- cial 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 rickett- sial and bacterial stages, organelles developed inside the organism, representing physical structures of chemi- cal 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 organ- elles 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 repre- sentative functional systems of the cell that make it a liv- ing 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 flu- ids. 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 mole- cules of the substance; substances move either through cell membrane pores or, in the case of lipid-soluble sub- stances, through the lipid matrix of the membrane.
Active transport involves the actual carrying of a sub- stance through the membrane by a physical protein struc- ture 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 func- tion of the cell membrane called endocytosis. The princi- pal 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. 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 mac- romolecules, 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 attach- ing to the membrane. These molecules usually attach to
15 nm- Small virus
150 nm- Large virus
350 nm- Rickettsia
1 mm Bacterium
5–10 mm+
Cell
Figure 2-10 Comparison of sizes of precellular organisms with that of the average cell in the human body.
Receptors
Actin and myosin Dissolving clathrin
Proteins
Coated pit Clathrin
A B
C D
Figure 2-11 Mechanism of pinocytosis.
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specialized protein receptors on the surface of the mem- brane 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 includ- ing 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 fibril- lar 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 pino- cytotic vesicle inside the cytoplasm of the cell.
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 cal- cium 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. Phagocytosis occurs in much the same way as pinocytosis, except that it involves large particles rather than molecules. Only certain cells have the capabil- ity of phagocytosis, most notably the tissue macrophages and some of the white blood cells.
Phagocytosis is initiated when a particle such as a bac- terium, 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 phago- cyte 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, progres- sively more and more membrane receptors attach to the particle ligands. All this occurs suddenly in a zip- per-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 inte- rior 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 phago- cytic vesicle appears inside a cell, one or more lyso- somes 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, rep- resents indigestible substances. In most instances, this is finally excreted through the cell membrane by a pro- cess called exocytosis, which is essentially the opposite of endocytosis.
Thus, the pinocytotic and phagocytic vesicles contain- ing 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 dur- ing 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 activ- ity in a tissue causes the lysosomes to increase their activ- ity is unknown.
Another special role of the lysosomes is removal of damaged cells or damaged portions of cells from tis- sues. 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
Pinocytotic or phagocytic vesicle
Lysosomes
Digestive vesicle
Residual body
Excretion
Figure 2-12 Digestion of substances in pinocytotic or phagocytic vesicles by enzymes derived from lysosomes.
Unit I Introduction to Physiology: The Cell and General Physiology
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digested, a process called autolysis. In this way, the cell is completely removed and a new cell of the same type ordi- narily 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 cellu- lar damage. These agents include (1) lysozyme, which dis- solves 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 meta- bolic 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 reticu- lum. 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 ribo- somes attached to the outer surfaces of the endoplas- mic reticulum membrane. As discussed in Chapter 3, protein molecules are synthesized within the struc- tures 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 endo- plasmic 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 endoplas- mic reticulum itself, thus causing the endoplasmic reticu- lum 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 ves- icles 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 break- down 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 coagula- tion, 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 syn- thesizing 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 col- lagen 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 migra- tion and proliferation.
Processing of Endoplasmic Secretions by the Golgi Apparatus—Formation of Vesicles. Figure 2-13 sum- marizes the major functions of the endoplasmic reticu- lum 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
Ribosomes Lysosomes Secretory vesicles
Protein formation
Glycosylation Transport vesicles
Smooth endoplasmic
reticulum
Golgi apparatus
Granular endoplasmic
reticulum
Lipid formation
Figure 2-13 Formation of proteins, lipids, and cellular vesicles by the endoplasmic reticulum and Golgi apparatus.
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Golgi apparatus. At this point, small transport vesicles composed of small envelopes of smooth endoplasmic reticulum continually break away and diffuse to the deep- est layer of the Golgi apparatus. Inside these vesicles are the synthesized proteins and other products from the endoplasmic reticulum.
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 com- pact the endoplasmic reticular secretions into highly concentrated packets. As the secretions pass toward the outermost layers of the Golgi apparatus, the com- paction 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 pres- ent in the Golgi apparatus, and within 1 to 2 hours, radioac- tive 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, open- ing 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 con- tinually replenish the cell membrane.
In summary, the membranous system of the endoplas- mic 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—carbohy- drates, 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 food- stuffs—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.
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
~~ ~~ PO
O
O-
O-O-
OH OH
H
NH2
H
NN
N C
C
C
N
C C
C
O
C
HH
O-
O O
POPOCH2
CH HC
Phosphate
Adenosine triphosphate
Adenine
Ribose
ATP is a nucleotide composed of (1) the nitrogenous base adenine, (2) the pentose sugar ribose, and (3) three phos- phate radicals. The last two phosphate radicals are con- nected 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 phos- phate 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 rad- ical is split away and adenosine diphosphate (ADP) is formed. This released energy is used to energize virtu- ally many of the cell’s other functions, such as synthesis of substances and muscular contraction.
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To reconstitute the cellular ATP as it is used up, energy derived from the cellular nutrients causes ADP and phos- phoric acid to recombine to form new ATP, and the entire process repeats over and over again. For these rea- sons, 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 car- bohydrates, 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 reac- tive, and they combine instantly with oxygen that has also diffused into the mitochondria. This releases a tre- mendous amount of energy, which is used by the mito- chondria 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 pro- trude 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 mito- chondrial 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 chemi- cal and physical details of this mechanism are presented in Chapter 67, and many of the detailed metabolic func- tions 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 func- tions: (1) transport of substances through multiple mem- branes 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 ribo- somes, and (3) to supply the energy needed during muscle contraction.
In addition to membrane transport of sodium, energy from ATP is required for membrane transport of potas- sium ions, calcium ions, magnesium ions, phosphate ions,
O2
Amino acids
Cell membrane Cytoplasm
Fatty acids
Glucose
AA
FA
Gl
Pyruvic acid
Acetoacetic acid
Mitochondrion
CO2
H2O H2O
O2
CO2
Acetyl-CoA ADP
ATP
2ADP 2ATP
36 ATP
36 ADP
O2
CO2 + H2O
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.
Mitochondrion
ADP
Na+ Na+
ATP ATP ADP
ATP
Muscle contraction
ADP
ATP ADP Protein synthesis
Ribosomes
Membrane transport Endoplasmic
reticulum
Figure 2-15 Use of adenosine triphosphate (ATP) (formed in the mitochondrion) to provide energy for three major cellular func- tions: membrane transport, protein synthesis, and muscle contrac- tion. ADP, adenosine diphosphate.
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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 phos- pholipids, cholesterol, purines, pyrimidines, and a host of other substances. Synthesis of almost any chemical com- pound requires energy. For instance, a single protein mol- ecule 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 dur- ing 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 mito- chondria 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 per- cent 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 cili- ary 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 protru- sion 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 elon-
gated 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.
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 attach- ment. 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 enlarg- ing pseudopodium. Contraction also occurs in the ecto- plasm of the cell body, where a preexisting actin network is already present beneath the cell membrane.
Endocytosis
Surrounding tissue Receptor binding
Pseudopodium
Exocytosis
Movement of cell
Figure 2-16 Ameboid motion by a cell.
Unit I Introduction to Physiology: The Cell and General Physiology
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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 locomo- tion 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 com- pletely 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 dur- ing 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 tis- sues. 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 concen- tration—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 ame- boid 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 sur- faces 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, bend- ing 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 back- ward direction has almost no effect on fluid movement. As a result, the fluid is continually propelled in the direc- tion 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.
Tip
Cross section
Forward stroke
Backward stroke
Membrane
Filament
Cell membrane
Basal body
Rootlet
Basal plate
C ili
ar y
st al
k
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.)
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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 sin- gle 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 ele- ments 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 cil- ium: (1) the availability of ATP and (2) appropriate ionic conditions, especially appropriate concentrations of mag- nesium 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 enzy- matic 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
chapter 3
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 sub- stances 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 deoxy- ribonucleic acid (DNA), automatically controls the for- mation 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 pro- teins in the cell cytoplasm, is often referred to as gene expression.
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 carbo hydrates, form the structures of the various intracellular organ- elles 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.
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.
Nucleotides. The first stage in the formation of DNA is to combine one molecule of phosphoric acid, one mol- ecule 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
Gene (DNA)
RNA formation
Protein formation
Cell function
Cell structure
Cell enzymes
Transcription
Translation
Plasma membrane
Nuclear envelope
DNA transcription
DNA
RNA
Nucleus
Cytosol
RNA splicing
RNA transport
Translation of messenger RNA
Protein
Ribosomes
Figure 3-1. General schema by which the genes control cell function.
Unit I Introduction to Physiology: The Cell and General Physiology
28
acid, and Figure 3-5 shows simple symbols for the four nucleotides that form DNA.
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 deoxyri- bose 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 pyrimi- dine base cytosine.
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.
Phosphoric acid
Deoxyribose
Bases
Purines Pyrimidines Guanine Cytosine
ThymineAdenine
O HP
O
O
H
OH
C C
O
C
C C H
H H
H N
N
N N
N
H
C C
N
C
C C H
C C
C C
H
H
O
H
H H
O O
H
H
N
H H
N N
N
O
C
H
C C H
C
H
C
O
N
HN
N
H
H
H H
C CO
O
N C
N
H
H H
H
H
H
C H
C
H
Figure 3-3. The basic building blocks of DNA.
C C
N
C
C C H
H
N
H H
N N
N
P
O
O
OH C C
C C
H
H
O
H
H
O
O
C
H H
H
H
H
Adenine
Phosphate
Deoxyribose
Figure 3-4. Deoxyadenylic acid, one of the nucleotides that make up DNA.
Chapter 3 Genetic Control of Protein Synthesis, Cell Function, and Cell Reproduction
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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 pres- ent 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.
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 sep- arated 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.
DP P
D PP
D P
D P
D P
D P
D P
D P
D P
D P
D P
D P
D P
D P
D P
D P
D
D P
C
G
C
G
G
C
T
A
C
G
T
A
G
C
A
T
A
T Figure 3-6. Arrangement of deoxyribose nucleotides in a double strand of DNA.
D
A
P
D
G
P
D
T
P
D
C
P
Deoxyadenylic acid
Deoxyguanylic acid
Deoxythymidylic acid
Deoxycytidylic acid
Figure 3-5. Symbols for the four nucleotides that combine to form DNA. Each nucleotide contains phosphoric acid (P), deoxyri- bose (D), and one of the four nucleotide bases: A, adenine; T, thy- mine; G, guanine; or C, cytosine.
D
G
P D
G
P D
C
P D
A
P D
G
P D
A
P D P D P D
T
P
C T
DNA strand
RNA molecule
RNA polymerase
Triphosphate R
C
P P R
C
P R
G
R
U
P P R
C
P R
U
P
P
R
G
P
P
P
R
A
P
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.
Proline Serine Glutamic acid
R
C
P R
C
P R
G
P R
U
P R
C
P R
U
P R
G
P R
A
P R
A
P
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.
Unit I Introduction to Physiology: The Cell and General Physiology
30
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 interme- diary of another type of nucleic acid, RNA, the forma- tion 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 mol- ecule. 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 struc- ture. Second, thymine is replaced by another pyrimidine, uracil.
Formation of RNA Nucleotides. The basic build- ing blocks of RNA form RNA nucleotides, exactly as pre- viously 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 nucle- otides 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 nucle- otide by high-energy phosphate bonds derived from ATP in the cell.
The result of this activation process is that large quanti- ties of ATP energy are made available to each of the nucle- otides, 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 forma- tion 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 comple- mentary 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 por- tions 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 nucle- otide 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 link- age 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 hydro- gen 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:
Chapter 3 Genetic Control of Protein Synthesis, Cell Function, and Cell Reproduction
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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 pro- tein 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 com- posed of several hundred to several thousand RNA nucle- otides 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, ser- ine, 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;
Amino Acid RNA Codons
Alanine GCU GCC GCA GCG
Arginine CGU CGC CGA CGG AGA AGG
Asparagine AAU AAC
Aspartic acid GAU GAC
Cysteine UGU UGC
Glutamic acid GAA GAG
Glutamine CAA CAG
Glycine GGU GGC GGA GGG
Histidine CAU CAC
Isoleucine AUU AUC AUA
Leucine CUU CUC CUA CUG UUA UUG
Lysine AAA AAG
Methionine AUG
Phenylalanine UUU UUC
Proline CCU CCC CCA CCG
Serine UCU UCC UCA UCG AGC AGU
Threonine ACU ACC ACA ACG
Tryptophan UGG
Tyrosine UAU UAC
Valine GUU GUC GUA GUG
Start (CI) AUG
Stop (CT) UAA UAG UGA
Table 3-1. RNA Codons for Amino Acids and for Start and Stop
CI, chain-initiating; CT, chain-terminating.
Unit I Introduction to Physiology: The Cell and General Physiology
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also, one codon represents the signal “start manufactur- ing 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.”
Transfer RNA—The Anticodons
Another type of RNA that plays an essential role in pro- tein synthesis is called tRNA because it transfers amino acid molecules to protein molecules as the protein is being synthesized. Each type of tRNA combines specifi- cally with 1 of the 20 amino acids that are to be incorpo- rated 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 deliv- ers the appropriate amino acid to the appropriate place in the chain of the newly forming protein molecule.
Transfer RNA, which contains only about 80 nucle- otides, 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.
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 mol- ecule (at the bottom of the cloverleaf configuration shown in Figure 3-9). During formation of the protein mole- cule, 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 appropri- ate 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 remain- der 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 pro- tein 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 nucleo- lus, a specialized structure lying adjacent to the chromo- somes. 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 nucle- olus 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.
MicroRNA
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
Forming protein
Alanine Cysteine
Histidine Alanine
Phenylalanine
Serine ProlineTransfer RNA
Ribosome Ribosome Messenger
RNA movement
Start codon
UG GCC UGU CAU GCC UUU UCC CCC AA AG GAC UAU GGG
A A C
Figure 3-9. A messenger RNA strand is moving through two ribo- somes. 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.
Chapter 3 Genetic Control of Protein Synthesis, Cell Function, and Cell Reproduction
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RNAs called pri-miRNAs, which are the primary tran- scripts 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.
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 regula- tion 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 mol- ecules, 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 trans- lation of any mRNA and therefore expression by any gene for which the nucleotide sequence is known. Some researchers have proposed that siRNAs may become use- ful 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, begin- ning at a predetermined end of the RNA molecule speci- fied 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 transla- tion. Thus, the ribosome reads the codons of the messen- ger 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.
Polyribosomes. 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 develop- ment 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.
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 physi- cal manufacturing plant in which the chemical reactions take place.
Many Ribosomes Attach to the Endoplasmic Reticulum. In Chapter 2, it was noted that many ribo- somes become attached to the endoplasmic reticulum. This occurs because the initial ends of many forming pro- tein molecules have amino acid sequences that immedi- ately attach to specific receptor sites on the endoplasmic reticulum; this causes these molecules to penetrate the
Protein-coding gene
Transcription of mRNA
Transport of pre-miRNA into
cytoplasm
Microprocessor complex
Transcription of pri-miRNA
Pri-miRNA
Pre-miRNA
Dicer
RISC
RISC-miRNA complex
Processing of pre-miRNA into
small RNA duplexes
Cytoplasm
miRNA
mRNA degradation Translational repression
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 miR- NAs regulate gene expression by binding to the complementary region of the RNA and repressing translation or promoting degra- dation of the mRNA before it can be translated by the ribosome.
Unit I Introduction to Physiology: The Cell and General Physiology
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reticulum wall and enter the endoplasmic reticulum matrix. This gives a granular appearance to those por- tions 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 ribo- somes attach to the membrane of the endoplasmic reticu- lum. 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 poly- peptide (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 ves- icles are formed, most proteins synthesized by the ribo- somes 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 mol- ecule are shown in Figure 3-12. This figure shows repre- sentative reactions for three separate amino acids, AA1,
AA2, and AA20. The stages of the reactions are the follow- ing: (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 pro- cess. (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 ribo- some, 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 succes- sive 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.
Transfer RNA Messenger
RNA Ribosome
Amino acid Polypeptide chain
Endoplasmic reticulum
Large subunit
Small subunit
Figure 3-11. Physical structure of the ribosomes, as well as their func- tional relation to messenger RNA, transfer RNA, and the endoplas- mic reticulum during the formation of protein molecules. (Courtesy Dr. Don W. Fawcett, Montana.)
Protein chain
Messenger RNA
Activated amino acid
Amino acid
RNA-amino acyl complex
Complex between tRNA, messenger RNA, and amino acid
AA1 AA5 AA3 AA9 AA2 AA13 AA20
A A
1
A A
5
A A
3
A A
9
A A
2
A A
13
A A
20
GTP GTP GTP GTP GTP GTP GTP
tR N
A 1
tR N
A 5
tR N
A 3
tR N
A 9
tR N
A 2
tR N
A 13
tR N
A 20
+
GCC
GCC
tRNA2
+
tRNA20
+
UGU
UGU
AAU
AAU
CAU
CAU
CGU
CGU
AUG
AUG
GUU
GUU
tRNA1
+ ATP
+ ATP
+ ATP
tRNA1
AMP
tRNA2
AMP
tRNA20
AMP
AA1
AA1
AA1
AA2
AA2
AA2
AA20
AA20
AA20
+ ++
Figure 3-12. Chemical events in the formation of a protein molecule.
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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 NH2 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 con- trol 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 over- act until they kill the cell. Each cell has powerful internal feedback control mechanisms that keep the various func- tional 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 biochem- ical activities in the cell are controlled: (1) genetic regula- tion, 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 mea- sure 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 pro- moter before it can begin traveling along the DNA strand to synthesize RNA. The upstream promoter is located far- ther 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 pro- moter vary from gene to gene to give rise to the different expression patterns of genes in different tissues.
Enhancer
Insulator
TATA INR
RNA polymerase 2
Transcription inhibitors
Transcription factors
Proximal promoter elements Basal promoter
Condensed chromatin
Upstream
RE RE
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).
O
R C
NH2
C OH H+
+
N
H R
C COOH
R C
NH2 O H R
C N C COOH H2O
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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 impor- tant 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 insu- lators. These insulators are gene sequences that provide a barrier so that a specific gene is isolated against tran- scriptional 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 rapid- ity 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 spe- cific 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 factors 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 control- ling 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 regula- tion, some cell activities are controlled by intracellular inhibitors or activators that act directly on specific intra- cellular enzymes. Thus, enzyme regulation represents a second category of mechanisms by which cellular bio- chemical functions can be controlled.
Enzyme Inhibition. Some chemical substances formed in the cell have direct feedback effects in inhibiting the spe- cific 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 conforma- tional change that inactivates it. One can readily recognize the importance of inactivating the first enzyme: this pre- vents buildup of intermediary products that are not used.
Enzyme inhibition is another example of negative feed- back control; it is responsible for controlling intracellular concentrations of multiple amino acids, purines, pyrimi- dines, vitamins, and other substances.
Enzyme Activation. Enzymes that are normally inac- tive 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 break- down product of the ATP; the presence of this cAMP, in turn, immediately activates the glycogen-splitting enzyme phosphorylase, liberating glucose molecules that are rap- idly metabolized and their energy used for replenishment of the ATP stores. Thus, cAMP acts as an enzyme activa- tor for the enzyme phosphorylase and thereby helps con- trol intracellular ATP concentration.
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Another interesting instance of both enzyme inhibi- tion 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 forma- tion of pyrimidines. Conversely, the pyrimidines inhibit their own enzymes but activate the purine enzymes. In this way, there is continual cross-feed between the syn- thesizing systems for these two substances, resulting in almost exactly equal amounts of the two substances in the cells at all times.
Summary. In summary, there are two principal meth- ods by which cells control proper proportions and proper quantities of different cellular constituents: (1) the mech- anism 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 con- tinually 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 intracel- lular 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 devel- opment 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 reproduc- tion. 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 dis- tinct physical events called mitosis that cause division of the cell into two new daughter cells. The events of mito- sis 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.
Except in special conditions of rapid cellular reproduc- tion, inhibitory factors almost always slow or stop the unin- hibited 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 rep- licated, not simply one of them.
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.)
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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 com- plex 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 simultane- ously 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 approx- imately 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 peri- odically 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, spe- cial 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 for- mation of some abnormal protein in the cell rather than a needed protein, often leading to abnormal cellular func- tion 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 muta- tions in the passage of the genome from parent to child. As a further protection, however, each human genome is rep- resented 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 chromo- somes. 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 num- bers 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 regula- tion of DNA activity because as long as the DNA is pack- aged 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 compo- nents of chromosomes, functioning both as chromo- somal 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 chromo- somes 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 cyto- plasm, 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 inter- phase, usually shortly before replication of the DNA. Each centriole is a small cylindrical body about 0.4 microm- eter long and about 0.15 micrometer in diameter, con- sisting 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
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actually pushing them apart. At the same time, other microtubules grow radially away from each of the cen- triole 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.
Prophase. The first stage of mitosis, called prophase, is shown in Figure 3-14A, B, and C. While the spindle is forming, the chromosomes of the nucleus (which in interphase consist of loosely coiled strands) become con- densed into well-defined chromosomes.
Prometaphase. During this stage (see Figure 3-14D), 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.
Metaphase. During metaphase (see Figure 3-14E), 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.
Anaphase. During this phase (see Figure 3-14F), 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.
Telophase. In telophase (see Figure 3-14G and H), the two sets of daughter chromosomes are pushed com- pletely 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 com- posed of actin and probably myosin (the two contractile proteins of muscle) at the juncture of the newly develop- ing 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 mar- row, the germinal layers of the skin, and the epithelium of the gut. Many other cells, however, such as smooth mus- cle 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 appro- priate 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 epi- thelial cells of some glands, such as the pancreas, fail to grow without a growth factor from the sublying connec- tive 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.
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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 infor- mation 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 repres- sion of different gene promoters. In fact, electron micro- graphs suggest that some segments of DNA helixes wound around histone cores become so condensed that they no longer uncoil to form RNA molecules. One expla- nation for this is as follows: It has been supposed that the cellular genome begins at a certain stage of cell differen- tiation 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 differ- entiates into a mesodermal axis that contains segmentally arranged somites and, as a result of inductions in the sur- rounding tissues, causes formation of essentially all the organs of the body.
Another instance of induction occurs when the devel- oping 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 differentia- tion 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 organ- ism, they undergo a suicidal programmed cell death, or apoptosis. This process involves a specific proteolytic cas- cade that causes the cell to shrink and condense, to disas- semble 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 inflamma- tion 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 pro- teases called caspases. These are enzymes that are syn- thesized 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 exces- sively. Recent studies suggest that abnormalities of apop- tosis may play a key role in neurodegenerative diseases such as Alzheimer’s disease, as well as in cancer and auto- immune disorders. Some drugs that have been used suc- cessfully for chemotherapy appear to induce apoptosis in cancer cells.
Cancer
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 sup- press 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 capa- bility than normal cells and simply die. Second, only a few of the mutated cells that do survive become cancer- ous, 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
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of this is the fact that in people whose immune systems have been suppressed, such as in those taking immuno- suppressant 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 simulta- neous 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 signifi- cant 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 sub- stances, and even ultraviolet light can predispose indi- viduals 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 produc- ing such substances, if unprotected, have a special pre- disposition 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 quar- ter of all cancer deaths.
3. Physical irritants can also lead to cancer, such as con- tinued 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 can- cers require not one mutation but two or more muta- tions 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 nor- mal 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 nor- mal 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 can- cers 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 tis- sue 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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Fuda NJ, Ardehali MB, Lis JT: Defining mechanisms that regulate RNA polymerase II transcription in vivo, Nature 461:186, 2009.
Hahn S: Structure and mechanism of the RNA polymerase II transcription machinery, Nat Struct Mol Biol 11:394, 2004.
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Hastings PJ, Lupski JR, Rosenberg SM, et al: Mechanisms of change in gene copy number, Nat Rev Genet 10:551, 2009.
Hoeijmakers JH: DNA damage, aging, and cancer, N Engl J Med 361:1475, 2009.
Hotchkiss RS, Strasser A, McDunn JE, et al: Cell death, N Engl J Med 361:1570, 2009.
Jinek M, Doudna JA: A three-dimensional view of the molecular machinery of RNA interference, Nature 457:40, 2009.
Jockusch BM, Hüttelmaier S, Illenberger S: From the nucleus toward the cell periphery: a guided tour for mRNAs, News Physiol Sci 18:7, 2003.
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Misteli T, Soutoglou E: The emerging role of nuclear architecture in DNA repair and genome maintenance, Nat Rev Mol Cell Biol 10:243, 2009.
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Nerve, and Muscle 4. Transport of Substances Through Cell
Membranes
5. Membrane Potentials and Action Potentials
6. Contraction of Skeletal Muscle
7. Excitation of Skeletal Muscle: Neuromuscular Transmission and Excitation-Contraction Coupling
8. Excitation and Contraction of Smooth Muscle
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chapter 4
Transport of Substances Through Cell Membranes
chapter 4
Figure 4-1 gives the approx- imate concentrations of important electrolytes and other substances in the extracellular fluid and intra- cellular fluid. Note that the extracellular fluid contains a
large amount of sodium but only a small amount of potas- sium. 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 chap- ter is to explain how the differences are brought about by the transport mechanisms of the cell membranes.
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 illus- trated 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.
The lipid bilayer is not miscible with either the extra- cellular fluid or the intracellular fluid. Therefore, it con- stitutes a barrier against movement of water molecules and water-soluble substances between the extracellular and intracellular fluid compartments. However, as dem- onstrated in Figure 4-2 by the leftmost arrow, a few sub- stances 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 pro- tein to the other side of the membrane. Both the channel proteins and the carrier proteins are usually highly selec- tive 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.
Na+ --------------- K+ ----------------- Ca++ -------------- Mg++ -------------- Cl– ---------------- HCO3
– ------------ Phosphates----- SO4
= -------------- Glucose --------- Amino acids ----
142 mEq/L --------- 4 mEq/L ------------ 2.4 mEq/L ---------- 1.2 mEq/L ---------- 103 mEq/L --------- 28 mEq/L ----------- 4 mEq/L ------------- 1 mEq/L ------------- 90 mg/dl ------------ 30 mg/dl ------------
10 mEq/L 140 mEq/L 0.0001 mEq/L 58 mEq/L 4 mEq/L 10 mEq/L 75 mEq/L 2 mEq/L 0 to 20 mg/dl 200 mg/dl ?
Cholesterol Phospholipids Neutral fat
0.5 g/dl-------------- 2 to 95 g/dl
PO2 --------------- PCO2 ------------- pH ----------------- Proteins ----------
35 mm Hg --------- 46 mm Hg --------- 7.4 ------------------- 2 g/dl ---------------- (5 mEq/L)
20 mm Hg ? 50 mm Hg ? 7.0 16 g/dl (40 mEq/L)
EXTRACELLULAR FLUID
INTRACELLULAR FLUID
Figure 4-1 Chemical compositions of extracellular and intracel- lular fluids.
Unit II Membrane Physiology, Nerve, and Muscle
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Although there are many variations of these basic mechanisms, diffusion means random molecular move- ment of substances molecule by molecule, either through intermolecular spaces in the membrane or in combina- tion with a carrier protein. The energy that causes diffu- sion 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 combina- tion with a carrier protein in such a way that the car- rier 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.
Diffusion
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 diffu- sion. 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 diffu- sion 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 mole- cules 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 deter- mines 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 pene- trate all the way through the membrane. The rapidity with which water molecules can move through most cell mem- branes 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.
Figure 4-3 Diffusion of a fluid molecule during a thousandth of a second.
Channel protein
Simple diffusion
Facilitated diffusion
Energy
Active transportDiffusion
Carrier proteins
Figure 4-2 Transport pathways through the cell membrane, and the basic mechanisms of transport.
Chapter 4 Transport of Substances Through Cell Membranes
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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 pro- tein pores and channels have demonstrated tubular path- ways 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 pro- teins 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 mol- ecules 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 impor- tant characteristics: (1) They are often selectively perme- able 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 diam- eter, 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 mech- anism for this remarkable ion selectivity? This question was partially answered when the structure of a bacterial potassium channel was determined by x-ray crystallogra- phy. Potassium channels were found to have a tetrameric
structure consisting of four identical protein subunits sur- rounding a central pore (Figure 4-4). At the top of the channel pore are pore loops that form a narrow selectiv- ity 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 carbo- nyl 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.
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 diame- ter, but more important, the inner surfaces of this chan- nel are lined with amino acids that are strongly negatively charged, as shown by the negative signs inside the chan- nel 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 chan- nel, 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.
Outside
Inside
Pore helix
Pore loop
Selectivity filter
Potassium ion
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, per- mitting the dehydrated potassium ions to pass through the pore.
Unit II Membrane Physiology, Nerve, and Muscle
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Gating of Protein Channels. Gating of protein chan- nels 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 exten- sions 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 confor- mation 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 mem- brane, 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 tremen- dous quantities of sodium to pass inward through the sodium pores. This is the basic mechanism for elicit- ing 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 termi- nating 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 4-6A shows an especially interest- ing characteristic of most voltage-gated channels. This figure shows two recordings of electrical current flow- ing 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 intermit- tently, giving an average current flow somewhere between the minimum and the maximum.
Patch-Clamp Method for Recording Ion Current Flow Through Single Channels. One might won- der how it is technically possible to record ion current flow through single protein channels as shown in Figure 4-6A. This has been achieved by using the “patch-clamp” method illustrated in Figure 4-6B. Very simply, a micropi- pette, 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 4-6B, 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 con- centrations of ions both inside the micropipette and in the outside solution to be altered as desired. Also, the volt- age 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 mem- brane patch being studied. By varying the concentrations of different ions, as well as the voltage across the mem- brane, one can determine the transport characteristics of the single channel and also its gating properties.
Outside
– – – – – –
– –
–
– – –
–
– –
–
Gate closed
Gate open
Inside
Na+ Na+
Gate closed
Gate open
Outside
Inside K+ K+
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.
Chapter 4 Transport of Substances Through Cell Membranes
49
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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 proportion- ately with the concentration of the diffusing substance,
in facilitated diffusion the rate of diffusion approaches a maximum, called Vmax, 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 Vmax level.
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
2 4
Milliseconds
Open sodium channel
P ic
oa m
pe re
s
6 8 10
Recorder
To recorder
0
A
B
3
3
0
0
Membrane “patch”
Figure 4-6 A, Record of current flow through a single voltage- gated sodium channel, demonstrating the “all or none” princi- ple 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.
Concentration of substance
R at
e of
d iff
us io
n
Simple diffusion
Facilitated diffusion
Vmax
Figure 4-7 Effect of concentration of a substance on rate of diffu- sion through a membrane by simple diffusion and facilitated diffu- sion. This shows that facilitated diffusion approaches a maximum rate called the V
max .
Transported molecule
Binding point
Carrier protein and
conformational change
Release of binding
Figure 4-8 Postulated mechanism for facilitated diffusion.
Unit II Membrane Physiology, Nerve, and Muscle
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it to break away and to be released on the opposite side of the membrane. The rate at which molecules can be trans- ported 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 spe- cifically, though, that this mechanism allows the trans- ported molecule to move—that is, to “diffuse”—in either direction through the membrane.
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 monosac- charides 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 4-9A shows a cell membrane with a substance in high con- centration 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:
Net diffusion ∝ (Co−Ci)
in which Co is concentration outside and Ci is concentra- tion 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 4-9B, 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 4-9B, the concentration of negative ions is the same on both sides of the mem- brane, 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 quan- tities of negative ions have moved to the right, creating the condition shown in the right panel of Figure 4-9B, in which a concentration difference of the ions has devel- oped 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 differ- ence of univalent ions—such as sodium (Na+) ions—can be determined from the following formula, called the Nernst equation:
( ) 1
2
C EMF in millivolts = 61 log(EMF in millivolts = 61 log( )EMF in millivolts = 61 log)
C EMF in millivolts = 61 log±EMF in millivolts = 61 log
in which EMF is the electromotive force (voltage) between side 1 and side 2 of the membrane, C1 is the concentra- tion on side 1, and C2 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.
−− −
− −
−
− − −
− −
−
−
−
− −
− −
− −
− −
−
−
−
−−
− −
− −
−
− − −
− − − −
− −
−
−
− −
– + +–
Outside
A
B
C
Inside
Membrane
Piston P2P1
Co Ci
Figure 4-9 Effect of concentration difference (A), electrical poten- tial difference affecting negative ions (B), and pressure differ- ence (C) to cause diffusion of molecules and ions through a cell membrane.
Chapter 4 Transport of Substances Through Cell Membranes
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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 4-9C, 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 dif- fuses 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 move- ment 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 con- ditions shown in Figure 4-10, with pure water on one side of the cell membrane and a solution of sodium chlo- ride 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.
Osmotic Pressure
If in Figure 4-10 pressure were applied to the sodium chlo- ride 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 osmo- sis is demonstrated in Figure 4-11, which shows a selec- tively permeable membrane separating two columns of fluid, one containing pure water and the other contain- ing a solution of water and any solute that will not pen- etrate 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 pres- sure difference develops between the two sides of the membrane great enough to oppose the osmotic effect.
Osmosis
Water NaCl 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.
cm H2O
Semipermeable membrane
A B
Figure 4-11 Demonstration of osmotic pressure caused by osmosis at a semipermeable membrane.
Unit II Membrane Physiology, Nerve, and Muscle
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The pressure difference across the membrane at this point is equal to the osmotic pressure of the solution that con- tains the nondiffusible solute.
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 mem- brane. 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
k= mv
2
2
are the same for each small particle as for each large parti- cle. 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 concen- tration 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 osmoti- cally 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 millios- moles per kilogram of water.
Relation of Osmolality to Osmotic Pressure. At nor- mal 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 flu- ids, 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 con- centration 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 osmo- lality, 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 extra- cellular fluid are great. This is especially true for sodium ions. Neither of these two effects could occur by sim- ple diffusion because simple diffusion eventually equili- brates concentrations on the two sides of the membrane. Instead, some energy source must cause excess move- ment 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, hydro- gen 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 triphos- phate (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 second- ary 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
Chapter 4 Transport of Substances Through Cell Membranes
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electrochemical gradient. Following are some examples of primary active transport and secondary active trans- port, with more detailed explanations of their principles of function.
Primary Active Transport
Sodium-Potassium Pump
Among the substances that are transported by pri- mary active transport are sodium, potassium, calcium, hydrogen, chloride, and a few other ions.
The active transport mechanism that has been stud- ied in greatest detail is the sodium-potassium (Na+-K+) pump, a transport process that pumps sodium ions out- ward 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, trans- mitting 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 pro- tein 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 function- ing 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.
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 acti- vated. 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 concen- tration 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 con- formation 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+, deter- mine 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 control- ling the volume is as follows: Inside the cell are large num- bers 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 automati- cally 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
ATP
Inside
Outside
ATPase
ADP + Pi
3Na+
3Na+
2K+
2K+
Figure 4-12 Postulated mechanism of the sodium-potassium pump. ADP, adenosine diphosphate; ATP, adenosine triphosphate; Pi, phosphate ion.
Unit II Membrane Physiology, Nerve, and Muscle
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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 mus- cle signals.
Primary Active Transport of Calcium Ions
Another important primary active transport mechanism is the calcium pump. Calcium ions are normally main- tained 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 cal- cium ions into one or more of the intracellular vesicular organelles of the cell, such as the sarcoplasmic reticu- lum 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 col- lecting ducts of the kidneys.
In the gastric glands, the deep-lying parietal cells have the most potent primary active mechanism for trans- porting 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 gas- tric 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 hydro- chloric 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 elimi- nating 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 sub- stance 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:
Energy(in calories per osmole)=1400 log C
C 1
2
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 con- centrating substances in cells or for removing substances from cells against a concentration gradient can be tremen- dous. 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 pri- mary 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 appro- priate 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 mem- brane. 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 exte- rior 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 conforma- tional 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 mecha- nism 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
Chapter 4 Transport of Substances Through Cell Membranes
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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 move- ment to the interior will not occur until a glucose mol- ecule 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-transport- ers are especially important mechanisms in transporting glucose across renal and intestinal epithelial cells, as dis- cussed in Chapters 27 and 65.
Sodium co-transport of the amino acids occurs in the same manner as for glucose, except that it uses a differ- ent 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 intes- tinal tract and the renal tubules of the kidneys to promote absorption of these substances into the blood, as is dis- cussed 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).
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 prox- imal tubules of the kidneys, where sodium ions move
from the lumen of the tubule to the interior of the tubu- lar cell, while hydrogen ions are counter-transported into the tubule lumen. As a mechanism for concentrating hydrogen ions, counter-transport is not nearly as power- ful as the primary active transport of hydrogen ions that occurs in the more distal renal tubules, but it can trans- port extremely large numbers of hydrogen ions, thus mak- ing 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 trans- ported 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 diffu- sion 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 intes- tines, 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
Na-binding site
Na+
Na+
Glucose
Glucose
Glucose-binding site
Figure 4-13 Postulated mechanism for sodium co-transport of glucose.
Outside
Inside
Na+ Na+
Ca++ H+
Figure 4-14 Sodium counter-transport of calcium and hydrogen ions.
Active transport
Brush border
Basement membrane
C onnective tissue
Osmosis
Osmosis
Lu m
en
Active transport Active
transport
Osmosis
Diffusion
Na+
Na+
and H2O
Na+
Na+
Na+
Figure 4-15 Basic mechanism of active transport across a layer of cells.
56
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 trans- ported into the extracellular fluid of the surrounding connective tissue and blood vessels. This creates a high sodium ion concentration gradient across these mem- branes, 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.
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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Blaustein MP, Zhang J, Chen L, et al: The pump, the exchanger, and endog- enous ouabain: signaling mechanisms that link salt retention to hyper- tension, Hypertension 53:291, 2009.
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DeCoursey TE: Voltage-gated proton channels and other proton transfer pathways, Physiol Rev 83:475, 2003.
DiPolo R, Beaugé L: Sodium/calcium exchanger: influence of metabolic reg- ulation on ion carrier interactions, Physiol Rev 86:155, 2006.
Drummond HA, Jernigan NL, Grifoni SC: Sensing tension: epithelial sodium channel/acid-sensing ion channel proteins in cardiovascular homeosta- sis, Hypertension 51:1265, 2008.
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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 gen- erating 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 glan- dular 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 5-1A, 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 per- meable 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 out- side the membrane and electronegativity inside because of negative anions that remain behind and do not diffuse out- ward 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 fur- ther net potassium diffusion to the exterior, despite the high potassium ion concentration gradient. In the normal mam- malian nerve fiber, the potential difference required is about 94 millivolts, with negativity inside the fiber membrane.
Figure 5-1B shows the same phenomenon as in Figure 5-1A, 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 oppo- site polarity to that in Figure 5-1A, with negativity outside and positivity inside. Again, the membrane potential rises high enough within milliseconds to block further net dif- fusion of sodium ions to the inside; however, this time, in the mammalian nerve fiber, the potential is about 61 mil- livolts positive inside the fiber.
Thus, in both parts of Figure 5-1, we see that a concen- tration difference of ions across a selectively permeable membrane can, under appropriate conditions, create a mem- brane potential. Later in this chapter, we show that many of the rapid changes in membrane potentials observed dur- ing 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.
+ – + – + – + – + – + – + –
+ –
– + – + – + – + – + – + – + – + – +
+ – – +
Nerve fiber
(Anions)–
(Anions)–
(–94 mV)
K+K+
– + – + – + – + – + – + – +
– +
+ – + – + – + – + – + – + – + – + –
– + + –
Nerve fiber(Anions)–
(Anions)–
(+61 mV)
Na+Na+
DIFFUSION POTENTIALS
A B 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 perme- able 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 potas- sium ions diffuse and positive when sodium ions diffuse because of opposite concentration gradients of these two ions.
Unit II Membrane Physiology, Nerve, and Muscle
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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 equa- tion, 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):
EMF (millivolts) = ± 61 × log Concentration inside
Concentration outside
where EMF is electromotive force. When using this formula, it is usually assumed that
the potential in the extracellular fluid outside the mem- brane 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 potas- sium 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 fac- tors: (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.
EMF (millivolts)
= -61 × log CNa
+ i PNa+ + CK
+ i PK+ +CCl
- o PCl-
CNa + o PNa+ + CK
+ o PK+ +CCl
- i PCl-
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 concen- tration 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 mem- brane 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 selec- tively 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 pos- itive 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 nega- tive 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 per- meability 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 elec- tronic apparatus that is capable of measuring small volt- ages 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 oscillo- scope, as explained later in the chapter.
The lower part of Figure 5-2 shows the electrical poten- tial that is measured at each point in or near the nerve fiber membrane, beginning at the left side of the figure and
— +
0
+ + + + + + + + + + + + + + + +
+ + + + + + + + + + + + + + + + +
Silver–silver chloride electrode
KCI
– – – – – – – – – – – – – – – – (–90 mV)
– – – – – – – – – – – – – – –
Figure 5-2 Measurement of the membrane potential of the nerve fiber using a microelectrode.
Chapter 5 Membrane Potentials and Action Potentials
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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 record- ing electrode passes through the voltage change area at the cell membrane (called the electrical dipole layer), the poten- tial decreases abruptly to −90 millivolts. Moving across the center of the fiber, the potential remains at a steady −90-mil- livolt level but reverses back to zero the instant it passes through the membrane on the opposite side 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 out- ward. 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 estab- lish 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 sig- nals 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 milli- volts. 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 out- side of the cell and potassium ions to the inside, as illus- trated 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.
The Na+-K+ pump also causes large concentration gra- dients for sodium and potassium across the resting nerve membrane. These gradients are the following:
Na+ (outside): 142 mEq/L
Na+ (inside): 14 mEq/L
K+ (outside): 4 mEq/L
K+ (inside): 140 mEq/L
The ratios of these two respective ions from the inside to the outside are
Na+ inside
/Na+ outside
= 0.1
K+ inside
/K+ outside
= 35.0
Leakage of Potassium Through the Nerve Membrane. The right side of Figure 5-4 shows a chan- nel 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 chan- nels 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 dif- ferential in permeability is a key factor in determining the level of the normal resting membrane potential.
–90
E le
ct ri
ca l p
o te
n ti
al (m
ill iv
o lt
s)
0
– + – + – + – + – + – + – + – + – + + – – + – + – – + + – + – + – + – + – + – + – + – + – + – + + – – + – + – – + + – + – + – + – + – + – + – + – + – + – + + – – + – + – – + + – + – + – + – + – + – + – + – + – + – + + – – + – + – – + + – + – + – + – + – + – + – + – + – + – + + – – + – + – – + + – +
Nerve 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.
3Na+ 2K+ K+
Na+
Selectivity filter
K+
Na+
Na+-K+ pump K+ "leak" channels
ATP ADP
Outside
K+
Figure 5-4 Functional characteristics of the Na+-K+ pump and of the K+ “leak” channels. ADP, adenosine diphosphate; ATP, adenos- ine triphosphate. The K+ “leak” channels also leak Na+ ions into the cell slightly, but are much more permeable to K+.
Unit II Membrane Physiology, Nerve, and Muscle
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Origin of the Normal Resting Membrane Potential
Figure 5-5 shows the important factors in the establish- ment of the normal resting membrane potential of −90 millivolts. They are as follows.
Contribution of the Potassium Diffusion Potential. In Figure 5-5A, 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 corre- sponding to this ratio is −94 millivolts because the loga- rithm 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 5-5B 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 cal- culated Nernst potential for the inside of the membrane of +61 millivolts. But also shown in Figure 5-5B 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 potas- sium 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 5-5C, the Na+-K+ pump is shown to provide an additional contri- bution to the resting potential. In this figure, there is con- tinuous pumping of three sodium ions to the outside for each two potassium ions pumped to the inside of the mem- brane. The fact that more sodium ions are being pumped to the outside than potassium to the inside causes con- tinual 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 5-5C, 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.
+ + +
+ + + +
+
- - -
- - - -
-
+ + + + + +
- - - - - -
+ + + + + + + + + + + + + + + + + + +
- - - - - - - - - - - - - - - - - - -
142 mEq/L 14 mEq/L
142 mEq/L 4 mEq/L
4 mEq/L 140 mEq/L
(Anions)- (Anions)-
(–90 mV)
Na+
Na+ K+
4 mEq/L
K+
Na+ pump
A
B
C
Diffusion
K+ K+ pump
Diffusion
140 mEq/L
(−94 mV)
(–94 mV)
K+
14 mEq/L
(+61 mV)
(–86 mV)
Na+
140 mEq/L
(–94 mV)
K+
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.
Chapter 5 Membrane Potentials and Action Potentials
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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 succes- sive 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.
The successive stages of the action potential are as follows.
Resting Stage. This is the resting membrane poten- tial 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 “polar- ized” 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 depo- larization 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
KCI
0.70.5 0.60.3 0.40.1 0.20
+35
0
–90
M ill
iv o
lt s
Overshoot
Milliseconds
D ep
ol ar
iz at
io n
R epolarization
Resting
— +
0
+ + + + +
+ + + + + + + + + +
Silver–silver chloride electrode
+ + + + – – – – – – – – – – – – – –
+ + + + + + + +
– – – – – – – – – – – – –
Figure 5-6 Typical action potential recorded by the method shown in the upper panel of the figure.
Activation gate
Selectivity filter
Inactivation gate
Inside
Resting (−90 mV)
Activated (−90 to +35 mV)
Inactivated (+35 to −90 mV,
delayed)
Resting (−90 mV)
Slow activation (+35 to −90 mV)
Na+ Na+
K+ K+
Na+
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.
Unit II Membrane Physiology, Nerve, and Muscle
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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.
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 conforma- tional 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 con- formational 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 repolar- ization 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 mem- brane 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 quanti- tative 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 stud- ies is shown in Figures 5-8 and 5-9.
Figure 5-8 shows an experimental apparatus called a volt- age 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 posi- tive 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
Current electrode
Voltage electrode
Amplifier
Electrode in fluid
Figure 5-8 “Voltage clamp” method for studying flow of ions through specific channels.
C o
n d
u ct
an ce
( m
m h
o /c
m 2 )
Time (milliseconds) 0 1 2 3
20
10
0
30
–90 mV +10 mV –90 mV Membrane potential
A ct
iv at
io n
Na+ channel
K+ channel
Inactivation
Figure 5-9 Typical changes in conductance of sodium and potas- sium ion channels when the membrane potential is suddenly increased from the normal resting value of −90 millivolts to a pos- itive value of +10 millivolts for 2 milliseconds. This figure shows that the sodium channels open (activate) and then close (inacti- vate) 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.
Chapter 5 Membrane Potentials and Action Potentials
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the effect of these ion movements on the desired setting of the intracellular voltage, electrical current is injected auto- matically 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 inverte- brates, 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 volt- age 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 poten- tial 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 con- ductance 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 millisec- ond. 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 origi- nal status, but again, only after an additional millisecond or more delay.
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 conduc- tance. 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.
C o
n d
u ct
an ce
(m m
h o
/c m
2 ) N
a+ co
n d
u ct
an ce
K +
co n
d u
ct an
ce
Milliseconds 0.50 1.51.0
0.1
0.01 0.005
1
10
100 0.001
0.01
0.1
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Overshoot
Positive afterpotential
+60 +40 +20 0
–20 –40 –60 –80 –100
M em
b ra
n e
p o
te n
ti al
( m
V )
Action potential
Ratio of conductances
Na+
K+
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.)
Unit II Membrane Physiology, Nerve, and Muscle
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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 chan- nels. 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 posi- tive 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 endoplas- mic 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 per- meable to sodium ions and calcium ions, but their perme- ability 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 chan- nels 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 acti- vation as for the sodium channels. For this reason they are often called slow channels, in contrast to the sodium chan- nels, which are called fast channels. Therefore, the opening of calcium channels provides a more sustained depolariza- tion, whereas the sodium channels play a key role in initiat- ing 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 cal- cium 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, some- times 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 exte- rior surfaces of the sodium channel protein molecule. The positive charges of these calcium ions in turn alter the elec- trical 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 fur- ther 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 feed- back 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 chan- nels 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 posi- tive 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
Chapter 5 Membrane Potentials and Action Potentials
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potential along the membrane. This mechanism is dem- onstrated in Figure 5-11. Figure 5-11A shows a normal resting nerve fiber, and Figure 5-11B 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 adja- cent resting membrane areas. That is, positive electrical charges are carried by the inward-diffusing sodium ions through the depolarized membrane and then for sev- eral 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 5-11C 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 depolariza- tion process along a nerve or muscle fiber is called a nerve or muscle impulse.
Direction of Propagation. As demonstrated in Figure 5-11, an excitable membrane has no single direc- tion 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 excit- able tissues. Occasionally, the action potential reaches a point on the membrane at which it does not generate suf- ficient 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 excita- tion 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 depo- larization and potassium ions diffuse to the outside dur- ing 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 estab- lishment 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 adeno- sine triphosphate (ATP) energy system of the cell. Figure 5-12 shows that the nerve fiber produces excess heat dur- ing recharging, which is a measure of energy expenditure when the nerve impulse frequency increases.
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 propor- tion to the third power of this intracellular sodium concen- tration. 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.”
+ + + + + + + + + + + + – – + + + + + + + + +
+ + + + + + + + + + + + – – + + + + + + + + +
+ + + + + + + + + + – – – – + + + + + + + +
+ + + + + + + + + + – – – – + + + + + + + +
– – + + + + + + + + + + + + + + + + + + – –
– – + + + + + + + + + + + + + + + + + + – –
+ + – – – – – – – – – – – – – – – – – – + +
+ + – – – – – – – – – – – – – – – – – – + +
– – – – – – – – – – – – + + – – – – – – – – –
+ + + + + + + + + + + + + + + + + + + + + + +
+ + + + + + + + + + + + + + + + + + + + + + +
– – – – – – – – – – – – – – – – – – – – – – –
– – – – – – – – – – – – – – – – – – – – – – –
– – – – – – – – – – – – + + – – – – – – – – –
– – – – – – – – – – + + + + – – – – – – – –
– – – – – – – – – – + + + + – – – – – – – –
A
B
C
D Figure 5-11 Propagation of action potentials in both directions along a conductive fiber.
Unit II Membrane Physiology, Nerve, and Muscle
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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.
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 chan- nels, and (2) voltage-activated calcium-sodium chan- nels, 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 pro- longed 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 mem- brane 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 neu- rons 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 neu- ronal 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 Sponta- neous Rhythmicity. For spontaneous rhythmicity to occur, the membrane even in its natural state must be per- meable 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 rhyth- mical control center of the heart is only −60 to −70 mil- livolts. 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;
0.90.80.70.60.50.40.30.20.10
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Figure 5-13 Action potential (in millivolts) from a Purkinje fiber of the heart, showing a “plateau.”
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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.
H ea
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io n
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Figure 5-12 Heat production in a nerve fiber at rest and at progressively increasing rates of stimulation.
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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 excitabil- ity causes depolarization again and a new action poten- tial occurs spontaneously. This cycle continues over and over and causes self-induced rhythmical excitation of the excitable tissue.
Why does the membrane of the heart control center not depolarize immediately after it has become repolar- ized, 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 conduc- tance” 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 consid- erably 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-sec- tional 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 aver- age nerve trunk contains about twice as many unmyelinated fibers as myelinated 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 poten- tial. 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.
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 sub- stance sphingomyelin. This substance is an excellent electri- cal 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 micrometers in length remains where ions still can flow with ease through the axon membrane between the extracel- lular fluid and the intracellular fluid inside the axon. This area is called the node of Ranvier.
Figure 5-15 Cross section of a small nerve trunk containing both myelinated and unmyelinated fibers.
Axon
A
B
Node of Ranvier
Unmyelinated axons
Schwann cell nucleus
Schwann cell cytoplasm
Schwann cell nucleus
Schwann cell cytoplasm
Myelin sheath
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.)
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“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.”
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 con- serves energy for the axon because only the nodes depolar- ize, allowing perhaps 100 times less loss of ions than would otherwise be necessary, and therefore requiring little metab- olism for re-establishing the sodium and potassium con- centration 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 dif- fuse 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 succes- sive muscle cells in the heart and intestine. For the purpose of understanding the excitation process, let us begin by dis- cussing 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 electric- ity to the nerve or muscle surface through two small elec- trodes, one of which is negatively charged and the other positively charged. When this is done, the excitable mem- brane 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 poten- tial 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 subthresh- old potentials.
Myelin sheath
1 2 3
Node of RanvierAxoplasm
Figure 5-17 Saltatory conduction along a myelinated axon. Flow of electrical current from node to node is illustrated by the arrows.
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Figure 5-18 Effect of stimuli of increasing voltages to elicit an action potential. Note development of “acute subthreshold poten- tials” when the stimuli are below the threshold value required for eliciting an action potential.
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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 stim- ulus 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 pre- ceding 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 poten- tial 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 can- not be elicited, even with a strong stimulus, is called the abso- lute refractory period. This period for large myelinated nerve fibers is about 1/2500 second. Therefore, one can readily cal- culate 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 excitabil- ity, still others, called membrane-stabilizing factors, can decrease excitability. For instance, a high extracellular fluid calcium ion concentration decreases membrane permeabil- ity to sodium ions and simultaneously reduces excitability. Therefore, calcium ions are said to be a “stabilizer.”
Local Anesthetics. Among the most important stabiliz- ers are the many substances used clinically as local anes- thetics, including procaine and tetracaine. Most of these act directly on the activation gates of the sodium chan- nels, 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 rap- idly 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 poten- tial 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.
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 record- ing 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 fig- ure, 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 fur- ther to the right is the recorded action potential itself.
Bibliography
Alberts B, Johnson A, Lewis J, et al: Molecular Biology of the Cell, ed 3, New York, 2008, Garland Science.
Biel M, Wahl-Schott C, Michalakis S, Zong X: Hyperpolarization-activated cation channels: from genes to function, Physiol Rev 89:847, 2009.
Blaesse P, Airaksinen MS, Rivera C, Kaila K: Cation-chloride cotransporters and neuronal function, Neuron 61:820, 2009.
Dai S, Hall DD, Hell JW: Supramolecular assemblies and localized regulation of voltage-gated ion channels, Physiol Rev 89:411, 2009.
Hodgkin AL, Huxley AF: Quantitative description of membrane current and its application to conduction and excitation in nerve, J Physiol (Lond) 117:500, 1952.
Kandel ER, Schwartz JH, Jessell TM: Principles of Neural Science, ed 4, New York, 2000, McGraw-Hill.
Electrical stimulator
Electronic sweep circuit
Horizontal plates
Vertical plates
Electron gun
Plugs
Nerve
Electronic amplifier
Recorded action potential
Electron beam
Stimulus artifact
Figure 5-19 Cathode ray oscilloscope for recording transient action potentials.
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Kleber AG, Rudy Y: Basic mechanisms of cardiac impulse propagation and associated arrhythmias, Physiol Rev 84:431, 2004.
Luján R, Maylie J, Adelman JP: New sites of action for GIRK and SK channels, Nat Rev Neurosci 10:475, 2009.
Mangoni ME, Nargeot J: Genesis and regulation of the heart automaticity, Physiol Rev 88:919, 2008.
Perez-Reyes E: Molecular physiology of low-voltage-activated T-type calcium channels, Physiol Rev 83:117, 2003.
Poliak S, Peles E: The local differentiation of myelinated axons at nodes of Ranvier, Nat Rev Neurosci 12:968, 2003.
Schafer DP, Rasband MN: Glial regulation of the axonal membrane at nodes of Ranvier, Curr Opin Neurobiol 16:508, 2006.
Vacher H, Mohapatra DP, Trimmer JS: Localization and targeting of voltage-dependent ion channels in mammalian central neurons, Physiol Rev 88:1407, 2008.
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chapter 6
Contraction of Skeletal Muscle
chapter 6
About 40 percent of the body is skeletal muscle, and perhaps another 10 per- cent is smooth and cardiac muscle. Some of the same basic principles of contrac- tion apply to all three differ-
ent 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.
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 sar- colemma 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 6-1C. Each myofibril (Figure 6-1D and E) is composed of about 1500 adjacent myosin filaments and
3000 actin filaments, which are large polymerized pro- tein 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.
Note in Figure 6-1E that the myosin and actin fila- ments partially interdigitate and thus cause the myofi- brils 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 polar- ized light. The dark bands contain myosin filaments, as well as the ends of the actin filaments where they over- lap the myosin, and are called A bands because they are anisotropic to polarized light. Note also the small pro- jections from the sides of the myosin filaments in Figure 6-1E and L. These are cross-bridges. It is the interaction between these cross-bridges and the actin filaments that causes contraction.
Figure 6-1E also shows that the ends of the actin fila- ments are attached to a so-called Z disc. From this disc, these filaments extend in both directions to interdigi- tate 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 stri- ated 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 sar- comere 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.
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Muscle fasciculus
SKELETAL MUSCLE
B
C
A Muscle
G-Actin molecules
F-Actin filament
Myofilaments
Muscle fiber
Myofibril
Sarcomere ZZ
H
H band
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Z disc
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A band
I band
Myosin filament
Myosin molecule
Light meromyosin
Heavy meromyosin
E
F G H I N
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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.
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Titin Filamentous Molecules Keep the Myosin and Actin Filaments in Place. The side-by-side relation- ship between the myosin and actin filaments is difficult to maintain. This is achieved by a large number of filamen- tous 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 sar- comere 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.
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 potas- sium, magnesium, and phosphate, plus multiple protein enzymes. Also present are tremendous numbers of mito- chondria 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 sar- coplasmic reticulum. This reticulum has a special organi- zation that is extremely important in controlling muscle contraction, as discussed in Chapter 7. The rapidly con- tracting types of muscle fibers have especially extensive sarcoplasmic reticula.
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 inte- rior of the muscle fiber membrane. This causes a local depolarization that in turn leads to opening of
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.)
Myosin (thick filament)
Actin (thin filament)
Titin
Z disc
M line
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.
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.)
Unit II Membrane Physiology, Nerve, and Muscle
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Tail
Head
A
B
Two heavy chains
Light chains
Actin filaments
Hinges
Myosin filament
BodyCross-bridges
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 of the cross-bridges with adjacent actin filaments.
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 mem- brane, 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 reticu- lum 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 mus- cle contractile process.
Molecular Mechanism of Muscle Contraction
Sliding Filament Mechanism of Muscle Con- t raction. Figure 6-5 demonstrates the basic mecha- nism 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 extend- ing 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 myo- sin 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 fila- ments. Thus, muscle contraction occurs by a sliding fila- ment 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 cal- cium ions that rapidly surround the myofibrils. The cal- cium 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 6-6A, has a molecular weight of about 480,000. Figure 6-6B 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.
The myosin molecule (see Figure 6-6A) 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
I A I
Z Z
I A
Relaxed
Contracted
I
Z Z
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 mem- branes toward each other.
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helix, which is called the tail of the myosin molecule. One end of each of these chains is folded bilaterally into a globu- lar 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 individ- ual myosin molecules. The central portion of one of these filaments is shown in Figure 6-6B, 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 myo- sin 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 fila- ment 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 direc- tions around the filament.
ATPase Activity of the Myosin Head. Another fea- ture of the myosin head that is essential for muscle contrac- tion 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, rep- resented 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.
Each strand of the double F-actin helix is composed of polymerized G-actin molecules, each having a molecu- lar 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 fil- aments 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 fila- ment 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 direc- tions 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 Contra c tion. Attached intermittently along the sides of the tropomy- osin 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 sub- units (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 tro- ponin for calcium ions is believed to initiate the contrac- tion 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 tro- ponin-tropomyosin complex is added to the actin fila- ment, 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-tropomy- osin 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.
Active sites
F-actin Tropomyosin
Troponin complex
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.
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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 nor- mal relation between the troponin-tropomyosin complex and actin is altered by calcium ions, producing a new con- dition 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 simul- taneously 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 direc- tion. 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 fila- ments 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 conforma- tional 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 mol- ecule was cleaved earlier.
4. Once the head of the cross-bridge tilts, this allows release of the ADP and phosphate ion that were pre- viously 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 condi- tion, 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.
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.
Actin filamentActive sites
Myosin filament
Hinges Power stroke
Movement
Figure 6-8 “Walk-along” mechanism for contraction of the muscle.
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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 devel- oped by the activated muscle is zero. Then, as the sarco- mere 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 fur- ther 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 fil- aments begin to overlap each other in addition to overlap- ping 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 sarco- mere has now contracted to its shortest length.
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 dif- ferent 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.
Note in Figure 6-10 that when the muscle is at its nor- mal 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
3 4210
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A
B C D
C
B
A
D
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er ce
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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 dif- ferent 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.)
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Figure 6-10 Relation of muscle length to tension in the muscle both before and during muscle contraction.
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V el
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Figure 6-11 Relation of load to velocity of contraction in a skele- tal muscle with a cross section of 1 square centimeter and a length of 8 centimeters.
Unit II Membrane Physiology, Nerve, and Muscle
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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.
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 avail- able 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 follow- ing equation:
W = L × D
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 chem- ical 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 sarco- plasm into the sarcoplasmic reticulum after the contrac- tion is over and (2) pumping sodium and potassium ions through the muscle fiber membrane to maintain appro- priate 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 mol- ecule to the contracting machinery of the muscle fiber. Then, as described in Chapter 2, the ADP is rephospho- rylated to form new ATP within another fraction of a second, which allows the muscle to continue its con- traction. 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 phos- phocreatine 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 “gly- colysis” 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 ener- gize additional muscle contraction and also to re-form the stores of phosphocreatine.
The importance of this glycolysis mechanism is two- fold. First, the glycolytic reactions can occur even in the absence of oxygen, so muscle contraction can be sus- tained 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 for- mation 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 cellu- lar 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 effi- ciency 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 con- verted into work.
Maximum efficiency can be realized only when the mus- cle contracts at a moderate velocity. If the muscle contracts slowly or without any movement, small amounts of main- tenance heat are released during contraction, even though little or no work is performed, thereby decreasing the con-
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version efficiency to as little as zero. Conversely, if contrac- tion 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, maxi- mum efficiency is developed when the velocity of contrac- tion 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 contrac- tion 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 con- traction. Systems for recording the two types of muscle con- traction are shown in Figure 6-12.
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 sys- tem, the muscle shortens against a fixed load; this is illus- trated 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 sys- tem 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 Rec orded 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 ener- getics 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 contrac- tion 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 func- tions 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 suffi- cient 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.
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 tibia- lis, are composed mainly of “fast” fibers with only small num- bers of the slow variety. Conversely, muscles such as soleus that respond slowly but with prolonged contraction are com- posed 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 pro- tein 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 con- traction. (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.
Kymograph Muscle
Stimulating electrodes
Stimulating electrodes
Weights
Isotonic system Isometric system
Electronic force transducer
To electronic recorder
Figure 6-12 Isotonic and isometric systems for recording muscle contractions.
Milliseconds
F o
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Ocular muscle
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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.
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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 Sum mation. Summation means the adding together of indi- vidual twitch contractions to increase the intensity of overall muscle contraction. Summation occurs in two ways: (1) by increasing the number of motor units con- tracting simultaneously, which is called multiple fiber summation, and (2) by increasing the frequency of con- traction, 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 grada- tions 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 natu- rally 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 tetaniza- tion. 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 con- traction 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.
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 ten- dons 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 increas- ing 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 sarco- plasm 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 mus- cle 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, exper- iments have also shown that transmission of the nerve signal
Rate of stimulation (times per second) 5 10 15 20 25 30 35 40 45 50 55
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Figure 6-14 Frequency summation and tetanization.
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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 contract- ing 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 apply- ing 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 ful- crum 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.
In short, an analysis of the lever systems of the body depends on knowledge of (1) the point of muscle inser- tion, (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 dif- ferent 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 mus- cles, lever systems, and their movements is called kinesi- ology 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 ago- nist 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 func- tional 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 mus- cle increases until the two strengths equal each other. At this point, movement of the arm or leg stops. Thus, by vary- ing the ratios of the degree of activation of the agonist and antagonist muscles, the nervous system directs the posi- tioning of the arm or leg.
We learn in Chapter 54 that the motor nervous system has additional important mechanisms to compensate for dif- ferent 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 diam- eters 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 remodel- ing process is often quite rapid, within a few weeks. Indeed, experiments in animals have shown that muscle contrac- tile 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 individ- ual 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 signifi- cant hypertrophy within 6 to 10 weeks.
The manner in which forceful contraction leads to hyper- trophy 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.Figure 6-15 Lever system activated by the biceps muscle.
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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 pro- tein 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 hyper- trophy 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, illus- trating 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 remod- eled 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 hypertro- phy 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 contrac- tile signals that are required to maintain normal mus- cle size. Therefore, atrophy begins almost immediately. After about 2 months, degenerative changes also begin to appear in the muscle fibers themselves. If the nerve sup- ply to the muscle grows back rapidly, full return of func- tion 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 dur- ing 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 macromo- tor 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 mus- cles contract and become rigid, even without action poten- tials. 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.
Bibliography
Allen DG, Lamb GD, Westerblad H: Skeletal muscle fatigue: cellular mecha- nisms, Physiol Rev 88:287, 2008.
Berchtold MW, Brinkmeier H, Muntener M: Calcium ion in skeletal muscle: its crucial role for muscle function, plasticity, and disease, Physiol Rev 80:1215, 2000.
Cheng H, Lederer WJ: Calcium sparks, Physiol Rev 88:1491, 2008. Clanton TL, Levine S: Respiratory muscle fiber remodeling in chronic
hyperinflation: dysfunction or adaptation? J Appl Physiol 107:324, 2009.
Clausen T: Na+-K+ pump regulation and skeletal muscle contractility, Physiol Rev 83:1269, 2003.
Dirksen RT: Checking your SOCCs and feet: the molecular mechanisms of Ca2+ entry in skeletal muscle, J Physiol 587:3139, 2009.
Fitts RH: The cross-bridge cycle and skeletal muscle fatigue, J Appl Physiol 104:551, 2008.
Glass DJ: Signalling pathways that mediate skeletal muscle hypertrophy and atrophy, Nat Cell Biol 5:87, 2003.
Gordon AM, Regnier M, Homsher E: Skeletal and cardiac muscle contractile activation: tropomyosin “rocks and rolls”, News Physiol Sci 16:49, 2001.
Gunning P, O’Neill G, Hardeman E: Tropomyosin-based regulation of the actin cytoskeleton in time and space, Physiol Rev 88:1, 2008.
Huxley AF, Gordon AM: Striation patterns in active and passive shortening of muscle, Nature (Lond) 193:280, 1962.
Kjær M: Role of extracellular matrix in adaptation of tendon and skeletal muscle to mechanical loading, Physiol Rev 84:649, 2004.
Lynch GS, Ryall JG: Role of beta-adrenoceptor signaling in skeletal muscle: implications for muscle wasting and disease, Physiol Rev 88:729, 2008.
MacIntosh BR: Role of calcium sensitivity modulation in skeletal muscle performance, News Physiol Sci 18:222, 2003.
Phillips SM, Glover EI, Rennie MJ: Alterations of protein turnover underlying disuse atrophy in human skeletal muscle, J Appl Physiol 107:645, 2009.
Powers SK, Jackson MJ: Exercise-induced oxidative stress: cellular mech- anisms and impact on muscle force production, Physiol Rev 88:1243, 2008.
Sandri M: Signaling in muscle atrophy and hypertrophy, Physiology (Bethesda) 160, 2008.
Sieck GC, Regnier M: Plasticity and energetic demands of contraction in skeletal and cardiac muscle, J Appl Physiol 90:1158, 2001.
Treves S, Vukcevic M, Maj M, et al: Minor sarcoplasmic reticulum mem- brane components that modulate excitation-contraction coupling in striated muscles, J Physiol 587:3071, 2009.
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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, myeli- nated 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 7-1A and B shows the neuromuscular junction from a large, myeli- nated nerve fiber to a skeletal muscle fiber. The nerve fiber forms a complex of branching nerve terminals that invagi- nate 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 7-1C 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 numer- ous smaller folds of the muscle membrane called subneu- ral clefts, which greatly increase the surface area at which the synaptic transmitter can act.
In the axon terminal are many mitochondria that sup- ply adenosine triphosphate (ATP), the energy source that is used for synthesis of an excitatory transmitter, acetyl- choline. 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 junc- tion, 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 neu- ral membrane above and the muscle membrane and its subneural clefts below.
On the inside surface of the neural membrane are lin- ear dense bars, shown in cross section in Figure 7-2. To each side of each dense bar are protein particles that pen- etrate the neural membrane; these are voltage-gated cal- cium 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, draw- ing 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 spec- ulative, 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 mus- cle 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.
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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 mol- ecules penetrate all the way through the membrane, lying side by side in a circle to form a tubular channel, illus-
trated 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.
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 neg- ative 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 pre- venting efflux of the positively charged potassium ions when they attempt to pass outward.
As shown in Figure 7-3B, 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 mem- brane, 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.
Calcium channels
Neural membrane
Muscle membrane
Release sites
Vesicles
Dense bar
Basal lamina and acetylcholinesterase
Acetylcholine receptors
Voltage activated Na+ channels
Subneural cleft
Figure 7-2 Release of acetylcholine from synaptic vesicles at the neural membrane of the neuromuscular junction. Note the prox- imity of the release sites in the neural membrane to the acetyl- choline receptors in the muscle membrane, at the mouths of the subneural clefts.
Axon
Myofibrils
A
C
B
Teloglial cell
Terminal nerve branches
Muscle nuclei
Myelin sheath
Synaptic vesicles Axon terminal in synaptic trough
Subneural clefts
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.)
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Destruction of the Released Acetylcholine by Acetylcholinesterase. The acetylcholine, once released into the synaptic space, continues to activate the acetyl- choline 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 syn- aptic 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 chan- nels open causes the electrical potential inside the fiber at the local area of the end plate to increase in the posi- tive 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 suf- ficient to initiate more and more sodium channel open- ing, thus initiating an action potential at the muscle fiber membrane.
Figure 7-4 shows the principle of an end plate poten- tial 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 bac- terial poison that decreases the quantity of acetylcholine release by the nerve terminals.
Safety Factor for Transmission at the Neuro muscular 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 min- utes often diminishes the number of acetylcholine ves- icles so much that impulses fail to pass into the muscle
AchNa+A
– – – – –
– – – – – –
–
B 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.
Milliseconds
M ill
iv o
lt s
0 15 30 45 60 75
CBA
+60
+40
+20
0
–20
–40 Threshold
–60
–80
–100
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 mus- cle 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.
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fiber. This is called fatigue of the neuromuscular junc- tion, and it is the same effect that causes fatigue of syn- apses 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 sys- tem for which most of the details of chemical transmission have been worked out. The formation and release of acetyl- choline 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 mol- ecules 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 syn- aptic space. About 125 vesicles usually rupture with each action potential. Then, after a few milliseconds, the ace- tylcholine 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 func- tion 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 termi- nal 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 ves- icles. Within about 20 seconds, the proteins contract and cause the pits to break away to the interior of the mem- brane, thus forming new vesicles. Within another few sec- onds, acetylcholine is transported to the interior of these vesicles, and they are then ready for a new cycle of acetyl- choline 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 mus- cle fiber membrane at the motor end plate where the acetyl- choline 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 poten- tial, thereby causing a state of muscle spasm.
Drugs That Stimulate the Neuromuscular Junction by Inactivating Acetylcholinesterase. Three particularly well-known drugs, neostigmine, physostigmine, and diiso- propyl fluorophosphate, inactivate the acetylcholinesterase in the synapses so that it no longer hydrolyzes acetylcho- line. Therefore, with each successive nerve impulse, addi- tional 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 acetyl- cholinesterase 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 acetyl- cholinesterase 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, antibod- ies that attack the acetylcholine receptors have been demon- strated in the blood of most patients with myasthenia gravis. Therefore, it is believed that myasthenia gravis is an autoim- mune disease in which the patients have developed antibod- ies 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, paraly- sis of the respiratory muscles. The disease can usually be
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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 ini- tiation and conduction of action potentials in nerve fibers applies equally to skeletal muscle fibers, except for quan- titative differences. Some of the quantitative aspects of muscle potentials are the following:
1. Resting membrane potential: about −80 to −90 milli- volts 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 poten- tials spreading along its surface membrane cause almost no current flow deep within the fiber. Yet to cause maxi- mum muscle contraction, current must penetrate deeply into the muscle fiber to the vicinity of the separate myo- fibrils. This is achieved by transmission of action poten- tials 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 contrac- tion. This overall process is called excitation- contraction coupling.
A band
I band
Sarcotubules
Myofibrils
Z line
Triad of the reticulum
Sarcolemma
Transverse tubule
Transverse tubule
Terminal cisternae
Mitochondrion
Sarcoplasmic reticulum
Figure 7-5 Transverse (T) tubule–sarcoplasmic reticulum system. Note that the T tubules communicate with the outside of the cell mem- brane, 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.
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+ +
+
+ +
+
+ + +
+
+ +
+
Action potential
Ca++
Ca++
Ca++ Release Channel (open)
DHP receptor
Ca++ Release Channel (closed)
Terminal Cisterne
Sarcoplasmic reticulum
Repolarization
Ca++
Calsequestrin
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 repolar- ization (bottom panel) the conformational change in the DHP recep- tor closes the Ca++ release channels and Ca++ is transported from the sarcoplasm into the sarcoplasmic reticulum by an ATP-dependent calcium pump.
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 origi- nate 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 yel- low. This is composed of two major parts: (1) large cham- bers 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 reticu- lum is that within its vesicular tubules is an excess of cal- cium 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 reticu- lar 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 chan- nels remain open for a few milliseconds, releasing calcium ions into the sarcoplasm surrounding the myofibrils and causing contraction, as discussed in Chapter 6.
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 main- tains a relaxed state of the muscle.
Conversely, full excitation of the T tubule and sar- coplasmic reticulum system causes enough release of calcium ions to increase the concentration in the
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myofibrillar fluid to as high as 2 × 10−4 molar concen- tration, 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.
Bibliography
Also see references for Chapters 5 and 6. Brown RH Jr: Dystrophin-associated proteins and the muscular dystro-
phies, Annu Rev Med 48:457, 1997. Chaudhuri A, Behan PO: Fatigue in neurological disorders, Lancet 363:978,
2004. Cheng H, Lederer WJ: Calcium sparks, Physiol Rev 88:1491, 2008. Engel AG, Ohno K, Shen XM, Sine SM: Congenital myasthenic syndromes:
multiple molecular targets at the neuromuscular junction, Ann N Y Acad Sci 998:138, 2003.
Fagerlund MJ, Eriksson LI: Current concepts in neuromuscular transmission, Br J Anaesth 103:108, 2009.
Haouzi P, Chenuel B, Huszczuk A: Sensing vascular distension in skel- etal muscle by slow conducting afferent fibers: neurophysiological basis and implication for respiratory control, J Appl Physiol 96:407, 2004.
Hirsch NP: Neuromuscular junction in health and disease, Br J Anaesth 99:132, 2007.
Keesey JC: Clinical evaluation and management of myasthenia gravis, Muscle Nerve 29:484, 2004.
Korkut C, Budnik V: WNTs tune up the neuromuscular junction, Nat Rev Neurosci 10:627, 2009.
Leite JF, Rodrigues-Pinguet N, Lester HA: Insights into channel function via channel dysfunction, J Clin Invest 111:436, 2003.
Meriggioli MN, Sanders DB: Autoimmune myasthenia gravis: emerg- ing clinical and biological heterogeneity, Lancet Neurol 8:475, 2009.
Rekling JC, Funk GD, Bayliss DA, et al: Synaptic control of motoneuronal excitability, Physiol Rev 80:767, 2000.
Rosenberg PB: Calcium entry in skeletal muscle, J Physiol 587:3149, 2009.
Toyoshima C, Nomura H, Sugita Y: Structural basis of ion pumping by Ca2+-ATPase of sarcoplasmic reticulum, FEBS Lett 555:106, 2003.
Van der Kloot W, Molgo J: Quantal acetylcholine release at the vertebrate neuromuscular junction, Physiol Rev 74:899, 1994.
Vincent A: Unraveling the pathogenesis of myasthenia gravis, Nat Rev Immunol 10:797, 2002.
Vincent A, McConville J, Farrugia ME, et al: Antibodies in myasthenia gravis and related disorders, Ann N Y Acad Sci 998:324, 2003.
Action potential
Calcium pump
ATP required
Actin filaments
Sarcolemma
Myosin filaments
CaCa
Ca++ Ca++
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.
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chapter 8
Excitation and Contraction of Smooth Muscle
Contraction of Smooth Muscle
In Chapters 6 and 7, the dis- cussion 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 impor- tant, 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 dimen- sions, (2) organization into bundles or sheets, (3) response to different types of stimuli, (4) characteristics of inner- vation, 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.
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 mix- ture 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 indepen- dently 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 pilo- erector muscles that cause erection of the hairs when stim- ulated 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 hun- dreds 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 adher- ent to one another at multiple points so that force gener- ated in one muscle fiber can be transmitted to the next. In addition, the cell membranes are joined by many gap junc- tions 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.
c0040
Multi-unit smooth muscleA Unitary smooth muscle
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B
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Figure 8-1 Multi-unit (A) and unitary (B) smooth muscle.
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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 mecha- nism for control of contraction is different. This is dis- cussed 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 mus- cle. 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 skele- tal 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 arrange- ment of actin and myosin filaments as is found in skel- etal 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.
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 micro- graphs, one usually finds 5 to 10 times as many actin fila- ments 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 bod- ies. 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 mus- cle serve the same role as the Z discs in skeletal muscle.
There is another difference: Most of the myosin fila- ments 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 oppo- site direction. This allows the myosin to pull an actin fila- ment 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 per- cent 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 con- traction, sometimes lasting hours or even days. Therefore, it is to be expected that both the physical and the chemi- cal characteristics of smooth muscle versus skeletal mus- cle contraction would differ. Following are some of the differences.
Slow Cycling of the Myosin Cross-Bridges. The rapid- ity of cycling of the myosin cross-bridges in smooth mus- cle—that is, their attachment to actin, then release from the actin, and reattachment for the next cycle—is much slower
Actin filaments
Dense bodies
Cell membrane
Myosin filaments
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.
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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 fil- aments, 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 mus- cle 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 slow- ness of attachment and detachment of the cross-bridges with the actin filaments. In addition, the initiation of con- traction 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/ cm2 cross-sectional area for smooth muscle, in compari- son with 3 to 4 kilograms for skeletal muscle. This great force of smooth muscle contraction results from the pro- longed 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 con- traction. 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 impor- tant 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 contrac- tion 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 sec- onds 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 intracel- lular 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 reg- ulatory protein that is activated by calcium ions to cause skeletal muscle contraction. Instead, smooth muscle con- traction 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.
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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.
Myosin Phosphatase Is Important in Cessation of Contraction. When the calcium ion concentration falls below a critical level, the aforementioned processes auto- matically 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 con- traction 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 lon- ger proportion of the cycling period. Therefore, the num- ber 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, ten- sion 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 sig- nals, 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 con- tains 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 dis- cuss nervous control of smooth muscle contraction, fol- lowed by hormonal control and other means of control.
Neuromuscular Junctions of Smooth Muscle
Physiologic Anatomy of Smooth Muscle Neu- romuscular Junctions. Neuromuscular junctions of the highly structured type found on skeletal muscle fibers do not occur in smooth muscle. Instead, the auto- nomic nerve fibers that innervate smooth muscle gener- ally branch diffusely on top of a sheet of muscle fibers, as shown in Figure 8-4. In most instances, these fibers
Ca++
Ca++
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Phosphatase
Active MLCK
MLC Phosphorylated
MLC Dephosphorylated
Inactive MLCK
Ca++SR
Calmodulin
Outside
Contraction Relaxation 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 acti- vates 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 phos- phatase removes the phosphate from MLC, leading to relaxation.
Varicosities
Gap junctions
Visceral Multi-unit
Figure 8-4 Innervation of smooth muscle.
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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 mus- cle cells; the transmitter substance then diffuses to the cells. Furthermore, where there are many layers of mus- cle 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 transmit- ter substance.
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 sub- stance can be secreted through the walls of the varicosi- ties. In the varicosities are vesicles similar to those in the skeletal muscle end plate that contain transmitter sub- stance. But in contrast to the vesicles of skeletal muscle junctions, which always contain acetylcholine, the vesi- cles of the autonomic nerve fiber endings contain acetyl- choline 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 nano- meters—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 consid- erably 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 mus- cle 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, norepi- nephrine 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 recep- tor determines whether the smooth muscle is inhibited or excited and also determines which of the two trans- mitters, acetylcholine or norepinephrine, is effective in
causing the excitation or inhibition. These receptors are discussed in more detail in Chapter 60 in relation to func- tion 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 mus- cle. 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 vis- ceral 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 8-5A. Such action potentials can be elicited in many ways, for example, by electrical stimulation, by the
Milliseconds
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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.
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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.
Action Potentials with Plateaus. Figure 8-5C 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 repo- larization of the muscle fiber membrane, the repolariza- tion 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 mea- sure 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 mech- anism 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 mem- brane potential. A typical slow wave in a visceral smooth muscle of the gut is shown in Figure 8-5B. 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 mem-
brane; that is, the membrane potential becomes more negative when sodium is pumped rapidly and less nega- tive 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 contrac- tion. 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 approxi- mate threshold for eliciting action potentials in most vis- ceral smooth muscle), an action potential develops and spreads over the muscle mass and contraction occurs. Figure 8-5B demonstrates this effect, showing that at each peak of the slow wave, one or more action poten- tials 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 auto- matically and rhythmically. For instance, when the gut is overfilled by intestinal contents, local automatic con- tractions 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 acetylcho- line 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 con- traction. 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 depolariza- tion (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.
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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 mus- cle contractile machinery and without action potentials. Two types of non-nervous and nonaction potential stim- ulating 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 mus- cle relaxation and, therefore, vasodilatation.
2. Excess carbon dioxide causes vasodilatation. 3. Increased hydrogen ion concentration causes vaso-
dilatation.
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 mus- cle when the muscle cell membrane contains hormone- gated excitatory receptors for the respective hormone. Conversely, the hormone causes inhibition if the mem- brane 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 mem- brane, the same as after nerve stimulation. Sometimes action potentials result, or action potentials that are already occurring may be enhanced. In other cases, depo- larization occurs without action potentials and this depo- larization 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 chan- nels 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 nega- tivity inside the muscle cell, a state called hyperpolariza- tion, 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 inter- nal 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 phospho- rylation of several enzymes that indirectly inhibit con- traction. 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 sub- stances. In some instances, the same substance may cause either relaxation or contraction of smooth muscles in dif- ferent locations. For example, norepinephrine inhibits contraction of smooth muscle in the intestine but stimu- lates 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 mus- cle 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 cal- cium 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
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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 Reti c- ulum. Figure 8-6 shows a few slightly developed sarco- plasmic 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 sarcoplas- mic tubules in the same way that action potentials in skel- etal 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.
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 contrac- tion usually ceases. Therefore, the force of contraction of smooth muscle is usually highly dependent on extracellu- lar 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 extracellu- lar 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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Kuriyama H, Kitamura K, Itoh T, Inoue R: Physiological features of visceral smooth muscle cells, with special reference to receptors and ion chan- nels, Physiol Rev 78:811, 1998.
Morgan KG, Gangopadhyay SS: Cross-bridge regulation by thin filament- associated proteins, J Appl Physiol 91:953, 2001.
Somlyo AP, Somlyo AV: Ca2+ sensitivity of smooth muscle and nonmuscle myosin II: modulated by G proteins, kinases, and myosin phosphatase, Physiol Rev 83:1325, 2003.
Stephens NL: Airway smooth muscle, Lung 179:333, 2001. Touyz RM: Transient receptor potential melastatin 6 and 7 channels, mag-
nesium transport, and vascular biology: implications in hypertension, Am J Physiol Heart Circ Physiol 294:H1103, 2008.
Walker JS, Wingard CJ, Murphy RA: Energetics of crossbridge phosphoryla- tion and contraction in vascular smooth muscle, Hypertension 23:1106, 1994.
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Webb RC: Smooth muscle contraction and relaxation, Adv Physiol Educ 27:201, 2003.
Caveolae
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Figure 8-6 Sarcoplasmic tubules in a large smooth muscle fiber showing their relation to invaginations in the cell membrane called caveolae.
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The Heart 9. Cardiac Muscle; The Heart as a Pump
and Function of the Heart Valves
10. Rhythmical Excitation of the Heart
11. The Normal Electrocardiogram
12. Electrocardiographic Interpretation of Cardiac Muscle and Coronary Blood Flow Abnormalities: Vectorial Analysis
13. Cardiac Arrhythmias and Their Electrocardiographic Interpretation
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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.
Special mechanisms in the heart cause a continuing succession of heart contractions called cardiac rhythmic- ity, transmitting action potentials throughout the cardiac muscle to cause the heart’s rhythmical beat. This rhythmi- cal control system is explained in Chapter 10. In this chap- ter, 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 auto- matic 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, demon- strating 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 skel- etal 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.
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 indi- vidual 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 “com- municating” 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
Aorta
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Figure 9-1 Structure of the heart, and course of blood flow through the heart chambers and heart valves.
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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 atrio- ventricular (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 con- ducted only by way of a specialized conductive system called the A-V bundle, a bundle of conductive fibers sev- eral millimeters in diameter that is discussed in detail in Chapter 10.
This division of the muscle of the heart into two func- tional 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 nega- tive 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.
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 mus- cle. 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 num- bers 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 thou- sandths of a second and then abruptly close. At the end of this closure, repolarization occurs, and the action poten- tial 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 popula- tion 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 car- diac muscle fiber, and this maintains a prolonged period of depolarization, causing the plateau in the action poten- tial. Further, the calcium ions that enter during this pla- teau 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 car- diac 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 per- meability of the cardiac muscle membrane for potassium ions decreases about fivefold, an effect that does not occur
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Figure 9-3 Rhythmical action potentials (in millivolts) from a Purkinje fiber and from a ventricular muscle fiber, recorded by means of microelectrodes.
Figure 9-2 “Syncytial,” interconnecting nature of cardiac muscle fibers.
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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 1⁄250 the velocity in very large nerve fibers and about 1⁄10 the velocity in skeletal muscle fibers. The velocity of conduction in the special- ized 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 nor- mal 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).
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 dif- ferences 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 poten- tial 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 sarco- plasmic reticulum. In another few thousandths of a sec- ond, 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 them- selves 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 mem- brane, triggering the release of calcium into the sarco- plasm. Calcium ions in the sarcoplasm then interact with troponin to initiate cross-bridge formation and con- traction by the same basic mechanism as described for skeletal muscle in Chapter 6.
Without the calcium from the T tubules, the strength of cardiac muscle contraction would be reduced consider- ably because the sarcoplasmic reticulum of cardiac mus- cle 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 diam- eter 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 diffu- sion 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 cal- cium-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 extra- cellular spaces surrounding the cells, allowing the same
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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 contrac- tion is hardly affected by moderate changes in extra- cellular 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 poten- tial, the influx of calcium ions to the interior of the muscle fiber is suddenly cut off, and the calcium ions in the sar- coplasm 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 con- tract 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 spontane- ous 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 spe- cial 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 contrac- tion 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.
ATP
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spark Ca++
spark
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signal Ca++
signal
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stores Ca++
stores
Sarcoplasmic reticulum Sarcoplasmic reticulum
Sarcoplasmic reticulum Sarcoplasmic reticulum
T TubuleT Tubule
Figure 9-5 Mechanisms of excitation-contraction coupling and relaxation in cardiac muscle.
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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 sys- tole 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 car- diac cycle for the left side of the heart. The top three curves show the pressure changes in the aorta, left ventri- cle, and left atrium, respectively. The fourth curve depicts the changes in left ventricular volume, the fifth the elec- trocardiogram, 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.
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 contrac- tion, 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 ven- tricles 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 repo- larization 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 addi- tional 20 percent filling of the ventricles. Therefore, the atria simply function as primer pumps that increase the ventricu- lar pumping effectiveness as much as 20 percent. However, the heart can continue to operate under most conditions
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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.
Unit III The Heart
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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 bulg- ing 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 contrac- tion; it results from slow flow of blood into the atria from the veins while the A-V valves are closed during ventricular con- traction. 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 ven- tricular 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 mod- erately 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 ven- tricles, 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 ven- tricular 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 suf- ficient pressure to push the semilunar (aortic and pul- monary) 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 increas- ing in the muscle but little or no shortening of the muscle fibers is occurring.
Period of Ejection. When the left ventricular pres- sure rises slightly above 80 mm Hg (and the right ven- tricular 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 per- cent 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 dis- tended 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 rap- idly 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 dur- ing 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 vol- ume can be decreased to as little as 10 to 20 ml. Conversely, when large amounts of blood flow into the ventricles dur- ing 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
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blood backward, and they open when a forward pressure gradient forces blood in the forward direction. For ana- tomical 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.
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 mus- cles 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 contrac- tion. 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 differ- ently 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 aor- tic 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 pul- monary 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 con- structed 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 imme- diately 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 con- tinually 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 ventric- ular contraction.
The pressure curves in the right ventricle and pulmo- nary 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, giv- ing 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
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Figure 9-7 Mitral and aortic valves (the left ventricular valves).
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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 pro- portion 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 ven- tricles pump. The additional work output of each ventricle required to create kinetic energy of blood flow is propor- tional to the mass of blood ejected times the square of veloc- ity of ejection.
Ordinarily, the work output of the left ventricle required to create kinetic energy of blood flow is only about 1 per- cent 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 explain- ing 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.
The diastolic pressure curve is determined by filling the heart with progressively greater volumes of blood and then measuring the diastolic pressure immediately before ventric- ular 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 ven- tricular 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 max- imum 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 pres- sure 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 ver- sion of this loop is shown in Figure 9-9. It is divided into four phases.
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 dur- ing 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 isovolu- mic 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 vol- ume 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 intraven- tricular 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
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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,” demon- strating changes in intraventricular volume and pressure during the normal cardiac cycle. EW, net external work.
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volume-pressure diagram (the tan shaded area, labeled EW) represents the net external work output of the ventricle dur- ing 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 far- ther 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 spec- ify 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 consid- ered 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 cor- responds 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 after- load 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 ven- tricle 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 per- cent of this energy is normally derived from oxidative metab- olism 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 mea- sure of the chemical energy liberated while the heart per- forms its work. The different chemical reactions that liberate this energy are discussed in Chapters 67 and 68.
Experimental studies have shown that oxygen consump- tion 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 mus- cle 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, corre- spondingly 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 impor- tant 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.
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Figure 9-9 The “volume-pressure diagram” dem- onstrating 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.
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Efficiency of Cardiac Contraction. During heart mus- cle 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 regu- lation 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 com- bine 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 cir- cuit again.
This intrinsic ability of the heart to adapt to increas- ing 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 con- tract 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 char- acteristic 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 pump- ing ability.
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 ven- tricles 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.
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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 ven- tricular function curves. Physiol Rev 35:107, 1955.)
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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.
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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 stim- ulation. By contrast, the output can be decreased to as low as zero or almost zero by vagal (parasympathetic) stimulation.
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 stimula- tion 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, sym- pathetic stimulation often can increase the maximum cardiac output as much as twofold to threefold, in addi- tion 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 sym- pathetic 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 para- sympathetic 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 con- traction 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 Stimula- tion on the Cardiac Function Curve. Figure 9-13 shows four cardiac function curves. They are similar to the ven- tricular function curves of Figure 9-11. However, they represent function of the entire heart rather than of a sin- gle 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.
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.
Sympathetic nerves
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Figure 9-12 Cardiac sympathetic and parasympathetic nerves. (The vagus nerves to the heart are parasympathetic nerves.)
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Figure 9-13 Effect on the cardiac output curve of different degrees of sympathetic or parasympathetic stimulation.
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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 acti- vating 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 flu- ids decreases the resting membrane potential in the car- diac muscle fibers, as explained in Chapter 5. That is, high extracellular fluid potassium concentration partially depo- larizes 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 potas- sium ions, causing the heart to go toward spastic con- traction. 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 car- diac 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 tem- porarily by a moderate increase in temperature, as occurs during body exercise, but prolonged elevation of tem- perature 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.
Bibliography
Bers DM: Altered cardiac myocyte Ca regulation in heart failure, Physiology (Bethesda) 21:380, 2006.
Bers DM: Calcium cycling and signaling in cardiac myocytes, Annu Rev Physiol 70:23, 2008.
Brette F, Orchard C: T-tubule function in mammalian cardiac myocytes, Circ Res 92:1182, 2003.
Chantler PD, Lakatta EG, Najjar SS: Arterial-ventricular coupling: mechanis- tic insights into cardiovascular performance at rest and during exercise, J Appl Physiol 105:1342, 2008.
Cheng H, Lederer WJ: Calcium sparks, Physiol Rev 88:1491, 2008. Clancy CE, Kass RS: Defective cardiac ion channels: from mutations to clini-
cal syndromes, J Clin Invest 110:1075, 2002. Couchonnal LF, Anderson ME: The role of calmodulin kinase II in myocardial
physiology and disease, Physiology (Bethesda) 23:151, 2008. Fuchs F, Smith SH: Calcium, cross-bridges, and the Frank-Starling relation-
ship, News Physiol Sci 16:5, 2001. Guyton AC: Determination of cardiac output by equating venous return
curves with cardiac response curves, Physiol Rev 35:123, 1955. Guyton AC, Jones CE, Coleman TG: Circulatory Physiology: Cardiac Output
and Its Regulation, 2nd ed, Philadelphia, 1973, WB Saunders. Kang M, Chung KY, Walker JW: G-protein coupled receptor signaling in myo-
cardium: not for the faint of heart, Physiology (Bethesda) 22:174, 2007. Knaapen P, Germans T, Knuuti J, et al: Myocardial energetic and efficiency:
current status of the noninvasive approach, Circulation 115:918, 2007. Mangoni ME, Nargeot J: Genesis and regulation of the heart automaticity,
Physiol Rev 88:919, 2008. Korzick DH: Regulation of cardiac excitation-contraction coupling: a cellu-
lar update, Adv Physiol Educ 27:192, 2003. Olson EN: A decade of discoveries in cardiac biology, Nat Med 10:467,
2004.
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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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Rudy Y, Ackerman MJ, Bers DM, et al: Systems approach to understanding electromechanical activity in the human heart: a National Heart, Lung, and Blood Institute workshop summary, Circulation 118:1202, 2008.
Saks V, Dzeja P, Schlattner U, et al: Cardiac system bioenergetics: metabolic basis of the Frank-Starling law, J Physiol 571:253, 2006.
Sarnoff SJ: Myocardial contractility as described by ventricular function curves, Physiol Rev 35:107, 1955.
Starling EH: The Linacre Lecture on the Law of the Heart, London, 1918, Longmans Green.
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chapter 10
Rhythmical Excitation of the Heart
The heart is endowed with a special system for (1) gen- erating rhythmical electrical impulses to cause rhythmi- cal contraction of the heart muscle and (2) conduct- ing these impulses rapidly
through the heart. When this system functions normally, the atria contract about one sixth of a second ahead of ven- tricular contraction, which allows filling of the ventricles before they pump the blood through the lungs and periph- eral 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 effec- tive 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 coro- nary blood flow. The effect is often a bizarre heart rhythm or abnormal sequence of contraction of the heart cham- bers, 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 conduc- tive 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.
Sinus (Sinoatrial) Node
The sinus node (also called sinoatrial node) is a small, flat- tened, 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 diam- eter 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 immedi- ately 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 ordi- narily controls the rate of beat of the entire heart, as dis- cussed 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 nega- tivity 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 neutral- ize some of the intracellular negativity.
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 caus- ing the voltage changes of the action potential. They are (1) fast sodium channels, (2) slow sodium-calcium chan- nels, 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 mus- cle, because of rapid influx of positive sodium ions to the
Unit III The Heart
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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 chan- nels in the sinus nodal fiber because the “resting” poten- tial is much less negative—only −55 millivolts in the nodal fiber instead of the −90 millivolts in the ventricular mus- cle 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 nega- tive than about −55 millivolts for more than a few mil- liseconds, 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 extracel- lular fluid outside the nodal fiber, as well as a moder- ate 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 “rest- ing” potential gradually rises and becomes less nega- tive 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 open- ing, 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 poten- tial back to its negative resting level and therefore termi- nate 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 hyperpolariza- tion state initially carries the “resting” membrane poten- tial down to about −55 to −60 millivolts at the termination of the action potential.
Why is this new state of hyperpolarization not main- tained forever? The reason is that during the next few tenths of a second after the action potential is over, pro- gressively 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, hyperpo- larization 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.
A-V node
A-V bundle
Right bundle branch
Left bundle branch
Sinus node
Internodal pathways
Figure 10-1 Sinus node and the Purkinje system of the heart, showing also the A-V node, atrial internodal pathways, and ven- tricular bundle branches.
Sinus nodal fiber
Ventricular muscle fiber
Threshold for discharge
“Resting potential”
M ill
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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.
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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 conduc- tion in most atrial muscle is about 0.3 m/sec, but conduc- tion is more rapid, about 1 m/sec, in several small bands of atrial fibers. One of these, called the anterior inter- atrial 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.
Atrioventricular Node and Delay of Impulse Conduction from the Atria to the Ventricles
The atrial conductive system is organized so that the car- diac impulse does not travel from the atria into the ventri- cles 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 conduc- tive 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 exit- ing 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 sub- sequent appearance in the A-V nodal system. Note that the impulse, after traveling through the internodal path- ways, reaches the A-V node about 0.03 second after its ori- gin 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 bun- dle 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 conduc- tion in the transitional, nodal, and penetrating A-V bundle fibers is caused mainly by diminished numbers of gap junc- tions 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 por- tion 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 ventricu- lar 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 per- meability 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 transmis- sion. 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 inabil- ity, 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
Atrioventricular fibrous tissue
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(0.03)
Internodal pathways
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Penetrating portion of A-V bundle
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Figure 10-3 Organization of the A-V node. The numbers repre- sent the interval of time from the origin of the impulse in the sinus node. The values have been extrapolated to human beings.
Unit III The Heart
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the ventricles to the atria, allowing only forward con- duction 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 bar- rier, a portion of which is shown in Figure 10-3. This bar- rier normally acts as an insulator to prevent passage of the cardiac impulse between atrial and ventricular mus- cle through any other route besides forward conduc- tion 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 condi- tions, 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 mus- cle, the distal portion of the A-V bundle passes down- ward 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 bun- dle 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, pro- gressively 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 ter- minations of the Purkinje fibers, the total elapsed time aver- ages 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 dou- ble 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 ventricu- lar 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 appear- ance 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 endo- cardial surfaces of the ventricles. Then the impulse once again spreads slightly less rapidly through the ventricular muscle to the epicardial surfaces.
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
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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.
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abnormal conditions, this is not the case. Other parts of the heart can also exhibit intrinsic rhythmical excita- tion 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 dis- charge 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 rhythmic- ity? The answer derives from the fact that the discharge rate of the sinus node is considerably faster than the natu- ral 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 mem- branes. 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. Occasio- n ally some other part of the heart develops a rhythmi- cal 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 penetrat- ing 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 con- tinue 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 ventric- ular 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 “over- driven” 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 car- diac impulse should travel through the ventricles slowly, much of the ventricular mass would contract before contraction of the remainder, in which case the over- all 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 para- sympathetic 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 rep- resentation 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 transmis- sion 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.
Unit III The Heart
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And strong stimulation of the vagi can stop completely the rhythmical excitation by the sinus node or block com- pletely 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 bun- dle, 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 con- ductive 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 trans- mission 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 essen- tially 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 por- tions 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 over- all 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 norepineph-
rine at the sympathetic nerve endings. Norepinephrine in turn stimulates beta-1 adrenergic receptors, which medi- ate 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 poten- tial 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.
Bibliography
Barbuti A, DiFrancesco D: Control of cardiac rate by “funny” channels in health and disease, Ann N Y Acad Sci 1123:213, 2008.
Baruscotti M, Robinson RB: Electrophysiology and pacemaker func- tion of the developing sinoatrial node, Am J Physiol Heart Circ Physiol 293:H2613, 2007.
Cheng H, Lederer WJ: Calcium sparks, Physiol Rev 88:1491, 2008. Chien KR, Domian IJ, Parker KK: Cardiogenesis and the complex biology of
regenerative cardiovascular medicine, Science 322:1494, 2008. Dobrzynski H, Boyett MR, Anderson RH: New insights into pacemaker
activity: promoting understanding of sick sinus syndrome, Circulation 115:1921, 2007.
James TN: Structure and function of the sinus node, AV node and His bun- dle of the human heart: part I—structure, Prog Cardiovasc Dis 45:235, 2002.
James TN: Structure and function of the sinus node, AV node and His bun- dle of the human heart: part II—function, Prog Cardiovasc Dis 45:327, 2003.
Kléber AG, Rudy Y: Basic mechanisms of cardiac impulse propagation and associated arrhythmias, Physiol Rev 84:431, 2004.
Lakatta EG, Vinogradova TM, Maltsev VA: The missing link in the mystery of normal automaticity of cardiac pacemaker cells, Ann N Y Acad Sci 1123:41, 2008.
Leclercq C, Hare JM: Ventricular resynchronization: current state of the art, Circulation 109:296, 2004.
Mangoni ME, Nargeot J: Genesis and regulation of the heart automaticity, Physiol Rev 88:919, 2008.
Mazgalev TN, Ho SY, Anderson RH: Anatomic-electrophysiological cor- relations concerning the pathways for atrioventricular conduction, Circulation 103:2660, 2001.
Schram G, Pourrier M, Melnyk P, et al: Differential distribution of cardiac ion channel expression as a basis for regional specialization in electrical function, Circ Res 90:939, 2002.
Yasuma F, Hayano J: Respiratory sinus arrhythmia: why does the heartbeat synchronize with respiratory rhythm? Chest 125:683, 2004.
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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 adja- cent 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 gener- ated 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.
Characteristics of the Normal Electrocardiogram
The normal electrocardiogram (see Figure 11-1) is com- posed 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 electrocardi- ography 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.
In Figure 11-2A, 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 polar- ized. 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 posi- tively. To the right of the muscle fiber is shown a record
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Figure 11-1 Normal electrocardiogram.
Unit III The Heart
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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 11-2A, the record has risen to a maximum positive value.
In Figure 11-2B, 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 11-2C shows halfway repolarization of the same muscle fiber, with positivity returning to the out- side 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 11-2A. Consequently, the recording, as shown to the right, becomes negative.
In Figure 11-2D, the muscle fiber has completely repo- larized, and both electrodes are now in areas of positiv- ity 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.
Note in the lower half of the figure a simultaneous recording of the electrocardiogram from this same ven- tricle, 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 com- pletely depolarized. Only when the muscle is partly polar- ized and partly depolarized does current flow from one part of the ventricles to another part and therefore cur- rent 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 ter- mination of the P wave. This is also approximately when the QRS complex is being recorded in the electrocardio- gram. 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 mus- cle 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
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Figure 11-2 Recording the depolarization wave (A and B) and the repolarization wave (C and D) from a cardiac muscle fiber.
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Figure 11-3 Above, Monophasic action potential from a ventric- ular 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.
Chapter 11 The Normal Electrocardiogram
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extends over a long period, about 0.15 second. For this rea- son, the T wave in the normal electrocardiogram is a pro- longed 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 appro- priate 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 direc- tion 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 milli- meters 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 electro- cardiogram depend on the manner in which the elec- trodes 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 elec- trode 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 deter- mined easily from an electrocardiogram because the heart rate is the reciprocal of the time interval between two suc- cessive 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 per- son 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 car- diac muscle during each beat of the heart change electri- cal 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 capa- ble of responding rapidly to these changes in potentials.
Recorders for Electrocardiographs
Many modern clinical electrocardiographs use com- puter-based systems and electronic display, whereas others use a direct pen recorder that writes the elec- trocardiogram 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 mov- ing 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 sty- lus 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 stim- ulation, all the exteriors of the muscle cells had been posi- tive 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 depolar- ized, negative charges leak to the outsides of the depo- larized muscle fibers, making this part of the surface
Unit III The Heart
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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.
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 volt- age 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, con- duct 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.
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 nega- tive 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 direc- tion, 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
−
−
+
+ − + − +
0
− +
0
− +
0
+++++ +++++
+++++ +++++
++++++++ +++++
++ + +++ ++
+
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+
+++++++++++ ++++++++++
++++++ ++++++
+++++ +++++ ++++++
+++++
++++++
+ − − − − − − −
+
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+ − − − − − − − − − −
+++++ +++++
++++ + + +
− − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − +
+ +++++
+++++ ++++++ − − − − − − − −
− − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − −
Figure 11-4 Instantaneous potentials develop on the surface of a cardiac muscle mass that has been depolarized in its center.
+ −−−
−− −−−−−−−++++
+ ++
+ ++++++
+ + + +++++++++++
++
0
A
B
− −
+
+
Figure 11-5 Flow of current in the chest around partially depolar- ized ventricles.
Chapter 11 The Normal Electrocardiogram
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limb leads. The term “bipolar” means that the electrocar- diogram is recorded from two electrodes located on dif- ferent 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 electrocardio- graph. The electrocardiograph in each instance is repre- sented 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 electrocardio- gram. 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.
Einthoven’s Law. Einthoven’s law states that if the elec- trical potentials of any two of the three bipolar limb elec- trocardiographic 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 mil- livolts on the left arm. Similarly, lead III records a pos- itive 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 respec- tive 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 record- ings 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.
On analysis of the three electrocardiograms, it can be shown, with careful measurements and proper obser- vance 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 dif- ferent 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
+0.3 mV
+0.7 mV
+0.5 mV
+1.0 mV
Lead III
+1.2 mV
Lead II
Lead I
−0.2 mV
0 − +
− + − −
++
0 − +
0 − +
− +
− +− +
Figure 11-6 Conventional arrangement of electrodes for record- ing the standard electrocardiographic leads. Einthoven’s triangle is superimposed on the chest.
Unit III The Heart
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leads are recorded, because abnormalities of cardiac mus- cle 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 sepa- rately in Chapters 12 and 13.
Chest Leads (Precordial Leads)
Often electrocardiograms are recorded with one elec- trode 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 electri- cal 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 11-9 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 poten- tial 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.
In leads V1 and V2, the QRS recordings of the nor- mal 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 posi- tive 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 uni- polar limb lead. In this type of recording, two of the limbs are connected through electrical resistances to the nega- tive 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.
− +
0 − +
1 2 3 456
LARA
5000 ohms
5000 ohms
5000 ohms
Figure 11-8 Connections of the body with the electrocardiograph for recording chest leads. LA, left arm; RA, right arm.
V1 V2 V3 V4 V5 V6
Figure 11-9 Normal electrocardiograms recorded from the six standard chest leads.
I
II
III
Figure 11-7 Normal electrocardiograms recorded from the three standard electrocardiographic leads.
Chapter 11 The Normal Electrocardiogram
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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 electrocar- diograph to determine this.)
Bibliography
See bibliography for Chapter 13.
aVR aVL aVF
Figure 11-10 Normal electrocardiograms recorded from the three augmented unipolar limb leads.
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chapter 12
From the discussion in Chapter 10 of impulse trans mission through the heart, it is obvious that any change in the pattern of this trans mission can cause abnor mal electrical potentials
around the heart and, consequently, alter the shapes of the waves in the electrocardiogram. For this reason, most seri ous 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 abnor malities 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 121 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 nondepolar ized 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 ventri cles 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.
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 122. From this zero reference point, the scale of vectors rotates clock wise: 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.
In a normal heart, the average direction of the vector during spread of the depolarization wave through the ven tricles, called the mean QRS vector, is about +59 degrees, which is shown by vector A drawn through the center of Figure 122 in the +59degree 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.
− − −
− −
− −
− −
− − − − −−−−−−−−−−−−
− −
++++ +
++ +
+ ++
+ + +
+ +
+++ ++ + + + + + + + + + +
+ + + + +
+ +
+ +
+ +
++++ + + + + + + +
Figure 12-1 Mean vector through the partially depolarized ventricles.
Electrocardiographic Interpretation of Cardiac Muscle and Coronary Blood Flow
Abnormalities: Vectorial Analysis
Unit III The Heart
130
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 hori zontal 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 righthand corner and the left leg connects in the lower lefthand 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 123, 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.
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 vec tors and, second, the axes of the leads, it is possible to use these together to determine the instantaneous poten tial that will be recorded in the electrocardiogram of each lead for a given vector in the heart, as follows.
Figure 124 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 dia gram below the heart, vector A is shown again, and a line is drawn to represent the axis of lead I in the 0degree 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 socalled 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 125 shows another example of vectorial anal ysis. In this example, vector A represents the electrical potential and its axis at a given instant during ventricu lar 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 per pendicular 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
I
–
+
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+
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– –
–
+
–
90
60
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−30 210
aVR
aVR
aVF
aVL
aVL
IIIII
III
I
120
Figure 12-3 Axes of the three bipolar and three unipolar leads.
–
A
B II
A
+
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.
−90 +270
+90
180
−100
0
59120
A
Figure 12-2 Vectors drawn to represent potentials for several dif- ferent hearts, and the “axis” of the potential (expressed in degrees) for each heart.
Chapter 12 Electrocardiographic Interpretation of Cardiac Muscle and Coronary Blood Flow Abnormalities: Vectorial Analysis
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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.
Vectorial Analysis of Potentials in the Three Standard Bipolar Limb Leads. In Figure 126, 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 stan dard 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, pro jected vector C depicts the potential in lead II, and pro jected vec tor 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 onehalf that of the actual potential in the heart (vector A); in lead II (vec tor C), it is almost equal to that in the heart; and in lead III (vector D), it is about onethird that in the heart.
An identical analysis can be used to determine poten tials recorded in augmented limb leads, except that the respective axes of the augmented leads (see Figure 123) are used in place of the standard bipolar limb lead axes used for Figure 126.
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 ventri cles to become depolarized is the left endocardial surface of the septum. Then depolarization spreads rapidly to involve both endocardial surfaces of the septum, as dem onstrated by the darker shaded portion of the ventricle in Figure 127A. Next, depolarization spreads along the endocardial surfaces of the remainder of the two ventri cles, as shown in Figure 127B and C. Finally, it spreads through the ventricular muscle to the outside of the heart, as shown progressively in Figure 127C, D, and E.
At each stage in Figure 127, parts A to E, the instan taneous mean electrical potential of the ventricles is rep resented 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 deter mine 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 vec torial analysis, it is essential that this analysis of the suc cessive normal vectors presented in Figure 127 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 127A, the ventricular muscle has just begun to be depolarized, representing an instant about 0.01 sec ond after the onset of depolarization. At this time, the vector is short because only a small portion of the ven tricles—the septum—is depolarized. Therefore, all elec trocardiographic voltages are low, as recorded to the right of the ventricular muscle for each of the leads. The volt age 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.
– B II
A
+
Figure 12-5 Determination of the projected vector B along the axis of lead I when vector A represents the instantaneous poten- tial in the ventricles.
A
B
C
D
I
IIIII
IIIII
I
+ +
+
––
–
Figure 12-6 Determination of projected vectors in leads I, II, and III when vector A represents the instantaneous potential in the ventricles.
Unit III The Heart
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A B
C D
E
−
− −
+
++
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+
++
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++
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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).
In Figure 127B, 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 electrocardio graphic leads have increased.
In Figure 127C, 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 electronega tive, neutralizing much of the positivity on the other epi cardial 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 127D, about 0.05 second after onset of depo larization, 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 127E, about 0.06 second after onset of depo larization, the entire ventricular muscle mass is depolar ized 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 127; this depression is the Q wave. When it occurs, it is caused by initial depolariza tion 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 basetoapex vector occurs. The major positive deflection shown in Figure 127 is the R wave, and the final negative deflection is the S wave.
Chapter 12 Electrocardiographic Interpretation of Cardiac Muscle and Coronary Blood Flow Abnormalities: Vectorial Analysis
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Electrocardiogram during Repolarization—the T Wave
After the ventricular muscle has become depolarized, about 0.15 second later, repolarization begins and pro ceeds until complete at about 0.35 second. This repolar ization causes the T wave in the electrocardiogram.
Because the septum and endocardial areas of the ven tricular 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 repolariza tion 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 repo larize 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 polar- ity of most of the normal QRS complex.
In Figure 128, five stages of repolarization of the ven tricles 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 great est when about half the heart is in the polarized state and about half is depolarized.
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 129. 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 electrocardio grams recorded from the atria during depolarization are also usually positive in all three of these leads, as shown in Figure 129. This record of atrial depolarization is known as the atrial P wave.
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 sig nal. Therefore, the musculature around the sinus node becomes depolarized a long time before the muscula ture 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 depo- larized 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
I
−
− −
II +
++
II
IIIII
III
II
III
Figure 12-8 Generation of the T wave during repolarization of the ventricles, showing also vectorial analysis of the first stage of repo- larization. The total time from the beginning of the T wave to its end is approximately 0.15 second.
I
P T
−
−
− − −
− −
−− − −−
−
II +
+ +
+ + +
+ + +
+ ++
+ ++ + +
+ +
++++++
++
II
IIIII
III
SA
II
III
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.
Unit III The Heart
134
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 129, the socalled 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 stan dard 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.
Vectorcardiogram
It has been noted in the discussion up to this point that the vector of current flow through the heart changes rap idly 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 electri cal potential from the heart. The socalled vectorcardio- gram depicts these changes at different times during the cardiac cycle, as shown in Figure 1210.
In the large vectorcardiogram of Figure 1210, 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 vec tor 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 vec tor 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 pos itive 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, vec tor 2 of Figure 1210 represents the state of depolariza tion of the ventricles about 0.02 second after vector 1. After another 0.02 second, vector 3 represents the poten tial, 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 oscillo scope and connecting chest surface electrodes from each side of the heart to the horizontal plates. When the vec tor 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 1210 is that of a normal heart. Note from this vectorcardiogram that the preponderant direction of the vectors of the ventri cles during depolarization is mainly toward the apex of the heart. That is, during most of the cycle of ventricu lar depolarization, the direction of the electrical poten tial (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 elec- trical 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 esti mated from the standard bipolar limb lead electrocar diograms rather than from the vectorcardiogram. Figure 1211 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 1211, 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 neg- ative 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 plot ted 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 1211.
5
4 1 2 3 4 5
3
2Depolarization QRS
Repolarization T
1
Figure 12-10 QRS and T vectorcardiograms.
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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 rep resents 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 ven tricles 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 1211, is 59 degrees positive (+59 degrees).
Abnormal Ventricular Conditions That Cause Axis Deviation
Although the mean electrical axis of the ventricles aver ages about 59 degrees, this axis can swing even in the nor mal heart from about 20 degrees to about 100 degrees. The causes of the normal variations are mainly anatomi cal 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 devia tion 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 electri cal 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 devi ates toward the hypertrophied ventricle.
Vectorial Analysis of Left Axis Deviation Resulting from Hypertrophy of the Left Ventricle. Figure 1212 shows the three standard bipolar limb lead electro cardiograms. Vectorial analysis demonstrates left axis deviation with mean electrical axis pointing in the −15degree direction. This is a typical electrocardio gram caused by increased muscle mass of the left ven tricle. 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
I
III
III
III
I
+
+
–
– II
I
59
–60
0180
120
Figure 12-11 Plotting the mean electrical axis of the ventricles from two electrocardiographic leads (leads I and III).
I
I I
II III
III
III +
+–
–
Figure 12-12 Left axis deviation in a hypertensive heart (hyper- trophic left ventricle). Note the slightly prolonged QRS complex as well.
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left ventricle hypertrophies as a result of aortic valvular stenosis, aortic valvular regurgitation, or any number of congenital heart conditions in which the left ventri cle enlarges while the right ventricle remains relatively normal in size.
Vectorial Analysis of Right Axis Deviation Resulting from Hypertrophy of the Right Ventricle. The elec trocardiogram of Figure 1213 shows intense right axis deviation, to an electrical axis of 170 degrees, which is 111 degrees to the right of the normal 59degree mean ventricular QRS axis. The right axis deviation demon strated 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 con genital heart conditions that cause hypertrophy of the right ventricle, such as tetralogy of Fallot and interventricular septal defect.
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 bun dle 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 depolariza tion 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 elec tronegative, whereas the left ventricle remains electroposi tive during most of the depolarization process, and a strong vector projects from the right ventricle toward the left ven tricle. 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 1214, which shows typical left axis deviation result ing from left bundle branch block.
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 1214. 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 neg ative 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
I I
I
III +
+–
–
II III
III
Figure 12-13 High-voltage electrocardiogram in congenital pul- monary valve stenosis with right ventricular hypertrophy. Intense right axis deviation and a slightly prolonged QRS complex also are seen.
I I
I
III +
+–
–
II III
III
Figure 12-14 Left axis deviation caused by left bundle branch block. Note also the greatly prolonged QRS complex.
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analyzed, in Figure 1215, which shows an axis of about 105 degrees instead of the normal 59 degrees and a pro longed QRS complex because of slow conduction.
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 highvoltage electrocardiogram.
The cause of highvoltage QRS complexes most often is increased muscle mass of the heart, which ordinar ily 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 gen eration 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 1212 and 1213.
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 com plex along with the decreased voltage. Figure 1216 shows
a typical lowvoltage electrocardiogram with prolonga tion 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.
Decreased Voltage Caused by Conditions Surround- ing 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 “shortcircuits” the electri cal potentials generated by the heart, decreasing the elec trocardiographic voltages that reach the outside surfaces of the body. Pleural effusion, to a lesser extent, also can “shortcircuit” the electricity around the heart so that the voltages at the surface of the body and in the electrocar diograms are decreased.
Pulmonary emphysema can decrease the electrocar diographic 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 pre vent spread of electrical voltage from the heart to the surface of the body, and this results in decreased electro cardiographic 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.
I
II
III III +
I I+–
– III
Figure 12-15 Right axis deviation caused by right bundle branch block. Note also the greatly prolonged QRS complex.
I II
III
Figure 12-16 Low-voltage electrocardiogram following local damage throughout the ventricles caused by previous myocardial infarction.
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Therefore, prolonged conduction of the impulse through the ventricles always causes a prolonged QRS complex. Such prolongation often occurs when one or both ven tricles 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 hyper trophy 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 nor mal. Therefore, if complete block of one of the bundle branches occurs, the duration of the QRS complex is usu ally increased to 0.14 second or greater.
In general, a QRS complex is considered to be abnor mally 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 electro cardiograms for bundle branch block in Figures 1214 and 1215.
Conditions That Cause Bizarre QRS Complexes
Bizarre patterns of the QRS complex most frequently are caused by two conditions: (1) destruction of cardiac mus cle 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, car diac impulse conduction becomes irregular, causing rapid shifts in voltages and axis deviations. This often causes double or even triple peaks in some of the electrocardio graphic leads, such as those shown in Figure 1214.
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 patho logically 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 nega- tive, 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 1217, a small area in the base of the left ventricle is newly infarcted (loss of coronary blood flow). Therefore, during the TP interval—that is, when the normal ventric ular muscle is totally polarized—abnormal negative cur rent still flows from the infarcted area at the base of the left ventricle and spreads toward the rest of the ventricles.
The vector of this “current of injury,” as shown in the first heart in Figure 1217, 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 poten- tial 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 termi nal 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 vec tor 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 vecto rial analysis, the successive stages of electrocardiogram generation by the depolarization wave traveling through the ventricles can be constructed graphically, as demon strated in the lower part of Figure 1217.
When the heart becomes totally depolarized, at the end of the depolarization process (as noted by the nexttolast stage in Figure 1217), all the ventricular muscle is in a negative state. Therefore, at this instant in the electrocar diogram, no current flows from the ventricles to the elec trocardiographic 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 depolar ization in the injured base of the left ventricle. Thus, repo larization causes a return of the current of injury in each lead, as noted at the far right in Figure 1217.
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
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stray currents exist in the body, such as currents result ing 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, includ ing 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 electrocardio gram, as shown in Figure 1218.
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 1218 shows electrocardiograms (leads I and III) from an injured heart. Both records show injury poten tials. In other words, the J point of each of these two electrocardiograms is not on the same line as the TP 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 electro cardiogram 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 1218, the respective injury potentials in leads I and III are plotted on the coordi nates of these leads, and the resultant vector of the injury potential for the whole ventricular muscle mass
II
II
IIIII
III
Current of injury
Injured area
Current of injury
I
II
III
−
− −
+
++
J
J
J
Figure 12-17 Effect of a current of injury on the electrocardiogram.
I I
0
I
0
“J” point
“J” point
+−
+
+
−
− III
III
0
III
0+−
Figure 12-18 J point as the zero reference potential of the elec- trocardiograms for leads I and III. Also, the method for plotting the axis of the injury potential is shown by the lowermost panel.
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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 vec tor for the injury potential directly over the ventricles, the negative end of the vector points toward the perma- nently depolarized, “injured” area of the ventricles. In the example shown in Figure 1218, 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, repolar ization 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 repo larization of the membranes. As long as this state exists, an injury potential continues to flow during the diastolic portion (the TP 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 TP interval between heartbeats, as shown in Figures 1219 and 1220. Therefore, one of the most important diag nostic features of electrocardiograms recorded after acute coronary thrombosis is the current of injury.
Acute Anterior Wall Infarction. Figure 1219 shows the electrocardiogram in the three standard bipolar limb leads and in one chest lead (lead V2) recorded from a patient with acute anterior wall cardiac infarction. The most important diagnostic feature of this electrocardio gram is the intense injury potential in chest lead V2. If one draws a zero horizontal potential line through the J point of this electrocardiogram, a strong negative injury poten tial during the TP 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 poten tial 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 1220 shows the three standard bipolar limb leads and one chest lead (lead V2) 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 TP 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
I I
I II
V2
+
+
–
– III
III
III
Figure 12-19 Current of injury in acute anterior wall infarction. Note the intense injury potential in lead V
2 .
II
I II
II
++
III
III
III V2
– –
Figure 12-20 Injury potential in acute posterior wall, apical infarction.
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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 electrocardio gram is the basis for diagnosing posterior wall infarction.
If one analyzes the injury potentials from leads II and III of Figure 1220, it is readily apparent that the injury poten tial 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 point ing 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 1221 shows a V3 chest lead from a patient with acute pos terior wall infarction, demonstrating changes in the elec trocardiogram from the day of the attack to 1 week later, 3 weeks later, and finally 1 year later. From this electro cardiogram, one can see that the injury potential is strong immediately after the acute attack (TP segment displaced positively from the ST segment). However, after about 1 week, the injury potential has diminished considerably, and after 3 weeks, it is gone. After that, the electrocardio gram does not change greatly during the next year. This is the usual recovery pattern after acute myocardial infarc tion of moderate degree, showing that the new collat- eral coronary blood flow develops enough to reestablish appropriate nutrition to most of the infarcted area.
Conversely, in some patients with myocardial infarc tion, 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 1222 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 poste rior 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.
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 pec toris” means pain from the heart felt in the pectoral regions of the upper chest. This pain usually also radi ates 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 electro cardiogram 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.
Same day 1 week 3 weeks 1 year
Figure 12-21 Recovery of the myocardium after moderate pos- terior 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.
Anterior
I
Q
III
Posterior
I
Q
III
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.
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Effect of Slow Conduction of the Depolarization Wave on the Characteristics of the T Wave
Referring to Figure 1214, 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 vec tor 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 con duction of the depolarization impulse through the ven tricles 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 abnor mally 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 stan dard 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 1223.
Mild ischemia is by far the most common cause of shortening of depolarization of cardiac muscle because this increases current flow through the potassium chan nels. 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 elec trocardiogram, 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—inver sion, 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 car diac muscle contraction. But when overdosages of digi talis 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 1224. Therefore, changes in the T wave during digitalis administration are often the earliest signs of digi talis toxicity.
Bibliography
See bibliography for Chapter 13.
T T
Figure 12-24 Biphasic T wave caused by digitalis toxicity.
T T T T
Figure 12-23 Inverted T wave resulting from mild ischemia at the apex of the ventricles.
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chapter 13
Cardiac Arrhythmias and Their Electrocardiographic Interpretation
Some of the most distress- ing types of heart malfunc- tion occur not as a result of abnormal heart mus- cle but because of abnor- mal 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 physi- ology of common cardiac arrhythmias and their effects on heart pumping, as well as their diagnosis by electro- cardiography. The causes of the cardiac arrhythmias are usually one or a combination of the following abnor- malities 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
Tachycardia
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 tachy- cardia 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.
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 excit- ability and rate of rhythm.
Many factors can cause the sympathetic nervous sys- tem 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.
Bradycardia
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.
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
Figure 13-1 Sinus tachycardia (lead I).
Figure 13-2 Sinus bradycardia (lead III).
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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 cir- culatory 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 (barore- ceptors) in the carotid sinus region of the carotid artery walls are excessively sensitive. Therefore, even mild external pres- sure 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 cardiota- chometer 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 respira- tion (left half of the record). Then, during deep respira- tion, the heart rate increased and decreased with each respiratory cycle by as much as 30 percent.
Sinus arrhythmia can result from any one of many circu- latory 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 phe- nomenon 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.
Atrioventricular Block
The only means by which impulses ordinarily can pass from the atria into the ventricles is through the A-V bun- dle, 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 ven- tricles. 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 cal- cified portions of the heart can depress or block con- duction 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 rela- tion 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 sec- ond, 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 pro- longed P-R interval; the interval in this instance is about
60 70 80
100 120H
ea rt
r at
e
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.
SA block
Figure 13-4 Sinoatrial nodal block, with A-V nodal rhythm during the block period (lead III).
P P P P P
Figure 13-5 Prolonged P-R interval caused by first degree A-V heart block (lead II).
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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 con- duction. 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.
Second-Degree Block. When conduction through the A-V bundle is slowed enough to increase the P-R inter- val to 0.25 to 0.45 second, the action potential is sometimes strong enough to pass through the bundle into the ventri- cles 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.
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 elec- trocardiogram 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.
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 lon- ger 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 over- drive 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 dis- tal part of the A-V node beyond the blocked point in the node, or in the A-V bundle, begins discharging rhythmi- cally 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 electri- cal 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 ven- tricular Purkinje system. Figure 13-8 shows the condi- tion known as electrical alternans, which results from partial intraventricular block every other heartbeat.
PPPPPPPPPP
Figure 13-7 Complete A-V block (lead II).
P P P P P P
Dropped beat
Figure 13-6 Second degree A-V block, showing occasional failure of the ventricles to receive the excitatory signals (lead V
3 ).
Figure 13-8 Partial intraventricular block—“electrical alternans” (lead III).
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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.
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, pre- mature beat, or ectopic beat.
Causes of Premature Contractions. Most pre- mature 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 dif- ferent 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 sys- tem, or myocardium caused by drugs, nicotine, or caf- feine. Mechanical initiation of premature contractions is also frequent during cardiac catheterization; large num- bers 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 contrac- tion. 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 contrac- tion and the next succeeding contraction is slightly pro- longed, which is called a compensatory pause. One of the reasons for this is that the premature contraction origi- nated 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.
Premature atrial contractions occur frequently in oth- erwise healthy people. Indeed, they often occur in ath- letes whose hearts are in very healthy condition. Mild toxic conditions resulting from such factors as smok- ing, lack of sleep, ingestion of too much coffee, alco- holism, 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 con- traction is depressed or almost absent. Therefore, the pulse wave passing to the peripheral arteries after a pre- mature 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 origi- nated 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 super- imposed 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 sig- nificance and causes as atrial premature contractions.
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 simulta- neously; consequently, in the normal heart, the depo- larization waves of the two sides of the heart—mainly of opposite polarity to each other—partially neutral- ize each other in the electrocardiogram. When a PVC
PPP
Premature beat
PP TTTTT
Figure 13-10 A-V nodal premature contraction (lead III).
Premature beat
Figure 13-9 Atrial premature beat (lead I).
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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 poten- tials, 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.
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 irri- tability. 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 devel- oping spontaneous lethal ventricular fibrillation, pre- sumably 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 electrocar-
diogram in Figure 13-11 the point of origin of the PVC as follows: Note that the potentials of the premature con- tractions 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 ven- tricular 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 syn- drome (LQTS).
The major reason that the long QT syndrome is of concern is that delayed repolarization of ventricular mus- cle increases a person’s susceptibility to develop ventricu- lar 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 arrhyth- mia 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 ventricu- lar fibrillation.
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 muta- tions 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 antiarrhyth- mic 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 ven- tricular 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.
−
+ +
−II
II
III
III
III II
Figure 13-11 Premature ventricular contractions (PVCs) dem- onstrated 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.
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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 irri- table focus, this focus becomes the pacemaker of the heart.
The term “paroxysmal” means that the heart rate becomes rapid in paroxysms, with the paroxysm begin- ning suddenly and lasting for a few seconds, a few min- utes, 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
electro cardiogram 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 parox- ysmal tachycardia is in the atrium, but because the P wave is abnormal in shape, the origin is not near the sinus node.
A-V Nodal Paroxysmal Tachycardia. Paroxysmal tachycardia often results from an aberrant rhythm that involves the A-V node. This usually causes almost nor- mal 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 gen- erally 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 ventric- ular tachycardia. The electrocardiogram of ventricular paroxysmal tachycardia has the appearance of a series of
Figure 13-13 Atrial paroxysmal tachycardia—onset in middle of record (lead I).
Premature depolarization
Torsades de pointes
Pause
Postpause QT Postpause QT
Pause
Repetitive premature depolarization
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.)
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ventricular premature beats occurring one after another without any normal beats interspersed.
Ventricular paroxysmal tachycardia is usually a seri- ous condition for two reasons. First, this type of tachycar- dia 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 stimu- lation 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 tachy- cardia. Conversely, quinidine, which increases the refrac- tory 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 ventricu- lar fibrillation, which, if not stopped within 1 to 3 min- utes, 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 con- traction of all the ventricular muscle at once, which is required for a pumping cycle of the heart. Despite mas- sive 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 neg- ligible 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 ventricu- lar 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 dif- ferent 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.”
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 refrac- tory 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 orig- inally 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 con- tinue around and around the circle.
Figure 13-14 Ventricular paroxysmal tachycardia (lead III).
Absolutely refractory
Absolutely refractory
Relatively refractory
NORMAL PATHWAY
LONG PATHWAY
Figure 13-15 Circus movement, showing annihilation of the impulse in the short pathway and continued propagation of the impulse in the long pathway.
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All these conditions occur in different pathological states of the human heart, as follows: (1) A long path- way 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 short- ened refractory period commonly occurs in response to various drugs, such as epinephrine, or after repetitive elec- trical 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 dif- ferent 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 elec- trical 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 por- tions. 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 refrac- tory 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 tran- spire in rapid succession, all occurring simultaneously and eventuating in a state of fibrillation.
First, block of the impulses in some directions but suc- cessful 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 predis- pose 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, allow- ing 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 fibrilla- tion 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 refrac- tory 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 direc- tions at the same time. Furthermore, this irregular pat- tern 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
Stimulus point Dividing
impulses
Blocked impulse A B 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.
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of ventricular fibrillation, relatively large masses of muscle contract simultaneously, and this causes coarse, irregular waves in the electrocardiogram. After another few sec- onds, 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 ventricu- lar 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.
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 ventricu- lar 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 alternat- ing 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 cur- rent 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 ven- tricles 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 elec- trodes 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.
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 defi- brillation 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 min- utes 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 dam- age 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 sepa- rated from the ventricular muscle mass by fibrous tissue. Therefore, ventricular fibrillation often occurs without atrial fibrillation. Likewise, fibrillation often occurs in the
Figure 13-17 Ventricular fibrillation (lead II).
Several thousand volts for a few milliseconds
Handle for application of pressure
Electrode
Figure 13-18 Application of electrical current to the chest to stop ventricular fibrillation.
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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 enlarge- ment 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 ven- tricles will not pump blood during ventricular fibrilla- tion, 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 fibril- lation, although at reduced efficiency of overall heart pumping.
Electrocardiogram in Atrial Fibrillation. Figure 13-19 shows the electrocardiogram during atrial fibril- lation. Numerous small depolarization waves spread in all directions through the atria during atrial fibril- lation. 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.
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 ven- tricular 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, dem- onstrated 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 electro- shock. 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 fibril- lation, 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 min- ute. However, because one side of the atria is contract- ing 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 peri- ods 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 13-21 shows a typical electrocardiogram in atrial flutter. The P waves are strong because of contrac- tion of semicoordinate masses of muscle. However, note
Atrial flutter Atrial fibrillation
Figure 13-20 Pathways of impulses in atrial flutter and atrial fibrillation.
Figure 13-19 Atrial fibrillation (lead I). The waves that can be seen are ventricular QRS and T waves.
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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.
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 inade- quate respiration. The hypoxia prevents the muscle fibers and conductive fibers from maintaining normal electro- lyte concentration differentials across their membranes, and their excitability may be so affected that the auto- matic rhythmicity disappears.
In most instances of cardiac arrest from anesthesia, prolonged cardiopulmonary resuscitation (many min- utes or even hours) is quite successful in re-establishing a normal heart rhythm. In some patients, severe myocar- dial disease can cause permanent or semipermanent car- diac arrest, which can cause death. To treat the condition, rhythmical electrical impulses from an implanted elec- tronic cardiac pacemaker have been used successfully to keep patients alive for months to years.
Bibliography
Antzelevitch C: Role of spatial dispersion of repolarization in inherited and acquired sudden cardiac death syndromes, Am J Physiol Heart Circ Physiol 293:H2024, 2007.
Awad MM, Calkins H, Judge DP: Mechanisms of disease: molecular genetics of arrhythmogenic right ventricular dysplasia/cardiomyopathy, Nat Clin Pract Cardiovasc Med 5:258, 2008.
Barbuti A, DiFrancesco D: Control of cardiac rate by “funny” channels in health and disease, Ann N Y Acad Sci 1123:213, 2008.
Cheng H, Lederer WJ: Calcium sparks, Physiol Rev 88:1491, 2008. Dobrzynski H, Boyett MR, Anderson RH: New insights into pacemaker
activity: promoting understanding of sick sinus syndrome, Circulation 115:1921, 2007.
Elizari MV, Acunzo RS, Ferreiro M: Hemiblocks revisited, Circulation 115:1154, 2007.
Jalife J: Ventricular fibrillation: mechanisms of initiation and maintenance, Annu Rev Physiol 62:25, 2000.
Lubitz SA, Fischer A, Fuster V: Catheter ablation for atrial fibrillation, BMJ 336:819, 2008.
Maron BJ: Sudden death in young athletes, N Engl J Med 349:1064, 2003. Morita H, Wu J, Zipes DP: The QT syndromes: long and short, Lancet
372:750, 2008. Murray KT, Roden DM: Disorders of cardiac repolarization: the long QT syn-
dromes. In Crawford MG, DiMarco JP, editors: Cardiology, London, 2001, Mosby.
Myerburg RJ: Implantable cardioverter-defibrillators after myocardial infarction, N Engl J Med 359:2245, 2008.
Passman R, Kadish A: Sudden death prevention with implantable devices, Circulation 116:561, 2007.
Roden DM: Drug-induced prolongation of the QT interval, N Engl J Med 350:1013, 2004.
Sanguinetti MC: Tristani-Firouzi M: hERG potassium channels and cardiac arrhythmia, Nature 440:463, 2006.
Swynghedauw B, Baillard C, Milliez P: The long QT interval is not only inherited but is also linked to cardiac hypertrophy, J Mol Med 81:336, 2003.
Wang K, Asinger RW, Marriott HJ: ST-segment elevation in conditions other than acute myocardial infarction, N Engl J Med 349:2128, 2003.
Zimetbaum PJ, Josephson ME: Use of the electrocardiogram in acute myo- cardial infarction, N Engl J Med 348:933, 2003.
Figure 13-21 Atrial flutter—2:1 and 3:1 atrial to ventricle rhythm (lead I).
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The Circulation 14. Overview of the Circulation; Biophysics
of Pressure, Flow, and Resistance
15. Vascular Distensibility and Functions of the Arterial and Venous Systems
16. The Microcirculation and Lymphatic System: Capillary Fluid Exchange, Interstitial Fluid, and Lymph Flow
17. Local and Humoral Control of Tissue Blood Flow
18. Nervous Regulation of the Circulation, and Rapid Control of Arterial Pressure
19. Role of the Kidneys in Long-Term Control of Arterial Pressure and in Hypertension; The Integrated System for Arterial Pressure Regulation
20. Cardiac Output, Venous Return, and Their Regulation
21. Muscle Blood Flow and Cardiac Output During Exercise; the Coronary Circulation and Ischemic Heart Disease
22. Cardiac Failure
23. Heart Valves and Heart Sounds; Valvular and Congenital Heart Defects
24. Circulatory Shock and Its Treatment
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chapter 14
Overview of the Circulation; Biophysics of Pressure, Flow, and Resistance
The function of the circula- tion is to service the needs of the body tissues—to trans- port 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 appro- priate 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 con- trolled 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 exam- ple, 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 pres- sure 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.
Functional Parts of the Circulation. Before dis- cussing 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 arter- ies 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 numer- ous 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 con- tract or expand and thereby act as a controllable reser- voir 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 cir- culation and lists the percentage of the total blood vol- ume 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 capil- laries. It is here, however, that the most important func- tion of the circulation occurs, diffusion of substances back and forth between the blood and the tissues. This func- tion 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:
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Unit IV The Circulation
Vessel Cross-Sectional Area (cm2) 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 vas- cular cross-sectional area (A):
v = F/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 mil- limeter, 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 continu- ally into the aorta, the mean pressure in the aorta is high, averaging about 100 mm Hg. Also, because heart pump- ing is pulsatile, the arterial pressure alternates between a systolic pressure level of 120 mm Hg and a diastolic pres- sure level of 80 mm Hg, as shown on the left side of Figure 14-2.
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 “func- tional” 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 circula- tion. In the pulmonary arteries, the pressure is pulsatile, just as in the aorta, but the pressure is far less: pulmo- nary 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—occasion- ally 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 microves- sels of each tissue continuously monitor tissue needs, such as the availability of oxygen and other nutrients
Systemic vessels Arteries–13%
Arterioles and capillaries–7%
Heart–7%
Aorta
Pulmonary circulation–9%
Systemic circulation–84%
Veins, venules, and venous
sinuses–64%
Inferior vena cava
Superior vena cava
Figure 14-1 Distribution of blood (in percentage of total blood) in the different parts of the circulatory system.
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and the accumulation of carbon dioxide and other tis- sue 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 imme- diately back into the arteries. Thus, the heart acts as an automaton, responding to the demands of the tis- sues. 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 indepen- dent 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 pres- sure 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 sig- nals 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 gener- alized constriction of most of the arterioles through- out 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 kid- neys 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.
P1 represents the pressure at the origin of the vessel; at the other end, the pressure is P2. Resistance occurs as a result of friction between the flowing blood and the intra- vascular endothelium all along the inside of the vessel. The flow through the vessel can be calculated by the fol- lowing formula, which is called Ohm’s law :
in which F is blood flow, ∆P is the pressure difference (P1 − P2) 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.
P re
ss u
re (
m m
H g
)
0 Systemic Pulmonary
60
80
100
120
40
20
0
A or
ta
La rg
e ar
te rie
s
S m
al l a
rt er
ie s
A rt
er io
le s
C ap
ill ar
ie s
V en
ul es
S m
al l v
ei ns
La rg
e ve
in s
V en
ae c
av ae
P ul
m on
ar y
ar te
rie s
A rt
er io
le s
C ap
ill ar
ie s
V en
ul es
P ul
m on
ar y
ve in
s
Figure 14-2 Normal blood pressures in the different portions of the circulatory system when a person is lying in the horizontal position.
Pressure gradient P1 P2
Resistance
Blood flow
Figure 14-3 Interrelationships of pressure, resistance, and blood flow.
F = R
DP
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Unit IV The Circulation
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 exam- ple, 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 mechan- ical 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 impor- tant devices for measuring blood flow without opening the vessel is the electromagnetic flowmeter, the princi- ples of which are illustrated in Figure 14-4. Figure 14-4A 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 gen- erator. Figure 14-4B shows that the same principle applies for generation of electromotive force in blood that is mov- ing through a magnetic field. In this case, a blood vessel is placed between the poles of a strong magnet, and elec- trodes are placed on the two sides of the vessel perpen- dicular 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 elec- trodes, and this is recorded using an appropriate voltme- ter or electronic recording apparatus. Figure 14-4C 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.
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 electro- magnetic 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
0 0 N S
A B
N S
C
− + − +
+ +
− −
Figure 14-4 Flowmeter of the electromagnetic type, showing generation of an electrical voltage in a wire as it passes through an electro- magnetic 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).
DP = F R
R = F
DP
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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 mov- ing 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.)
For the flowmeter shown in Figure 14-5, the high-fre- quency ultrasound wave is intermittently cut off, and the reflected wave is received back onto the crystal and ampli- fied 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 mix- ing 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 14-6A, 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 14-6B; 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 dis- tance. This effect is called the “parabolic profile for veloc- ity of blood flow.”
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 ves- sel 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 14-6C). 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 stream- lined, because eddies add tremendously to the overall friction of flow in the vessel.
The tendency for turbulent flow increases in direct pro- portion 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 veloc- ity of blood flow (in centimeters/second), d is the vessel diameter (in centimeters), ρ is density, and η is the vis- cosity (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
Crystal
Reflected wave
Transmitted wave
Figure 14-5 Ultrasonic Doppler flowmeter.
A
B
C 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.
Re = h n. .d r
Unit IV The Circulation
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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, turbu- lence 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 con- ditions 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 pres- sure means the force exerted by the blood against any unit area of the vessel wall. When one says that the pres- sure 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 H2O). A pressure of 10 cm H2O means a pres- sure 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 spe- cific gravity of mercury is 13.6 times that of water, and 1 centimeter is 10 times as great as 1 millimeter.
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 calcu- lated from measurements of blood flow and pressure
A
B
C Figure 14-7 Principles of three types of electronic transducers for recording rapidly changing blood pressures (explained in the text).
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 manome- ter, 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 pres- sure 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 electri- cal 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.
In Figure 14-7A, 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 14-7B, a small iron slug rests on the mem- brane, 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 14-7C, a very thin, stretched resis- tance 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, pres- sure cycles up to 500 cycles per second have been recorded accurately. In common use are recorders capable of regis- tering pressure changes that occur as rapidly as 20 to 100 cycles per second, in the manner shown on the recording paper in Figure 14-7C.
Chapter 14 Overview of the Circulation; Biophysics of Pressure, Flow, and Resistance
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difference between two points in the vessel. If the pres- sure 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. Occa sionally, a basic physical unit called the CGS (centimeters, grams, seconds) unit is used to express resistance. This unit is dyne sec/cm5. 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 approxi- mately 100 ml/sec. The pressure difference from the sys- temic 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 periph- eral 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 pres- sure 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 dif- ference. 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 milli- meter 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 chan- ges 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 14-8A, 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:
Poiseuille’s Law. The cause of this great increase in con- ductance when the diameter increases can be explained by referring to Figure 14-8B, which shows cross sections of a large and a small vessel. The concentric rings inside the ves- sels indicate that the velocity of flow in each ring is differ- ent 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 ves- sel 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.
P = 100 mm
Hg
d = 1
d = 2
d = 4
1 ml/min
16 ml/min
256 ml/min
Large vessel
Small vessel
A
B Figure 14-8 A, Demonstration of the effect of vessel diameter on blood flow. B, Concentric rings of blood flowing at different veloci- ties; the farther away from the vessel wall, the faster the flow.
R in cm5
dyne sec
ml/sec
1333 mm Hg =
Conductance = Resistance
1
Conductance µ Diameter4
F = Pr4
8 1 pD PpD P
h8 1h8 1
Unit IV The Circulation
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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 diame- ter 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 sys- temic resistance to blood flow is arteriolar resistance in the small arterioles. The internal diameters of the arteri- oles 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 ves- sel 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 diam- eter 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 circula- tion (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, capillar- ies, 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 (Rtotal) 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 14-9A, the total vascular resistance is equal to the sum of R1 and R2.
Blood vessels branch extensively to form parallel cir- cuits 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, indepen- dently of flow to other tissues.
For blood vessels arranged in parallel (Figure 14-9B), 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 resis- tance of any single blood vessel. Flow through each of the parallel vessels in Figure 14-9B is determined by the pres- sure 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 ves- sels 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 pro- vides another pathway, or conductance, for blood flow. The total conductance (Ctotal) 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 tis- sue is a fraction of the total blood flow (cardiac output) and is determined by the resistance (the reciprocal of con- ductance) for blood flow in the tissue, as well as the pres- sure 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 viscos- ity 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.
A
B
R1
R1
R2
R2
R3 R4
Figure 14-9 Vascular resistances: A, in series and B, in parallel.
R total
R1 R2 R3 R4 . . .= +R= +R1= +1 + +R+ +R3+ +3
. . . Rtotal R1 R2 R3 R4
1 1 1 1 1+ + +=
C total = C1 + C2 + C3 + C4 . . .
Chapter 14 Overview of the Circulation; Biophysics of Pressure, Flow, and Resistance
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Hematocrit. 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 tremen- dously, 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 dis- cussed 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.
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.
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 hema- tocrit 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 vas- cular resistance within a few seconds through activation of the local control mechanisms discussed in Chapter 17. Conversely, with reductions in arterial pressure most vas- cular 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 main- tain normal blood flow during changes in arterial pres- sure between approximately 70 and 175 mm Hg is called blood flow autoregulation.
Normal Anemia Polycythemia
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Figure 14-10 Hematocrits in a healthy (normal) person and in patients with anemia and polycythemia.
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Figure 14-11 Effect of hematocrit on blood viscosity. (Water viscosity = 1.)
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Figure 14-12 Effect of changes in arterial pressure over a period of several minutes on blood flow in a tissue such as skeletal mus- cle. 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 nor- epinephrine, angiotensin II, vasopressin, or endothelin on this rela- tionship. Reduced tissue blood flow is rarely maintained for more than a few hours due to activation of local autoregulatory mecha- nisms that eventually return blood flow toward normal.
Unit IV The Circulation
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Note in Figure 14-12 that changes in blood flow can be caused by strong sympathetic stimulation, which constricts the blood vessels. Likewise, hormonal vaso- constrictors, such as norepinephrine, angiotensin II, vaso- pressin, 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 pre- dicted 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 pres- sure falls below a critical level, called the critical closing pressure, flow ceases as the blood vessels are completely collapsed.
Sympathetic stimulation and other vasoconstrictors can alter the passive pressure-flow relationship shown in Figure 14-13. Thus, inhibition of sympathetic activity
greatly 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 sec- onds despite high arterial pressure.
In reality, there are few physiological conditions in which tissues display the passive pressure-flow relation- ship 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 sus- tained, as discussed in Chapter 17.
Bibliography
See bibliography for Chapter 15.
B lo
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in )
0 0
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Arterial pressure (mm Hg)
Sympathetic inhibition
Normal
Sympathetic stimulation
Critical closing
pressure
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.
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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 disten-
sible. 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 quan- tities of extra blood that can be called into use whenever required elsewhere in the circulation.
Units of Vascular Distensibility. Vascular distensi- bility 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 con- tained 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 disten- sibilities 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 impor- tant to know the total quantity of blood that can be stored in a given portion of the circulation for each mm Hg pres- sure 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 disten- sibility 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 pres- sure to volume in a vessel or in any portion of the circu- lation is to use the so-called volume-pressure curve. The red and blue solid curves in Figure 15-1 represent, respec- tively, the volume-pressure curves of the normal systemic arterial system and venous system, showing that when the arterial system of the average adult person (includ- ing all the large arteries, small arteries, and arterioles) is filled with about 700 milliliters of blood, the mean arte- rial pressure is 100 mm Hg, but when it is filled with only 400 milliliters of blood, the pressure falls to zero.
In the entire systemic venous system, the volume nor- mally 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
Vascular distensibility = Increase in volume
Increase in pressure Original volume
Vascular compliance = Increase in volume
Increase in pressure
Unit IV The Circulation
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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 sym- pathetic nerves on the volume-pressure curves. It is evi- dent that increase in vascular smooth muscle tone caused by sympathetic stimulation increases the pressure at each volume of the arteries or veins, whereas sympathetic inhi- bition 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 ves- sel 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 ves- sel 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 sud- denly 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 mus- cle 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.
Delayed compliance is a valuable mechanism by which the circulation can accommodate extra blood when nec- essary, such as after too large a transfusion. Delayed com- pliance 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 seri- ous 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 sys- tem, all of this new blood would have to flow through the peripheral blood vessels almost instantaneously, only dur- ing cardiac systole, and no flow would occur during dias- tole. 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 sys- tolic 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.
Two major factors affect the pulse pressure: (1) the stroke volume output of the heart and (2) the compliance (total dis- tensibility) 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 dias- tole, thus causing a greater pulse pressure. Conversely, the less the compliance of the arterial system, the greater the
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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.
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Figure 15-1 “Volume-pressure curves” of the systemic arterial and venous systems, showing the effects of stimulation or inhibi- tion of the sympathetic nerves to the circulatory system.
Chapter 15 Vascular Distensibility and Functions of the Arterial and Venous Systems
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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.
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:
Pulse Pressure ª stroke volume/arterial compliance
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 regur- gitation, 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 dis- tended 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.
The velocity of pressure pulse transmission in the nor- mal 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 transmis- sion 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
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Sharp incisura
Exponential diastolic decline (may be distorted by
reflected wave)
Sharp upstroke
Figure 15-3 Pressure pulse contour in the ascending aorta.
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Aortic regurgitation
Arteriosclerosis Aortic stenosis
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Figure 15-4 Aortic pressure pulse contours in arteriosclerosis, aortic stenosis, patent ductus arteriosus, and aortic regurgitation.
Wave fronts
Figure 15-5 Progressive stages in transmission of the pressure pulse along the aorta.
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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 espe- cially in the three lower curves that the intensity of pulsa- tion 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.
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 resis- tance 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 ves- sel, 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 arte- rial 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 aus- cultatory method.
Auscultatory Method. Figure 15-7 shows the aus- cultatory method for determining systolic and diastolic arterial pressures. A stethoscope is placed over the ante- cubital artery and a blood pressure cuff is inflated around the upper arm. As long as the cuff continues to com- press the arm with too little pressure to close the bra- chial artery, no sounds are heard from the antecubital artery with the stethoscope. However, when the cuff pres- sure 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,
120
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80 240
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B
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C
C
D
D
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Figure 15-7 Auscultatory method for measuring systolic and diastolic arterial pressures.
0 1 2
Time (seconds)
Incisura
Radial artery
Arteriole
Capillary
Diastole
Proximal aorta
Femoral artery
Systole
Figure 15-6 Changes in the pulse pressure contour as the pulse wave travels toward the smaller vessels.
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named after Nikolai Korotkoff, a Russian physician who described them in 1905.
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 dur- ing any part of the pressure cycle. Therefore, no Korotkoff sounds are heard in the lower artery. But then the cuff pres- sure 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 con- nected 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 tap- ping quality and more of a rhythmical and harsher quality. Then, finally, when the pressure in the cuff falls near dia- stolic 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 muf- fled 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 can- not 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 approxi- mate 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 pres- sure 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.
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 millisec- ond 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 pres- sure 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 pres- sure (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 dia- stolic 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 func- tions that are necessary for operation of the circulation. Especially important, they are capable of constricting and enlarging and thereby storing either small or large quan- tities of blood and making this blood available when it is required by the remainder of the circulation. The periph- eral 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.
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Figure 15-8 Changes in systolic, diastolic, and mean arterial pres- sures with age. The shaded areas show the approximate normal ranges.
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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 ves- sels. 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 con- ditions, such as (1) serious heart failure or (2) after mas- sive 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 val- ues 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 tis- sues 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 periph- eral small veins in a person lying down is usually +4 to +6 mm Hg greater than the right atrial pressure.
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 pres- sure rises still further, the additional increase causes a corre- sponding 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 notice- ably 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 cav- ity of a recumbent person normally averages about +6 mm Hg, but it can rise to +15 to +30 mm Hg as a result of preg- nancy, 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
Atmospheric pressure collapse in neck
Axillary collapse
Abdominal pressure collapse
Intrathoracic pressure = −4 mm Hg
Rib collapse
Figure 15-9 Compression points that tend to collapse the veins entering the thorax.
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of distance below the surface. This pressure results from the weight of the water and therefore is called gravita- tional pressure or hydrostatic pressure.
Gravitational pressure also occurs in the vascular sys- tem of the human being because of weight of the blood in the vessels, as shown in Figure 15-10. When a per- son is standing, the pressure in the right atrium remains about 0 mm Hg because the heart pumps into the arter- ies 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.
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 atmo- spheric 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 “suc- tion” 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 ordi- nary circumstances, the venous pressure in the feet of a walking adult remains less than +20 mm Hg.
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 per- cent of the blood volume can be lost from the circulatory
Sagittal sinus −10 mm
0 mm 0 mm
+6 mm
+8 mm
+22 mm
+35 mm
+40 mm
+90 mm
Figure 15-10 Effect of gravitational pressure on the venous pres- sures throughout the body in the standing person.
Unit IV The Circulation
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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 com- pletely. 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 character- ized 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 capil- laries 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.
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 insert- ing 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 cath- eter 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 gravita- tional 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.
The reason for lack of gravitational effects at the tricus- pid valve is that the heart automatically prevents signifi- cant 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 there- fore 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
Right ventricle
Right atrium
Natural reference point
Figure 15-12 Reference point for circulatory pressure mea- surement (located near the tricuspid valve).
Deep vein
Perforating vein
Superficial vein
Valve
Figure 15-11 Venous valves of the leg.
Clinical Estimation of Venous Pressure. The venous pres- sure 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
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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 compli- ant, it is said that the venous system serves as a blood res- ervoir for the circulation.
When blood is lost from the body and the arterial pres- sure 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 circu- latory 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 compli- ant 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 millili- ters 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 sym- pathetic 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 pres- sures decrease to low values.
The Spleen as a Reservoir for Storing Red Blood Cells. Figure 15-13 shows that the spleen has two sepa- rate 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.
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 quan- tities of concentrated red blood cells. These can then be expelled into the general circulation whenever the sym- pathetic 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.
Pulp
Vein
Artery
Capillaries
Venous sinuses
Figure 15-13 Functional structures of the spleen. (Courtesy Dr. Don W. Fawcett, Montana.)
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-Cleansing Function of the Spleen—Removal of Old Cells Blood cells passing through the splenic pulp before enter- ing 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 hemo- globin and the cell stroma are digested by the reticuloen- dothelial 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 cleans- ing 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 func- tion of the circulation occurs in the microcirculation: This is transport of nutrients to the tissues and removal of cell excreta. The small arte- rioles control blood flow to
each tissue, and local conditions in the tissues in turn con- trol 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, con- structed 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 esti- mated 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 specifi- cally 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.
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 arrange- ment 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 nutri- ents, 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.
“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 inter- cellular cleft, which is the thin-slit, curving channel that lies at the bottom of the figure between adjacent endothe- lial cells. Each cleft is interrupted periodically by short ridges of protein attachments that hold the endothelial cells together, but between these ridges fluid can perco- late freely through the cleft. The cleft normally has a uni- form 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
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molecules, as well as most water-soluble ions and small sol- utes, 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 plas- malemmal 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 cave- olae 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 extra- cellular 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 demon- strated 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 characteris- tics 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 hav- ing 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 inter- mittency 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 oxy- gen concentration decreases below normal, the intermit- tent 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 quanti- ties of oxygen (as well as other nutrients) to the tissues.
Figure 16-1 Structure of the mesenteric capillary bed. (Redrawn from Zweifach BW: Factors Regulating Blood Pressure. New York: Josiah Macy, Jr., Foundation, 1950.)
Endothelial cell
Caveolae (Plasmalemmal vesicles)
Vesicular channel??
Basement membrane
Intercellular cleft
Caveolin
Phospholipid
Sphingolipid
Cholesterol
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 mem- brane 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.
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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 capil- lary bed, an average capillary pressure within the capillar- ies, 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 intermit- tently 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.
Lipid-Soluble Substances Can Diffuse Directly Through the Cell Membranes of the Capillary Endothelium. If a substance is lipid soluble, it can dif- fuse directly through the cell membranes of the capillary without having to go through the pores. Such substances include oxygen and carbon dioxide. Because these sub- stances 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 sub- stances, 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 mem- branes 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 tre- mendous diffusion of water and water-soluble substances through these cleft-pores. To give one an idea of the rapid- ity 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 permeabil- ity of the capillary pores for different substances varies according to their molecular diameters.
Table 16-1 gives the relative permeabilities of the cap- illary pores in skeletal muscle for substances commonly encountered, demonstrating, for instance, that the per- meability 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.
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 per- meability of the renal glomerular membrane for water
Arterial end Venous endBlood capillary
Lymphatic capillary
Figure 16-3 Diffusion of fluid molecules and dissolved substances between the capillary and interstitial fluid spaces.
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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 mem- brane is proportional to the concentration difference of the substance between the two sides of the membrane. That is, the greater the difference between the concen- trations 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 con- centration 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 mem- branes of most nutritionally important substances are so great that only slight concentration differences suf- fice to cause more than adequate transport between the plasma and interstitial fluid. For instance, the con- centration of oxygen in the interstitial fluid imme- diately 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 inter- stitium. 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 inter- stitium. They are extremely strong and therefore provide most of the tensional strength of the tissues. The pro- teoglycan 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 micro- scope and are difficult to demonstrate even with the elec- tron microscope. Nevertheless, they form a mat of very fine reticular filaments aptly described as a “brush pile.”
“Gel” in the Interstitium. The fluid in the intersti- tium is derived by filtration and diffusion from the capil- laries. 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.
Free fluid vesicles
Rivulets of free fluid
Proteoglycan filaments
Collagen fiber bundlesCapillary
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.
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
Table 16-1 Relative Permeability of Skeletal Muscle Capillary Pores to Different-Sized Molecules
Data from Pappenheimer JR: Passage of molecules through capillary walls. Physiol Rev 33:387, 1953.
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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 per- cent 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, car- bon 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 pres- sure 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 pres- sure 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 mech- anisms 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.
If the sum of these forces—the net filtration pressure—is positive, there will be a net fluid filtration across the cap- illaries. 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:
NFP = Pc – Pif – Pp + Pif
As discussed later, the NFP is slightly positive under nor- mal 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 flow- ing. These factors are usually expressed together as the capillary filtration coefficient (Kf). The Kf is therefore a measure of the capacity of the capillary membranes to fil- ter 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 deter- mined as:
Filtration = K f × NFP
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 capil- lary pressure of about 25 mm Hg in some tissues such as
Capillary pressure
(Pc)
Plasma colloid osmotic pressure
(p)
Interstitial fluid pressure
(Pif)
Interstitial fluid colloid osmotic pressure
(if)
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.
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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 cannu- lation, a microscopic glass pipette is thrust directly into the capillary, and the pressure is measured by an appro- priate micromanometer system. Using this method, cap- illary 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 micropi- pette 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 demon- strates 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 arte- rial pressure is decreased, the resulting decrease in capil- lary 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 immedi- ately 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 pres- sure is kept constant while simultaneously (1) decreas- ing the arterial pressure and (2) increasing the venous 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 capil- lary 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 deter- mines 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 differ- ent 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 differ- ent 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 inter- stitial pressure is generally positive (greater than atmo- spheric pressure). The methods most widely used have been (1) direct cannulation of the tissues with a micropi- pette, (2) measurement of the pressure from implanted perforated capsules, and (3) measurement of the pressure from a cotton wick inserted into the tissue.
Arterial pressure Gut
100
80
60 P
re ss
u re
40
20
0 100 50
Capillary pressure = 17 mm Hg
Arterial
Venous
Arterial pressure – venous pressure 0
Venous pressure
Figure 16-6 Isogravimetric method for measuring capillary pressure.
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Measurement of Interstitial Fluid Pressure Using the Micropipette. The same type of micropipette used for mea- suring capillary pressure can also be used in some tissues for measuring interstitial fluid pressure. The tip of the micropi- pette 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 atmo- spheric 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 aver- ages 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 encase- ments, 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 invari- ably are still less than the pressures exerted on the out- sides 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 pres- sure surrounding the kidney averages about +13 mm Hg, whereas the reported renal interstitial fluid pressures have averaged about +6 mm Hg. Thus, if one remem- bers 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 intersti- tial fluid pressure is usually several millimeters of mer- cury 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 subat- mospheric 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 posi- tive. 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 axil- lary space, and in the scrotum. Amounts of fluid cal- culated 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 observa- tions 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 surround- ing 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 arte- rial 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 Inter stitial 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 physiolo- gists that the true interstitial fluid pressure in loose sub- cutaneous 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 lym- phatic 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
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terminal lymphatic capillaries, the lymph vessel walls automatically contract for a few seconds and pump the fluid into the blood circulation. This overall process cre- ates 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 semiperme- able membrane exert osmotic pressure. Because the pro- teins are the only dissolved constituents in the plasma and interstitial fluids that do not readily pass through the cap- illary 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 mem- brane, 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 pres- sure 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 follow- ing 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 ’p (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 pres- sure of the plasma results from the albumin fraction, 20 per- cent 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 inter- stitial 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 aver- age protein concentration of the interstitial fluid is usu- ally 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 pro- teins 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 capil- lary 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, tend- ing to move fluid outward through the capillary pores.
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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 inter- stitial 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 reab- sorption, 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 numer- ous 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 cap- illaries to be reabsorbed at the venous ends. The remain- ing 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 fil- tering 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 imbal- ance 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 mem- branes of 0.3 mm Hg causes net fluid filtration in the entire body of 2 ml/min. Expressing this for each millime- ter 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 sepa- rate parts of the body in terms of rate of filtration per min- ute 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 dif- ferences in permeabilities of the capillary systems in dif- ferent 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 glomeru- lus 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 tis- sue 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
Unit IV The Circulation
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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 tis- sue spaces, neither of which can be removed by absorp- tion 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 inter- stitial channels called prelymphatics through which inter- stitial 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.
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 subcla- vian vein and internal jugular vein.
Cervical nodes Sentinel node
Subclavian vein R. lymphatic duct
Thoracic duct Axillary nodes
Cisterna chyli
Abdominal nodes
Inguinal nodes
Peripheral lymphatics
Tissue cell
Blood capillary
Lymphatic capillary
Lymphatic vessel
Interstitial fluid
Masses of lymphocytes and macrophages
Figure 16-7 Lymphatic system.
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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 cap- illaries; 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 unim- peded. 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 capil- lary 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.
Formation of Lymph
Lymph is derived from interstitial fluid that flows into the lymphatics. Therefore, lymph as it first enters the termi- nal 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, espe- cially 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 cap- illaries 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 inter- stitial fluid pressure on lymph flow as measured in dog legs. Note that normal lymph flow is very little at intersti- tial 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
Endothelial cells
Anchoring filaments
Valves
Figure 16-8 Special structure of the lymphatic capillaries that permits passage of substances of high molecular weight into the lymph.
R el
at iv
e ly
m p
h f
lo w
1
10
20
20−2−4−6 4
PT (mm Hg)
2 times/ mm Hg
7 times/ mm Hg
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.)
Unit IV The Circulation
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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
All of these cause a balance of fluid exchange at the blood capillary membrane to favor fluid movement into the inter- stitium, 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 intersti- tial 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 “maxi- mum 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 lym- phatic capillaries empty.
Motion pictures of exposed lymph vessels in animals and in human beings show that when a collecting lym- phatic 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 ves- sel between successive valves functions as a separate auto- matic 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 sub- sequent 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 Com- pression of the Lymphatics. In addition to the pumping caused by intrinsic intermittent contraction of the lymph vessel walls, any external factor that intermittently com- presses 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 cap- illary and fluid flows into the terminal lymphatic capil- lary 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 lym- phatic 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 lym- phatic capillaries in the same way that many of the small blood and larger lymphatic vessels also contract rhythmi- cally. Therefore, it is probable that at least part of lymph
Pores
Lymphatic capillaries
Collecting lymphatic
Valves
Figure 16-10 Structure of lymphatic capillaries and a collecting lymphatic, showing also the lymphatic valves.
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pumping results from lymph capillary endothelial cell contraction in addition to contraction of the larger mus- cular 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 lym- phatic pump. Therefore, one can state that, roughly, the rate of lymph flow is determined by the product of inter- stitial 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 cen- tral 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 intersti- tium. 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 accu- mulate 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 vol- ume 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 pres- sure, the return of protein and fluid by way of the lym- phatic 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 bal- anced 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 tis- sues of the body are held together entirely by connective tissue fibers. However, at many places in the body, con- nective 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 dis- cussed in Chapter 25.
Bibliography
Dejana E: Endothelial cell-cell junctions: happy together, Nat Rev Mol Cell Biol 5:261, 2004.
Gashev AA: Physiologic aspects of lymphatic contractile function: current perspectives, Ann N Y Acad Sci 979:178, 2002.
Gratton JP, Bernatchez P, Sessa WC: Caveolae and caveolins in the cardio- vascular system, Circ Res 94:1408, 2004.
Guyton AC: Concept of negative interstitial pressure based on pressures in implanted perforated capsules, Circ Res 12:399, 1963.
Guyton AC: Interstitial fluid pressure: II. Pressure-volume curves of intersti- tial space, Circ Res 16:452, 1965.
Guyton AC, Granger HJ, Taylor AE: Interstitial fluid pressure, Physiol Rev 51:527, 1971.
Michel CC, Curry FE: Microvascular permeability, Physiol Rev 79:703, 1999. Mehta D, Malik AB: Signaling mechanisms regulating endothelial perme-
ability, Physiol Rev 86:279, 2006. Miyasaka M, Tanaka T: Lymphocyte trafficking across high endothelial
venules: dogmas and enigmas, Nat Rev Immunol 4:360, 2004. Parker JC: Hydraulic conductance of lung endothelial phenotypes and
Starling safety factors against edema, Am J Physiol Lung Cell Mol Physiol 292:L378, 2007.
Parker JC, Townsley MI: Physiological determinants of the pulmonary filtra- tion coefficient, Am J Physiol Lung Cell Mol Physiol 295:L235, 2008.
Predescu SA, Predescu DN, Malik AB: Molecular determinants of endothe- lial transcytosis and their role in endothelial permeability, Am J Physiol Lung Cell Mol Physiol 293:L823, 2007.
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Taylor AE, Granger DN: Exchange of macromolecules across the micro- circulation. In Renkin EM, Michel CC, editors: Handbook of Physiology, Sec 2, vol IV, Bethesda, MD, 1984, American Physiological Society, pp 467.
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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 fundamen- tal 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 kid- neys 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 control- ling 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.
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 vas- cular 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 tis- sue 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, metar- terioles, 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
Unit IV The Circulation
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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 num- bers 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.
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 alti- tude at the top of a high mountain, (2) in pneumonia, (3) in carbon monoxide poisoning (which poisons the abil- ity 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 three- fold; 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.
Total cyanide poisoning of oxygen usage by a local tis- sue 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 metabo- lism 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 tis- sues to the precapillary sphincters, metarterioles, and arterioles to cause dilation. Some of the different vasodi- lator substances that have been suggested are adenosine,
B lo
o d
f lo
w (
x n
o rm
al )
0 0
2
3
4
7654321 8
Rate of metabolism (x normal)
1 Normal level
Figure 17-1 Effect of increasing rate of metabolism on tissue blood flow.
B lo
o d
f lo
w (
x n
o rm
al )
75100
0
2
3
50 25
Arterial oxygen saturation (percent)
1
Figure 17-2 Effect of decreasing arterial oxygen saturation on blood flow through an isolated dog leg.
Percent of Cardiac
Output
ml/min ml/min/100 g of Tissue Weight
Brain 14 700 50
Heart 4 200 70
Bronchi 2 100 25
Kidneys 22 1100 360
Liver 27 1350 95
Portal (21) 1050
Arterial (6) 300
Muscle (inactive state)
15 750 4
Bone 5 250 3
Skin (cool weather)
6 300 3
Thyroid gland 1 50 160
Adrenal glands 0.5 25 300
Other tissues 3.5 175 1.3
Total 100.0 5000
Table 17-1 Blood Flow to Different Organs and Tissues Under Basal Conditions
Chapter 17 Local and Humoral Control of Tissue Blood Flow
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carbon dioxide, adenosine phosphate compounds, hista- mine, potassium ions, and hydrogen ions.
Vasodilator substances may be released from the tis- sue in response to oxygen deficiency. For instance, experi- ments 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 par- tially 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 contin- ues at the same rate, or when cell metabolism is suddenly increased. As the concentration of vasodilator metabo- lites 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 impor- tant 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 triphos- phate (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, provid- ing increased coronary blood flow to supply the increased nutrient demands of the active heart.
Although research evidence is less clear, many physiol- ogists also have suggested that the same adenosine mech- anism 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 quan- tities of any single vasodilator substance, including ade- nosine, are indeed formed in the tissues to cause all the measured increase in blood flow. It is likely that a combi- nation of several different vasodilators released by the tis- sues 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 physiolo- gists favor still another theory, which can be called either the oxygen lack theory or, more accurately, the nutri- ent lack theory (because other nutrients besides oxy- gen 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 mus- cle 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 capil- lary 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 com- pletely open or completely closed. The number of precap- illary sphincters that are open at any given time is roughly proportional to the requirements of the tissue for nutri- tion. The precapillary sphincters and metarterioles open and close cyclically several times per minute, with the dura- tion of the open phases being proportional to the metabolic needs of the tissues for oxygen. The cyclical opening and closing is called vasomotion.
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 con- centration. 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 meta- bolic needs of the tissues. Probably the truth lies in a com- bination of the two mechanisms.
Precapillary sphincter Metarteriole
Sidearm capillary
Figure 17-3 Diagram of a tissue unit area for explanation of acute local feedback control of blood flow, showing a metarteriole pass- ing through the tissue and a sidearm capillary with its precapillary sphincter for controlling capillary blood flow.
Unit IV The Circulation
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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 hyper- emia and active hyperemia.
Reactive Hyperemia. When the blood supply to a tis- sue is blocked for a few seconds to as long as an hour or more and then is unblocked, blood flow through the tis- sue usually increases immediately to four to seven times normal; this increased flow will continue for a few sec- onds if the block has lasted only a few seconds but some- times continues for as long as many hours if the blood flow has been stopped for an hour or more. This phenom- enon is called reactive hyperemia.
Reactive hyperemia is another manifestation of the local “metabolic” blood flow regulation mechanism; that is, lack of flow sets into motion all of those factors that cause vasodilation. After short periods of vascular occlu- sion, the extra blood flow during the reactive hyperemia phase lasts long enough to repay almost exactly the tis- sue oxygen deficit that has accrued during the period of occlusion. This mechanism emphasizes the close connec- tion between local blood flow regulation and delivery of oxygen and other nutrients to the tissues.
Active Hyperemia. When any tissue becomes highly active, such as an exercising muscle, a gastrointesti- nal gland during a hypersecretory period, or even the brain during rapid mental activity, the rate of blood flow through the tissue increases. Here again, by simply apply- ing the basic principles of local blood flow control, one can easily understand this active hyperemia. The increase in local metabolism causes the cells to devour tissue fluid nutrients rapidly and also to release large quanti- ties of vasodilator substances. The result is to dilate the local blood vessels and, therefore, to increase local blood flow. In this way, the active tissue receives the additional
nutrients required to sustain its new level of function. As pointed out earlier, active hyperemia in skeletal muscle can increase local muscle blood flow as much as 20-fold during intense exercise.
“Autoregulation” of Blood Flow When the Arterial Pressure Changes from Normal—“Metabolic” and “Myogenic” Mechanisms
In any tissue of the body, a rapid increase in arterial pres- sure causes an immediate rise in blood flow. But, within less than a minute, the blood flow in most tissues returns almost to the normal level, even though the arterial pres- sure is kept elevated. This return of flow toward normal is called “autoregulation” of blood flow. After autoregulation has occurred, the local blood flow in most body tissues will be related to arterial pressure approximately in accord with the solid “acute” curve in Figure 17-4. Note that between arterial pressures of about 70 mm Hg and 175 mm Hg the blood flow increases only 20 to 30 percent even though the arterial pressure increases 150 percent.
For almost a century, two views have been proposed to explain this acute autoregulation mechanism. They have been called (1) the metabolic theory and (2) the myogenic theory.
The metabolic theory can be understood easily by applying the basic principles of local blood flow regula- tion discussed in previous sections. Thus, when the arte- rial pressure becomes too great, the excess flow provides too much oxygen and too many other nutrients to the tis- sues and “washes out” the vasodilators released by the tis- sues. These nutrients (especially oxygen) and decreased tissue levels of vasodilators then cause the blood vessels to constrict and the flow to return nearly to normal despite the increased pressure.
The myogenic theory, however, suggests that still another mechanism not related to tissue metabolism explains the phenomenon of autoregulation. This theory is based on the observation that sudden stretch of small blood vessels causes the smooth muscle of the vessel wall
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Figure 17-4 Effect of different levels of arterial pressure on blood flow through a muscle. The solid red curve shows the effect if the arterial pressure is raised over a period of a few minutes. The dashed green curve shows the effect if the arterial pressure is raised slowly over a period of many weeks.
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to contract. Therefore, it has been proposed that when high arterial pressure stretches the vessel, this in turn causes reactive vascular constriction that reduces blood flow nearly back to normal. Conversely, at low pres- sures, the degree of stretch of the vessel is less, so that the smooth muscle relaxes, reducing vascular resistance and helping to return flow toward normal.
The myogenic response is inherent to vascular smooth muscle and can occur in the absence of neural or hor- monal influences. It is most pronounced in arterioles but can also be observed in arteries, venules, veins, and even lymphatic vessels. Myogenic contraction is initiated by stretch-induced vascular depolarization, which then rapidly increases calcium ion entry from the extracellu- lar fluid into the cells, causing them to contract. Changes in vascular pressure may also open or close other ion channels that influence vascular contraction. The precise mechanisms by which changes in pressure cause opening or closing of vascular ion channels are still uncertain but likely involve mechanical effects of pressure on extracel- lular proteins that are tethered to cytoskeleton elements of the vascular wall or to the ion channels themselves.
The myogenic mechanism appears to be important in preventing excessive stretch of blood vessel when blood pressure is increased. However, the role of the myogenic mechanism in blood flow regulation is unclear because this pressure-sensing mechanism cannot directly detect changes in blood flow in the tissue. Indeed, metabolic fac- tors appear to override the myogenic mechanism in cir- cumstances where the metabolic demands of the tissues are significantly increased, such as during vigorous mus- cle exercise, which can cause dramatic increases in skel- etal muscle blood flow.
Special Mechanisms for Acute Blood Flow Control in Specific Tissues
Although the general mechanisms for local blood flow control discussed thus far are present in almost all tissues of the body, distinctly different mechanisms operate in a few special areas. All mechanisms are discussed through- out this text in relation to specific organs, but two notable ones are as follows:
1. In the kidneys, blood flow control is vested to a great extent in a mechanism called tubuloglomeru- lar feedback, in which the composition of the fluid in the early distal tubule is detected by an epithelial structure of the distal tubule itself called the mac- ula densa. This is located where the distal tubule lies adjacent to the afferent and efferent arterioles at the nephron juxtaglomerular apparatus. When too much fluid filters from the blood through the glomerulus into the tubular system, feedback sig- nals from the macula densa cause constriction of the afferent arterioles, in this way reducing both renal blood flow and glomerular filtration rate back to or near to normal. The details of this mechanism are discussed in Chapter 26.
2. In the brain, in addition to control of blood flow by tis- sue oxygen concentration, the concentrations of car- bon dioxide and hydrogen ions play prominent roles. An increase of either or both of these dilates the cere- bral vessels and allows rapid washout of the excess car- bon dioxide or hydrogen ions from the brain tissues. This is important because the level of excitability of the brain itself is highly dependent on exact control of both carbon dioxide concentration and hydrogen ion con- centration. This special mechanism for cerebral blood flow control is presented in Chapter 61.
3. In the skin, blood flow control is closely linked to regulation of body temperature. Cutaneous and sub- cutaneous flow regulates heat loss from the body by metering the flow of heat from the core to the surface of the body, where heat is lost to the environment. Skin blood flow is controlled largely by the central nervous system through the sympathetic nerves, as discussed in Chapter 73. Although skin blood flow is only about 3 ml/min/100 g of tissue in cool weather, large changes from that value can occur as needed. When humans are exposed to body heating, skin blood flow may increase manyfold, to as high as 7 to 8 L/min for the entire body. When body temperature is reduced, skin blood flow decreases, falling to barely above zero at very low temperatures. Even with severe vasoconstric- tion, skin blood flow is usually great enough to meet the basic metabolic demands of the skin.
Control of Tissue Blood Flow by Endothelial-Derived Relaxing or Constricting Factors
The endothelial cells lining the blood vessels synthesize several substances that, when released, can affect the degree of relaxation or contraction of the arterial wall. For many of these endothelial-derived relaxing or constric- tor factors, the physiological roles are just beginning to be understood and clinical applications have, in most cases, not yet been developed.
Nitric Oxide—A Vasodilator Released from Healthy Endothelial Cells. The most important of the endothelial- derived relaxing factors is nitric oxide (NO), a lipophilic gas that is released from endothelial cells in response to a variety of chemical and physical stimuli. Nitric oxide syn- thase (NOS) enzymes in endothelial cells synthesize NO from arginine and oxygen and by reduction of inorganic nitrate. After diffusing out of the endothelial cell, NO has a half-life in the blood of only about 6 seconds and acts mainly in the local tissues where it is released. NO activates soluble guanylate cyclases in vascular smooth muscle cells (Figure 17-5), resulting in conversion of cyclic guanosine triphosphate (cGTP) to cyclic guanosine monophosphate (cGMP) and activation of cGMP-depen- dent protein kinase (PKG), which has several actions that cause the blood vessels to relax.
When blood flows through the arteries and arterioles, this causes shear stress on the endothelial cells because of viscous drag of the blood against the vascular walls.
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Shear stress
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Figure 17-5 Nitric oxide synthase (eNOS) enzyme in endothelial cells synthesizes nitric oxide (NO) from arginine and oxygen. NO activates soluble guanylate cyclases in vascular smooth muscle cells, resulting in conversion of cyclic guanosine triphosphate (cGTP) to cyclic guanos- ine monophosphate (cGMP) which ultimately causes the blood vessels to relax.
This stress contorts the endothelial cells in the direction of flow and causes significant increase in the release of NO. The NO then relaxes the blood vessels. This is for- tunate because the local metabolic mechanisms for con- trolling tissue blood flow dilate mainly the very small arteries and arterioles in each tissue. Yet, when blood flow through a microvascular portion of the circulation increases, this secondarily stimulates the release of NO from larger vessels due to increased flow and shear stress in these vessels. The released NO increases the diame- ters of the larger upstream blood vessels whenever micro- vascular blood flow increases downstream. Without such a response, the effectiveness of local blood flow control would be decreased because a significant part of the resis- tance to blood flow is in the upstream small arteries.
NO synthesis and release from endothelial cells are also stimulated by some vasoconstrictors, such as angio- tensin II, which bind to specific receptors on endothelial cells. The increased NO release protects against excessive vasoconstriction.
When endothelial cells are damaged by chronic hyper- tension or atherosclerosis, impaired NO synthesis may con- tribute to excessive vasoconstriction and worsening of the hypertension and endothelial damage, which, if untreated, may eventually cause vascular injury and damage to vul- nerable tissues such as the heart, kidneys, and brain.
Even before NO was discovered, clinicians used nitro- glycerin, amyl nitrates, and other nitrate derivatives to treat patients suffering from angina pectoris, severe chest pain caused by ischemia of the heart muscle. These drugs, when broken down chemically, release NO and evoke dilation of blood vessels throughout the body, including the coronary blood vessels.
Other important applications of NO physiology and pharmacology are the development and clinical use of drugs (e.g., sildenafil) that inhibit cGMP specific phospho- diesterase-5 (PDE-5), an enzyme that degrades cGMP. By preventing the degradation of cGMP the PDE-5 inhibi-
tors effectively prolong the actions of NO to cause vaso- dilation. The primary clinical use of the PDE-5 inhibitors is to treat erectile dysfunction. Penile erection is caused by parasympathetic nerve impulses through the pelvic nerves to the penis, where the neurotransmitters acetyl- choline and NO are released. By preventing the degrada- tion of NO, the PDE-5 inhibitors enhance the dilation of the blood vessels in the penis and aid in erection, as dis- cussed in Chapter 80.
Endothelin—A Powerful Vasoconstrictor Released from Damaged Endothelium. Endothelial cells also release vasoconstrictor substances. The most impor- tant of these is endothelin, a large 21 amino acid peptide that requires only nanogram quantities to cause pow- erful vasoconstriction. This substance is present in the endothelial cells of all or most blood vessels but greatly increases when the vessels are injured. The usual stimu- lus for release is damage to the endothelium, such as that caused by crushing the tissues or injecting a traumatizing chemical into the blood vessel. After severe blood vessel damage, release of local endothelin and subsequent vaso- constriction helps to prevent extensive bleeding from arteries as large as 5 millimeters in diameter that might have been torn open by crushing injury.
Increased endothelin release is also believed to contrib- ute to vasoconstriction when the endothelium is damaged by hypertension. Drugs that block endothelin receptors have been used to treat pulmonary hypertension but have not generally been used for lowering blood pressure in patients with systemic arterial hypertension.
Long-Term Blood Flow Regulation
Thus far, most of the mechanisms for local blood flow regulation that we have discussed act within a few sec- onds to a few minutes after the local tissue conditions have changed. Yet, even after full activation of these acute mechanisms, the blood flow usually is adjusted only about three quarters of the way to the exact additional
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requirements of the tissues. For instance, when the arte- rial pressure suddenly increases from 100 to 150 mm Hg, the blood flow increases almost instantaneously about 100 percent. Then, within 30 seconds to 2 minutes, the flow decreases back to about 10 to 15 percent above the original control value. This illustrates the rapidity of the acute mechanisms for local blood flow regulation, but at the same time, it demonstrates that the regulation is still incomplete because there remains a 10 to 15 percent excess blood flow.
However, over a period of hours, days, and weeks, a long-term type of local blood flow regulation develops in addition to the acute control. This long-term regula- tion gives far more complete control of blood flow. For instance, in the aforementioned example, if the arte- rial pressure remains at 150 mm Hg indefinitely, within a few weeks the blood flow through the tissues gradually approaches almost exactly the normal flow level. Figure 17-4 shows by the dashed green curve the extreme effec- tiveness of this long-term local blood flow regulation. Note that once the long-term regulation has had time to occur, long-term changes in arterial pressure between 50 and 250 mm Hg have little effect on the rate of local blood flow.
Long-term regulation of blood flow is especially important when the metabolic demands of a tissue change. Thus, if a tissue becomes chronically overactive and therefore requires increased quantities of oxygen and other nutrients, the arterioles and capillary vessels usually increase both in number and size within a few weeks to match the needs of the tissue—unless the circulatory sys- tem has become pathological or too old to respond.
Mechanism of Long-Term Regulation—Change in “Tissue Vascularity”
The mechanism of long-term local blood flow regula- tion is principally to change the amount of vascularity of the tissues. For instance, if the metabolism in a tissue is increased for a prolonged period, vascularity increases, a process generally called angiogenesis; if the metabolism is decreased, vascularity decreases. Figure 17-6 shows the large increase in the number of capillaries in a rat ante- rior tibialis muscle that was stimulated electrically to contract for short periods of time each day for 30 days, compared with the unstimulated muscle in the other leg of the animal.
Thus, there is actual physical reconstruction of the tissue vasculature to meet the needs of the tissues. This reconstruction occurs rapidly (within days) in young ani- mals. It also occurs rapidly in new growth tissue, such as in scar tissue and cancerous tissue; however, it occurs much slower in old, well-established tissues. Therefore, the time required for long-term regulation to take place may be only a few days in the neonate or as long as months in the elderly person. Furthermore, the final degree of response is much better in younger tissues than in older, so that in the neonate, the vascularity will adjust to match almost exactly the needs of the tissue for blood flow, whereas in
older tissues, vascularity frequently lags far behind the needs of the tissues.
Role of Oxygen in Long-Term Regulation. Oxygen is important not only for acute control of local blood flow but also for long-term control. One example of this is increased vascularity in tissues of animals that live at high altitudes, where the atmospheric oxygen is low. A second example is that fetal chicks hatched in low oxygen have up to twice as much tissue blood vessel conductivity as is normally true. This same effect is also dramatically dem- onstrated in premature human babies put into oxygen tents for therapeutic purposes. The excess oxygen causes almost immediate cessation of new vascular growth in the retina of the premature baby’s eyes and even causes degeneration of some of the small vessels that already have formed. Then when the infant is taken out of the oxygen tent, there is explosive overgrowth of new vessels
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Figure 17-6 Large increase in the number of capillaries (white dots) in a rat anterior tibialis muscle that was stimulated electri- cally to contract for short periods of time each day for 30 days (B), compared with the unstimulated muscle (A).The 30 days of intermittent electrical stimulation converted the predominantly fast twitch, glycolytic anterior tibialis muscle to a predominantly slow twitch, oxidative muscle with increased numbers of cap- illaries and decreased fiber diameter as shown. (Photo courtesy Dr. Thomas Adair.)
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to make up for the sudden decrease in available oxygen; indeed, there is often so much overgrowth that the reti- nal vessels grow out from the retina into the eye’s vitreous humor and eventually cause blindness. (This condition is called retrolental fibroplasia.)
Importance of Vascular Endothelial Growth Factor in Formation of New Blood Vessels
A dozen or more factors that increase growth of new blood vessels have been found, almost all of which are small peptides. Three of those that have been best char- acterized are vascular endothelial growth factor (VEGF), fibroblast growth factor, and angiogenin, each of which has been isolated from tissues that have inadequate blood supply. Presumably, it is deficiency of tissue oxy- gen or other nutrients, or both, that leads to formation of the vascular growth factors (also called “angiogenic factors”).
Essentially all the angiogenic factors promote new ves- sel growth in the same way. They cause new vessels to sprout from other small vessels. The first step is dissolution of the basement membrane of the endothelial cells at the point of sprouting. This is followed by rapid reproduction of new endothelial cells that stream outward through the vessel wall in extended cords directed toward the source of the angiogenic factor. The cells in each cord continue to divide and rapidly fold over into a tube. Next, the tube con- nects with another tube budding from another donor ves- sel (another arteriole or venule) and forms a capillary loop through which blood begins to flow. If the flow is great enough, smooth muscle cells eventually invade the wall, so some of the new vessels eventually grow to be new arteri- oles or venules or perhaps even larger vessels. Thus, angio- genesis explains the manner in which metabolic factors in local tissues can cause growth of new vessels.
Certain other substances, such as some steroid hor- mones, have exactly the opposite effect on small blood vessels, occasionally even causing dissolution of vascular cells and disappearance of vessels. Therefore, blood vessels can also be made to disappear when not needed. Peptides produced in the tissues can also block the growth of new blood vessels. For example, angiostatin, a fragment of the protein plasminogen, is a naturally occurring inhibitor of angiogenesis. Endostatin is another antiangiogenic peptide that is derived from the breakdown of col- lagen type XVII. Although the precise physiological functions of these antiangiogenic substances are still unknown, there is great interest in their potential use in arresting blood vessel growth in cancerous tumors and therefore preventing the large increases in blood flow needed to sustain the nutrient supply of rapidly growing tumors.
Vascularity Is Determined by Maximum Blood Flow Need, Not by Average Need. An especially valu- able characteristic of long-term vascular control is that vascularity is determined mainly by the maximum level of blood flow need rather than by average need. For instance, during heavy exercise the need for whole
body blood flow often increases to six to eight times the resting blood flow. This great excess of flow may not be required for more than a few minutes each day. Nevertheless, even this short need can cause enough VEGF to be formed by the muscles to increase their vas- cularity as required. Were it not for this capability, every time that a person attempted heavy exercise, the muscles would fail to receive the required nutrients, especially the required oxygen, so that the muscles simply would fail to contract.
However, after extra vascularity does develop, the extra blood vessels normally remain mainly vasocon- stricted, opening to allow extra flow only when appropri- ate local stimuli such as oxygen lack, nerve vasodilatory stimuli, or other stimuli call forth the required extra flow.
Development of Collateral Circulation— a Phenomenon of Long-Term Local Blood Flow Regulation
When an artery or a vein is blocked in virtually any tis- sue of the body, a new vascular channel usually develops around the blockage and allows at least partial resupply of blood to the affected tissue. The first stage in this process is dilation of small vascular loops that already connect the vessel above the blockage to the vessel below. This dilation occurs within the first minute or two, indicating that the dilation is likely mediated by metabolic factors that relax the muscle fibers of the small vessels involved. After this initial opening of collateral vessels, the blood flow often is still less than one quarter that is needed to supply all the tissue needs. However, further opening occurs within the ensuing hours, so within 1 day as much as half the tissue needs may be met, and within a few days the blood flow is usually sufficient to meet the tissue needs.
The collateral vessels continue to grow for many months thereafter, almost always forming multiple small collateral channels rather than one single large vessel. Under rest- ing conditions, the blood flow usually returns very near to normal, but the new channels seldom become large enough to supply the blood flow needed during strenuous tissue activity. Thus, the development of collateral vessels follows the usual principles of both acute and long-term local blood flow control, the acute control being rapid metabolic dilation, followed chronically by growth and enlargement of new vessels over a period of weeks and months.
The most important example of the development of collateral blood vessels occurs after thrombosis of one of the coronary arteries. Almost all people by the age of 60 years have had at least one of the smaller branch coronary vessels closed, or at least partially occluded. Yet most people do not know that this has happened because collaterals have developed rapidly enough to prevent myocardial damage. It is in those other instances in which coronary insufficiency occurs too rapidly or too severely for collaterals to develop that serious heart attacks occur.
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Humoral Control of the Circulation
Humoral control of the circulation means control by substances secreted or absorbed into the body fluids— such as hormones and locally produced factors. Some of these substances are formed by special glands and trans- ported in the blood throughout the entire body. Others are formed in local tissue areas and cause only local circu- latory effects. Among the most important of the humoral factors that affect circulatory function are the following.
Vasoconstrictor Agents
Norepinephrine and Epinephrine. Norepinephrine is an especially powerful vasoconstrictor hormone; epi- nephrine is less so and in some tissues even causes mild vasodilation. (A special example of vasodilation caused by epinephrine occurs to dilate the coronary arteries during increased heart activity.)
When the sympathetic nervous system is stimulated in most or all parts of the body during stress or exercise, the sympathetic nerve endings in the individual tissues release norepinephrine, which excites the heart and con- tracts the veins and arterioles. In addition, the sympa- thetic nerves to the adrenal medullae cause these glands to secrete both norepinephrine and epinephrine into the blood. These hormones then circulate to all areas of the body and cause almost the same effects on the circula- tion as direct sympathetic stimulation, thus providing a dual system of control: (1) direct nerve stimulation and (2) indirect effects of norepinephrine and/or epinephrine in the circulating blood.
Angiotensin II. Angiotensin II is another powerful vasoconstrictor substance. As little as one millionth of a gram can increase the arterial pressure of a human being 50 mm Hg or more.
The effect of angiotensin II is to constrict powerfully the small arterioles. If this occurs in an isolated tissue area, the blood flow to that area can be severely depressed. However, the real importance of angiotensin II is that it normally acts on many of the arterioles of the body at the same time to increase the total peripheral resis- tance, thereby increasing the arterial pressure. Thus, this hormone plays an integral role in the regulation of arterial pressure, as is discussed in detail in Chapter 19.
Vasopressin. Vasopressin, also called antidiuretic hor- mone, is even more powerful than angiotensin II as a vaso- constrictor, thus making it one of the body’s most potent vascular constrictor substances. It is formed in nerve cells in the hypothalamus of the brain (see Chapters 28 and 75) but is then transported downward by nerve axons to the posterior pituitary gland, where it is finally secreted into the blood.
It is clear that vasopressin could have enormous effects on circulatory function. Yet normally, only minute amounts of vasopressin are secreted, so most physiologists have
thought that vasopressin plays little role in vascular control. However, experiments have shown that the concentration of circulating blood vasopressin after severe hemorrhage can increase enough to raise the arterial pressure as much as 60 mm Hg. In many instances, this can, by itself, bring the arterial pressure almost back up to normal.
Vasopressin has a major function to increase greatly water reabsorption from the renal tubules back into the blood (discussed in Chapter 28), and therefore to help control body fluid volume. That is why this hormone is also called antidiuretic hormone.
Vasodilator Agents
Bradykinin. Several substances called kinins cause powerful vasodilation when formed in the blood and tis- sue fluids of some organs.
The kinins are small polypeptides that are split away by proteolytic enzymes from alpha2-globulins in the plasma or tissue fluids. A proteolytic enzyme of particular impor- tance for this purpose is kallikrein, which is present in the blood and tissue fluids in an inactive form. This inactive kallikrein is activated by maceration of the blood, by tis- sue inflammation, or by other similar chemical or physical effects on the blood or tissues. As kallikrein becomes acti- vated, it acts immediately on alpha2-globulin to release a kinin called kallidin that is then converted by tissue enzymes into bradykinin. Once formed, bradykinin per- sists for only a few minutes because it is inactivated by the enzyme carboxypeptidase or by converting enzyme, the same enzyme that also plays an essential role in activat- ing angiotensin, as discussed in Chapter 19. The activated kallikrein enzyme is destroyed by a kallikrein inhibitor also present in the body fluids.
Bradykinin causes both powerful arteriolar dilation and increased capillary permeability. For instance, injec- tion of 1 microgram of bradykinin into the brachial artery of a person increases blood flow through the arm as much as sixfold, and even smaller amounts injected locally into tissues can cause marked local edema resulting from increase in capillary pore size.
There is reason to believe that kinins play special roles in regulating blood flow and capillary leakage of fluids in inflamed tissues. It also is believed that bradykinin plays a normal role to help regulate blood flow in the skin, as well as in the salivary and gastrointestinal glands.
Histamine. Histamine is released in essentially every tissue of the body if the tissue becomes damaged or inflamed or is the subject of an allergic reaction. Most of the histamine is derived from mast cells in the damaged tissues and from basophils in the blood.
Histamine has a powerful vasodilator effect on the arterioles and, like bradykinin, has the ability to increase greatly capillary porosity, allowing leakage of both fluid and plasma protein into the tissues. In many pathological condi- tions, the intense arteriolar dilation and increased capillary porosity produced by histamine cause tremendous quanti- ties of fluid to leak out of the circulation into the tissues,
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inducing edema. The local vasodilatory and edema-pro- ducing effects of histamine are especially prominent dur- ing allergic reactions and are discussed in Chapter 34.
Vascular Control by Ions and Other Chemical Factors
Many different ions and other chemical factors can either dilate or constrict local blood vessels. Most of them have little function in overall regulation of the circulation, but some specific effects are:
1. An increase in calcium ion concentration causes vaso- constriction. This results from the general effect of calcium to stimulate smooth muscle contraction, as discussed in Chapter 8.
2. An increase in potassium ion concentration, within the physiological range, causes vasodilation. This results from the ability of potassium ions to inhibit smooth muscle contraction.
3. An increase in magnesium ion concentration causes powerful vasodilation because magnesium ions inhibit smooth muscle contraction.
4. An increase in hydrogen ion concentration (decrease in pH) causes dilation of the arterioles. Conversely, slight decrease in hydrogen ion concentration causes arterio- lar constriction.
5. Anions that have significant effects on blood vessels are acetate and citrate, both of which cause mild degrees of vasodilation.
6. An increase in carbon dioxide concentration causes moderate vasodilation in most tissues but marked vaso- dilation in the brain. Also, carbon dioxide in the blood, acting on the brain vasomotor center, has an extremely powerful indirect effect, transmitted through the sym- pathetic nervous vasoconstrictor system, to cause widespread vasoconstriction throughout the body.
Most Vasodilators or Vasoconstrictors Have Little Effect on Long-Term Blood Flow Unless They Alter Metabolic Rate of the Tissues. In most cases, tissue blood flow and cardiac output (the sum of flow to all of the body’s tissues) are not substantially altered, except for a day or two, in experimental studies when one chronically infuses large amounts of powerful vasoconstrictors such as angiotensin II or vasodilators such as bradykinin. Why is blood flow not significantly altered in most tissues even in the presence of very large amounts of these vasoactive agents?
To answer this question we must return to one of the fundamental principles of circulatory function that we previously discussed—the ability of each tissue to auto- regulate its own blood flow according to the metabolic needs and other functions of the tissue. Administration of a powerful vasoconstrictor, such as angiotensin II, may cause transient decreases in tissue blood flow and cardiac output but usually has little long-term effect if it does not alter metabolic rate of the tissues. Likewise, most vasodilators cause only short-term changes in tis- sue blood flow and cardiac output if they do not alter tissue metabolism. Therefore, blood flow is generally regulated according to the specific needs of the tissues as long as the arterial pressure is adequate to perfuse the tissues.
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chapter 18
Nervous Regulation of the Circulation, and Rapid Control of Arterial Pressure
Nervous Regulation of the Circulation
As discussed in Chapter 17, adjustment of blood flow in the tissues and organs of the
body is mainly the function of local tissue control mecha- nisms. In this chapter we discuss how nervous control of the circulation has more global functions, such as redistrib- uting blood flow to different areas of the body, increasing or decreasing pumping activity by the heart, and providing very rapid control of systemic arterial pressure.
The nervous system controls the circulation almost entirely through the autonomic nervous system. The total function of this system is presented in Chapter 60, and this subject was also introduced in Chapter 17. For our present discussion, we will consider additional specific anatomical and functional characteristics, as follows.
Autonomic Nervous System
By far the most important part of the autonomic nervous system for regulating the circulation is the sympathetic nervous system. The parasympathetic nervous system, however, contributes importantly to regulation of heart function, as described later in the chapter.
Sympathetic Nervous System. Figure 18-1 shows the anatomy of sympathetic nervous control of the circulation. Sympathetic vasomotor nerve fibers leave the spinal cord through all the thoracic spinal nerves and through the first one or two lumbar spinal nerves. They then pass immedi- ately into a sympathetic chain, one of which lies on each side of the vertebral column. Next, they pass by two routes to the circulation: (1) through specific sympathetic nerves that innervate mainly the vasculature of the internal viscera and the heart, as shown on the right side of Figure 18-1, and (2) almost immediately into peripheral portions of the spinal nerves distributed to the vasculature of the peripheral areas. The precise pathways of these fibers in the spinal cord and in the sympathetic chains are discussed in Chapter 60.
Sympathetic Innervation of the Blood Vessels. Figure 18-2 shows distribution of sympathetic nerve fibers to the blood vessels, demonstrating that in most
tissues all the vessels except the capillaries are innervated. Precapillary sphincters and metarterioles are innervated in some tissues, such as the mesenteric blood vessels, although their sympathetic innervation is usually not as dense as in the small arteries, arterioles, and veins.
The innervation of the small arteries and arterioles allows sympathetic stimulation to increase resistance to blood flow and thereby to decrease rate of blood flow through the tissues.
The innervation of the large vessels, particularly of the veins, makes it possible for sympathetic stimulation to decrease the volume of these vessels. This can push blood into the heart and thereby play a major role in regulation of heart pumping, as we explain later in this and subse- quent chapters.
Sympathetic Nerve Fibers to the Heart. Sympathetic fibers also go directly to the heart, as shown in Figure 18-1 and as discussed in Chapter 9. It should be recalled that sympathetic stimulation markedly increases the activity of the heart, both increasing the heart rate and enhancing its strength and volume of pumping.
Parasympathetic Control of Heart Function, Especially Heart Rate. Although the parasympathetic nervous system is exceedingly important for many other autonomic functions of the body, such as control of mul- tiple gastrointestinal actions, it plays only a minor role in regulation of vascular function in most tissues. Its most important circulatory effect is to control heart rate by way of parasympathetic nerve fibers to the heart in the vagus nerves, shown in Figure 18-1 by the dashed red line from the brain medulla directly to the heart.
The effects of parasympathetic stimulation on heart function were discussed in detail in Chapter 9. Principally, parasympathetic stimulation causes a marked decrease in heart rate and a slight decrease in heart muscle contractility.
Sympathetic Vasoconstrictor System and Its Control by the Central Nervous System
The sympathetic nerves carry tremendous numbers of vasoconstrictor nerve fibers and only a few vasodilator fibers. The vasoconstrictor fibers are distributed to essentially all
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segments of the circulation, but more to some tissues than others. This sympathetic vasoconstrictor effect is especially powerful in the kidneys, intestines, spleen, and skin but much less potent in skeletal muscle and the brain.
Vasomotor Center in the Brain and Its Control of the Vasoconstrictor System. Located bilaterally mainly in the reticular substance of the medulla and of the lower third of the pons is an area called the vaso- motor center, shown in Figures 18-1 and 18-3. This center transmits parasympathetic impulses through the vagus nerves to the heart and transmits sympa- thetic impulses through the spinal cord and peripheral sympathetic nerves to virtually all arteries, arterioles, and veins of the body.
Although the total organization of the vasomotor center is still unclear, experiments have made it possi- ble to identify certain important areas in this center, as follows:
1. A vasoconstrictor area located bilaterally in the ante- rolateral portions of the upper medulla. The neurons originating in this area distribute their fibers to all lev- els of the spinal cord, where they excite preganglionic vasoconstrictor neurons of the sympathetic nervous system.
Arteries
Sympathetic vasoconstriction
Arterioles
Capillaries
VenulesVeins
Figure 18-2 Sympathetic innervation of the systemic circulation.
Vasoconstrictor
Sympathetic chain
Cardioinhibitor Vasodilator
Blood vessels
Vasomotor center
Blood vessels
Heart
Vagus
Figure 18-1 Anatomy of sympathetic nervous control of the circulation. Also shown by the dashed red line, a vagus nerve that carries para- sympathetic signals to the heart.
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2. A vasodilator area located bilaterally in the ante- rolateral portions of the lower half of the medulla. The fibers from these neurons project upward to the vasoconstrictor area just described; they inhibit the vasoconstrictor activity of this area, thus causing vasodilation.
3. A sensory area located bilaterally in the tractus soli- tarius in the posterolateral portions of the medulla and lower pons. The neurons of this area receive sen- sory nerve signals from the circulatory system mainly through the vagus and glossopharyngeal nerves, and output signals from this sensory area then help to control activities of both the vasoconstrictor and vasodilator areas of the vasomotor center, thus pro- viding “reflex” control of many circulatory functions. An example is the baroreceptor reflex for control- ling arterial pressure, which we describe later in this chapter.
Continuous Partial Constriction of the Blood Vessels Is Normally Caused by Sympathetic Vasoconstrictor Tone. Under normal conditions, the vasoconstrictor area of the vasomotor center transmits signals contin- uously to the sympathetic vasoconstrictor nerve fibers over the entire body, causing slow firing of these fibers at a rate of about one half to two impulses per second. This continual firing is called sympathetic vasoconstrictor tone. These impulses normally maintain a partial state of contraction in the blood vessels, called vasomotor tone.
Figure 18-4 demonstrates the significance of vasocon- strictor tone. In the experiment of this figure, total spinal anesthesia was administered to an animal. This blocked all transmission of sympathetic nerve impulses from the spinal cord to the periphery. As a result, the arterial pressure fell from 100 to 50 mm Hg, demonstrating the effect of losing vasoconstrictor tone throughout the body. A few minutes later, a small amount of the hormone nor- epinephrine was injected into the blood (norepineph- rine is the principal vasoconstrictor hormonal substance secreted at the endings of the sympathetic vasoconstric- tor nerve fibers throughout the body). As this injected hormone was transported in the blood to blood vessels, the vessels once again became constricted and the arte- rial pressure rose to a level even greater than normal for 1 to 3 minutes, until the norepinephrine was destroyed.
Control of Heart Activity by the Vasomotor Center. At the same time that the vasomotor cen- ter regulates the amount of vascular constriction, it also controls heart activity. The lateral portions of the vasomotor center transmit excitatory impulses through the sympathetic nerve fibers to the heart when there is need to increase heart rate and contrac- tility. Conversely, when there is need to decrease heart pumping, the medial portion of the vasomotor cen- ter sends signals to the adjacent dorsal motor nuclei of the vagus nerves, which then transmit parasympathetic impulses through the vagus nerves to the heart to decrease heart rate and heart contractility. Therefore,
the vasomotor center can either increase or decrease heart activity. Heart rate and strength of heart con- traction ordinarily increase when vasoconstriction occurs and ordinarily decrease when vasoconstriction is inhibited.
Control of the Vasomotor Center by Higher Nervous Centers. Large numbers of small neurons located throughout the reticular substance of the pons, mesen- cephalon, and diencephalon can either excite or inhibit the vasomotor center. This reticular substance is shown in Figure 18-3 by the rose-colored area. In general, the neurons in the more lateral and superior portions of the reticular substance cause excitation, whereas the more medial and inferior portions cause inhibition.
The hypothalamus plays a special role in controlling the vasoconstrictor system because it can exert either powerful excitatory or inhibitory effects on the vaso- motor center. The posterolateral portions of the hypo- thalamus cause mainly excitation, whereas the anterior portion can cause either mild excitation or inhibition, depending on the precise part of the anterior hypothala- mus stimulated.
Many parts of the cerebral cortex can also excite or inhibit the vasomotor center. Stimulation of the motor cortex, for instance, excites the vasomotor center because of impulses transmitted downward into the hypothala- mus and then to the vasomotor center. Also, stimula- tion of the anterior temporal lobe, the orbital areas of the frontal cortex, the anterior part of the cingulate gyrus, the amygdala, the septum, and the hippocampus can all either excite or inhibit the vasomotor center, depending on the precise portions of these areas that are stimulated and on the intensity of stimulus. Thus, widespread basal areas of the brain can have profound effects on cardiovascular function.
Reticular substance
Motor Cingulate
Orbital
Temporal
Pons
Medulla VASOMOTOR CENTER
VASOCONSTRICTOR VASODILATOR
Mesencephalon
{
Figure 18-3 Areas of the brain that play important roles in the nervous regulation of the circulation. The dashed lines represent inhibitory pathways.
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Norepinephrine—The Sympathetic Vasoconstrictor Transmitter Substance. The substance secreted at the endings of the vasoconstrictor nerves is almost entirely norepinephrine, which acts directly on the alpha adrener- gic receptors of the vascular smooth muscle to cause vaso- constriction, as discussed in Chapter 60.
Adrenal Medullae and Their Relation to the Sympathetic Vasoconstrictor System. Sympathetic impulses are transmitted to the adrenal medullae at the same time that they are transmitted to the blood vessels. They cause the medullae to secrete both epi- nephrine and norepinephrine into the circulating blood. These two hormones are carried in the blood stream to all parts of the body, where they act directly on all blood vessels, usually to cause vasoconstriction. In a few tissues epinephrine causes vasodilation because it also has a “beta” adrenergic receptor stimulatory effect, which dilates rather than constricts certain vessels, as discussed in Chapter 60.
Sympathetic Vasodilator System and Its Control by the Central Nervous System. The sympathetic nerves to skeletal muscles carry sympathetic vasodilator fibers, as well as con- strictor fibers. In some animals such as the cat, these dilator fibers release acetylcholine, not norepinephrine, at their end- ings, although in primates, the vasodilator effect is believed to be caused by epinephrine exciting specific beta-adrenergic receptors in the muscle vasculature.
The pathway for central nervous system control of the vasodilator system is shown by the dashed lines in Figure 18-3. The principal area of the brain controlling this system is the anterior hypothalamus.
Possible Unimportance of the Sympathetic Vasodilator System. It is doubtful that the sympathetic vasodilator system plays a major role in the control of the circulation in the human being because complete block of the sym- pathetic nerves to the muscles hardly affects the ability of
these muscles to control their own blood flow in response to their needs. Yet some experiments suggest that at the onset of exercise, the sympathetic vasodilator system might cause initial vasodilation in skeletal muscles to allow anticipa- tory increase in blood flow even before the muscles require increased nutrients.
Emotional Fainting—Vasovagal Syncope. A particu- larly interesting vasodilatory reaction occurs in people who experience intense emotional disturbances that cause faint- ing. In this case, the muscle vasodilator system becomes acti- vated, and at the same time, the vagal cardioinhibitory center transmits strong signals to the heart to slow the heart rate markedly. The arterial pressure falls rapidly, which reduces blood flow to the brain and causes the person to lose con- sciousness. This overall effect is called vasovagal syncope. Emotional fainting begins with disturbing thoughts in the cerebral cortex. The pathway probably then goes to the vaso- dilatory center of the anterior hypothalamus next to the vagal centers of the medulla, to the heart through the vagus nerves, and also through the spinal cord to the sympathetic vasodila- tor nerves of the muscles.
Role of the Nervous System in Rapid Control of Arterial Pressure
One of the most important functions of nervous con- trol of the circulation is its capability to cause rapid increases in arterial pressure. For this purpose, the entire vasoconstrictor and cardioaccelerator func- tions of the sympathetic nervous system are stimulated together. At the same time, there is reciprocal inhibi- tion of parasympathetic vagal inhibitory signals to the heart. Thus, three major changes occur simultaneously, each of which helps to increase arterial pressure. They are as follows:
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Minutes 0 5 10 15 20 25
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125
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75
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Total spinal anesthesia
Injection of norepinephrine
Figure 18-4 Effect of total spinal anesthesia on the arterial pressure, showing marked decrease in pressure resulting from loss of “vasomotor tone.”
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1. Most arterioles of the systemic circulation are con- stricted. This greatly increases the total peripheral resistance, thereby increasing the arterial pressure.
2. The veins especially (but the other large vessels of the circulation as well) are strongly constricted. This dis- places blood out of the large peripheral blood ves- sels toward the heart, thus increasing the volume of blood in the heart chambers. The stretch of the heart then causes the heart to beat with far greater force and therefore to pump increased quantities of blood. This, too, increases the arterial pressure.
3. Finally, the heart itself is directly stimulated by the autonomic nervous system, further enhancing car- diac pumping. Much of this is caused by an increase in the heart rate, the rate sometimes increasing to as great as three times normal. In addition, sympa- thetic nervous signals have a significant direct effect to increase contractile force of the heart muscle, this, too, increasing the capability of the heart to pump larger volumes of blood. During strong sympathetic stimulation, the heart can pump about two times as much blood as under normal conditions. This contributes still more to the acute rise in arterial pressure.
Rapidity of Nervous Control of Arterial Pressure. An especially important characteristic of nervous con- trol of arterial pressure is its rapidity of response, begin- ning within seconds and often increasing the pressure to two times normal within 5 to 10 seconds. Conversely, sudden inhibition of nervous cardiovascular stimulation can decrease the arterial pressure to as little as one-half normal within 10 to 40 seconds. Therefore, nervous con- trol of arterial pressure is by far the most rapid of all our mechanisms for pressure control.
Increase in Arterial Pressure During Muscle Exercise and Other Types of Stress
An important example of the ability of the nervous system to increase the arterial pressure is the increase in pressure that occurs during muscle exercise. During heavy exercise, the muscles require greatly increased blood flow. Part of this increase results from local vasodilation of the muscle vasculature caused by increased metabolism of the mus- cle cells, as explained in Chapter 17. Additional increase results from simultaneous elevation of arterial pressure caused by sympathetic stimulation of the overall circula- tion during exercise. In most heavy exercise, the arterial pressure rises about 30 to 40 percent, which increases blood flow almost an additional twofold.
The increase in arterial pressure during exercise results mainly from the following effect: At the same time that the motor areas of the brain become activated to cause exer- cise, most of the reticular activating system of the brain stem is also activated, which includes greatly increased stimulation of the vasoconstrictor and cardioacceleratory areas of the vasomotor center. These increase the arterial
pressure instantaneously to keep pace with the increase in muscle activity.
In many other types of stress besides muscle exercise, a similar rise in pressure can also occur. For instance, dur- ing extreme fright, the arterial pressure sometimes rises by as much as 75 to 100 mm Hg within a few seconds. This is called the alarm reaction, and it provides an excess of arterial pressure that can immediately supply blood to the muscles of the body that might need to respond instantly to cause flight from danger.
Reflex Mechanisms for Maintaining Normal Arterial Pressure
Aside from the exercise and stress functions of the auto- nomic nervous system to increase arterial pressure, there are multiple subconscious special nervous control mech- anisms that operate all the time to maintain the arterial pressure at or near normal. Almost all of these are nega- tive feedback reflex mechanisms, which we explain in the following sections.
Baroreceptor Arterial Pressure Control System— Baroreceptor Reflexes
By far the best known of the nervous mechanisms for arte- rial pressure control is the baroreceptor reflex. Basically, this reflex is initiated by stretch receptors, called either baroreceptors or pressoreceptors, located at specific points in the walls of several large systemic arteries. A rise in arterial pressure stretches the baroreceptors and causes them to transmit signals into the central nervous system. “Feedback” signals are then sent back through the auto- nomic nervous system to the circulation to reduce arterial pressure downward toward the normal level.
Physiologic Anatomy of the Baroreceptors and Their Innervation. Baroreceptors are spray-type nerve end- ings that lie in the walls of the arteries; they are stimu- lated when stretched. A few baroreceptors are located in the wall of almost every large artery of the thoracic and neck regions; but, as shown in Figure 18-5, barorecep- tors are extremely abundant in (1) the wall of each inter- nal carotid artery slightly above the carotid bifurcation, an area known as the carotid sinus, and (2) the wall of the aortic arch.
Figure 18-5 shows that signals from the “carotid barore- ceptors” are transmitted through small Hering’s nerves to the glossopharyngeal nerves in the high neck, and then to the tractus solitarius in the medullary area of the brain stem. Signals from the “aortic baroreceptors” in the arch of the aorta are transmitted through the vagus nerves also to the same tractus solitarius of the medulla.
Response of the Baroreceptors to Arterial Pressure. Figure 18-6 shows the effect of different arte- rial pressure levels on the rate of impulse transmission in a Hering’s carotid sinus nerve. Note that the carotid sinus baroreceptors are not stimulated at all by pressures between 0 and 50 to 60 mm Hg, but above these levels, they respond progressively more rapidly and reach a
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maximum at about 180 mm Hg. The responses of the aortic baroreceptors are similar to those of the carotid receptors except that they operate, in general, at arterial pressure levels about 30 mm Hg higher.
Note especially that in the normal operating range of arterial pressure, around 100 mm Hg, even a slight change in pressure causes a strong change in the baroreflex signal to readjust arterial pressure back toward normal. Thus, the baroreceptor feedback mechanism functions most effectively in the pressure range where it is most needed.
The baroreceptors respond rapidly to changes in arte- rial pressure; in fact, the rate of impulse firing increases in the fraction of a second during each systole and decreases again during diastole. Furthermore, the baroreceptors respond much more to a rapidly changing pressure than to a stationary pressure. That is, if the mean arterial pressure is 150 mm Hg but at that moment is rising rapidly, the rate of impulse transmission may be as much as twice that when the pressure is stationary at 150 mm Hg.
Circulatory Reflex Initiated by the Baroreceptors. After the baroreceptor signals have entered the tractus solitarius of the medulla, secondary signals inhibit the vasoconstrictor center of the medulla and excite the vagal parasympathetic center. The net effects are (1) vasodilation of the veins and arterioles throughout the peripheral cir- culatory system and (2) decreased heart rate and strength of heart contraction. Therefore, excitation of the barore- ceptors by high pressure in the arteries reflexly causes the arterial pressure to decrease because of both a decrease in peripheral resistance and a decrease in cardiac output. Conversely, low pressure has opposite effects, reflexly caus- ing the pressure to rise back toward normal.
Figure 18-7 shows a typical reflex change in arte- rial pressure caused by occluding the two common carotid arteries. This reduces the carotid sinus pressure; as a result, signals from the baroreceptors decrease and cause less inhibitory effect on the vasomotor center. The vasomotor center then becomes much more active than usual, causing the aortic arterial pressure to rise and remain elevated during the 10 minutes that the carotids are occluded. Removal of the occlusion allows the pres- sure in the carotid sinuses to rise, and the carotid sinus reflex now causes the aortic pressure to fall immediately to slightly below normal as a momentary overcompensa- tion and then return to normal in another minute.
Function of the Baroreceptors During Changes in Body Posture. The ability of the baroreceptors to main- tain relatively constant arterial pressure in the upper body
Glossopharyngeal nerve
Hering’s nerve
Vagus nerve
Aortic baroreceptors
Carotid body
Carotid sinus
Figure 18-5 The baroreceptor system for controlling arterial pressure.
N u
m b
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f im
p u
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f ro
m c
ar o
ti d
si n
u s
n er
ve s
p er
s ec
o n
d
800 160 240
DI DP
= maximum
Arterial blood pressure (mm Hg)
Figure 18-6 Activation of the baroreceptors at different levels of arterial pressure. ∆I, change in carotid sinus nerve impulses per second; ∆P, change in arterial blood pressure in mm Hg.
150
Both common carotids clamped
Carotids released
100
50
A rt
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m m
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0 0 2 4 6 8
Minutes
10 12 14
Figure 18-7 Typical carotid sinus reflex effect on aortic arterial pressure caused by clamping both common carotids (after the two vagus nerves have been cut).
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is important when a person stands up after having been lying down. Immediately on standing, the arterial pres- sure in the head and upper part of the body tends to fall, and marked reduction of this pressure could cause loss of consciousness. However, the falling pressure at the barore- ceptors elicits an immediate reflex, resulting in strong sym- pathetic discharge throughout the body. This minimizes the decrease in pressure in the head and upper body.
Pressure “Buffer” Function of the Baroreceptor Control System. Because the baroreceptor system opposes either increases or decreases in arterial pressure, it is called a pressure buffer system and the nerves from the baroreceptors are called buffer nerves.
Figure 18-8 shows the importance of this buffer func- tion of the baroreceptors. The upper record in this figure shows an arterial pressure recording for 2 hours from a normal dog, and the lower record shows an arterial pres- sure recording from a dog whose baroreceptor nerves from both the carotid sinuses and the aorta had been removed. Note the extreme variability of pressure in the denervated dog caused by simple events of the day, such as lying down, standing, excitement, eating, defecation, and noises.
Figure 18-9 shows the frequency distributions of the mean arterial pressures recorded for a 24-hour day in both the normal dog and the denervated dog. Note that when the baroreceptors were functioning normally the mean
arterial pressure remained throughout the day within a narrow range between 85 and 115 mm Hg—indeed, during most of the day at almost exactly 100 mm Hg. Conversely, after denervation of the baroreceptors, the frequency distribution curve became the broad, low curve of the figure, showing that the pressure range increased 2.5-fold, frequently falling to as low as 50 mm Hg or rising to over 160 mm Hg. Thus, one can see the extreme vari- ability of pressure in the absence of the arterial barorecep- tor system.
In summary, a primary purpose of the arte- rial baroreceptor system is to reduce the minute-by- minute variation in arterial pressure to about one-third that which would occur if the baroreceptor system was not present.
Are the Baroreceptors Important in Long-Term Regulation of Arterial Pressure? Although the arterial baroreceptors provide powerful moment-to-moment control of arterial pressure, their importance in long-term blood pressure regulation has been controversial. One rea- son that the baroreceptors have been considered by some physiologists to be relatively unimportant in chronic reg- ulation of arterial pressure chronically is that they tend to reset in 1 to 2 days to the pressure level to which they are exposed. That is, if the arterial pressure rises from the normal value of 100 mm Hg to 160 mm Hg, a very high rate of baroreceptor impulses are at first transmitted. During the next few minutes, the rate of firing diminishes considerably; then it diminishes much more slowly during the next 1 to 2 days, at the end of which time the rate of firing will have returned to nearly normal despite the fact that the mean arterial pressure still remains at 160 mm Hg. Conversely, when the arterial pressure falls to a very low level, the baroreceptors at first transmit no impulses,
NORMAL
24
BARORECEPTORS DENERVATED
Time (min)
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200
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Figure 18-8 Two-hour records of arterial pressure in a normal dog (above) and in the same dog (below) several weeks after the baroreceptors had been denervated. (Redrawn from Cowley AW Jr, Liard JF, Guyton AC: Role of baroreceptor reflex in daily control of arterial blood pressure and other variables in dogs. Circ Res 32:564, 1973. By permission of the American Heart Association, Inc.)
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Normal
Denervated
Figure 18-9 Frequency distribution curves of the arterial pressure for a 24-hour period in a normal dog and in the same dog sev- eral weeks after the baroreceptors had been denervated. (Redrawn from Cowley AW Jr, Liard JP, Guyton AC: Role of baroreceptor reflex in daily control of arterial blood pressure and other variables in dogs. Circ Res 32:564, 1973. By permission of the American Heart Association, Inc.)
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but gradually, over 1 to 2 days, the rate of baroreceptor fir- ing returns toward the control level.
This “resetting” of the baroreceptors may attenuate their potency as a control system for correcting disturbances that tend to change arterial pressure for longer than a few days at a time. Experimental studies, however, have sug- gested that the baroreceptors do not completely reset and may therefore contribute to long-term blood pressure reg- ulation, especially by influencing sympathetic nerve activ- ity of the kidneys. For example, with prolonged increases in arterial pressure, the baroreceptor reflexes may mediate decreases in renal sympathetic nerve activity that promote increased excretion of sodium and water by the kidneys. This, in turn, causes a gradual decrease in blood volume, which helps to restore arterial pressure toward normal. Thus, long-term regulation of mean arterial pressure by the baroreceptors requires interaction with additional sys- tems, principally the renal–body fluid–pressure control system (along with its associated nervous and hormonal mechanisms), discussed in Chapters 19 and 29.
Control of Arterial Pressure by the Carotid and Aortic Chemoreceptors—Effect of Oxygen Lack on Arterial Pressure. Closely associated with the baroreceptor pres- sure control system is a chemoreceptor reflex that operates in much the same way as the baroreceptor reflex except that chemoreceptors, instead of stretch receptors, initiate the response.
The chemoreceptors are chemosensitive cells sensitive to oxygen lack, carbon dioxide excess, and hydrogen ion excess. They are located in several small chemoreceptor organs about 2 millimeters in size (two carotid bodies, one of which lies in the bifurcation of each common carotid artery, and usually one to three aortic bodies adjacent to the aorta). The chemoreceptors excite nerve fibers that, along with the baroreceptor fibers, pass through Hering’s nerves and the vagus nerves into the vasomotor center of the brain stem.
Each carotid or aortic body is supplied with an abun- dant blood flow through a small nutrient artery, so the chemoreceptors are always in close contact with arterial blood. Whenever the arterial pressure falls below a criti- cal level, the chemoreceptors become stimulated because diminished blood flow causes decreased oxygen, as well as excess buildup of carbon dioxide and hydrogen ions that are not removed by the slowly flowing blood.
The signals transmitted from the chemoreceptors excite the vasomotor center, and this elevates the arterial pressure back toward normal. However, this chemorecep- tor reflex is not a powerful arterial pressure controller until the arterial pressure falls below 80 mm Hg. Therefore, it is at the lower pressures that this reflex becomes important to help prevent further decreases in arterial pressure.
The chemoreceptors are discussed in much more detail in Chapter 41 in relation to respiratory control, in which they play a far more important role than in blood pressure control.
Atrial and Pulmonary Artery Reflexes Regulate Arte- rial Pressure. Both the atria and the pulmonary arteries
have in their walls stretch receptors called low-pressure receptors. They are similar to the baroreceptor stretch receptors of the large systemic arteries. These low-pres- sure receptors play an important role, especially in mini- mizing arterial pressure changes in response to changes in blood volume. For example, if 300 milliliters of blood suddenly are infused into a dog with all receptors intact, the arterial pressure rises only about 15 mm Hg. With the arterial baroreceptors denervated, the pressure rises about 40 mm Hg. If the low-pressure receptors also are dener- vated, the arterial pressure rises about 100 mm Hg.
Thus, one can see that even though the low-pressure receptors in the pulmonary artery and in the atria can- not detect the systemic arterial pressure, they do detect simultaneous increases in pressure in the low-pressure areas of the circulation caused by increase in volume, and they elicit reflexes parallel to the baroreceptor reflexes to make the total reflex system more potent for control of arterial pressure.
Atrial Reflexes That Activate the Kidneys—The “Volume Reflex.” Stretch of the atria also causes signifi- cant reflex dilation of the afferent arterioles in the kid- neys. Signals are also transmitted simultaneously from the atria to the hypothalamus to decrease secretion of antidiuretic hormone (ADH). The decreased afferent arteriolar resistance in the kidneys causes the glomeru- lar capillary pressure to rise, with resultant increase in filtration of fluid into the kidney tubules. The diminu- tion of ADH diminishes the reabsorption of water from the tubules. Combination of these two effects—increase in glomerular filtration and decrease in reabsorption of the fluid—increases fluid loss by the kidneys and reduces an increased blood volume back toward normal. (We will also see in Chapter 19 that atrial stretch caused by increased blood volume also elicits a hormonal effect on the kidneys—release of atrial natriuretic peptide—that adds still further to the excretion of fluid in the urine and return of blood volume toward normal.)
All these mechanisms that tend to return the blood volume back toward normal after a volume overload act indirectly as pressure controllers, as well as blood vol- ume controllers, because excess volume drives the heart to greater cardiac output and leads, therefore, to greater arterial pressure. This volume reflex mechanism is dis- cussed again in Chapter 29, along with other mechanisms of blood volume control.
Atrial Reflex Control of Heart Rate (the Bainbridge Reflex). An increase in atrial pressure also causes an increase in heart rate, sometimes increasing the heart rate as much as 75 percent. A small part of this increase is caused by a direct effect of the increased atrial volume to stretch the sinus node; it was pointed out in Chapter 10 that such direct stretch can increase the heart rate as much as 15 percent. An additional 40 to 60 percent increase in rate is caused by a ner- vous reflex called the Bainbridge reflex. The stretch receptors of the atria that elicit the Bainbridge reflex transmit their afferent signals through the vagus
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nerves to the medulla of the brain. Then efferent signals are transmitted back through vagal and sympa- thetic nerves to increase heart rate and strength of heart contraction. Thus, this reflex helps prevent damming of blood in the veins, atria, and pulmonary circulation.
Central Nervous System Ischemic Response— Control of Arterial Pressure by the Brain’s Vasomotor Center in Response to Diminished Brain Blood Flow
Most nervous control of blood pressure is achieved by reflexes that originate in the baroreceptors, the chemore- ceptors, and the low-pressure receptors, all of which are located in the peripheral circulation outside the brain. However, when blood flow to the vasomotor center in the lower brain stem becomes decreased severely enough to cause nutritional deficiency—that is, to cause cerebral ischemia—the vasoconstrictor and cardioaccelerator neu- rons in the vasomotor center respond directly to the isch- emia and become strongly excited. When this occurs, the systemic arterial pressure often rises to a level as high as the heart can possibly pump. This effect is believed to be caused by failure of the slowly flowing blood to carry car- bon dioxide away from the brain stem vasomotor center: At low levels of blood flow to the vasomotor center, the local concentration of carbon dioxide increases greatly and has an extremely potent effect in stimulating the sym- pathetic vasomotor nervous control areas in the brain’s medulla.
It is possible that other factors, such as buildup of lactic acid and other acidic substances in the vasomotor center, also contribute to the marked stimulation and elevation in arterial pressure. This arterial pressure elevation in response to cerebral ischemia is known as the central ner- vous system (CNS) ischemic response.
The ischemic effect on vasomotor activity can elevate the mean arterial pressure dramatically, sometimes to as high as 250 mm Hg for as long as 10 minutes. The degree of sympathetic vasoconstriction caused by intense cerebral ischemia is often so great that some of the peripheral ves- sels become totally or almost totally occluded. The kidneys, for instance, often entirely cease their production of urine because of renal arteriolar constriction in response to the sympathetic discharge. Therefore, the CNS is chemic response is one of the most powerful of all the activators of the sympathetic vasoconstrictor system.
Importance of the CNS Ischemic Response as a Regulator of Arterial Pressure. Despite the power- ful nature of the CNS ischemic response, it does not become significant until the arterial pressure falls far below normal, down to 60 mm Hg and below, reaching its greatest degree of stimulation at a pressure of 15 to 20 mm Hg. Therefore, it is not one of the normal mecha- nisms for regulating arterial pressure. Instead, it oper- ates principally as an emergency pressure control system that acts rapidly and very powerfully to prevent further
decrease in arterial pressure whenever blood flow to the brain decreases dangerously close to the lethal level. It is sometimes called the “last ditch stand” pressure control mechanism.
Cushing Reaction to Increased Pressure Around the Brain. The so-called Cushing reaction is a spe- cial type of CNS ischemic response that results from increased pressure of the cerebrospinal fluid around the brain in the cranial vault. For instance, when the cerebrospinal fluid pressure rises to equal the arterial pressure, it compresses the whole brain, as well as the arteries in the brain, and cuts off the blood supply to the brain. This initiates a CNS ischemic response that causes the arterial pressure to rise. When the arterial pressure has risen to a level higher than the cerebro- spinal fluid pressure, blood will flow once again into the vessels of the brain to relieve the brain ischemia. Ordinarily, the blood pressure comes to a new equilib- rium level slightly higher than the cerebrospinal fluid pressure, thus allowing blood to begin again to flow through the brain. The Cushing reaction helps protect the vital centers of the brain from loss of nutrition if ever the cerebrospinal fluid pressure rises high enough to compress the cerebral arteries.
Special Features of Nervous Control of Arterial Pressure
Role of the Skeletal Nerves and Skeletal Muscles in Increasing Cardiac Output and Arterial Pressure
Although most rapidly acting nervous control of the cir- culation is effected through the autonomic nervous sys- tem, at least two conditions in which the skeletal nerves and muscles also play major roles in circulatory responses are the following.
Abdominal Compression Reflex. When a barore- ceptor or chemoreceptor reflex is elicited, nerve sig- nals are transmitted simultaneously through skeletal nerves to skeletal muscles of the body, particularly to the abdominal muscles. This compresses all the venous res- ervoirs of the abdomen, helping to translocate blood out of the abdominal vascular reservoirs toward the heart. As a result, increased quantities of blood are made avail- able for the heart to pump. This overall response is called the abdominal compression reflex. The resulting effect on the circulation is the same as that caused by sympa- thetic vasoconstrictor impulses when they constrict the veins: an increase in both cardiac output and arterial pressure. The abdominal compression reflex is probably much more important than has been realized in the past because it is well known that people whose skeletal mus- cles have been paralyzed are considerably more prone to hypotensive episodes than are people with normal skel- etal muscles.
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Increased Cardiac Output and Arterial Pressure Caused by Skeletal Muscle Contraction During Exercise. When the skeletal muscles contract during exercise, they compress blood vessels throughout the body. Even anticipation of exercise tightens the muscles, thereby compressing the vessels in the muscles and in the abdo- men. The resulting effect is to translocate blood from the peripheral vessels into the heart and lungs and, therefore, to increase the cardiac output. This is an essential effect in helping to cause the fivefold to sevenfold increase in car- diac output that sometimes occurs in heavy exercise. The increase in cardiac output in turn is an essential ingredi- ent in increasing the arterial pressure during exercise, an increase usually from a normal mean of 100 mm Hg up to 130 to 160 mm Hg.
Respiratory Waves in the Arterial Pressure
With each cycle of respiration, the arterial pressure usu- ally rises and falls 4 to 6 mm Hg in a wavelike manner, causing respiratory waves in the arterial pressure. The waves result from several different effects, some of which are reflex in nature, as follows:
1. Many of the “breathing signals” that arise in the respi- ratory center of the medulla “spill over” into the vaso- motor center with each respiratory cycle.
2. Every time a person inspires, the pressure in the tho- racic cavity becomes more negative than usual, causing the blood vessels in the chest to expand. This reduces the quantity of blood returning to the left side of the heart and thereby momentarily decreases the cardiac output and arterial pressure.
3. The pressure changes caused in the thoracic vessels by respiration can excite vascular and atrial stretch receptors.
Although it is difficult to analyze the exact relations of all these factors in causing the respiratory pressure waves, the net result during normal respiration is usually an increase in arterial pressure during the early part of expi- ration and a decrease in pressure during the remainder of the respiratory cycle. During deep respiration, the blood pressure can rise and fall as much as 20 mm Hg with each respiratory cycle.
Arterial Pressure “Vasomotor” Waves—Oscillation of Pressure Reflex Control Systems
Often while recording arterial pressure from an animal, in addition to the small pressure waves caused by respira- tion, some much larger waves are also noted—as great as 10 to 40 mm Hg at times—that rise and fall more slowly than the respiratory waves. The duration of each cycle varies from 26 seconds in the anesthetized dog to 7 to 10 seconds in the unanesthetized human. These waves are called vasomotor waves or “Mayer waves.” Such records are demonstrated in Figure 18-10, showing the cyclical rise and fall in arterial pressure.
The cause of vasomotor waves is “reflex oscillation” of one or more nervous pressure control mechanisms, some of which are the following.
Oscillation of the Baroreceptor and Chemoreceptor Reflexes. The vasomotor waves of Figure 18-10B are often seen in experimental pressure recordings, although usually much less intense than shown in the figure. They are caused mainly by oscillation of the baroreceptor reflex. That is, a high pressure excites the baroreceptors; this then inhibits the sympathetic nervous system and lowers the pressure a few seconds later. The decreased pressure in turn reduces the baroreceptor stimulation and allows the vasomotor center to become active once again, ele- vating the pressure to a high value. The response is not instantaneous, and it is delayed until a few seconds later. This high pressure then initiates another cycle, and the oscillation continues on and on.
The chemoreceptor reflex can also oscillate to give the same type of waves. This reflex usually oscillates simul- taneously with the baroreceptor reflex. It probably plays the major role in causing vasomotor waves when the arte- rial pressure is in the range of 40 to 80 mm Hg because in this low range, chemoreceptor control of the circula- tion becomes powerful, whereas baroreceptor control becomes weaker.
Oscillation of the CNS Ischemic Response. The record in Figure 18-10A resulted from oscillation of the CNS ischemic pressure control mechanism. In this experiment, the cerebrospinal fluid pressure was raised to 160 mm Hg, which compressed the cerebral vessels and initiated a CNS ischemic pressure response up to 200 mm Hg. When the arterial pressure rose to such a high value, the brain ischemia was relieved and the sym- pathetic nervous system became inactive. As a result, the arterial pressure fell rapidly back to a much lower value, causing brain ischemia once again. The ischemia then initiated another rise in pressure. Again the ischemia was relieved and again the pressure fell. This repeated itself cyclically as long as the cerebrospinal fluid pressure remained elevated.
Thus, any reflex pressure control mechanism can oscil- late if the intensity of “feedback” is strong enough and if there is a delay between excitation of the pressure recep- tor and the subsequent pressure response. The vasomotor
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waves are of considerable theoretical importance because they show that the nervous reflexes that control arterial pressure obey the same principles as those applicable to mechanical and electrical control systems. For instance, if the feedback “gain” is too great in the guiding mechanism of an automatic pilot for an airplane and there is also delay in response time of the guiding mechanism, the plane will oscillate from side to side instead of following a straight course.
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DiBona GF: Physiology in perspective: the wisdom of the body. Neural con- trol of the kidney, Am J Physiol Regul Integr Comp Physiol 289:R633, 2005.
Esler M, Lambert G, Brunner-La Rocca HP, et al: Sympathetic nerve activity and neurotransmitter release in humans: translation from pathophysi- ology into clinical practice, Acta Physiol Scand 177:275, 2003.
Freeman R: Clinical practice. Neurogenic orthostatic hypotension, N Engl J Med 358:615, 2008.
Goldstein DS, Robertson D, Esler M, et al: Dysautonomias: clinical disorders of the autonomic nervous system, Ann Intern Med 137:753, 2002.
Guyton AC: Arterial pressure and hypertension, Philadelphia, 1980, WB Saunders.
Guyenet PG: The sympathetic control of blood pressure, Nat Rev Neurosci 7:335, 2006.
Joyner MJ: Baroreceptor function during exercise: resetting the record, Exp Physiol 91:27, 2006.
Lohmeier TE, Dwyer TM, Irwin ED, et al: Prolonged activation of the barore- flex abolishes obesity-induced hypertension, Hypertension 49:1307, 2007.
Lohmeier TE, Hildebrandt DA, Warren S, et al: Recent insights into the inter- actions between the baroreflex and the kidneys in hypertension, Am J Physiol Regul Integr Comp Physiol 288:R828, 2005.
Ketch T, Biaggioni I, Robertson R, Robertson D: Four faces of baroreflex fail- ure: hypertensive crisis, volatile hypertension, orthostatic tachycardia, and malignant vagotonia, Circulation 105:2518, 2002.
Mifflin SW: What does the brain know about blood pressure? News Physiol Sci 16:266, 2001.
Olshansky B, Sabbah HN, Hauptman PJ, et al: Parasympathetic nervous system and heart failure: pathophysiology and potential implications for therapy, Circulation 118:863, 2008.
Schultz HD, Li YL, Ding Y: Arterial chemoreceptors and sympathetic nerve activity: implications for hypertension and heart failure, Hypertension 50:6, 2007.
Zucker IH: Novel mechanisms of sympathetic regulation in chronic heart failure, Hypertension 48:1005, 2006.
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chapter 19
Role of the Kidneys in Long-Term Control of Arterial Pressure and in Hypertension: The
Integrated System for Arterial Pressure Regulation
Short-term control of arte- rial pressure by the sympa- thetic nervous system, as discussed in Chapter 18, occurs primarily through the effects of the nervous system on total peripheral
vascular resistance and capacitance, as well as on cardiac pumping ability.
The body, however, also has powerful mechanisms for regulating arterial pressure week after week and month after month. This long-term control of arterial pressure is closely intertwined with homeostasis of body fluid vol- ume, which is determined by the balance between the fluid intake and output. For long-term survival, fluid intake and output must be precisely balanced, a task that is performed by multiple nervous and hormonal controls, and by local control systems within the kidneys that regu- late their excretion of salt and water. In this chapter we discuss these renal–body fluid systems that play a domi- nant role in long-term blood pressure regulation.
Renal–Body Fluid System for Arterial Pressure Control
The renal–body fluid system for arterial pressure con- trol acts slowly but powerfully as follows: If blood vol- ume increases and vascular capacitance is not altered, arterial pressure will also increase. The rising pressure in turn causes the kidneys to excrete the excess volume, thus returning the pressure back toward normal.
In the phylogenetic history of animal development, this renal–body fluid system for pressure control is a primitive one. It is fully operative in one of the lowest of vertebrates, the hagfish. This animal has a low arterial pressure, only 8 to 14 mm Hg, and this pressure increases almost directly in proportion to its blood volume. The hagfish continually drinks sea water, which is absorbed into its blood, increas- ing the blood volume and blood pressure. However, when the pressure rises too high, the kidney simply excretes the excess volume into the urine and relieves the pres- sure. At low pressure, the kidney excretes less fluid than
is ingested. Therefore, because the hagfish continues to drink, extracellular fluid volume, blood volume, and pres- sure all build up again to the higher levels.
Throughout the ages, this primitive mechanism of pressure control has survived almost as it functions in the hagfish; in the humans, kidney output of water and salt is just as sensitive to pressure changes as in the hagfish, if not more so. Indeed, an increase in arterial pressure in the human of only a few mm Hg can double renal output of water, which is called pressure diuresis, as well as double the output of salt, which is called pressure natriuresis.
In the human being, the renal–body fluid system for arterial pressure control, just as in the hagfish, is a funda- mental mechanism for long-term arterial pressure con- trol. However, through the stages of evolution, multiple refinements have been added to make this system much more exact in its control in the human being. An espe- cially important refinement, as discussed later, has been the addition of the renin-angiotensin mechanism.
Quantitation of Pressure Diuresis as a Basis for Arterial Pressure Control
Figure 19-1 shows the approximate average effect of dif- ferent arterial pressure levels on urinary volume output by an isolated kidney, demonstrating markedly increased urine volume output as the pressure rises. This increased urinary output is the phenomenon of pressure diuresis. The curve in this figure is called a renal urinary output curve or a renal function curve. In the human being, at an arterial pressure of 50 mm Hg, the urine output is essentially zero. At 100 mm Hg it is normal, and at 200 mm Hg it is about six to eight times normal. Furthermore, not only does increas- ing the arterial pressure increase urine volume output, but it causes approximately equal increase in sodium output, which is the phenomenon of pressure natriuresis.
An Experiment Demonstrating the Renal–Body Fluid System for Arterial Pressure Control. Figure 19-2 shows the results of an experiment in dogs in which all the nervous reflex mechanisms for blood pressure control were first blocked. Then the arterial pressure was suddenly elevated by infusing about 400 ml of blood intravenously. Note the rapid increase in cardiac output
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to about double normal and increase in mean arterial pressure to 205 mm Hg, 115 mm Hg above its resting level. Shown by the middle curve is the effect of this increased arterial pressure on urine output, which increased 12-fold. Along with this tremendous loss of fluid in the urine, both the cardiac output and the arterial pressure returned to normal during the subsequent hour. Thus, one sees an extreme capability of the kidneys to eliminate fluid volume from the body in response to high arterial pressure and in so doing to return the arterial pressure back to normal.
Arterial Pressure Control by the Renal–Body Fluid Mechanism—“Near Infinite Feedback Gain” Feature. Figure 19-3 shows a graphical method that
can be used for analyzing arterial pressure control by the renal–body fluid system. This analysis is based on two separate curves that intersect each other: (1) the renal output curve for water and salt in response to rising arterial pressure, which is the same renal output curve as that shown in Figure 19-1, and (2) the line that represents the net water and salt intake.
Over a long period, the water and salt output must equal the intake. Furthermore, the only place on the graph in Figure 19-3 at which output equals intake is where the two curves intersect, which is called the equilibrium point. Now, let us see what happens if the arterial pres- sure increases above, or decreases below, the equilibrium point.
First, assume that the arterial pressure rises to 150 mm Hg. At this level, the renal output of water and salt is about three times as great as the intake. Therefore, the body loses fluid, the blood volume decreases, and the arterial pressure decreases. Furthermore, this “negative balance” of fluid will not cease until the pressure falls all the way back exactly to the equilibrium level. Indeed, even when the arterial pressure is only 1 mm Hg greater than the equilibrium level, there still is slightly more loss of water and salt than intake, so the pressure continues to fall that last 1 mm Hg until the pressure eventually returns exactly to the equilibrium point.
If the arterial pressure falls below the equilibrium point, the intake of water and salt is greater than the out- put. Therefore, body fluid volume increases, blood volume increases, and the arterial pressure rises until once again it returns exactly to the equilibrium point. This return of the arterial pressure always back to the equilibrium point is the near infinite feedback gain principle for control of arterial pressure by the renal–body fluid mechanism.
Two Determinants of the Long-Term Arterial Pressure Level. In Figure 19-3, one can also see that two basic long-term factors determine the long-term arterial pressure level. This can be explained as follows.
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Figure 19-2 Increases in cardiac output, urinary output, and arte- rial pressure caused by increased blood volume in dogs whose ner- vous pressure control mechanisms had been blocked. This figure shows return of arterial pressure to normal after about an hour of fluid loss into the urine. (Courtesy Dr. William Dobbs.)
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Figure 19-3 Analysis of arterial pressure regulation by equating the “renal output curve” with the “salt and water intake curve.” The equilibrium point describes the level to which the arterial pressure will be regulated. (That small portion of the salt and water intake that is lost from the body through nonrenal routes is ignored in this and similar figures in this chapter.)
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Figure 19-1 Typical renal urinary output curve measured in a per- fused isolated kidney, showing pressure diuresis when the arterial pressure rises above normal.
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As long as the two curves representing (1) renal output of salt and water and (2) intake of salt and water remain exactly as they are shown in Figure 19-3, the mean arterial pressure level will eventually readjust to 100 mm Hg, which is the pressure level depicted by the equilibrium point of this figure. Furthermore, there are only two ways in which the pressure of this equilibrium point can be changed from the 100 mm Hg level. One of these is by shifting the pressure level of the renal output curve for salt and water, and the other is by changing the level of the water and salt intake line. Therefore, expressed simply, the two primary determinants of the long-term arterial pressure level are as follows:
1. The degree of pressure shift of the renal output curve for water and salt
2. The level of the water and salt intake
Operation of these two determinants in the control of arterial pressure is demonstrated in Figure 19-4. In Figure 19-4A, some abnormality of the kidneys has caused the renal output curve to shift 50 mm Hg in the high-pres- sure direction (to the right). Note that the equilibrium point has also shifted to 50 mm Hg higher than normal. Therefore, one can state that if the renal output curve shifts to a new pressure level, the arterial pressure will follow to this new pressure level within a few days.
Figure 19-4B shows how a change in the level of salt and water intake also can change the arterial pressure. In this case, the intake level has increased fourfold and the equi- librium point has shifted to a pressure level of 160 mm Hg,
60 mm Hg above the normal level. Conversely, a decrease in the intake level would reduce the arterial pressure.
Thus, it is impossible to change the long-term mean arterial pressure level to a new value without changing one or both of the two basic determinants of long-term arte- rial pressure—either (1) the level of salt and water intake or (2) the degree of shift of the renal function curve along the pressure axis. However, if either of these is changed, one finds the arterial pressure thereafter to be regulated at a new pressure level, the arterial pressure at which the two new curves intersect.
The Chronic Renal Output Curve Is Much Steeper than the Acute Curve. An important characteristic of pressure natriuresis (and pressure diuresis) is that chronic changes in arterial pressure, lasting for days or months, have much greater effect on renal output of salt and water than observed during acute changes in pressure (Figure 19-5). Thus, when the kidneys are functioning normally, the chronic renal output curve is much steeper than the acute curve.
The powerful effects of chronic increases in arterial pressure on urine output are because increased pressure not only has direct hemodynamic effects on the kidney to increase excretion, but also indirect effects mediated by nervous and hormonal changes that occur when blood pressure is increased. For example, increased arterial pressure decreases activity of the sympathetic nervous system and various hormones such as angiotensin II and aldosterone that tend to reduce salt and water excretion by the kidneys. Reduced activity of these antinatriuretic systems therefore amplifies the effectiveness of pressure natriuresis and diuresis in raising salt and water excre- tion during chronic increases in arterial pressure (see Chapters 27 and 29 for further discussion).
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Figure 19-4 Two ways in which the arterial pressure can be increased: A, by shifting the renal output curve in the right-hand direction toward a higher pressure level or B, by increasing the intake level of salt and water.
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Figure 19-5 Acute and chronic renal output curves. Under steady- state conditions renal output of salt and water is equal to intake of salt and water. A and B represent the equilibrium points for long-term regulation of arterial pressure when salt intake is nor- mal or six times normal, respectively. Because of the steepness of the chronic renal output curve, increased salt intake causes only small changes in arterial pressure. In persons with impaired kidney function, the steepness of the renal output curve may be reduced, similar to the acute curve, resulting in increased sensitivity of arterial pressure to changes in salt intake.
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Conversely, when blood pressure is reduced, the sym- pathetic nervous system is activated and formation of antinatriuretic hormones is increased, adding to the direct effects of reduced pressure to decrease renal out- put of salt and water. This combination of direct effects of pressure on the kidneys and indirect effects of pressure on the sympathetic nervous system and various hormone systems make pressure natriuresis and diuresis extremely powerful for long-term control of arterial pressure and body fluid volumes.
The importance of neural and hormonal influences on pressure natriuresis is especially evident during chronic changes in sodium intake. If the kidneys and the nervous and hormonal mechanisms are functioning normally, chronic increases in intakes of salt and water to as high as six times normal are usually associated with only small increases in arterial pressure. Note that the blood pres- sure equilibrium point B on the curve is nearly the same as point A, the equilibrium point at normal salt intake. Conversely, decreases in salt and water intake to as low as one-sixth normal typically have little effect on arterial pressure. Thus, many persons are said to be salt insensi- tive because large variations in salt intake do not change blood pressure more than a few mm Hg.
Individuals with kidney injury or excessive secretion of antinatriuretic hormones such as angiotensin II or aldos- terone, however, may be salt sensitive with an attenuated renal output curve similar to the acute curve shown in Figure 19-5. In these cases, even moderate increases in salt intake may cause significant increases in arterial pressure.
Some of the factors include loss of functional nephrons due to kidney injury, or excessive formation of antinatri- uretic hormones such as angiotensin II or aldosterone. For example, surgical reduction of kidney mass or injury to the kidney due to hypertension, diabetes, and various kid- ney diseases all cause blood pressure to be more sensitive to changes in salt intake. In these instances, greater than normal increases in arterial pressure are required to raise renal output sufficiently to maintain a balance between the intake and output of salt and water.
There is some evidence that long-term high salt intake, lasting for several years, may actually damage the kid- neys and eventually make blood pressure more salt sen- sitive. We will discuss salt sensitivity of blood pressure in patients with hypertension later in this chapter.
Failure of Increased Total Peripheral Resistance to Elevate the Long-Term Level of Arterial Pressure if Fluid Intake and Renal Function Do Not Change
Now is the chance for the reader to see whether he or she really understands the renal–body fluid mechanism for arterial pressure control. Recalling the basic equation for arterial pressure—arterial pressure equals cardiac out- put times total peripheral resistance—it is clear that an increase in total peripheral resistance should elevate the arterial pressure. Indeed, when the total peripheral resis- tance is acutely increased, the arterial pressure does rise
immediately. Yet if the kidneys continue to function nor- mally, the acute rise in arterial pressure usually is not maintained. Instead, the arterial pressure returns all the way to normal within a day or so. Why?
The answer to this is the following: Increasing resis- tance in the blood vessels everywhere else in the body besides in the kidneys does not change the equilibrium point for blood pressure control as dictated by the kid- neys (see again Figures 19-3 and 19-4). Instead, the kid- neys immediately begin to respond to the high arterial pressure, causing pressure diuresis and pressure natriure- sis. Within hours, large amounts of salt and water are lost from the body, and this continues until the arterial pres- sure returns to the pressure level of the equilibrium point. At this point blood pressure is normalized and extracellu- lar fluid volume and blood volume are decreased to levels below normal.
As proof of this principle that changes in total periph- eral resistance do not affect the long-term level of arterial pressure if function of the kidneys is still normal, carefully study Figure 19-6. This figure shows the approximate car- diac outputs and the arterial pressures in different clini- cal conditions in which the long-term total peripheral resistance is either much less than or much greater than normal, but kidney excretion of salt and water is normal. Note in all these different clinical conditions that the arte- rial pressure is also exactly normal.
A word of caution is necessary at this point in our dis- cussion. Many times when the total peripheral resistance increases, this also increases the intrarenal vascular resis- tance at the same time, which alters the function of the kidney and can cause hypertension by shifting the renal
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Figure 19-6 Relations of total peripheral resistance to the long- term levels of arterial pressure and cardiac output in differ- ent clinical abnormalities. In these conditions, the kidneys were functioning normally. Note that changing the whole-body total peripheral resistance caused equal and opposite changes in cardiac output but in all cases had no effect on arterial pressure. (Redrawn from Guyton AC: Arterial Pressure and Hypertension. Philadelphia: WB Saunders, 1980.)
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function curve to a higher pressure level, in the manner shown in Figure 19-4A. We see an example of this later in this chapter when we discuss hypertension caused by vasoconstrictor mechanisms. But it is the increase in renal resistance that is the culprit, not the increased total peripheral resistance—an important distinction.
Increased Fluid Volume Can Elevate Arterial Pressure by Increasing Cardiac Output or Total Peripheral Resistance
The overall mechanism by which increased extracellular fluid volume may elevate arterial pressure, if vascular capac- ity is not simultaneously increased, is shown in Figure 19-7. The sequential events are (1) increased extracellular fluid volume (2) increases the blood volume, which (3) increases the mean circulatory filling pressure, which (4) increases venous return of blood to the heart, which (5) increases cardiac output, which (6) increases arterial pressure. The increased arterial pressure, in turn, increases real excretion of salt and water and may return extracellular fluid volume to nearly normal if kidney function is normal.
Note especially in this schema the two ways in which an increase in cardiac output can increase the arterial pressure. One of these is the direct effect of increased car- diac output to increase the pressure, and the other is an indirect effect to raise total peripheral vascular resistance
through autoregulation of blood flow. The second effect can be explained as follows.
Referring to Chapter 17, let us recall that whenever an excess amount of blood flows through a tissue, the local tissue vasculature constricts and decreases the blood flow back toward normal. This phenomenon is called “autoreg- ulation,” which means simply regulation of blood flow by the tissue itself. When increased blood volume increases the cardiac output, the blood flow increases in all tissues of the body, so this autoregulation mechanism constricts blood vessels all over the body. This in turn increases the total peripheral resistance.
Finally, because arterial pressure is equal to cardiac output times total peripheral resistance, the second- ary increase in total peripheral resistance that results from the autoregulation mechanism helps greatly in increasing the arterial pressure. For instance, only a 5 to 10 percent increase in cardiac output can increase the arterial pressure from the normal mean arterial pres- sure of 100 mm Hg up to 150 mm Hg. In fact, the slight increase in cardiac output is often not measurable.
Importance of Salt (NaCl) in the Renal–Body Fluid Schema for Arterial Pressure Regulation
Although the discussions thus far have emphasized the importance of volume in regulation of arterial pressure, experimental studies have shown that an increase in salt intake is far more likely to elevate the arterial pressure than is an increase in water intake. The reason for this is that pure water is normally excreted by the kidneys almost as rapidly as it is ingested, but salt is not excreted so easily. As salt accumulates in the body, it also indirectly increases the extracellular fluid volume for two basic reasons:
1. When there is excess salt in the extracellular fluid, the osmolality of the fluid increases, and this in turn stim- ulates the thirst center in the brain, making the person drink extra amounts of water to return the extracel- lular salt concentration to normal. This increases the extracellular fluid volume.
2. The increase in osmolality caused by the excess salt in the extracellular fluid also stimulates the hypothalamic-posterior pituitary gland secretory mechanism to secrete increased quantities of antidi- uretic hormone. (This is discussed in Chapter 28.) The antidiuretic hormone then causes the kidneys to reab- sorb greatly increased quantities of water from the renal tubular fluid, thereby diminishing the excreted volume of urine but increasing the extracellular fluid volume.
Thus, for these important reasons, the amount of salt that accumulates in the body is the main determi- nant of the extracellular fluid volume. Because only small increases in extracellular fluid and blood volume can often increase the arterial pressure greatly if the vascular capacity is not simultaneously increased, accumulation of even a small amount of extra salt in the body can lead to considerable elevation of arterial pressure.
Increased extracellular fluid volume
Increased blood volume
Increased mean circulatory filling pressure
Increased venous return of blood to the heart
Increased cardiac output
Autoregulation
Increased total peripheral resistance
Increased arterial pressure
Increased urine output
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Figure 19-7 Sequential steps by which increased extracellular fluid volume increases the arterial pressure. Note especially that increased cardiac output has both a direct effect to increase arte- rial pressure and an indirect effect by first increasing the total peripheral resistance.
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As discussed previously, raising salt intake in the absence of impaired kidney function or excessive for- mation of antinatriuretic hormones usually does not increase arterial pressure much because the kidneys rap- idly eliminate the excess salt and blood volume is hardly altered.
Chronic Hypertension (High Blood Pressure) Is Caused by Impaired Renal Fluid Excretion
When a person is said to have chronic hypertension (or “high blood pressure”), it is meant that his or her mean arterial pressure is greater than the upper range of the accepted normal measure. A mean arterial pressure greater than 110 mm Hg (normal is about 90 mm Hg) is considered to be hypertensive. (This level of mean pres- sure occurs when the diastolic blood pressure is greater than about 90 mm Hg and the systolic pressure is greater than about 135 mm Hg.) In severe hypertension, the mean arterial pressure can rise to 150 to 170 mm Hg, with dia- stolic pressure as high as 130 mm Hg and systolic pressure occasionally as high as 250 mm Hg.
Even moderate elevation of arterial pressure leads to shortened life expectancy. At severely high pressures— mean arterial pressures 50 percent or more above nor- mal—a person can expect to live no more than a few more years unless appropriately treated. The lethal effects of hypertension are caused mainly in three ways:
1. Excess workload on the heart leads to early heart fail- ure and coronary heart disease, often causing death as a result of a heart attack.
2. The high pressure frequently damages a major blood vessel in the brain, followed by death of major por- tions of the brain; this is a cerebral infarct. Clinically it is called a “stroke.” Depending on which part of
the brain is involved, a stroke can cause paralysis, dementia, blindness, or multiple other serious brain disorders.
3. High pressure almost always causes injury in the kid- neys, producing many areas of renal destruction and, eventually, kidney failure, uremia, and death.
Lessons learned from the type of hypertension called “volume-loading hypertension” have been crucial in understanding the role of the renal–body fluid volume mechanism for arterial pressure regulation. Volume- loading hypertension means hypertension caused by excess accumulation of extracellular fluid in the body, some examples of which follow.
Experimental Volume-Loading Hypertension Caused by Reduced Renal Mass Along with Simul- taneous Increase in Salt Intake. Figure 19-8 shows a typical experiment demonstrating volume-loading hypertension in a group of dogs with 70 percent of their kidney mass removed. At the first circled point on the curve, the two poles of one of the kidneys were removed, and at the second circled point, the entire opposite kidney was removed, leaving the animals with only 30 percent of normal renal mass. Note that removal of this amount of kidney mass increased the arterial pressure an average of only 6 mm Hg. Then, the dogs were given salt solution to drink instead of water. Because salt solution fails to quench the thirst, the dogs drank two to four times the normal amounts of volume, and within a few days, their average arterial pressure rose to about 40 mm Hg above normal. After 2 weeks, the dogs were given tap water again instead of salt solution; the pressure returned to normal within 2 days. Finally, at the end of the experiment, the dogs were given salt solution again, and this time the pressure
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Figure 19-8 Average effect on arterial pressure of drinking 0.9 percent saline solution instead of water in four dogs with 70 percent of their renal tissue removed. (Redrawn from Langston JB, Guyton AC, Douglas BH, et al: Effect of changes in salt intake on arterial pressure and renal function in partially nephrectomized dogs. Circ Res 12:508, 1963. By permission of the American Heart Association, Inc.)
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rose much more rapidly to an even higher level because the dogs had already learned to tolerate the salt solution and therefore drank much more. Thus, this experiment demonstrates volume-loading hypertension.
If the reader considers again the basic determinants of long-term arterial pressure regulation, he or she can immediately understand why hypertension occurred in the volume-loading experiment of Figure 19-8. First, reduction of the kidney mass to 30 percent of normal greatly reduced the ability of the kidneys to excrete salt and water. Therefore, salt and water accumulated in the body and in a few days raised the arterial pressure high enough to excrete the excess salt and water intake.
Sequential Changes in Circulatory Function During the Development of Volume-Loading Hyper- tension. It is especially instructive to study the sequential changes in circulatory function during progressive development of volume-loading hypertension. Figure 19-9 shows these sequential changes. A week or so before the point labeled “0” days, the kidney mass had already been decreased to only 30 percent of normal. Then, at this point, the intake of salt and water was increased to about six times normal and kept at this high intake thereafter. The acute effect was to increase extracellular fluid volume, blood volume, and cardiac output to 20 to 40 percent above normal. Simultaneously, the arterial pressure began to rise but not nearly so much at first as did the fluid volumes and cardiac output. The reason for this slower rise in pressure can be discerned by studying the total peripheral resistance curve, which shows an initial decrease in total peripheral resistance. This decrease was caused by the baroreceptor mechanism discussed in Chapter 18, which tried to prevent the rise in pressure. However, after 2 to 4 days, the baroreceptors adapted (reset) and were no longer able to prevent the rise in pressure. At this time, the arterial pressure had risen almost to its full height because of the increase in cardiac output, even though the total peripheral resistance was still almost at the normal level.
After these early acute changes in the circulatory vari- ables had occurred, more prolonged secondary changes occurred during the next few weeks. Especially important was a progressive increase in total peripheral resistance, while at the same time the cardiac output decreased almost all the way back to normal, mainly as a result of the long- term blood flow autoregulation mechanism that is dis- cussed in detail in Chapter 17 and earlier in this chapter. That is, after the cardiac output had risen to a high level and had initiated the hypertension, the excess blood flow through the tissues then caused progressive constriction of the local arterioles, thus returning the local blood flows in all the body tissues and also the cardiac output almost all the way back to normal, while simultaneously causing a secondary increase in total peripheral resistance.
Note, too, that the extracellular fluid volume and blood volume returned almost all the way back to normal along with the decrease in cardiac output. This resulted from
two factors: First, the increase in arteriolar resistance decreased the capillary pressure, which allowed the fluid in the tissue spaces to be absorbed back into the blood. Second, the elevated arterial pressure now caused the kid- neys to excrete the excess volume of fluid that had initially accumulated in the body.
Last, let us take stock of the final state of the circula- tion several weeks after the initial onset of volume load- ing. We find the following effects:
1. Hypertension 2. Marked increase in total peripheral resistance 3. Almost complete return of the extracellular fluid vol-
ume, blood volume, and cardiac output back to normal
Therefore, we can divide volume-loading hyperten- sion into two separate sequential stages: The first stage results from increased fluid volume causing increased cardiac output. This increase in cardiac output mediates the hypertension. The second stage in volume-loading hypertension is characterized by high blood pressure and high total peripheral resistance but return of the cardiac output so near to normal that the usual measuring tech- niques frequently cannot detect an abnormally elevated cardiac output.
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Figure 19-9 Progressive changes in important circulatory sys- tem variables during the first few weeks of volume-loading hyper- tension. Note especially the initial increase in cardiac output as the basic cause of the hypertension. Subsequently, the autoregu- lation mechanism returns the cardiac output almost to normal while simultaneously causing a secondary increase in total periph- eral resistance. (Modified from Guyton AC: Arterial Pressure and Hypertension. Philadelphia: WB Saunders, 1980.)
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Thus, the increased total peripheral resistance in vol- ume-loading hypertension occurs after the hypertension has developed and, therefore, is secondary to the hyper- tension rather than being the cause of the hypertension.
Volume-Loading Hypertension in Patients Who Have No Kidneys but Are Being Maintained on an Artificial Kidney
When a patient is maintained on an artificial kidney, it is especially important to keep the patient’s body fluid vol- ume at a normal level—that is, it is important to remove an appropriate amount of water and salt each time the patient is dialyzed. If this is not done and extracellular fluid volume is allowed to increase, hypertension almost invariably develops in exactly the same way as shown in Figure 19-9. That is, the cardiac output increases at first and causes hypertension. Then the autoregulation mech- anism returns the cardiac output back toward normal while causing a secondary increase in total peripheral resistance. Therefore, in the end, the hypertension is a high peripheral resistance type of hypertension.
Hypertension Caused by Primary Aldosteronism
Another type of volume-loading hypertension is caused by excess aldosterone in the body or, occasionally, by excesses of other types of steroids. A small tumor in one of the adrenal glands occasionally secretes large quan- tities of aldosterone, which is the condition called “pri- mary aldosteronism.” As discussed in Chapters 27 and 29, aldosterone increases the rate of reabsorption of salt and water by the tubules of the kidneys, thereby reducing the loss of these in the urine while at the same time causing an increase in blood volume and extracellular fluid volume. Consequently, hypertension occurs. And, if salt intake is increased at the same time, the hypertension becomes even greater. Furthermore, if the condition persists for months or years, the excess arterial pressure often causes pathological changes in the kidneys that make the kidneys retain even more salt and water in addition to that caused directly by the aldosterone. Therefore, the hypertension often finally becomes lethally severe.
Here again, in the early stages of this type of hyperten- sion, the cardiac output is increased, but in later stages, the cardiac output generally returns almost to normal while the total peripheral resistance becomes secondarily elevated, as explained earlier in the chapter for primary volume-loading hypertension.
The Renin-Angiotensin System: Its Role in Arterial Pressure Control
Aside from the capability of the kidneys to control arterial pressure through changes in extracellular fluid volume, the kidneys also have another powerful mechanism for controlling pressure. It is the renin-angiotensin system.
Renin is a protein enzyme released by the kidneys when the arterial pressure falls too low. In turn, it raises
the arterial pressure in several ways, thus helping to cor- rect the initial fall in pressure.
Components of the Renin-Angiotensin System
Figure 19-10 shows the functional steps by which the renin-angiotensin system helps to regulate arterial pressure.
Renin is synthesized and stored in an inactive form called prorenin in the juxtaglomerular cells (JG cells) of the kidneys. The JG cells are modified smooth muscle cells located in the walls of the afferent arterioles imme- diately proximal to the glomeruli. When the arterial pres- sure falls, intrinsic reactions in the kidneys themselves cause many of the prorenin molecules in the JG cells to split and release renin. Most of the renin enters the renal blood and then passes out of the kidneys to circulate throughout the entire body. However, small amounts of the renin do remain in the local fluids of the kidney and initiate several intrarenal functions.
Renin itself is an enzyme, not a vasoactive substance. As shown in the schema of Figure 19-10, renin acts enzy- matically on another plasma protein, a globulin called renin substrate (or angiotensinogen), to release a 10-amino acid peptide, angiotensin I. Angiotensin I has mild vaso- constrictor properties but not enough to cause significant changes in circulatory function. The renin persists in the blood for 30 minutes to 1 hour and continues to cause for- mation of still more angiotensin I during this entire time.
Within a few seconds to minutes after formation of angiotensin I, two additional amino acids are split from
Decreased arterial pressure
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Figure 19-10 Renin-angiotensin vasoconstrictor mechanism for arterial pressure control.
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the angiotensin I to form the 8-amino acid peptide angio- tensin II. This conversion occurs to a great extent in the lungs while the blood flows through the small vessels of the lungs, catalyzed by an enzyme called angiotensin con- verting enzyme that is present in the endothelium of the lung vessels. Other tissues such as the kidneys and blood vessels also contain converting enzyme and therefore form angiotensin II locally.
Angiotensin II is an extremely powerful vasoconstric- tor, and it also affects circulatory function in other ways as well. However, it persists in the blood only for 1 or 2 minutes because it is rapidly inactivated by multiple blood and tissue enzymes collectively called angiotensinases.
During its persistence in the blood, angiotensin II has two principal effects that can elevate arterial pressure. The first of these, vasoconstriction in many areas of the body, occurs rapidly. Vasoconstriction occurs intensely in the arterioles and much less so in the veins. Constriction of the arterioles increases the total peripheral resistance, thereby raising the arterial pressure, as demonstrated at the bottom of the schema in Figure 19-10. Also, the mild constriction of the veins promotes increased venous return of blood to the heart, thereby helping the heart pump against the increasing pressure.
The second principal means by which angiotensin II increases the arterial pressure is to decrease excretion of both salt and water by the kidneys. This slowly increases the extracellular fluid volume, which then increases the arterial pressure during subsequent hours and days. This long-term effect, acting through the extracellular fluid volume mechanism, is even more powerful than the acute vasoconstrictor mechanism in eventually raising the arte- rial pressure.
Rapidity and Intensity of the Vasoconstrictor Pressure Response to the Renin-Angiotensin System
Figure 19-11 shows a typical experiment demonstrating the effect of hemorrhage on the arterial pressure under two separate conditions: (1) with the renin-angiotensin system functioning and (2) without the system func- tioning (the system was interrupted by a renin-blocking antibody). Note that after hemorrhage—enough to cause
acute decrease of the arterial pressure to 50 mm Hg—the arterial pressure rose back to 83 mm Hg when the renin- angiotensin system was functional. Conversely, it rose to only 60 mm Hg when the renin-angiotensin system was blocked. This shows that the renin-angiotensin sys- tem is powerful enough to return the arterial pressure at least halfway back to normal within a few minutes after severe hemorrhage. Therefore, sometimes it can be of lifesaving service to the body, especially in circulatory shock.
Note also that the renin-angiotensin vasoconstrictor system requires about 20 minutes to become fully active. Therefore, it is somewhat slower to act for blood pressure control than are the nervous reflexes and the sympathetic norepinephrine-epinephrine system.
Effect of Angiotensin II in the Kidneys to Cause Renal Retention of Salt and Water—An Important Means for Long-Term Control of Arterial Pressure
Angiotensin II causes the kidneys to retain both salt and water in two major ways:
1. Angiotensin II acts directly on the kidneys to cause salt and water retention.
2. Angiotensin II causes the adrenal glands to secrete aldosterone, and the aldosterone in turn increases salt and water reabsorption by the kidney tubules.
Thus, whenever excess amounts of angiotensin II circulate in the blood, the entire long-term renal–body fluid mechanism for arterial pressure control automati- cally becomes set to a higher arterial pressure level than normal.
Mechanisms of the Direct Renal Effects of Angiotensin II to Cause Renal Retention of Salt and Water. Angiotensin has several direct renal effects that make the kidneys retain salt and water. One major effect is to con- strict the renal arterioles, thereby diminishing blood flow through the kidneys. The slow flow of blood reduces the pressure in the peritubular capillaries, which causes rapid reabsorption of fluid from the tubules. Angiotensin II also has important direct actions on the tubular cells them- selves to increase tubular reabsorption of sodium and water. The total result of all these effects is significant, sometimes decreasing urine output to less than one fifth of normal.
Stimulation of Aldosterone Secretion by Angiotensin II, and the Effect of Aldosterone to Increase Salt and Water Retention by the Kidneys. Angiotensin II is also one of the most powerful stimulators of aldosterone secre- tion by the adrenal glands, as we shall discuss in relation to body fluid regulation in Chapter 29 and in relation to adrenal gland function in Chapter 77. Therefore, when the renin-angiotensin system becomes activated, the rate of aldosterone secretion usually also increases; and an important subsequent function of aldosterone is to cause marked increase in sodium reabsorption by the kid- ney tubules, thus increasing the total body extracellular
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Figure 19-11 Pressure-compensating effect of the renin- angiotensin vasoconstrictor system after severe hemorrhage. (Drawn from experiments by Dr. Royce Brough.)
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fluid sodium. This increased sodium then causes water retention, as already explained, increasing the extracel- lular fluid volume and leading secondarily to still more long-term elevation of the arterial pressure.
Thus both the direct effect of angiotensin on the kidney and its effect acting through aldosterone are important in long-term arterial pressure control. However, research in our laboratory has suggested that the direct effect of angiotensin on the kidneys is perhaps three or more times as potent as the indirect effect acting through aldosterone—even though the indirect effect is the one most widely known.
Quantitative Analysis of Arterial Pressure Changes Caused by Angiotensin II. Figure 19-12 shows a quantitative analy- sis of the effect of angiotensin in arterial pressure control. This figure shows two renal output curves, as well as a line depicting a normal level of sodium intake. The left-hand renal output curve is that measured in dogs whose renin- angiotensin system had been blocked by an angiotensin- converting enzyme inhibitor drug that blocks the conversion of angiotensin I to angiotensin II. The right-hand curve was measured in dogs infused continuously with angiotensin II at a level about 2.5 times the normal rate of angiotensin for- mation in the blood. Note the shift of the renal output curve toward higher pressure levels under the influence of angio- tensin II. This shift is caused by both the direct effects of angiotensin II on the kidney and the indirect effect acting through aldosterone secretion, as explained earlier.
Finally, note the two equilibrium points, one for zero angiotensin showing an arterial pressure level of 75 mm Hg, and one for elevated angiotensin showing a pressure level of 115 mm Hg. Therefore, the effect of angiotensin to cause renal retention of salt and water can have a powerful effect in promoting chronic elevation of the arterial pressure.
Role of the Renin-Angiotensin System in Maintaining a Normal Arterial Pressure Despite Large Variations in Salt Intake
One of the most important functions of the renin-angio- tensin system is to allow a person to eat either very small or very large amounts of salt without causing great changes in either extracellular fluid volume or arte- rial pressure. This function is explained by the schema in Figure 19-13, which shows that the initial effect of increased salt intake is to elevate the extracellular fluid volume, in turn elevating the arterial pressure. Then, the increased arterial pressure causes increased blood flow through the kidneys, as well as other effects, which reduce the rate of secretion of renin to a much lower level and lead sequentially to decreased renal retention of salt and water, return of the extracellular fluid volume almost to normal, and, finally, return of the arterial pres- sure also almost to normal. Thus, the renin- angiotensin system is an automatic feedback mechanism that helps maintain the arterial pressure at or near the normal level even when salt intake is increased. Or, when salt intake is decreased below normal, exactly opposite effects take place.
To emphasize the efficacy of the renin-angiotensin system in controlling arterial pressure, when the system functions normally, the pressure rises no more than 4 to 6 mm Hg in response to as much as a 50-fold increase in salt intake. Conversely, when the renin-angiotensin sys- tem is blocked, the same increase in salt intake sometimes causes the pressure to rise 10 times the normal increase, often as much as 50 to 60 mm Hg.
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Figure 19-12 Effect of two angiotensin II levels in the blood on the renal output curve, showing regulation of the arterial pressure at an equilibrium point of 75 mm Hg when the angiotensin II level is low and at 115 mm Hg when the angiotensin II level is high.
Increased salt intake
Increased extracellular volume
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Figure 19-13 Sequential events by which increased salt intake increases the arterial pressure, but feedback decrease in activity of the renin angiotensin system returns the arterial pressure almost to the normal level.
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Types of Hypertension in Which Angiotensin Is Involved: Hypertension Caused by a Renin- Secreting Tumor or by Infusion of Angiotensin II
Occasionally a tumor of the renin-secreting juxtaglo- merular cells (the JG cells) occurs and secretes tremen- dous quantities of renin; in turn, equally large quantities of angiotensin II are formed. In all patients in whom this has occurred, severe hypertension has developed. Also, when large amounts of angiotensin II are infused continu- ously for days or weeks into animals, similar severe long- term hypertension develops.
We have already noted that angiotensin II can increase the arterial pressure in two ways:
1. By constricting the arterioles throughout the entire body, thereby increasing the total peripheral resistance and arterial pressure; this effect occurs within seconds after one begins to infuse angiotensin.
2. By causing the kidneys to retain salt and water; over a period of days, this, too, causes hypertension and is the principal cause of the long-term continuation of the elevated pressure.
“One-Kidney” Goldblatt Hypertension. When one kidney is removed and a constrictor is placed on the renal artery of the remaining kidney, as shown in Figure 19-14, the immediate effect is greatly reduced pressure in the renal artery beyond the constrictor, as demonstrated by the dashed curve in the figure. Then, within seconds or minutes, the systemic arterial pressure begins to rise and continues to rise for several days. The pressure usually rises rapidly for the first hour or so, and this is followed by a slower additional rise during the next several days. When the systemic arterial pressure reaches its new stable pressure level, the renal arterial pressure (the dashed curve in the figure) will have returned almost all the way back to normal. The hypertension produced in this way is called “one-kidney” Goldblatt hypertension in honor of Dr. Harry Goldblatt, who first studied the important quantitative features of hypertension caused by renal artery constriction.
The early rise in arterial pressure in Goldblatt hyper- tension is caused by the renin-angiotensin vasoconstric- tor mechanism. That is, because of poor blood flow through the kidney after acute constriction of the renal artery, large quantities of renin are secreted by the kidney, as demonstrated by the lowermost curve in Figure 19-14, and this increases angiotensin II and aldosterone in the blood. The angiotensin in turn raises the arterial pressure acutely. The secretion of renin rises to a peak in an hour or so but returns nearly to normal in 5 to 7 days because the renal arterial pressure by that time has also risen back to normal, so the kidney is no longer ischemic.
The second rise in arterial pressure is caused by reten- tion of salt and water by the constricted kidney (that is also stimulated by angiotensin II and aldosterone). In 5 to 7 days, the body fluid volume will have increased enough
to raise the arterial pressure to its new sustained level. The quantitative value of this sustained pressure level is deter- mined by the degree of constriction of the renal artery. That is, the aortic pressure must rise high enough so that renal arterial pressure distal to the constrictor is enough to cause normal urine output.
A similar scenario occurs in patients with stenosis of the renal artery of a single remaining kidney, as some- times occurs after a person receives a kidney transplant. Also, functional or pathological increases in resistance of the renal arterioles, due to atherosclerosis or exces- sive levels of vasoconstrictors, can cause hypertension through the same mechanisms as constriction of the main renal artery.
“Two-Kidney” Goldblatt Hypertension. Hyper- tension also can result when the artery to only one kidney is constricted while the artery to the other kidney
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Figure 19-14 Effect of placing a constricting clamp on the renal artery of one kidney after the other kidney has been removed. Note the changes in systemic arterial pressure, renal artery pressure distal to the clamp, and rate of renin secretion. The resulting hypertension is called “one-kidney” Goldblatt hypertension.
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is normal. This hypertension results from the following mechanism: The constricted kidney secretes renin and also retains salt and water because of decreased renal arterial pressure in this kidney. Then the “normal” opposite kidney retains salt and water because of the renin produced by the ischemic kidney. This renin causes formation of angiotension II and aldosterone, both of which circulate to the opposite kidney and cause it also to retain salt and water. Thus, both kidneys, but for different reasons, become salt and water retainers. Consequently, hypertension develops.
The clinical counterpart of “two-kidney Goldblatt” hypertension occurs when there is stenosis of a single renal artery, for example caused by atherosclerosis, in a person who has two kidneys.
Hypertension Caused by Diseased Kidneys That Secrete Renin Chronically. Often, patchy areas of one or both kidneys are diseased and become ischemic because of local vascular constrictions, whereas other areas of the kidneys are normal. When this occurs, almost identical effects occur as in the two-kidney type of Goldblatt hypertension. That is, the patchy ischemic kidney tissue secretes renin, and this in turn, acting through the formation of angiotensin II, causes the remaining kidney mass also to retain salt and water. Indeed, one of the most common causes of renal hypertension, especially in older persons, is such patchy ischemic kidney disease.
Other Types of Hypertension Caused by Combinations of Volume Loading and Vasoconstriction
Hypertension in the Upper Part of the Body Caused by Coarctation of the Aorta. One out of every few thousand babies is born with pathological constriction or blockage of the aorta at a point beyond the aortic arterial branches to the head and arms but proximal to the renal arteries, a condition called coarctation of the aorta. When this occurs, blood flow to the lower body is carried by multiple, small collateral arteries in the body wall, with much vascular resistance between the upper aorta and the lower aorta. As a consequence, the arterial pressure in the upper part of the body may be 40 to 50 percent higher than that in the lower body.
The mechanism of this upper-body hypertension is almost identical to that of one-kidney Goldblatt hyper- tension. That is, when a constrictor is placed on the aorta above the renal arteries, the blood pressure in both kid- neys at first falls, renin is secreted, angiotensin and aldos- terone are formed, and hypertension occurs in the upper body. The arterial pressure in the lower body at the level of the kidneys rises approximately to normal, but high pres- sure persists in the upper body. The kidneys are no longer ischemic, so secretion of renin and formation of angiotensin and aldosterone return to normal. Likewise, in coarctation of the aorta, the arterial pressure in the lower body is usually almost normal, whereas the pressure in the upper body is far higher than normal.
Role of Autoregulation in the Hypertension Caused by Aortic Coarctation. A significant feature of hypertension caused by aortic coarctation is that blood flow in the arms, where the pressure may be 40 to 60 percent above normal, is almost exactly normal. Also, blood flow in the legs, where the pressure is not elevated, is almost exactly normal. How could this be, with the pressure in the upper body 40 to 60 percent greater than in the lower body? The answer is not that there are differences in vasoconstrictor substances in the blood of the upper and lower body, because the same blood flows to both areas. Likewise, the nervous system innervates both areas of the circulation similarly, so there is no reason to believe that there is a difference in nervous control of the blood vessels. The only reasonable answer is that long-term autoregulation develops so nearly completely that the local blood flow control mechanisms have com- pensated almost 100 percent for the differences in pressure. The result is that, in both the high-pressure area and the low-pressure area, the local blood flow is controlled almost exactly in accord with the needs of the tissue and not in accord with the level of the pressure. One of the reasons these observations are so important is that they demon- strate how nearly complete the long-term autoregulation process can be.
Hypertension in Preeclampsia (Toxemia of Preg nancy). Approximately 5 to 10 percent of expectant mothers develop a syndrome called preeclampsia (also called tox- emia of pregnancy). One of the manifestations of preeclamp- sia is hypertension that usually subsides after delivery of the baby. Although the precise causes of preeclampsia are not completely understood, ischemia of the placenta and sub- sequent release by the placenta of toxic factors are believed to play a role in causing many of the manifestations of this disorder, including hypertension in the mother. Substances released by the ischemic placenta, in turn, cause dysfunction of vascular endothelial cells throughout the body, including the blood vessels of the kidneys. This endothelial dysfunc- tion decreases release of nitric oxide and other vasodila- tor substances, causing vasoconstriction, decreased rate of fluid filtration from the glomeruli into the renal tubules, impaired renal-pressure natriuresis, and development of hypertension.
Another pathological abnormality that may contrib- ute to hypertension in preeclampsia is thickening of the kidney glomerular membranes (perhaps caused by an autoimmune process), which also reduces the rate of glomerular fluid filtration. For obvious reasons, the arte- rial pressure level required to cause normal formation of urine becomes elevated, and the long-term level of arte- rial pressure becomes correspondingly elevated. These patients are especially prone to extra degrees of hyperten- sion when they have excess salt intake.
Neurogenic Hypertension. Acute neurogenic hyper tension can be caused by strong stimulation of the sympathetic nervous system. For instance, when a person becomes excited for any reason or at times during states of anxiety, the sympathetic sys- tem becomes excessively stimulated, peripheral vasoconstric- tion occurs everywhere in the body, and acute hypertension ensues.
Acute Neurogenic Hypertension Caused by Sectio ning the Baroreceptor Nerves. Another type of acute neuro- genic hypertension occurs when the nerves leading from the baroreceptors are cut or when the tractus solitarius
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hypertension is caused mainly by increased renal tubular reabsorption of salt and water due to increased sympa- thetic nerve activity and increased levels of angiotensin II and aldosterone. However, if hypertension is not effectively treated, there may also be vascular damage in the kidneys that can reduce the glomerular filtration rate and increase the severity of the hypertension. Eventually uncontrolled hypertension associated with obesity can lead to severe vascular injury and complete loss of kidney function.
Graphical Analysis of Arterial Pressure Control in Essential Hypertension. Figure 19-15 is a graphical analysis of essential hypertension. The curves of this figure are called sodium-loading renal function curves because the arterial pressure in each instance is increased very slowly, over many days or weeks, by gradually increasing the level of sodium intake. The sodium-loading type of curve can be determined by increasing the level of sodium intake to a new level every few days, then waiting for the renal output of sodium to come into balance with the intake, and at the same time recording the changes in arterial pressure.
When this procedure is used in essential hypertensive patients, two types of curves, shown to the right in Figure 19-15, can be recorded in essential hypertensive patients, one called (1) salt-insensitive hypertension and the other (2) salt-sensitive hypertension. Note in both instances that the curves are shifted to the right, to a higher pressure level than for normal people. Now, let us plot on this same graph (1) a normal level of salt intake and (2) a high level of salt intake representing 3.5 times the normal intake. In the case of the person with salt-insensitive essen- tial hypertension, the arterial pressure does not increase significantly when changing from normal salt intake to high salt intake. Conversely, in those patients who have
salt-sensitive essential hypertension, the high salt intake significantly exacerbates the hypertension.
Two additional points should be emphasized: (1) Salt sensitivity of blood pressure is not an all-or-none char- acteristic—it is a quantitative characteristic, with some individuals being more salt sensitive than others. (2) Salt sensitivity of blood pressure is not a fixed characteristic; instead, blood pressure usually becomes more salt sensi- tive as a person ages, especially after 50 or 60 years of age.
The reason for the difference between salt-insensitive essential hypertension and salt-sensitive hypertension is presumably related to structural or functional differences in the kidneys of these two types of hypertensive patients. For example, salt-sensitive hypertension may occur with different types of chronic renal disease due to gradual loss of the functional units of the kidneys (the nephrons) or to normal aging as discussed in Chapter 31. Abnormal func- tion of the renin-angiotensin system can also cause blood pressure to become salt sensitive, as discussed previously in this chapter.
Treatment of Essential Hypertension. Current guide lines for treating hypertension recommend, as a first step, lifestyle modifications that are aimed at increasing physical activity and weight loss in most patients. Unfortunately, many patients are unable to lose weight, and pharmacological treatment with antihypertensive drugs must be initiated.
Two general classes of drugs are used to treat hyper- tension: (1) vasodilator drugs that increase renal blood flow and (2) natriuretic or diuretic drugs that decrease tubular reabsorption of salt and water.
Vasodilator drugs usually cause vasodilation in many other tissues of the body, as well as in the kidneys. Different ones act in one of the following ways: (1) by inhibiting sympathetic nervous signals to the kidneys or by block- ing the action of the sympathetic transmitter substance on the renal vasculature and renal tubules, (2) by directly relaxing the smooth muscle of the renal vasculature, or (3) by blocking the action of the renin-angiotensin system on the renal vasculature or renal tubules.
Those drugs that reduce reabsorption of salt and water by the renal tubules include especially drugs that block active transport of sodium through the tubular wall; this blockage in turn also prevents the reabsorption of water, as explained earlier in the chapter. These natri- uretic or diuretic drugs are discussed in greater detail in Chapter 31.
Summary of the Integrated, Multifaceted System for Arterial Pressure Regulation
By now, it is clear that arterial pressure is regulated not by a single pressure controlling system but instead by sev- eral interrelated systems, each of which performs a spe- cific function. For instance, when a person bleeds severely
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is destroyed in each side of the medulla oblongata (these are the areas where the nerves from the carotid and aortic baroreceptors connect in the brain stem). The sudden cessation of normal nerve signals from the barorecep- tors has the same effect on the nervous pressure control mechanisms as a sudden reduction of the arterial pres- sure in the aorta and carotid arteries. That is, loss of the normal inhibitory effect on the vasomotor center caused by normal baroreceptor nervous signals allows the vaso- motor center suddenly to become extremely active and the mean arterial pressure to increase from 100 mm Hg to as high as 160 mm Hg. The pressure returns to nearly normal within about 2 days because the response of the vasomotor center to the absent baroreceptor sig- nal fades away, which is called central “resetting” of the baroreceptor pressure control mechanism. Therefore, the neurogenic hypertension caused by sectioning the barore- ceptor nerves is mainly an acute type of hypertension, not a chronic type.
Genetic Causes of Hypertension. Spontaneous heredi- tary hypertension has been observed in several strains of animals, including different strains of rats, rabbits, and at least one strain of dogs. In the strain of rats that has been studied to the greatest extent, the Okamoto spontaneously hypertensive rat strain, there is evidence that in early devel- opment of the hypertension, the sympathetic nervous sys- tem is considerably more active than in normal rats. In the later stages of this type of hypertension, structural changes have been observed in the nephrons of the kidneys: (1) increased preglomerular renal arterial resistance and (2) decreased permeability of the glomerular membranes. These structural changes could also contribute to the long- term continuance of the hypertension. In other strains of hypertensive rats, impaired renal function also has been observed.
In humans, several different gene mutations have been identified that can cause hypertension. These forms of hypertension are called monogenic hypertension because they are caused by mutation of a single gene. An interest- ing feature of these genetic disorders is that they all cause excessive salt and water reabsorption by the renal tubules. In some cases the increased reabsorption is due to gene muta- tions that directly increase transport of sodium or chloride in the renal tubular epithelial cells. In other instances, the gene mutations cause increased synthesis or activity of hormones that stimulate renal tubular salt and water reabsorption. Thus, in all monogenic hypertensive disorders discovered thus far, the final common pathway to hypertension appears to be increased salt reabsorption and expansion of extracel- lular fluid volume. Monogenic hypertension, however, is rare and all of the known forms together account for less than 1% of human hypertension.
“Primary (Essential) Hypertension”
About 90 to 95 percent of all people who have hyperten- sion are said to have “primary hypertension,” also widely known as “essential hypertension” by many clinicians. These terms mean simply that the hypertension is of unknown origin, in contrast to those forms of hyperten- sion that are secondary to known causes, such as renal artery stenosis or monogenic forms of hypertension.
In most patients, excess weight gain and sedentary lifestyle appear to play a major role in causing hyperten- sion. The majority of patients with hypertension are over- weight, and studies of different populations suggest that excess weight gain and obesity may account for as much as 65 to 75 percent of the risk for developing primary hyper- tension. Clinical studies have clearly shown the value of weight loss for reducing blood pressure in most patients with hypertension. In fact, clinical guidelines for treating hypertension recommend increased physical activity and weight loss as a first step in treating most patients with hypertension.
Some of the characteristics of primary hypertension caused by excess weight gain and obesity include:
1. Cardiac output is increased due, in part, to the addi- tional blood flow required for the extra adipose tissue. However, blood flow in the heart, kidneys, gastroin- testinal tract, and skeletal muscle also increases with weight gain due to increased metabolic rate and growth of the organs and tissues in response to their increased metabolic demands. As the hypertension is sustained for many months and years, total peripheral vascular resistance may be increased.
2. Sympathetic nerve activity, especially in the kidneys, is increased in overweight patients. The causes of increased sympathetic activity in obesity are not fully understood, but recent studies suggest that hormones, such as leptin, released from fat cells may directly stim- ulate multiple regions of the hypothalamus, which, in turn, have an excitatory influence on the vasomotor centers of the brain medulla.
3. Angiotensin II and aldosterone levels are increased two- fold to threefold in many obese patients. This may be caused partly by increased sympathetic nerve stimula- tion, which increases renin release by the kidneys and therefore formation of angiotensin II, which, in turn, stimulates the adrenal gland to secrete aldosterone.
4. The renal-pressure natriuresis mechanism is impaired, and the kidneys will not excrete adequate amounts of salt and water unless the arterial pressure is high or unless kidney function is somehow improved. In other words, if the mean arterial pressure in the essen- tial hypertensive person is 150 mm Hg, acute reduc- tion of the mean arterial pressure artificially to the normal value of 100 mm Hg (but without otherwise altering renal function except for the decreased pres- sure) will cause almost total anuria, and the person will retain salt and water until the pressure rises back to the elevated value of 150 mm Hg. Chronic reduc- tions in arterial pressure with effective antihyperten- sive therapies, however, usually do not cause marked salt and water retention by the kidneys because these therapies also improve renal-pressure natriuresis, as discussed later.
Experimental studies in obese animals and obese patients suggest that impaired renal-pressure natriuresis in obesity
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so that the pressure falls suddenly, two problems confront the pressure control system. The first is survival, that is, to return the arterial pressure immediately to a high enough level that the person can live through the acute episode. The second is to return the blood volume and arterial eventually to their normal levels so that the circulatory system can reestablish full normality, not merely back to the levels required for survival.
In Chapter 18, we saw that the first line of defense against acute changes in arterial pressure is the nervous control system. In this chapter, we have emphasized a second line of defense achieved mainly by kidney mecha- nisms for long-term control of arterial pressure. However, there are other pieces to the puzzle. Figure 19-16 helps to put these together.
Figure 19-16 shows the approximate immediate (sec- onds and minutes) and long-term (hours and days) con- trol responses, expressed as feedback gain, of eight arterial pressure control mechanisms. These mechanisms can be divided into three groups: (1) those that react rapidly, within seconds or minutes; (2) those that respond over an intermediate time period, minutes or hours; and (3) those that provide long-term arterial pressure regulation, days, months, and years. Let us see how they fit together as a total, integrated system for pressure control.
Rapidly Acting Pressure Control Mechanisms, Acting Within Seconds or Minutes. The rapidly acting pressure control mechanisms are almost entirely acute nervous reflexes or other nervous responses. Note in Figure 19-16 the three mechanisms that show responses within seconds. They are (1) the baroreceptor feedback mechanism, (2) the central nervous system ischemic
mechanism, and (3) the chemoreceptor mechanism. Not only do these mechanisms begin to react within seconds, but they are also powerful. After any acute fall in pressure, as might be caused by severe hemorrhage, the nervous mechanisms combine (1) to cause constriction of the veins and transfer of blood into the heart, (2) to cause increased heart rate and contractility of the heart to provide greater pumping capacity by the heart, and (3) to cause constriction of most peripheral arterioles to impede flow of blood out of the arteries; all these effects occur almost instantly to raise the arterial pressure back into a survival range.
When the pressure suddenly rises too high, as might occur in response to rapid transfusion of excess blood, the same control mechanisms operate in the reverse direc- tion, again returning the pressure back toward normal.
Pressure Control Mechanisms That Act After Many Minutes. Several pressure control mechanisms exhibit significant responses only after a few minutes following acute arterial pressure change. Three of these, shown in Figure 19-16, are (1) the renin-angiotensin vasoconstrictor mechanism, (2) stress-relaxation of the vasculature, and (3) shift of fluid through the tissue capillary walls in and out of the circulation to readjust the blood volume as needed.
We have already described at length the role of the renin-angiotensin vasoconstrictor system to provide a semiacute means for increasing the arterial pressure when this is necessary. The stress-relaxation mecha- nism is demonstrated by the following example: When the pressure in the blood vessels becomes too high, they become stretched and keep on stretching more and more for minutes or hours; as a result, the pressure in the ves- sels falls toward normal. This continuing stretch of the vessels, called stress-relaxation, can serve as an interme- diate-term pressure “buffer.”
The capillary fluid shift mechanism means simply that any time capillary pressure falls too low, fluid is absorbed from the tissues through the capillary membranes and into the circulation, thus building up the blood volume and increasing the pressure in the circulation. Conversely, when the capillary pressure rises too high, fluid is lost out of the circulation into the tissues, thus reducing the blood volume, as well as virtually all the pressures throughout the circulation.
These three intermediate mechanisms become mostly activated within 30 minutes to several hours. During this time, the nervous mechanisms usually become less and less effective, which explains the importance of these non- nervous, intermediate time pressure control measures.
Long-Term Mechanisms for Arterial Pressure Regulation. The goal of this chapter has been to explain the role of the kidneys in long-term control of arterial pressure. To the far right in Figure 19-16 is shown the renal–blood volume pressure control mechanism (which is the same as the renal–body fluid pressure control
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mechanism), demonstrating that it takes a few hours to begin showing significant response. Yet it eventually develops a feedback gain for control of arterial pressure nearly equal to infinity. This means that this mechanism can eventually return the arterial pressure nearly all the way back, not merely partway back, to that pressure level that provides normal output of salt and water by the kidneys. By now, the reader should be familiar with this concept, which has been the major point of this chapter.
Many factors can affect the pressure-regulating level of the renal–body fluid mechanism. One of these, shown in Figure 19-16, is aldosterone. A decrease in arterial pressure leads within minutes to an increase in aldos- terone secretion, and over the next hour or days, this plays an important role in modifying the pressure con- trol characteristics of the renal–body fluid mechanism.
Especially important is interaction of the renin- angiotensin system with the aldosterone and renal fluid mechanisms. For instance, a person’s salt intake varies tre- mendously from one day to another. We have seen in this chapter that the salt intake can decrease to as little as one- tenth normal or can increase to 10 to 15 times normal and yet the regulated level of the mean arterial pressure will change only a few mm Hg if the renin-angiotensin-aldos- terone system is fully operative. But, without a functional renin- angiotensin-aldosterone system, blood pressure becomes very sensitive to changes in salt intake.
Thus, arterial pressure control begins with the life- saving measures of the nervous pressure controls, then continues with the sustaining characteristics of the inter- mediate pressure controls, and, finally, is stabilized at the long-term pressure level by the renal–body fluid mecha- nism. This long-term mechanism in turn has multiple interactions with the renin-angiotensin- aldosterone sys- tem, the nervous system, and several other factors that
provide special blood pressure control capabilities for special purposes.
Bibliography
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Coffman TM, Crowley SD: Kidney in hypertension: Guyton redux, Hypertension 51:811, 2008.
Cowley AW Jr: Long-term control of arterial blood pressure, Physiol Rev 72:231, 1992.
Guyton AC: Arterial pressure and hypertension, Philadelphia, 1980, WB Saunders.
Guyton AC: Blood pressure control—special role of the kidneys and body fluids, Science 252:1813, 1991.
Hall JE: The kidney, hypertension, and obesity, Hypertension 41:625, 2003. Hall JE, Brands MW, Henegar JR: Angiotensin II and long-term arterial
pressure regulation: the overriding dominance of the kidney, J Am Soc Nephrol 10(Suppl 12):S258, 1999.
Hall JE, Granger JP, Hall ME, et al: Pathophysiology of hypertension. In Fuster V, O’Rourke RA, Walsh RA, et al, eds.: Hurst’s The Heart, ed 12, New York, 2008, McGraw-Hill Medical, pp 1570.
Hall JE, da Silva AA, Brandon E, et al: Pathophysiology of obesity hyperten- sion and target organ injury. In Lip GYP, Hall JE, eds.: Comprehensive Hypertension, New York, 2007, Elsevier, pp 447.
LaMarca BD, Gilbert J, Granger JP: Recent progress toward the under- standing of the pathophysiology of hypertension during preeclampsia, Hypertension 51:982, 2008.
Lohmeier TE, Hildebrandt DA, Warren S, et al: Recent insights into the inter- actions between the baroreflex and the kidneys in hypertension, Am J Physiol Regul Integr Comp Physiol 288:R828, 2005.
Oparil S, Zaman MA, Calhoun DA: Pathogenesis of hypertension, Ann Intern Med 139:761, 2003.
Reckelhoff JF, Fortepiani LA: Novel mechanisms responsible for postmeno- pausal hypertension, Hypertension 43:918, 2004.
Rossier BC, Schild L: Epithelial sodium channel: mendelian versus essential hypertension, Hypertension 52:595, 2008.
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chapter 20
Cardiac Output, Venous Return, and Their Regulation
Cardiac output is the quan- tity of blood pumped into the aorta each minute by the heart. This is also the quantity of blood that flows through the circulation. Cardiac output is one of the
most important factors that we have to consider in rela- tion to the circulation because it is the sum of the blood flows to all of the tissues of the body.
Venous return is the quantity of blood flowing from the veins into the right atrium each minute. The venous return and the cardiac output must equal each other except for a few heartbeats at a time when blood is temporarily stored in or removed from the heart and lungs.
Normal Values for Cardiac Output at Rest and During Activity
Cardiac output varies widely with the level of activity of the body. The following factors, among others, directly affect cardiac output: (1) the basic level of body metabo- lism, (2) whether the person is exercising, (3) the person’s age, and (4) size of the body.
For young, healthy men, resting cardiac output averages about 5.6 L/min. For women, this value is about 4.9 L/min. When one considers the factor of age as well—because with increasing age, body activity and mass of some tis- sues (e.g., skeletal muscle) diminish—the average cardiac output for the resting adult, in round numbers, is often stated to be about 5 L/min.
Cardiac Index Experiments have shown that the cardiac output increases approximately in proportion to the surface area of the body. Therefore, cardiac output is frequently stated in terms of the cardiac index, which is the cardiac output per square meter of body surface area. The normal human being weighing 70 kilograms has a body surface area of about 1.7 square meters, which means that the normal average cardiac index for adults is about 3 L/min/m2 of body surface area.
Effect of Age on Cardiac Output. Figure 20-1 shows the cardiac output, expressed as cardiac index, at different ages. Rising rapidly to a level greater than 4 L/min/m2 at age 10 years, the cardiac index declines to about 2.4 L/min/m2 at age 80 years. We explain later in the chapter that the cardiac output is regulated throughout life almost directly in proportion to the overall bodily metabolic activity. Therefore, the declin- ing cardiac index is indicative of declining activity or declining muscle mass with age.
Control of Cardiac Output by Venous Return—Role of the Frank-Starling Mechanism of the Heart
When one states that cardiac output is controlled by venous return, this means that it is not the heart itself that is normally the primary controller of cardiac output. Instead, it is the various factors of the peripheral circula- tion that affect flow of blood into the heart from the veins, called venous return, that are the primary controllers.
The main reason peripheral factors are usually more important than the heart itself in controlling cardiac out- put is that the heart has a built-in mechanism that nor- mally allows it to pump automatically whatever amount of blood that flows into the right atrium from the veins. This mechanism, called the Frank-Starling law of the heart, was discussed in Chapter 9. Basically, this law states that when increased quantities of blood flow into the heart, the increased blood stretches the walls of the heart chambers. As a result of the stretch, the cardiac muscle contracts with increased force, and this empties the extra blood that has entered from the systemic cir- culation. Therefore, the blood that flows into the heart is automatically pumped without delay into the aorta and flows again through the circulation.
Another important factor, discussed in Chapter 10, is that stretching the heart causes the heart to pump faster—at an increased heart rate. That is, stretch of the sinus node in the wall of the right atrium has a direct effect on the rhythmicity of the node itself to increase heart rate as much as 10 to 15 percent. In addition, the
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stretched right atrium initiates a nervous reflex called the Bainbridge reflex, passing first to the vasomotor cen- ter of the brain and then back to the heart by way of the sympathetic nerves and vagi, also to increase the heart rate.
Under most normal unstressful conditions, the cardiac output is controlled almost entirely by peripheral factors that determine venous return. However, we discuss later in the chapter that if the returning blood does become more than the heart can pump, then the heart becomes the limiting factor that determines cardiac output.
Cardiac Output Regulation Is the Sum of Blood Flow Regulation in All the Local Tissues of the Body—Tissue Metabolism Regulates Most Local Blood Flow
The venous return to the heart is the sum of all the local blood flows through all the individual tissue segments of the peripheral circulation. Therefore, it follows that car- diac output regulation is the sum of all the local blood flow regulations.
The mechanisms of local blood flow regulation were discussed in Chapter 17. In most tissues, blood flow increases mainly in proportion to each tissue’s metab- olism. For instance, local blood flow almost always increases when tissue oxygen consumption increases; this effect is demonstrated in Figure 20-2 for different levels of exercise. Note that at each increasing level of work output during exercise, the oxygen consump- tion and the cardiac output increase in parallel to each other.
To summarize, cardiac output is determined by the sum of all the various factors throughout the body that con- trol local blood flow. All the local blood flows summate
to form the venous return, and the heart automatically pumps this returning blood back into the arteries to flow around the system again.
Effect of Total Peripheral Resistance on the Long-Term Cardiac Output Level. Figure 20-3 is the same as Figure 19-6. It is repeated here to illustrate an extremely important principle in cardiac output control: Under many conditions, the long-term cardiac output level varies reciprocally with changes in total peripheral resistance, as long as the arterial pressure is unchanged. Note in Figure 20-3 that when the total peripheral resistance is exactly normal (at the 100 percent mark in the figure), the cardiac output is also normal. Then, when
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the total peripheral resistance increases above normal, the cardiac output falls; conversely, when the total peripheral resistance decreases, the cardiac output increases. One can easily understand this by reconsidering one of the forms of Ohm’s law, as expressed in Chapter 14:
Cardiac Output = Arterial Pressure
Total Peripheral Resistance
The meaning of this formula, and of Figure 20-3, is simply the following: Any time the long-term level of total peripheral resistance changes (but no other functions of the circulation change), the cardiac output changes quan- titatively in exactly the opposite direction.
The Heart Has Limits for the Cardiac Output That It Can Achieve
There are definite limits to the amount of blood that the heart can pump, which can be expressed quantitatively in the form of cardiac output curves.
Figure 20-4 demonstrates the normal cardiac output curve, showing the cardiac output per minute at each level of right atrial pressure. This is one type of cardiac function curve, which was discussed in Chapter 9. Note that the plateau level of this normal cardiac output curve is about 13 L/min, 2.5 times the normal cardiac output of about 5 L/min. This means that the normal human heart, functioning without any special stimulation, can pump an amount of venous return up to about 2.5 times the normal venous return before the heart becomes a limiting factor in the control of cardiac output.
Shown in Figure 20-4 are several other cardiac output curves for hearts that are not pumping normally. The uppermost curves are for hypereffective hearts that are
pumping better than normal. The lowermost curves are for hypoeffective hearts that are pumping at levels below normal.
Factors That Cause a Hypereffective Heart
Two types of factors can make the heart a better pump than normal: (1) nervous stimulation and (2) hypertrophy of the heart muscle.
Effect of Nervous Excitation to Increase Heart Pumping. In Chapter 9, we saw that a combination of (1) sympathetic stimulation and (2) parasympathetic inhi- bition does two things to increase the pumping effec- tiveness of the heart: (1) It greatly increases the heart rate—sometimes, in young people, from the normal level of 72 beats/min up to 180 to 200 beats/min—and (2) it increases the strength of heart contraction (which is called increased “contractility”) to twice its normal strength. Combining these two effects, maximal ner- vous excitation of the heart can raise the plateau level of the cardiac output curve to almost twice the plateau of the normal curve, as shown by the 25-L/min level of the uppermost curve in Figure 20-4.
Increased Pumping Effectiveness Caused by Heart Hypertrophy. A long-term increased workload, but not so much excess load that it damages the heart, causes the heart muscle to increase in mass and contractile strength in the same way that heavy exercise causes skeletal mus- cles to hypertrophy. For instance, it is common for the hearts of marathon runners to be increased in mass by 50 to 75 percent. This increases the plateau level of the cardiac output curve, sometimes 60 to 100 percent, and therefore allows the heart to pump much greater than usual amounts of cardiac output.
When one combines nervous excitation of the heart and hypertrophy, as occurs in marathon runners, the total effect can allow the heart to pump as much 30 to 40 L/min, about 2½ times the level that can be achieved in the average person; this increased level of pumping is one of the most important factors in determining the runner’s running time.
Factors That Cause a Hypoeffective Heart
Any factor that decreases the heart’s ability to pump blood causes hypoeffectivity. Some of the factors that can do this are the following: • Increased arterial pressure against which the heart
must pump, such as in hypertension • Inhibition of nervous excitation of the heart • Pathological factors that cause abnormal heart rhythm
or rate of heartbeat • Coronary artery blockage, causing a “heart attack” • Valvular heart disease • Congenital heart disease • Myocarditis, an inflammation of the heart muscle • Cardiac hypoxia
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Figure 20-4 Cardiac output curves for the normal heart and for hypoeffective and hypereffective hearts. (Redrawn from Guyton AC, Jones CE, Coleman TB: Circulatory Physiology: Cardiac Output and Its Regulation, 2nd ed. Philadelphia: WB Saunders, 1973.)
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Role of the Nervous System in Controlling Cardiac Output
Importance of the Nervous System in Maintaining Arterial Pressure When Peripheral Blood Vessels Are Dilated and Venous Return and Cardiac Output Increase
Figure 20-5 shows an important difference in cardiac output control with and without a functioning auto- nomic nervous system. The solid curves demonstrate the effect in the normal dog of intense dilation of the peripheral blood vessels caused by administering the drug dinitrophenol, which increased the metabolism of virtually all tissues of the body about fourfold. Note that with nervous control to keep the arterial pressure from falling, dilating all the peripheral blood vessels caused almost no change in arterial pressure but increased the cardiac output almost fourfold. However, after auto- nomic control of the nervous system had been blocked, none of the normal circulatory reflexes for maintain- ing the arterial pressure could function. Vasodilation of the vessels with dinitrophenol (dashed curves) then caused a profound fall in arterial pressure to about one- half normal, and the cardiac output rose only 1.6-fold instead of 4-fold.
Thus, maintenance of a normal arterial pressure by the nervous reflexes, by mechanisms explained in Chapter 18, is essential to achieve high cardiac outputs when the peripheral tissues dilate their vessels to increase the venous return.
Effect of the Nervous System to Increase the Arterial Pressure During Exercise. During exercise, intense increase in metabolism in active skeletal muscles acts directly on the muscle arterioles to relax them and to allow adequate oxygen and other nutrients needed
to sustain muscle contraction. Obviously, this greatly decreases the total peripheral resistance, which normally would decrease the arterial pressure as well. However, the nervous system immediately compensates. The same brain activity that sends motor signals to the muscles sends simultaneous signals into the autonomic nervous centers of the brain to excite circulatory activity, caus- ing large vein constriction, increased heart rate, and increased contractility of the heart. All these changes acting together increase the arterial pressure above nor- mal, which in turn forces still more blood flow through the active muscles.
In summary, when local tissue blood vessels dilate and thereby increase venous return and cardiac output above normal, the nervous system plays an exceedingly impor- tant role in preventing the arterial pressure from falling to disastrously low levels. In fact, during exercise, the nervous system goes even further, providing additional signals to raise the arterial pressure even above normal, which serves to increase the cardiac output an extra 30 to 100 percent.
Pathologically High or Low Cardiac Outputs
In healthy humans, the average cardiac outputs are sur- prisingly constant from one person to another. However, multiple clinical abnormalities can cause either high or low cardiac outputs. Some of the more important of these are shown in Figure 20-6.
High Cardiac Output Caused by Reduced Total Peripheral Resistance
The left side of Figure 20-6 identifies conditions that com- monly cause cardiac outputs higher than normal. One of the distinguishing features of these conditions is that they all result from chronically reduced total peripheral resistance. None of them result from excessive excitation of the heart itself, which we will explain subsequently. For the present, let us look at some of the conditions that can decrease the peripheral resistance and at the same time increase the car- diac output to above normal.
1. Beriberi. This disease is caused by insufficient quan- tity of the vitamin thiamine (vitamin B1) in the diet. Lack of this vitamin causes diminished ability of the tissues to use some cellular nutrients, and the local tis- sue blood flow mechanisms in turn cause marked com- pensatory peripheral vasodilation. Sometimes the total peripheral resistance decreases to as little as one-half normal. Consequently, the long-term levels of venous return and cardiac output also often increase to twice normal.
2. Arteriovenous fistula (shunt). Earlier, we pointed out that whenever a fistula (also called an AV shunt) occurs between a major artery and a major vein, tremendous amounts of blood flow directly from the artery into the vein. This, too, greatly decreases the total peripheral resistance and, likewise, increases the venous return and cardiac output.
C ar
d ia
c o
u tp
u t
( L
/m in
)
0
2 3
4
5 6
A rt
er ia
l p re
ss u
re (
m m
H g
)
0
50
75
100
With nervous control
Dinitrophenol
Without nervous control
0 10 20 30
Minutes
Figure 20-5 Experiment in a dog to demonstrate the importance of nervous maintenance of the arterial pressure as a prerequi- site for cardiac output control. Note that with pressure control, the metabolic stimulant dinitrophenol increases cardiac output greatly; without pressure control, the arterial pressure falls and the cardiac output rises very little. (Drawn from experiments by Dr. M. Banet.)
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3. Hyperthyroidism. In hyperthyroidism, the metabolism of most tissues of the body becomes greatly increased. Oxygen usage increases, and vasodilator products are released from the tissues. Therefore, the total peripheral resistance decreases markedly because of the local tissue blood flow control reactions throughout the body; con- sequently, the venous return and cardiac output often increase to 40 to 80 percent above normal.
4. Anemia. In anemia, two peripheral effects greatly decrease the total peripheral resistance. One of these is reduced viscosity of the blood, resulting from the decreased concentration of red blood cells. The other is diminished delivery of oxygen to the tissues, which causes local vasodilation. As a consequence, the cardiac output increases greatly.
Any other factor that decreases the total peripheral resis- tance chronically also increases the cardiac output if arterial pressure does not decrease too much.
Low Cardiac Output
Figure 20-6 shows at the far right several conditions that cause abnormally low cardiac output. These conditions fall into two categories: (1) those abnormalities that cause the pumping effectiveness of the heart to fall too low and (2) those that cause venous return to fall too low.
Decreased Cardiac Output Caused by Cardiac Factors. Whenever the heart becomes severely dam- aged, regardless of the cause, its limited level of pump- ing may fall below that needed for adequate blood flow to the tissues. Some examples of this include (1) severe coronary blood vessel blockage and consequent myocar- dial infarction, (2) severe valvular heart disease, (3) myo- carditis, (4) cardiac tamponade, and (5) cardiac metabolic
derangements. The effects of several of these are shown on the right in Figure 20-6, demonstrating the low cardiac outputs that result.
When the cardiac output falls so low that the tissues throughout the body begin to suffer nutritional defi- ciency, the condition is called cardiac shock. This is dis- cussed fully in Chapter 22 in relation to cardiac failure.
Decrease in Cardiac Output Caused by Noncardiac Peripheral Factors—Decreased Venous Return. Anything that interferes with venous return also can lead to decreased cardiac output. Some of these fac- tors are the following:
1. Decreased blood volume. By far, the most common noncardiac peripheral factor that leads to decreased cardiac output is decreased blood volume, result- ing most often from hemorrhage. It is clear why this condition decreases the cardiac output: Loss of blood decreases the filling of the vascular system to such a low level that there is not enough blood in the periph- eral vessels to create peripheral vascular pressures high enough to push the blood back to the heart.
2. Acute venous dilation. On some occasions, the peripheral veins become acutely vasodilated. This results most often when the sympathetic nervous system suddenly becomes inactive. For instance, fainting often results from sudden loss of sympathetic nervous system activity, which causes the peripheral capacitative vessels, especially the veins, to dilate markedly. This decreases the filling pressure of the vascular system because the blood volume can no lon- ger create adequate pressure in the now flaccid peripheral blood vessels. As a result, the blood “pools” in the vessels and does not return to the heart.
B er
ib er
i ( 5)
A V
s h
u n
ts (
33 )
H yp
er th
yr o
id is
m (
29 )
A n
em ia
( 75
)
A n
xi et
y (2
1)
P u
lm o
n ar
y d
is ea
se (
29 )
P re
g n
an cy
( 46
)
P ag
et ’s
d is
ea se
( 9)
C o
n tr
o l (
yo u
n g
a d
u lt
s) (
30 8)
H yp
er te
n si
o n
( 47
)
M ild
v al
ve d
is ea
se (
31 )
M yo
ca rd
ia l i
n fa
rc ti
o n
( 22
)
M ild
s h
o ck
( 4)
S ev
er e
va lv
e d
is ea
se (
29 )
Tr au
m at
ic s
h o
ck (
4)
C ar
d ia
c sh
o ck
( 7)
Control (young adults)
Average 45-year-old adult
200
175
150
125
100
75
50
25
0
C ar
d ia
c o
u tp
u t
(p er
ce n
t o
f co
n tr
o l)
7
6
5
4
3
2
1
0
C ar
d ia
c in
d ex
(L /m
in /m
2 )
Figure 20-6 Cardiac output in different pathological conditions. The numbers in parentheses indicate number of patients studied in each condition. (Redrawn from Guyton AC, Jones CE, Coleman TB: Circulatory Physiology: Cardiac Output and Its Regulation, 2nd ed. Philadelphia: WB Saunders, 1973.)
Unit IV The Circulation
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3. Obstruction of the large veins. On rare occasions, the large veins leading into the heart become obstructed, so the blood in the peripheral vessels cannot flow back into the heart. Consequently, the cardiac output falls markedly.
4. Decreased tissue mass, especially decreased skeletal muscle mass. With normal aging or with prolonged periods of physical inactivity, there is usually a reduc- tion in the size of the skeletal muscles. This, in turn, decreases the total oxygen consumption and blood flow needs of the muscles, resulting in decreases in skeletal muscle blood flow and cardiac output.
5. Decreased metabolic rate of the tissues. If tissue meta- bolic rate is reduced, such as occurs in skeletal muscle during prolonged bed rest, the oxygen consumption and nutrition needs of the tissues will also be lower. This decreases blood flow to the tissues, resulting in reduced cardiac output. Other conditions, such as hypothyroidism, may also reduce metabolic rate and therefore tissue blood flow and cardiac output. Regardless of the cause of low cardiac output, whether
it be a peripheral factor or a cardiac factor, if ever the cardiac output falls below that level required for ade- quate nutrition of the tissues, the person is said to suffer circulatory shock. This condition can be lethal within a few minutes to a few hours. Circulatory shock is such an important clinical problem that it is discussed in detail in Chapter 24.
A More Quantitative Analysis of Cardiac Output Regulation
Our discussion of cardiac output regulation thus far is adequate for understanding the factors that control car- diac output in most simple conditions. However, to understand cardiac output regulation in especially stress- ful situations, such as the extremes of exercise, cardiac failure, and circulatory shock, a more complex quantita- tive analysis is presented in the following sections.
To perform the more quantitative analysis, it is nec- essary to distinguish separately the two primary factors concerned with cardiac output regulation: (1) the pump- ing ability of the heart, as represented by cardiac output curves, and (2) the peripheral factors that affect flow of blood from the veins into the heart, as represented by venous return curves. Then one can put these curves together in a quantitative way to show how they inter- act with each other to determine cardiac output, venous return, and right atrial pressure at the same time.
Cardiac Output Curves Used in the Quantitative Analysis
Some of the cardiac output curves used to depict quan- titative heart pumping effectiveness have already been shown in Figure 20-4. However, an additional set of curves is required to show the effect on cardiac output caused by changing external pressures on the outside of the heart, as explained in the next section.
Effect of External Pressure Outside the Heart on Cardiac Output Curves. Figure 20-7 shows the effect of changes in external cardiac pressure on the cardiac out- put curve. The normal external pressure is equal to the normal intrapleural pressure (the pressure in the chest cavity), which is −4 mm Hg. Note in the figure that a rise in intrapleural pressure, to −2 mm Hg, shifts the entire cardiac output curve to the right by the same amount. This shift occurs because to fill the cardiac chambers with blood requires an extra 2 mm Hg right atrial pres- sure to overcome the increased pressure on the outside of the heart. Likewise, an increase in intrapleural pres- sure to +2 mm Hg requires a 6 mm Hg increase in right atrial pressure from the normal −4 mm Hg, which shifts the entire cardiac output curve 6 mm Hg to the right.
Some of the factors that can alter the external pressure on the heart and thereby shift the cardiac output curve are the following:
1. Cyclical changes of intrapleural pressure during res- piration, which are about ±2 mm Hg during normal breathing but can be as much as ±50 mm Hg during strenuous breathing.
2. Breathing against a negative pressure, which shifts the curve to a more negative right atrial pressure (to the left).
3. Positive pressure breathing, which shifts the curve to the right.
4. Opening the thoracic cage, which increases the intra- pleural pressure to 0 mm Hg and shifts the cardiac out- put curve to the right 4 mm Hg.
5. Cardiac tamponade, which means accumulation of a large quantity of fluid in the pericardial cavity around the heart with resultant increase in external cardiac pressure and shifting of the curve to the right. Note in Figure 20-7 that cardiac tamponade shifts the upper parts of the curves farther to the right than the lower parts because the external “tamponade” pressure rises to higher values as the chambers of the heart fill to increased volumes during high cardiac output.
In tr
ap le
ur al
p re
ss ur
e =
–5 .5
m m Hg
In
tra pl
eu ra
l p re
ss ur
e =
–2 m
m
H g
I nt
ra pl
eu ra
l p re
ss ur
e =
+2 m
m H
g
N or
m al
( in
tr ap
le ur
al p
re ss
ur
e = –4)
C ar
d ia
c o
u tp
u t
(L /m
in )
0 –4
Right atrial pressure (mm Hg)
5
10
15
0 +4 +8 +12
Cardiac tamponade
Figure 20-7 Cardiac output curves at different levels of intra- pleural pressure and at different degrees of cardiac tamponade. (Redrawn from Guyton AC, Jones CE, Coleman TB: Circulatory Physiology: Cardiac Output and Its Regulation, 2nd ed. Philadelphia: WB Saunders, 1973.)
Chapter 20 Cardiac Output, Venous Return, and Their Regulation
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Combinations of Different Patterns of Cardiac Output Curves. Figure 20-8 shows that the final car- diac output curve can change as a result of simultaneous changes in (a) external cardiac pressure and (b) effective- ness of the heart as a pump. For example, the combina- tion of a hypereffective heart and increased intrapleural pressure would lead to increased maximum level of car- diac output due to the increased pumping capability of the heart but the cardiac output curve would be shifted to the right (to higher atrial pressures) due to the increased intrapleural pressure. Thus, by knowing what is happen- ing to the external pressure, as well as to the capability of the heart as a pump, one can express the momentary abil- ity of the heart to pump blood by a single cardiac output curve.
Venous Return Curves
There remains the entire systemic circulation that must be considered before total analysis of cardiac regulation can be achieved. To analyze the function of the systemic circulation, we first remove the heart and lungs from the circulation of an animal and replace them with a pump and artificial oxygenator system. Then, different factors, such as blood volume, vascular resistances, and central venous pressure in the right atrium, are altered to deter- mine how the systemic circulation operates in different circulatory states. In these studies, one finds three princi- pal factors that affect venous return to the heart from the systemic circulation. They are as follows:
1. Right atrial pressure, which exerts a backward force on the veins to impede flow of blood from the veins into the right atrium.
2. Degree of filling of the systemic circulation (measured by the mean systemic filling pressure), which forces the sys- temic blood toward the heart (this is the pressure mea- sured everywhere in the systemic circulation when all flow of blood is stopped and is discussed in detail later).
3. Resistance to blood flow between the peripheral vessels and the right atrium. These factors can all be expressed quantitatively by the
venous return curve, as we explain in the next sections.
Normal Venous Return Curve
In the same way that the cardiac output curve relates pumping of blood by the heart to right atrial pressure, the venous return curve relates venous return also to right atrial pressure—that is, the venous flow of blood into the heart from the systemic circulation at different levels of right atrial pressure.
The curve in Figure 20-9 is the normal venous return curve. This curve shows that when heart pumping capability becomes diminished and causes the right atrial pressure to rise, the backward force of the ris- ing atrial pressure on the veins of the systemic circu- lation decreases venous return of blood to the heart. If all nervous circulatory reflexes are prevented from act- ing, venous return decreases to zero when the right atrial pressure rises to about +7 mm Hg. Such a slight rise in right atrial pressure causes a drastic decrease in venous return because the systemic circulation is a distensible bag, so any increase in back pressure causes blood to dam up in this bag instead of returning to the heart.
At the same time that the right atrial pressure is ris- ing and causing venous stasis, pumping by the heart also approaches zero because of decreasing venous return. Both the arterial and the venous pressures come to equi- librium when all flow in the systemic circulation ceases at a pressure of 7 mm Hg, which, by definition, is the mean systemic filling pressure (Psf ).
Plateau in the Venous Return Curve at Negative Atrial Pressures Caused by Collapse of the Large Veins. When the right atrial pressure falls below zero—that is, below atmospheric pressure—further increase in venous return almost ceases. And by the time the right atrial pressure has fallen to about −2 mm Hg, the venous return will have reached a plateau. It remains at this plateau level even though the right atrial pressure falls to −20 mm Hg, −50 mm Hg, or even further. This plateau is caused by collapse of the veins entering the chest. Negative pressure
C ar
d ia
c o
u tp
u t
(L /m
in )
0 –4
Right atrial pressure (mm Hg)
5
10
15
0 +4 +8 +12
Normal
Hypereffective + increased intrapleural pressure
Hypoeffective + reduced intrapleural pressure
Figure 20-8 Combinations of two major patterns of cardiac output curves showing the effect of alterations in both extracar- diac pressure and effectiveness of the heart as a pump. (Redrawn from Guyton AC, Jones CE, Coleman TB: Circulatory Physiology: Cardiac Output and Its Regulation, 2nd ed. Philadelphia: WB Saunders, 1973.)
V en
o u
s re
tu rn
( L
/m in
)
0
5
–8 –4 0 +4 +8
Transitional zonePlateau
Down slope
Mean systemic
filling pressure
Right atrial pressure (mm Hg)
Figure 20-9 Normal venous return curve. The plateau is caused by collapse of the large veins entering the chest when the right atrial pressure falls below atmospheric pressure. Note also that venous return becomes zero when the right atrial pressure rises to equal the mean systemic filling pressure.
Unit IV The Circulation
236
in the right atrium sucks the walls of the veins together where they enter the chest, which prevents any additional flow of blood from the peripheral veins. Consequently, even very negative pressures in the right atrium cannot increase venous return significantly above that which exists at a normal atrial pressure of 0 mm Hg.
Mean Circulatory Filling Pressure and Mean Systemic Filling Pressure, and Their Effect on Venous Return
When heart pumping is stopped by shocking the heart with electricity to cause ventricular fibrillation or is stop ped in any other way, flow of blood everywhere in the circulation ceases a few seconds later. Without blood flow, the pressures everywhere in the circulation become equal. This equilibrated pressure level is called the mean circulatory filling pressure.
Effect of Blood Volume on Mean Circulatory Filling Pressure. The greater the volume of blood in the circu- lation, the greater is the mean circulatory filling pres- sure because extra blood volume stretches the walls of the vasculature. The red curve in Figure 20-10 shows the approximate normal effect of different levels of blood volume on the mean circulatory filling pressure. Note that at a blood volume of about 4000 milliliters, the mean circulatory filling pressure is close to zero because this is the “unstressed volume” of the circula- tion, but at a volume of 5000 milliliters, the filling pres- sure is the normal value of 7 mm Hg. Similarly, at still higher volumes, the mean circulatory filling pressure increases almost linearly.
Effect of Sympathetic Nervous Stimulation of the Circulation on Mean Circulatory Filling Pressure. The green curve and blue curve in Figure 20-10 show the effects, respectively, of high and low levels of sympathetic nervous activity on the mean circulatory filling pressure.
Strong sympathetic stimulation constricts all the systemic blood vessels, as well as the larger pulmonary blood ves- sels and even the chambers of the heart. Therefore, the capacity of the system decreases so that at each level of blood volume, the mean circulatory filling pressure is increased. At normal blood volume, maximal sympa- thetic stimulation increases the mean circulatory filling pressure from 7 mm Hg to about 2.5 times that value, or about 17 mm Hg.
Conversely, complete inhibition of the sympathetic nervous system relaxes both the blood vessels and the heart, decreasing the mean circulatory filling pres- sure from the normal value of 7 mm Hg down to about 4 mm Hg. Before leaving Figure 20-10, note specifically how steep the curves are. This means that even slight changes in blood volume or slight changes in the capac- ity of the system caused by various levels of sympathetic activity can have large effects on the mean circulatory filling pressure.
Mean Systemic Filling Pressure and Its Relation to Mean Circulatory Filling Pressure. The mean systemic filling pressure, Psf, is slightly different from the mean circulatory filling pressure. It is the pressure measured everywhere in the systemic circulation after blood flow has been stopped by clamping the large blood vessels at the heart, so the pressures in the systemic circula- tion can be measured independently from those in the pulmonary circulation. The mean systemic pres- sure, although almost impossible to measure in the liv- ing animal, is the important pressure for determining venous return. The mean systemic filling pressure, how- ever, is almost always nearly equal to the mean circula- tory filling pressure because the pulmonary circulation has less than one eighth as much capacitance as the systemic circulation and only about one tenth as much blood volume.
Effect on the Venous Return Curve of Changes in Mean Systemic Filling Pressure. Figure 20-11 shows the effects on the venous return curve caused by increasing or decreasing the mean systemic filling pressure (Psf ). Note in Figure 20-11 that the normal mean systemic fill- ing pressure is 7 mm Hg. Then, for the uppermost curve
0
2
4
6
8
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14
0 1000 2000 3000 4000 5000 6000 7000
Volume (ml)
M ea
n c
ir cu
la to
ry f
ill in
g p
re ss
u re
( m
m H
g )
Strong sympathetic stimulation
Normal circulatory system Complete sympathetic inhibition
Normal volume
Figure 20-10 Effect of changes in total blood volume on the mean circulatory filling pressure (i.e., “volume-pressure curves” for the entire circulatory system). These curves also show the effects of strong sympathetic stimulation and complete sympathetic inhibition.
V en
o u
s re
tu rn
( L
/m in
)
–4 0 +4 +8 +12 0
5
10
Right atrial pressure (mm Hg)
Normal
Psf = 3.5
Psf = 7 Psf = 14
Figure 20-11 Venous return curves showing the normal curve when the mean systemic filling pressure (Psf) is 7 mm Hg and the effect of altering the Psf to either 3.5 or 14 mm Hg. (Redrawn from Guyton AC, Jones CE, Coleman TB: Circulatory Physiology: Cardiac Output and Its Regulation, 2nd ed. Philadelphia: WB Saunders, 1973.)
Chapter 20 Cardiac Output, Venous Return, and Their Regulation
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in the figure, the mean systemic filling pressure has been increased to 14 mm Hg, and for the lowermost curve, has been decreased to 3.5 mm Hg. These curves demonstrate that the greater the mean systemic filling pressure (which also means the greater the “tightness” with which the cir- culatory system is filled with blood), the more the venous return curve shifts upward and to the right. Conversely, the lower the mean systemic filling pressure, the more the curve shifts downward and to the left.
To express this another way, the greater the system is filled, the easier it is for blood to flow into the heart. The less the filling, the more difficult it is for blood to flow into the heart.
“Pressure Gradient for Venous Return”—When This Is Zero, There Is No Venous Return. When the right atrial pressure rises to equal the mean systemic filling pressure, there is no longer any pressure difference between the peripheral vessels and the right atrium. Consequently, there can no longer be any blood flow from any periph- eral vessels back to the right atrium. However, when the right atrial pressure falls progressively lower than the mean systemic filling pressure, the flow to the heart increases proportionately, as one can see by studying any of the venous return curves in Figure 20-11. That is, the greater the difference between the mean systemic filling pressure and the right atrial pressure, the greater becomes the venous return. Therefore, the difference between these two pressures is called the pressure gradient for venous return.
Resistance to Venous Return
In the same way that mean systemic filling pressure repre- sents a pressure pushing venous blood from the periphery toward the heart, there is also resistance to this venous flow of blood. It is called the resistance to venous return. Most of the resistance to venous return occurs in the veins, although some occurs in the arterioles and small arteries as well.
Why is venous resistance so important in determin- ing the resistance to venous return? The answer is that when the resistance in the veins increases, blood begins to be dammed up, mainly in the veins themselves. But the venous pressure rises very little because the veins are highly distensible. Therefore, this rise in venous pres- sure is not very effective in overcoming the resistance, and blood flow into the right atrium decreases drasti- cally. Conversely, when arteriolar and small artery resis- tances increase, blood accumulates in the arteries, which have a capacitance only one thirtieth as great as that of the veins. Therefore, even slight accumulation of blood in the arteries raises the pressure greatly—30 times as much as in the veins—and this high pressure does over- come much of the increased resistance. Mathematically, it turns out that about two thirds of the so-called “resis- tance to venous return” is determined by venous resis- tance, and about one third by the arteriolar and small artery resistance.
Venous return can be calculated by the following formula:
VR = Psf-PRA
RVR
in which VR is venous return, Psf is mean systemic filling pressure, PRA is right atrial pressure, and RVR is resis- tance to venous return. In the healthy human adult, the values for these are as follows: venous return equals 5 L/ min, mean systemic filling pressure equals 7 mm Hg, right atrial pressure equals 0 mm Hg, and resistance to venous return equals 1.4 mm Hg per L/min of blood flow.
Effect of Resistance to Venous Return on the Venous Return Curve. Figure 20-12 demonstrates the effect of dif- ferent levels of resistance to venous return on the venous return curve, showing that a decrease in this resistance to one-half normal allows twice as much flow of blood and, therefore, rotates the curve upward to twice as great a slope. Conversely, an increase in resistance to twice normal rotates the curve downward to one half as great a slope.
Note also that when the right atrial pressure rises to equal the mean systemic filling pressure, venous return becomes zero at all levels of resistance to venous return because when there is no pressure gradient to cause flow of blood, it makes no difference what the resistance is in the circulation; the flow is still zero. Therefore, the highest level to which the right atrial pressure can rise, regardless of how much the heart might fail, is equal to the mean systemic filling pressure.
Combinations of Venous Return Curve Patterns. Figure 20-13 shows effects on the venous return curve caused by simultaneous changes in mean systemic pres- sure (Psf ) and resistance to venous return, demonstrat- ing that both these factors can operate simultaneously.
V en
o u
s re
tu rn
( L
/m in
)
–4 0 +4 +8 0
5
10
15
20
Right atrial pressure (mm Hg)
1/2 resistanceNormal resistance 2 resistance
Psf = 7
Figure 20-12 Venous return curves depicting the effect of alter- ing the “resistance to venous return.” Psf, mean systemic fill- ing pressure. (Redrawn from Guyton AC, Jones CE, Coleman TB: Circulatory Physiology: Cardiac Output and Its Regulation, 2nd ed. Philadelphia: WB Saunders, 1973.)
Unit IV The Circulation
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Analysis of Cardiac Output and Right Atrial Pressure Using Simultaneous Cardiac Output and Venous Return Curves
In the complete circulation, the heart and the systemic circulation must operate together. This means that (1) the venous return from the systemic circulation must equal the cardiac output from the heart and (2) the right atrial pressure is the same for both the heart and the systemic circulation.
Therefore, one can predict the cardiac output and right atrial pressure in the following way: (1) Determine the momentary pumping ability of the heart and depict this in the form of a cardiac output curve; (2) determine the momentary state of flow from the systemic circulation into the heart and depict this in the form of a venous return curve; and (3) “equate” these curves against each other, as shown in Figure 20-14.
Two curves in the figure depict the normal cardiac out- put curve (red line) and the normal venous return curve (blue line). There is only one point on the graph, point A, at which the venous return equals the cardiac output and at which the right atrial pressure is the same for both the heart and the systemic circulation. Therefore, in the nor- mal circulation, the right atrial pressure, cardiac output, and venous return are all depicted by point A, called the equilibrium point, giving a normal value for cardiac out- put of 5 L/min and a right atrial pressure of 0 mm Hg.
Effect of Increased Blood Volume on Cardiac Output. A sudden increase in blood volume of about 20 percent increases the cardiac output to about 2.5 to 3 times normal. An analysis of this effect is shown in Figure 20-14. Immediately on infusing the large quantity of extra blood, the increased filling of the system causes the mean systemic filling pressure (Psf ) to increase to 16 mm Hg, which shifts the venous return curve to the right. At the
same time, the increased blood volume distends the blood vessels, thus reducing their resistance and thereby reduc- ing the resistance to venous return, which rotates the curve upward. As a result of these two effects, the venous return curve of Figure 20-14 is shifted to the right. This new curve equates with the cardiac output curve at point B, showing that the cardiac output and venous return increase 2.5 to 3 times, and that the right atrial pressure rises to about +8 mm Hg.
Further Compensatory Effects Initiated in Response to Increased Blood Volume. The greatly increased car- diac output caused by increased blood volume lasts for only a few minutes because several compensatory effects immediately begin to occur: (1) The increased car- diac output increases the capillary pressure so that fluid begins to transude out of the capillaries into the tissues, thereby returning the blood volume toward normal. (2) The increased pressure in the veins causes the veins to continue distending gradually by the mechanism called stress-relaxation, especially causing the venous blood reservoirs, such as the liver and spleen, to distend, thus reducing the mean systemic pressure. (3) The excess blood flow through the peripheral tissues causes autoregula- tory increase in the peripheral vascular resistance, thus increasing the resistance to venous return. These factors cause the mean systemic filling pressure to return back toward normal and the resistance vessels of the sys- temic circulation to constrict. Therefore, gradually, over a period of 10 to 40 minutes, the cardiac output returns almost to normal.
Effect of Sympathetic Stimulation on Cardiac Output. Sympathetic stimulation affects both the heart and the systemic circulation: (1) It makes the heart a stronger pump. (2) In the systemic circulation, it increases
V en
o u
s re
tu rn
( L
/m in
)
–4 0 +4 +8 +12 0
5
10
15
Right atrial pressure (mm Hg)
Normal resistance
2 resistance
1/2 resistance
1/3 resistance
Psf = 10
Psf = 7 Psf = 2.3
Psf = 10.5
Figure 20-13 Combinations of the major patterns of venous return curves, showing the effects of simultaneous changes in mean systemic filling pressure (Psf) and in “resistance to venous return.” (Redrawn from Guyton AC, Jones CE, Coleman TB: Circulatory Physiology: Cardiac Output and Its Regulation, 2nd ed. Philadelphia: WB Saunders, 1973.)
C ar
d ia
c o
u tp
u t
an d
v en
o u
s re
tu rn
( L
/m in
)
−4 0 +4 +8 +12 +16 0
5
10
15
20
Right atrial pressure (mm Hg)
B
A Psf = 7 Psf = 16
Figure 20-14 The two solid curves demonstrate an analysis of car- diac output and right atrial pressure when the cardiac output (red line) and venous return (blue line) curves are normal. Transfusion of blood equal to 20 percent of the blood volume causes the venous return curve to become the dashed curve; as a result, the cardiac output and right atrial pressure shift from point A to point B. Psf, mean systemic filling pressure.
Chapter 20 Cardiac Output, Venous Return, and Their Regulation
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the mean systemic filling pressure because of contrac- tion of the peripheral vessels, especially the veins, and it increases the resistance to venous return.
In Figure 20-15, the normal cardiac output and venous return curves are depicted; these equate with each other at point A, which represents a normal venous return and cardiac output of 5 L/min and a right atrial pressure of 0 mm Hg. Note in the figure that maximal sympathetic stimulation (green curves) increases the mean systemic filling pressure to 17 mm Hg (depicted by the point at which the venous return curve reaches the zero venous return level). And the sympathetic stimulation also increases pumping effectiveness of the heart by nearly 100 percent. As a result, the cardiac output rises from the normal value at equilibrium point A to about double normal at equilibrium point D—and yet the right atrial pressure hardly changes. Thus, different degrees of sym- pathetic stimulation can increase the cardiac output pro- gressively to about twice normal for short periods of time, until other compensatory effects occur within seconds or minutes.
Effect of Sympathetic Inhibition on Cardiac Output. The sympathetic nervous system can be blocked by inducing total spinal anesthesia or by using some drug, such as hexamethonium, that blocks trans- mission of nerve signals through the autonomic ganglia. The lowermost curves in Figure 20-15 show the effect of sympathetic inhibition caused by total spinal anesthesia, demonstrating that (1) the mean systemic filling pressure falls to about 4 mm Hg and (2) the effectiveness of the heart as a pump decreases to about 80 percent of normal. The cardiac output falls from point A to point B, which is a decrease to about 60 percent of normal.
Effect of Opening a Large Arteriovenous Fistula. Figure 20-16 shows various stages of circulatory changes that occur after opening a large arteriovenous fistula, that is, after making an opening directly between a large artery and a large vein.
1. The two red curves crossing at point A show the nor- mal condition.
2. The curves crossing at point B show the circulatory condition immediately after opening the large fistula. The principal effects are (1) a sudden and precipitous rotation of the venous return curve upward caused by the large decrease in resistance to venous return when blood is allowed to flow with almost no impediment directly from the large arteries into the venous sys- tem, bypassing most of the resistance elements of the peripheral circulation, and (2) a slight increase in the level of the cardiac output curve because opening the fistula decreases the peripheral resistance and allows an acute fall in arterial pressure against which the heart can pump more easily. The net result, depicted by point B, is an increase in cardiac output from 5 L/ min up to 13 L/min and an increase in right atrial pres- sure to about +3 mm Hg.
3. Point C represents the effects about 1 minute later, after the sympathetic nerve reflexes have restored the arte- rial pressure almost to normal and caused two other effects: (1) an increase in the mean systemic filling pressure (because of constriction of all veins and arter- ies) from 7 to 9 mm Hg, thus shifting the venous return
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Spinal anesthesia
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Figure 20-15 Analysis of the effect on cardiac output of (1) moderate sympathetic stimulation (from point A to point C), (2) maximal sympathetic stimulation (point D), and (3) sympathetic inhibition caused by total spinal anesthesia (point B). (Redrawn from Guyton AC, Jones CE, Coleman TB: Circulatory Physiology: Cardiac Output and Its Regulation, 2nd ed. Philadelphia: WB Saunders, 1973.)
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Figure 20-16 Analysis of successive changes in cardiac output and right atrial pressure in a human being after a large arterio- venous (AV) fistula is suddenly opened. The stages of the analysis, as shown by the equilibrium points, are A, normal conditions; B, immediately after opening the AV fistula; C, 1 minute or so after the sympathetic reflexes have become active; and D, several weeks after the blood volume has increased and the heart has begun to hypertrophy. (Redrawn from Guyton AC, Jones CE, Coleman TB: Circulatory Physiology: Cardiac Output and Its Regulation, 2nd ed. Philadelphia: WB Saunders, 1973.)
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curve 2 mm Hg to the right, and (2) further elevation of the cardiac output curve because of sympathetic ner- vous excitation of the heart. The cardiac output now rises to almost 16 L/min, and the right atrial pressure to about 4 mm Hg.
4. Point D shows the effect after several more weeks. By this time, the blood volume has increased because the slight reduction in arterial pressure and the sympathetic stimulation have both reduced kidney output of urine. The mean systemic filling pressure has now risen to +12 mm Hg, shifting the venous return curve another 3 mm Hg to the right. Also, the prolonged increased workload on the heart has caused the heart muscle to hypertrophy slightly, raising the level of the cardiac out- put curve still further. Therefore, point D shows a car- diac output now of almost 20 L/min and a right atrial pressure of about 6 mm Hg.
Other Analyses of Cardiac Output Regulation. In Chapter 21, analysis of cardiac output regulation during exercise is presented, and in Chapter 22, analyses of car- diac output regulation at various stages of congestive heart failure are shown.
Methods for Measuring Cardiac Output
In animal experiments, one can cannulate the aorta, pulmo- nary artery, or great veins entering the heart and measure the cardiac output using any type of flowmeter. An electro- magnetic or ultrasonic flowmeter can also be placed on the aorta or pulmonary artery to measure cardiac output.
In the human, except in rare instances, cardiac out- put is measured by indirect methods that do not require surgery. Two of the methods that have been used for experimental studies are the oxygen Fick method and the indicator dilution method.
Cardiac output can also be estimated by echocardiog- raphy, a method that uses ultrasound waves from a trans- ducer placed on the chest wall or passed into the patient’s esophagus to measure the size of the heart’s chambers, as well as the velocity of blood flowing from the left ventricle into the aorta. Stroke volume is calculated from the velocity of blood flowing into the aorta and the aorta cross-sectional area determined from the aorta diameter that is measured by ultrasound imaging. Cardiac output is then calculated from the product of the stroke volume and the heart rate.
Pulsatile Output of the Heart as Measured by an Electromagnetic or Ultrasonic Flowmeter
Figure 20-17 shows a recording in a dog of blood flow in the root of the aorta made using an electromagnetic flowmeter. It demonstrates that the blood flow rises rapidly to a peak during systole, and then at the end of sys- tole reverses for a fraction of a second. This reverse flow causes the aortic valve to close and the flow to return to zero.
Measurement of Cardiac Output Using the Oxygen Fick Principle
The Fick principle is explained by Figure 20-18. This fig- ure shows that 200 milliliters of oxygen are being absorbed from the lungs into the pulmonary blood each minute. It also shows that the blood entering the right heart has an oxygen concentration of 160 milliliters per liter of blood, whereas that leaving the left heart has an oxygen concen- tration of 200 milliliters per liter of blood. From these data, one can calculate that each liter of blood passing through the lungs absorbs 40 milliliters of oxygen.
Because the total quantity of oxygen absorbed into the blood from the lungs each minute is 200 milliliters, divid- ing 200 by 40 calculates a total of five 1-liter portions of blood that must pass through the pulmonary circulation each minute to absorb this amount of oxygen. Therefore, the quantity of blood flowing through the lungs each min- ute is 5 liters, which is also a measure of the cardiac out- put. Thus, the cardiac output can be calculated by the following formula:
Cardiac output (L/min) O2 absorbed per minute by the lungs (ml/min) Arteriovenous O2 difference (ml/L of blood)
=
In applying this Fick procedure for measuring cardiac output in the human being, mixed venous blood is usu- ally obtained through a catheter inserted up the brachial vein of the forearm, through the subclavian vein, down to the right atrium, and, finally, into the right ventricle or
0 1 2
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Figure 20-17 Pulsatile blood flow in the root of the aorta recorded using an electromagnetic flowmeter.
O2 = 160 ml/L
right heart
O2 = 200 ml/L left heart
Oxygen used = 200 ml/min
LUNGS
Cardiac output = 5000 ml/min
Figure 20-18 Fick principle for determining cardiac output.
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pulmonary artery. And systemic arterial blood can then be obtained from any systemic artery in the body. The rate of oxygen absorption by the lungs is measured by the rate of disappearance of oxygen from the respired air, using any type of oxygen meter.
Indicator Dilution Method for Measuring Cardiac Output
To measure cardiac output by the so-called “indicator dilution method,” a small amount of indicator, such as a dye, is injected into a large systemic vein or, preferably, into the right atrium. This passes rapidly through the right side of the heart, then through the blood vessels of the lungs, through the left side of the heart, and, finally, into the systemic arterial system. The concentration of the dye is recorded as the dye passes through one of the peripheral arteries, giving a curve as shown in Figure 20-19. In each of these instances, 5 milligrams of Cardio- Green dye was injected at zero time. In the top recording, none of the dye passed into the arterial tree until about 3 seconds after the injection, but then the arterial concen- tration of the dye rose rapidly to a maximum in about 6 to 7 seconds. After that, the concentration fell rapidly, but before the concentration reached zero, some of the dye had already circulated all the way through some of the peripheral systemic vessels and returned through the heart for a second time. Consequently, the dye con- centration in the artery began to rise again. For the pur- pose of calculation, it is necessary to extrapolate the early down-slope of the curve to the zero point, as shown by the dashed portion of each curve. In this way, the extrap- olated time-concentration curve of the dye in the sys- temic artery without recirculation can be measured in its first portion and estimated reasonably accurately in its latter portion.
Once the extrapolated time-concentration curve has been determined, one then calculates the mean concentration of dye in the arterial blood for the dura- tion of the curve. For instance, in the top example of Figure 20-19, this was done by measuring the area under the entire initial and extrapolated curve and then aver- aging the concentration of dye for the duration of the curve; one can see from the shaded rectangle straddling the curve in the upper figure that the average concentra - tion of dye was 0.25 mg/dl of blood and that the dura t ion of this average value was 12 seconds. A total of 5 milli - grams of dye had been injected at the beginning of the experiment. For blood carrying only 0.25 milligram of dye in each 100 milliliters to carry the entire 5 milligrams of dye through the heart and lungs in 12 seconds, a total of 20 portions each with 100 milliliters of blood would have passed through the heart during the 12 seconds, which would be the same as a cardiac output of 2 L/12 sec, or 10 L/min. We leave it to the reader to calculate the cardiac output from the bottom extrapolated curve of Figure 20-19. To summarize, the cardiac output can be determined using the following formula:
Bibliography
Gaasch WH, Zile MR: Left ventricular diastolic dysfunction and diastolic heart failure, Annu Rev Med 55:373, 2004.
Guyton AC: Venous return. In Hamilton WF, editor: Handbook of Physiology, Sec 2, vol 2, Baltimore, 1963, Williams & Wilkins, p 1099.
Guyton AC: Determination of cardiac output by equating venous return curves with cardiac response curves, Physiol Rev 35:123, 1955.
Guyton AC, Jones CE, Coleman TG: Circulatory physiology: cardiac output and its regulation, Philadelphia, 1973, WB Saunders.
Guyton AC, Lindsey AW, Kaufmann BN: Effect of mean circulatory filling pressure and other peripheral circulatory factors on cardiac output, Am J Physiol 180:463–468, 1955.
Hall JE: Integration and regulation of cardiovascular function, Am J Physiol 277:S174, 1999.
Hall JE: The pioneering use of systems analysis to study cardiac output reg- ulation, Am J Physiol Regul Integr Comp Physiol 287:R1009, 2004.
Klein I, Danzi S: Thyroid disease and the heart, Circulation 116:1725, 2007. Koch WJ, Lefkowitz RJ, Rockman HA: Functional consequences of altering
myocardial adrenergic receptor signaling, Annu Rev Physiol 62:237, 2000. Mathews L, Singh RK: Cardiac output monitoring, Ann Card Anaesth 11:56,
2008. Rothe CF: Mean circulatory filling pressure: its meaning and measurement,
J Appl Physiol 74:499, 1993. Rothe CF: Reflex control of veins and vascular capacitance, Physiol Rev
63:1281, 1983. Sarnoff SJ, Berglund E: Ventricular function. 1. Starling’s law of the heart,
studied by means of simultaneous right and left ventricular function curves in the dog, Circulation 9:706–718, 1953.
Uemura K, Sugimachi M, Kawada T, et al: A novel framework of circulatory equilibrium, Am J Physiol Heart Circ Physiol 286:H2376, 2004.
Vatner SF, Braunwald E: Cardiovascular control mechanisms in the con- scious state, N Engl J Med 293:970, 1975.
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Figure 20-19 Extrapolated dye concentration curves used to cal- culate two separate cardiac outputs by the dilution method. (The rectangular areas are the calculated average concentrations of dye in the arterial blood for the durations of the respective extrapo- lated curves.)
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chapter 21
Muscle Blood Flow and Cardiac Output During Exercise; the Coronary Circulation
and Ischemic Heart Disease
In this chapter we consider (1) blood flow to the skel- etal muscles and (2) coro- nary artery blood flow to the heart. Regulation of each of these is achieved mainly by local control of vascular
resistance in response to muscle tissue metabolic needs. We also discuss the physiology of related subjects such
as (1) cardiac output control during exercise, (2) character- istics of heart attacks, and (3) the pain of angina pectoris.
Blood Flow Regulation in Skeletal Muscle at Rest and During Exercise
Very strenuous exercise is one of the most stressful condi- tions that the normal circulatory system faces. This is true because there is such a large mass of skeletal muscle in the body, all of it requiring large amounts of blood flow. Also, the cardiac output often must increase in the nonathlete to four to five times normal, or in the well-trained ath- lete to six to seven times normal, to satisfy the metabolic needs of the exercising muscles.
Rate of Blood Flow Through the Muscles
During rest, blood flow through skeletal muscle averages 3 to 4 ml/min/100 g of muscle. During extreme exercise in the well-conditioned athlete, this can increase 25- to 50-fold, rising to 100 to 200 ml/min/100 g of muscle. Peak blood flows as high as 400 ml/min/100 g of muscle have been reported in thigh muscles of endurance-trained athletes.
Blood Flow During Muscle Contractions. Figure 21-1 shows a record of blood flow changes in a calf muscle of a human leg during strong rhythmical muscular exercise. Note that the flow increases and decreases with each muscle contraction. At the end of the contractions, the blood flow remains very high for a few seconds but then returns toward normal during the next few minutes.
The cause of the lower flow during the muscle contrac- tion phase of exercise is compression of the blood vessels
by the contracted muscle. During strong tetanic contrac- tion, which causes sustained compression of the blood vessels, the blood flow can be almost stopped, but this also causes rapid weakening of the contraction.
Increased Blood Flow in Muscle Capillaries During Exercise. During rest, some muscle capillaries have little or no flowing blood. But during strenuous exercise, all the capillaries open. This opening of dormant capillaries diminishes the distance that oxygen and other nutrients must diffuse from the capillaries to the contracting muscle fibers and sometimes contributes a twofold to threefold increased capillary surface area through which oxygen and nutrients can diffuse from the blood to the tissues.
Control of Blood Flow in Skeletal Muscles
Local Regulation—Decreased Oxygen in Muscle Greatly Enhances Flow. The tremendous increase in muscle blood flow that occurs during skeletal muscle activity is caused mainly by chemicals acting directly on the muscle arterioles to cause dilation. One of the most important chemical effects is reduction of oxygen in the muscle tissues. When muscles are active they use oxygen rapidly, thereby decreasing the oxygen concentration in the tissue fluids. This in turn causes local arteriolar vaso- dilation because the arteriolar walls cannot maintain contraction in the absence of oxygen and because oxy- gen deficiency causes release of vasodilator substances. Adenosine may be an important vasodilator substance, but experiments have shown that even large amounts of adenosine infused directly into a muscle artery cannot increase blood flow to the same extent as during intense exercise and cannot sustain vasodilation in skeletal muscle for more than about 2 hours.
Fortunately, even after the muscle blood vessels have become insensitive to the vasodilator effects of adeno- sine, still other vasodilator factors continue to maintain increased capillary blood flow as long as the exercise continues. These factors include (1) potassium ions, (2) adenosine triphosphate (ATP), (3) lactic acid, and (4) carbon dioxide. We still do not know quantitatively how great a role each of these plays in increasing muscle
Unit IV The Circulation
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blood flow during muscle activity; this subject was dis- cussed in additional detail in Chapter 17.
Nervous Control of Muscle Blood Flow. In addition to local tissue vasodilator mechanisms, skeletal muscles are provided with sympathetic vasoconstrictor nerves and (in some species of animals) sympathetic vasodilator nerves as well.
Sympathetic Vasoconstrictor Nerves. The sympa- thetic vasoconstrictor nerve fibers secrete norepineph- rine at their nerve endings. When maximally activated, this can decrease blood flow through resting muscles to as little as one-half to one-third normal. This vasocon- striction is of physiologic importance in circulatory shock and during other periods of stress when it is necessary to maintain a normal or even high arterial pressure.
In addition to the norepinephrine secreted at the sym- pathetic vasoconstrictor nerve endings, the medullae of the two adrenal glands also secrete large amounts of nor- epinephrine plus even more epinephrine into the circu- lating blood during strenuous exercise. The circulating norepinephrine acts on the muscle vessels to cause a vaso- constrictor effect similar to that caused by direct sympa- thetic nerve stimulation. The epinephrine, however, often has a slight vasodilator effect because epinephrine excites more of the beta-adrenergic receptors of the vessels, which are vasodilator receptors, in contrast to the alpha vasocon- strictor receptors excited especially by norepinephrine. These receptors are discussed in Chapter 60.
Total Body Circulatory Readjustments During Exercise
Three major effects occur during exercise that are essen- tial for the circulatory system to supply the tremendous blood flow required by the muscles. They are (1) mass discharge of the sympathetic nervous system through- out the body with consequent stimulatory effects on the entire circulation, (2) increase in arterial pressure, and (3) increase in cardiac output.
Effects of Mass Sympathetic Discharge
At the onset of exercise, signals are transmitted not only from the brain to the muscles to cause muscle contrac- tion but also into the vasomotor center to initiate sym- pathetic discharge throughout the body. Simultaneously, the parasympathetic signals to the heart are attenuated. Therefore, three major circulatory effects result.
First, the heart is stimulated to greatly increased heart rate and increased pumping strength as a result of the sympathetic drive to the heart plus release of the heart from normal parasympathetic inhibition.
Second, most of the arterioles of the peripheral circu- lation are strongly contracted, except for the arterioles in the active muscles, which are strongly vasodilated by the local vasodilator effects in the muscles, as noted ear- lier. Thus, the heart is stimulated to supply the increased blood flow required by the muscles, while at the same time blood flow through most nonmuscular areas of the body is temporarily reduced, thereby “lending” blood supply to the muscles. This accounts for as much as 2 L/min of extra blood flow to the muscles, which is exceedingly important when one thinks of a person running for his life—even a fractional increase in running speed may make the differ- ence between life and death. Two of the peripheral cir- culatory systems, the coronary and cerebral systems, are spared this vasoconstrictor effect because both these cir- culatory areas have poor vasoconstrictor innervation— fortunately so because both the heart and the brain are as essential to exercise as are the skeletal muscles.
Third, the muscle walls of the veins and other capaci- tative areas of the circulation are contracted powerfully, which greatly increases the mean systemic filling pres- sure. As we learned in Chapter 20, this is one of the most important factors in promoting increase in venous return of blood to the heart and, therefore, in increasing the cardiac output.
Increase in Arterial Pressure During Exercise Due to Sympathetic Stimulation
An important effect of increased sympathetic stimulation in exercise is to increase the arterial pressure. This results from multiple stimulatory effects, including (1) vasocon- striction of the arterioles and small arteries in most tis- sues of the body except the active muscles, (2) increased pumping activity by the heart, and (3) a great increase in mean systemic filling pressure caused mainly by venous contraction. These effects, working together, almost always increase the arterial pressure during exercise. This increase can be as little as 20 mm Hg or as great as 80 mm Hg, depending on the conditions under which the exercise is performed. When a person performs exercise under tense conditions but uses only a few muscles, the sympathetic nervous response still occurs everywhere in the body. In the few active muscles, vasodilation occurs, but everywhere else in the body the effect is mainly vaso- constriction, often increasing the mean arterial pressure to as high as 170 mm Hg. Such a condition might occur
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Figure 21-1 Effects of muscle exercise on blood flow in the calf of a leg during strong rhythmical contraction. The blood flow was much less during contractions than between contractions. (Adapted from Barcroft H, Dornhorst AC: The blood flow through the human calf during rhythmic exercise. J Physiol 109:402, 1949.)
Chapter 21 Muscle Blood Flow and Cardiac Output During Exercise; the Coronary Circulation and Ischemic Heart Disease
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in a person standing on a ladder and nailing with a ham- mer on the ceiling above. The tenseness of the situation is obvious.
Conversely, when a person performs massive whole- body exercise, such as running or swimming, the increase in arterial pressure is often only 20 to 40 mm Hg. This lack of a large increase in pressure results from the extreme vasodilation that occurs simultaneously in large masses of active muscle.
Why Is the Arterial Pressure Increase During Exercise Important? When muscles are stimulated maximally in a laboratory experiment but without allowing the arterial pressure to rise, muscle blood flow seldom rises more than about eightfold. Yet, we know from studies of marathon runners that muscle blood flow can increase from as little as 1 L/min for the whole body during rest to more than 20 L/min during maximal activity. Therefore, it is clear that muscle blood flow can increase much more than occurs in the aforementioned simple laboratory experiment. What is the difference? Mainly, the arterial pressure rises during normal exercise. Let us assume, for instance, that the arterial pressure rises 30 percent, a common increase during heavy exercise. This 30 percent increase causes 30 percent more force to push blood through the muscle tissue vessels. But this is not the only important effect; the extra pressure also stretches the walls of the vessels, and this effect, along with the locally released vasodilators and higher blood pressure, may increase muscle total flow to more than 20 times normal.
Importance of the Increase in Cardiac Output During Exercise
Many different physiologic effects occur at the same time during exercise to increase cardiac output approximately in proportion to the degree of exercise. In fact, the abil- ity of the circulatory system to provide increased cardiac output for delivery of oxygen and other nutrients to the muscles during exercise is equally as important as the strength of the muscles themselves in setting the limit for continued muscle work. For instance, marathon run- ners who can increase their cardiac outputs the most are generally the same persons who have record-breaking running times.
Graphical Analysis of the Changes in Cardiac Output During Heavy Exercise. Figure 21-2 shows a graphical analysis of the large increase in cardiac output that occurs during heavy exercise. The cardiac output and venous return curves crossing at point A give the analysis for the normal circulation, and the curves crossing at point B ana- lyze heavy exercise. Note that the great increase in cardiac output requires significant changes in both the cardiac output curve and the venous return curve, as follows.
The increased level of the cardiac output curve is easy to understand. It results almost entirely from sympathetic stimulation of the heart that causes (1) increased heart rate, often up to rates as high as 170 to 190 beats/min, and (2) increased strength of contraction of the heart, often to as much as twice normal. Without this increased level
of cardiac function, the increase in cardiac output would be limited to the plateau level of the normal heart, which would be a maximum increase of cardiac output of only about 2.5-fold rather than the 4-fold that can commonly be achieved by the untrained runner and the 7-fold that can be achieved in some marathon runners.
Now study the venous return curves. If no change occurred from the normal venous return curve, the car- diac output could hardly rise at all in exercise because the upper plateau level of the normal venous return curve is only 6 L/min. Yet two important changes do occur:
1. The mean systemic filling pressure rises tremen- dously at the onset of heavy exercise. This results partly from the sympathetic stimulation that contracts the veins and other capacitative parts of the circula- tion. In addition, tensing of the abdominal and other skeletal muscles of the body compresses many of the internal vessels, thus providing more compression of the entire capacitative vascular system, causing a still greater increase in mean systemic filling pressure. During maximal exercise, these two effects together can increase the mean systemic filling pressure from a normal level of 7 mm Hg to as high as 30 mm Hg.
2. The slope of the venous return curve rotates upward. This is caused by decreased resistance in virtually all the blood vessels in active muscle tissue, which also causes resistance to venous return to decrease, thus increas- ing the upward slope of the venous return curve.
Therefore, the combination of increased mean sys- temic filling pressure and decreased resistance to venous return raises the entire level of the venous return curve.
In response to the changes in both the venous return curve and the cardiac output curve, the new equilibrium point in Figure 21-2 for cardiac output and right atrial pressure is now point B, in contrast to the normal level at point A. Note especially that the right atrial pressure has hardly changed, having risen only 1.5 mm Hg. In fact, in a person with a strong heart, the right atrial pressure often falls below normal in very heavy exercise because of the greatly increased sympathetic stimulation of the heart during exercise.
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Figure 21-2 Graphical analysis of change in cardiac output and right atrial pressure with onset of strenuous exercise. Black curves, normal circulation. Red curves, heavy exercise.
Unit IV The Circulation
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Coronary Circulation
About one third of all deaths in industrialized countries of the Western world result from coronary artery disease, and almost all elderly people have at least some impair- ment of the coronary artery circulation. For this reason, understanding normal and pathological physiology of the coronary circulation is one of the most important sub- jects in medicine.
Physiologic Anatomy of the Coronary Blood Supply
Figure 21-3 shows the heart and its coronary blood supply. Note that the main coronary arteries lie on the surface of the heart and smaller arteries then pene- trate from the surface into the cardiac muscle mass. It is almost entirely through these arteries that the heart receives its nutritive blood supply. Only the inner 1/10 millimeter of the endocardial surface can obtain sig- nificant nutrition directly from the blood inside the cardiac chambers, so this source of muscle nutrition is minuscule.
The left coronary artery supplies mainly the anterior and left lateral portions of the left ventricle, whereas the right coronary artery supplies most of the right ventricle, as well as the posterior part of the left ventricle in 80 to 90 percent of people.
Most of the coronary venous blood flow from the left ventricular muscle returns to the right atrium of the heart by way of the coronary sinus, which is about 75 percent of the total coronary blood flow. And most of the coronary venous blood from the right ventricu- lar muscle returns through small anterior cardiac veins that flow directly into the right atrium, not by way of the coronary sinus. A very small amount of coronary venous blood also flows back into the heart through very minute thebesian veins, which empty directly into all chambers of the heart.
Normal Coronary Blood Flow—About 5 Percent of Cardiac Output
The resting coronary blood flow in the resting human being averages 70 ml/min/100 g heart weight, or about 225 ml/min, which is about 4 to 5 percent of the total car- diac output.
During strenuous exercise, the heart in the young adult increases its cardiac output fourfold to sevenfold, and it pumps this blood against a higher than normal arterial pressure. Consequently, the work output of the heart under severe conditions may increase sixfold to ninefold. At the same time, the coronary blood flow increases threefold to fourfold to supply the extra nutrients needed by the heart. This increase is not as much as the increase in workload, which means that the ratio of energy expenditure by the heart to coronary blood flow increases. Thus, the “effi- ciency” of cardiac utilization of energy increases to make up for the relative deficiency of coronary blood supply.
Phasic Changes in Coronary Blood Flow During Systole and Diastole—Effect of Cardiac Muscle Compression. Figure 21-4 shows the changes in blood flow through the nutrient capillaries of the left ventricu- lar coronary system in ml/min in the human heart dur- ing systole and diastole, as extrapolated from studies in experimental animals. Note from this diagram that the coronary capillary blood flow in the left ventricle muscle falls to a low value during systole, which is opposite to flow in vascular beds elsewhere in the body. The reason for this is strong compression of the left ventricular muscle around the intramuscular vessels during systolic contraction.
During diastole, the cardiac muscle relaxes and no lon- ger obstructs blood flow through the left ventricular mus- cle capillaries, so blood flows rapidly during all of diastole.
Blood flow through the coronary capillaries of the right ventricle also undergoes phasic changes during the cardiac cycle, but because the force of contraction of the right ventricular muscle is far less than that of the left ventricular muscle, the inverse phasic changes are only partial, in contrast to those in the left ventricular muscle.
Left coronary artery
Right coronary artery
Pulmonary artery
Aorta
Left circumflex branch
Left anterior descending branch
Figure 21-3 The coronary arteries.
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Figure 21-4 Phasic flow of blood through the coronary capillaries of the human left ventricle during cardiac systole and diastole (as extrapolated from measured flows in dogs).
Chapter 21 Muscle Blood Flow and Cardiac Output During Exercise; the Coronary Circulation and Ischemic Heart Disease
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Epicardial Versus Subendocardial Coronary Blood Flow—Effect of Intramyocardial Pressure. Figure 21-5 demonstrates the special arrangement of the coro- nary vessels at different depths in the heart muscle, show- ing on the outer surface epicardial coronary arteries that supply most of the muscle. Smaller, intramuscular arteries derived from the epicardial arteries penetrate the mus- cle, supplying the needed nutrients. Lying immediately beneath the endocardium is a plexus of subendocardial arteries. During systole, blood flow through the subendo- cardial plexus of the left ventricle, where the intramuscu- lar coronary vessels are compressed greatly by ventricular muscle contraction, tends to be reduced. But the extra vessels of the subendocardial plexus normally compensate for this. Later in the chapter, we explain how this pecu- liar difference between blood flow in the epicardial and subendocardial arteries plays an important role in certain types of coronary ischemia.
Control of Coronary Blood Flow
Local Muscle Metabolism Is the Primary Controller of Coronary Flow
Blood flow through the coronary system is regulated mostly by local arteriolar vasodilation in response to the nutritional needs of cardiac muscle. That is, whenever the vigor of cardiac contraction is increased, the rate of cor- onary blood flow also increases. Conversely, decreased heart activity is accompanied by decreased coronary flow. This local regulation of coronary blood flow is almost identical to that occurring in many other tissues of the body, especially in the skeletal muscles.
Oxygen Demand as a Major Factor in Local Coronary Blood Flow Regulation. Blood flow in the coronary arter- ies usually is regulated almost exactly in proportion to the need of the cardiac musculature for oxygen. Normally, about 70 percent of the oxygen in the coronary arterial blood is removed as the blood flows through the heart muscle. Because not much oxygen is left, very little addi- tional oxygen can be supplied to the heart musculature unless the coronary blood flow increases. Fortunately, the coronary blood flow does increase almost in direct proportion to any additional metabolic consumption of oxygen by the heart.
However, the exact means by which increased oxy- gen consumption causes coronary dilation has not been determined. It is speculated by many research workers that a decrease in the oxygen concentration in the heart
causes vasodilator substances to be released from the muscle cells and that these dilate the arterioles. A sub- stance with great vasodilator propensity is adenosine. In the presence of very low concentrations of oxygen in the muscle cells, a large proportion of the cell’s ATP degrades to adenosine monophosphate; then small portions of this are further degraded and release adenosine into the tissue fluids of the heart muscle, with resultant increase in local coronary blood flow. After the adenosine causes vasodila- tion, much of it is reabsorbed into the cardiac cells to be reused.
Adenosine is not the only vasodilator product that has been identified. Others include adenosine phosphate compounds, potassium ions, hydrogen ions, carbon dioxide, prostaglandins, and nitric oxide. Yet the mech- anisms of coronary vasodilation during increased car- diac activity have not been fully explained by adenosine. Pharmacologic agents that block or partially block the vasodilator effect of adenosine do not prevent coronary vasodilation caused by increased heart muscle activity. Studies in skeletal muscle have also shown that contin- ued infusion of adenosine maintains vascular dilation for only 1 to 3 hours, and yet muscle activity still dilates the local blood vessels even when the adenosine can no lon- ger dilate them. Therefore, the other vasodilator mecha- nisms listed earlier should be remembered.
Nervous Control of Coronary Blood Flow
Stimulation of the autonomic nerves to the heart can affect coronary blood flow both directly and indirectly. The direct effects result from action of the nervous trans- mitter substances acetylcholine from the vagus nerves and norepinephrine and epinephrine from the sympa- thetic nerves on the coronary vessels themselves. The indirect effects result from secondary changes in coro- nary blood flow caused by increased or decreased activity of the heart.
The indirect effects, which are mostly opposite to the direct effects, play a far more important role in normal control of coronary blood flow. Thus, sympathetic stim- ulation, which releases norepinephrine and epineph- rine, increases both heart rate and heart contractility and increases the rate of metabolism of the heart. In turn, the increased metabolism of the heart sets off local blood flow regulatory mechanisms for dilating the coronary vessels, and the blood flow increases approximately in propor- tion to the metabolic needs of the heart muscle. In con- trast, vagal stimulation, with its release of acetylcholine, slows the heart and has a slight depressive effect on heart contractility. These effects in turn decrease cardiac oxy- gen consumption and, therefore, indirectly constrict the coronary arteries.
Direct Effects of Nervous Stimuli on the Coronary Vasculature. The distribution of parasympathetic (vagal) nerve fibers to the ventricular coronary system is not very great. However, the acetylcholine released by parasympa- thetic stimulation has a direct effect to dilate the coronary arteries.
Cardiac muscle
Epicardial coronary arteries
Subendocardial arterial plexus
Figure 21-5 Diagram of the epicardial, intramuscular, and suben- docardial coronary vasculature.
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There is much more extensive sympathetic innervation of the coronary vessels. In Chapter 60, we see that the sym- pathetic transmitter substances norepinephrine and epi- nephrine can have either vascular constrictor or vascular dilator effects, depending on the presence or absence of constrictor or dilator receptors in the blood vessel walls. The constrictor receptors are called alpha receptors and the dilator receptors are called beta receptors. Both alpha and beta receptors exist in the coronary vessels. In gen- eral, the epicardial coronary vessels have a preponderance of alpha receptors, whereas the intramuscular arteries may have a preponderance of beta receptors. Therefore, sympathetic stimulation can, at least theoretically, cause slight overall coronary constriction or dilation, but usu- ally constriction. In some people, the alpha vasoconstric- tor effects seem to be disproportionately severe, and these people can have vasospastic myocardial ischemia during periods of excess sympathetic drive, often with resultant anginal pain.
Metabolic factors, especially myocardial oxygen con- sumption, are the major controllers of myocardial blood flow. Whenever the direct effects of nervous stimulation alter the coronary blood flow in the wrong direction, the metabolic control of coronary flow usually overrides the direct coronary nervous effects within seconds.
Special Features of Cardiac Muscle Metabolism
The basic principles of cellular metabolism, discussed in Chapters 67 through 72, apply to cardiac muscle the same as for other tissues, but there are some quantitative differ- ences. Most important, under resting conditions, cardiac muscle normally consumes fatty acids to supply most of its energy instead of carbohydrates (about 70 percent of the energy is derived from fatty acids). However, as is also true of other tissues, under anaerobic or ischemic con- ditions, cardiac metabolism must call on anaerobic gly- colysis mechanisms for energy. Unfortunately, glycolysis consumes tremendous quantities of the blood glucose and at the same time forms large amounts of lactic acid in the cardiac tissue, which is probably one of the causes of cardiac pain in cardiac ischemic conditions, as discussed later in this chapter.
As is true in other tissues, more than 95 percent of the metabolic energy liberated from foods is used to form ATP in the mitochondria. This ATP in turn acts as the conveyer of energy for cardiac muscular contraction and other cellular functions. In severe coronary ischemia, the ATP degrades first to adenosine diphosphate, then to adenosine monophosphate and adenosine. Because the cardiac muscle cell membrane is slightly permeable to adenosine, much of this can diffuse from the muscle cells into the circulating blood.
The released adenosine is believed to be one of the substances that causes dilation of the coronary arterioles during coronary hypoxia, as discussed earlier. However, loss of adenosine also has a serious cellular consequence. Within as little as 30 minutes of severe coronary is chemia, as occurs after a myocardial infarct, about one half of the
adenine base can be lost from the affected cardiac mus- cle cells. Furthermore, this loss can be replaced by new synthesis of adenine at a rate of only 2 percent per hour. Therefore, once a serious bout of coronary ischemia has persisted for 30 or more minutes, relief of the ischemia may be too late to prevent injury and death of the cardiac cells. This almost certainly is one of the major causes of cardiac cellular death during myocardial ischemia.
Ischemic Heart Disease
The most common cause of death in Western culture is ischemic heart disease, which results from insufficient coronary blood flow. About 35 percent of people in the United States die of this cause. Some deaths occur sud- denly as a result of acute coronary occlusion or fibrilla- tion of the heart, whereas other deaths occur slowly over a period of weeks to years as a result of progressive weak- ening of the heart pumping process. In this chapter, we discuss acute coronary ischemia caused by acute coronary occlusion and myocardial infarction. In Chapter 22, we dis- cuss congestive heart failure, the most frequent cause of which is slowly increasing coronary ischemia and weak- ening of the cardiac muscle.
Atherosclerosis as a Cause of Ischemic Heart Disease. The most frequent cause of diminished coronary blood flow is atherosclerosis. The atherosclerotic process is discussed in connection with lipid metabolism in Chapter 68. Briefly, this process is the following.
In people who have genetic predisposition to athero- sclerosis, who are overweight or obese and have a seden- tary lifestyle, or who have high blood pressure and damage to the endothelial cells of the coronary blood vessels, large quantities of cholesterol gradually become depos- ited beneath the endothelium at many points in arteries throughout the body. Gradually, these areas of deposit are invaded by fibrous tissue and frequently become calci- fied. The net result is the development of atherosclerotic plaques that actually protrude into the vessel lumens and either block or partially block blood flow. A common site for development of atherosclerotic plaques is the first few centimeters of the major coronary arteries.
Acute Coronary Occlusion
Acute occlusion of a coronary artery most frequently occurs in a person who already has underlying atheroscle- rotic coronary heart disease but almost never in a person with a normal coronary circulation. Acute occlusion can result from any one of several effects, two of which are the following:
1. The atherosclerotic plaque can cause a local blood clot called a thrombus, which in turn occludes the artery. The thrombus usually occurs where the arterioscle- rotic plaque has broken through the endothelium, thus coming in direct contact with the flowing blood. Because the plaque presents an unsmooth surface, blood platelets adhere to it, fibrin is deposited, and red
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blood cells become entrapped to form a blood clot that grows until it occludes the vessel. Or, occasionally, the clot breaks away from its attachment on the athero- sclerotic plaque and flows to a more peripheral branch of the coronary arterial tree, where it blocks the artery at that point. A thrombus that flows along the artery in this way and occludes the vessel more distally is called a coronary embolus.
2. Many clinicians believe that local muscular spasm of a coronary artery also can occur. The spasm might result from direct irritation of the smooth muscle of the arte- rial wall by the edges of an arteriosclerotic plaque, or it might result from local nervous reflexes that cause excess coronary vascular wall contraction. The spasm may then lead to secondary thrombosis of the vessel.
Lifesaving Value of Collateral Circulation in the Heart. The degree of damage to the heart muscle caused either by slowly developing atherosclerotic constriction of the coronary arteries or by sudden coronary occlusion is determined to a great extent by the degree of collateral circulation that has already developed or that can open within minutes after the occlusion.
In a normal heart, almost no large communications exist among the larger coronary arteries. But many anas- tomoses do exist among the smaller arteries sized 20 to 250 micrometers in diameter, as shown in Figure 21-6.
When a sudden occlusion occurs in one of the larger coronary arteries, the small anastomoses begin to dilate within seconds. But the blood flow through these minute collaterals is usually less than one-half that needed to keep alive most of the cardiac muscle that they now supply; the diameters of the collateral vessels do not enlarge much more for the next 8 to 24 hours. But then collateral flow does begin to increase, doubling by the second or third
day and often reaching normal or almost normal coro- nary flow within about 1 month. Because of these devel- oping collateral channels, many patients recover almost completely from various degrees of coronary occlusion when the area of muscle involved is not too great.
When atherosclerosis constricts the coronary arter- ies slowly over a period of many years rather than sud- denly, collateral vessels can develop at the same time while the atherosclerosis becomes more and more severe. Therefore, the person may never experience an acute epi- sode of cardiac dysfunction. But, eventually, the sclerotic process develops beyond the limits of even the collateral blood supply to provide the needed blood flow, and some- times the collateral blood vessels themselves develop ath- erosclerosis. When this occurs, the heart muscle becomes severely limited in its work output, often so much so that the heart cannot pump even normally required amounts of blood flow. This is one of the most common causes of the cardiac failure that occurs in vast numbers of older people.
Myocardial Infarction
Immediately after an acute coronary occlusion, blood flow ceases in the coronary vessels beyond the occlu- sion except for small amounts of collateral flow from sur- rounding vessels. The area of muscle that has either zero flow or so little flow that it cannot sustain cardiac mus- cle function is said to be infarcted. The overall process is called a myocardial infarction.
Soon after the onset of the infarction, small amounts of collateral blood begin to seep into the infarcted area, and this, combined with progressive dilation of local blood vessels, causes the area to become overfilled with stagnant blood. Simultaneously the muscle fibers use the last ves- tiges of the oxygen in the blood, causing the hemoglobin to become totally deoxygenated. Therefore, the infarcted area takes on a bluish-brown hue, and the blood vessels of the area appear to be engorged despite lack of blood flow. In later stages, the vessel walls become highly permeable and leak fluid; the local muscle tissue becomes edema- tous, and the cardiac muscle cells begin to swell because of diminished cellular metabolism. Within a few hours of almost no blood supply, the cardiac muscle cells die.
Cardiac muscle requires about 1.3 ml of oxygen per 100 grams of muscle tissue per minute just to remain alive. This is in comparison with about 8 ml of oxygen per 100 grams delivered to the normal resting left ventricle each minute. Therefore, if there is even 15 to 30 percent of normal resting coronary blood flow, the muscle will not die. In the central portion of a large infarct, however, where there is almost no collateral blood flow, the muscle does die.
Subendocardial Infarction. The subendocardial mus- cle frequently becomes infarcted even when there is no evidence of infarction in the outer surface portions of the heart. The reason for this is that the subendocar- dial muscle has extra difficulty obtaining adequate blood flow because the blood vessels in the subendocardium
Artery
Vein
Artery
Vein
Figure 21-6 Minute anastomoses in the normal coronary arterial system.
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are intensely compressed by systolic contraction of the heart, as explained earlier. Therefore, any condition that compromises blood flow to any area of the heart usually causes damage first in the subendocardial regions, and the damage then spreads outward toward the epicardium.
Causes of Death After Acute Coronary Occlusion
The most common causes of death after acute myocardial infarction are (1) decreased cardiac output; (2) damming of blood in the pulmonary blood vessels and then death resulting from pulmonary edema; (3) fibrillation of the heart; and, occasionally, (4) rupture of the heart.
Decreased Cardiac Output—Systolic Stretch and Cardiac Shock. When some of the cardiac muscle fibers are not functioning and others are too weak to contract with great force, the overall pumping ability of the affected ventricle is proportionately depressed. Indeed, the overall pumping strength of the infarcted heart is often decreased more than one might expect because of a phenomenon called systolic stretch, shown in Figure 21-7. That is, when the normal portions of the ventricular muscle contract, the ischemic portion of the muscle, whether it is dead or simply nonfunctional, instead of contracting is forced outward by the pressure that develops inside the ventricle. Therefore, much of the pumping force of the ventricle is dissipated by bulging of the area of nonfunctional cardiac muscle.
When the heart becomes incapable of contracting with sufficient force to pump enough blood into the peripheral arterial tree, cardiac failure and death of peripheral tissues ensue as a result of peripheral ischemia. This condition is called coronary shock, cardiogenic shock, cardiac shock, or
low cardiac output failure. It is discussed more fully in the next chapter. Cardiac shock almost always occurs when more than 40 percent of the left ventricle is infarcted. And death occurs in over 70 percent of patients once they develop cardiac shock.
Damming of Blood in the Body’s Venous System. When the heart is not pumping blood forward, it must be damming blood in the atria and in the blood vessels of the lungs or in the systemic circulation. This leads to increased capillary pressures, particularly in the lungs.
This damming of blood in the veins often causes lit- tle difficulty during the first few hours after myocardial infarction. Instead, symptoms develop a few days later for the following reason: The acutely diminished cardiac out- put leads to diminished blood flow to the kidneys. Then, for reasons that are discussed in Chapter 22, the kidneys fail to excrete enough urine. This adds progressively to the total blood volume and, therefore, leads to congestive symptoms. Consequently, many patients who seemingly are getting along well during the first few days after onset of heart failure will suddenly develop acute pulmonary edema and often will die within a few hours after appear- ance of the initial pulmonary symptoms.
Fibrillation of the Ventricles After Myocardial Infarction. Many people who die of coronary occlu- sion die because of sudden ventricular fibrillation. The tendency to develop fibrillation is especially great after a large infarction, but fibrillation can sometimes occur after small occlusions as well. Indeed, some patients with chronic coronary insufficiency die suddenly from fibrilla- tion without any acute infarction.
There are two especially dangerous periods after cor- onary infarction during which fibrillation is most likely to occur. The first is during the first 10 minutes after the infarction occurs. Then there is a short period of relative safety, followed by a second period of cardiac irritabil- ity beginning 1 hour or so later and lasting for another few hours. Fibrillation can also occur many days after the infarct but less likely so.
At least four factors enter into the tendency for the heart to fibrillate:
1. Acute loss of blood supply to the cardiac muscle causes rapid depletion of potassium from the ischemic musculature. This also increases the potassium concentration in the extracellular fluids surround- ing the cardiac muscle fibers. Experiments in which potassium has been injected into the coronary system have demonstrated that an elevated extracellular potas- sium concentration increases the irritability of the cardiac musculature and, therefore, its likelihood of fibrillating.
2. Ischemia of the muscle causes an “injury current,” which is described in Chapter 12 in relation to electrocardio- grams in patients with acute myocardial infarction.
Normal contraction
Nonfunctional muscle
Systolic stretch
Figure 21-7 Systolic stretch in an area of ischemic cardiac muscle.
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That is, the ischemic musculature often cannot com- pletely repolarize its membranes after a heartbeat, so the external surface of this muscle remains nega- tive with respect to normal cardiac muscle membrane potential elsewhere in the heart. Therefore, electric current flows from this ischemic area of the heart to the normal area and can elicit abnormal impulses that can cause fibrillation.
3. Powerful sympathetic reflexes often develop after mas- sive infarction, principally because the heart does not pump an adequate volume of blood into the arterial tree, which leads to reduced blood pressure. The sym- pathetic stimulation also increases irritability of the cardiac muscle and thereby predisposes to fibrillation.
4. Cardiac muscle weakness caused by the myocardial infarction often causes the ventricle to dilate excessively. This increases the pathway length for impulse conduc- tion in the heart and frequently causes abnormal con- duction pathways all the way around the infarcted area of the cardiac muscle. Both of these effects predispose to development of circus movements because, as discussed in Chapter 13, excess prolongation of conduction path- ways in the ventricles allows impulses to re-enter muscle that is already recovering from refractoriness, thereby initiating a “circus movement” cycle of new excitation and causing the process to continue on and on.
Rupture of the Infarcted Area. During the first day or so after an acute infarct, there is little danger of rupture of the ischemic portion of the heart, but a few days later, the dead muscle fibers begin to degen- erate, and the heart wall becomes stretched very thin. When this happens, the dead muscle bulges outward severely with each heart contraction, and this systolic stretch becomes greater and greater until finally the heart ruptures. In fact, one of the means used in assess- ing progress of severe myocardial infarction is to record by cardiac imaging (i.e., x-rays) whether the degree of systolic stretch is worsening.
When a ventricle does rupture, loss of blood into the pericardial space causes rapid development of car- diac tamponade—that is, compression of the heart from the outside by blood collecting in the pericardial cavity. Because of this compression of the heart, blood cannot flow into the right atrium, and the patient dies of suddenly decreased cardiac output.
Stages of Recovery from Acute Myocardial Infarction
The upper left part of Figure 21-8 shows the effects of acute coronary occlusion in a patient with a small area of muscle ischemia; to the right is shown a heart with a large area of ischemia. When the area of ischemia is small, little or no death of the muscle cells may occur, but part of the muscle often does become temporarily nonfunc- tional because of inadequate nutrition to support muscle contraction.
When the area of ischemia is large, some of the mus- cle fibers in the center of the area die rapidly, within 1 to 3 hours where there is total cessation of coronary blood supply. Immediately around the dead area is a nonfunc- tional area, with failure of contraction and usually failure of impulse conduction. Then, extending circumferentially around the nonfunctional area is an area that is still con- tracting but weakly so because of mild ischemia.
Replacement of Dead Muscle by Scar Tissue. In the lower part of Figure 21-8, the various stages of recov- ery after a large myocardial infarction are shown. Shortly after the occlusion, the muscle fibers in the center of the ischemic area die. Then, during the ensuing days, this area of dead fibers becomes bigger because many of the mar- ginal fibers finally succumb to the prolonged ischemia. At the same time, because of enlargement of collateral arterial channels supplying the outer rim of the infarcted area, much of the nonfunctional muscle recovers. After a few days to 3 weeks, most of the nonfunctional mus- cle becomes functional again or dies—one or the other. In the meantime, fibrous tissue begins developing among the dead fibers because ischemia can stimulate growth of fibroblasts and promote development of greater than normal quantities of fibrous tissue. Therefore, the dead muscle tissue is gradually replaced by fibrous tissue. Then, because it is a general property of fibrous tissue to undergo progressive contraction and dissolution, the fibrous scar may grow smaller over a period of several months to a year.
Finally, the normal areas of the heart gradually hyper- trophy to compensate at least partially for the lost dead cardiac musculature. By these means, the heart recov- ers either partially or almost completely within a few months.
Value of Rest in Treating Myocardial Infarction. The degree of cardiac cellular death is determined by the degree of ischemia and the workload on the heart muscle.
Mild ischemia
Non- functional
Mild ischemia
Non- functional
Dead fibers
Nonfunctional
Dead fibers Fibrous tissue
Figure 21-8 Top, Small and large areas of coronary ischemia. Bottom, Stages of recovery from myocardial infarction.
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When the workload is greatly increased, such as dur- ing exercise, in severe emotional strain, or as a result of fatigue, the heart needs increased oxygen and other nutri- ents for sustaining its life. Furthermore, anastomotic blood vessels that supply blood to ischemic areas of the heart must also still supply the areas of the heart that they normally supply. When the heart becomes excessively active, the vessels of the normal musculature become greatly dilated. This allows most of the blood flowing into the coronary vessels to flow through the normal muscle tissue, thus leaving little blood to flow through the small anastomotic channels into the ischemic area so that the ischemic condition worsens. This condition is called the “coronary steal” syndrome. Consequently, one of the most important factors in the treatment of a patient with myo- cardial infarction is observance of absolute body rest dur- ing the recovery process.
Function of the Heart After Recovery from Myocardial Infarction
Occasionally, a heart that has recovered from a large myocardial infarction returns almost to full functional capability, but more frequently its pumping capabil- ity is permanently decreased below that of a healthy heart. This does not mean that the person is necessar- ily a cardiac invalid or that the resting cardiac output is depressed below normal, because the normal heart is capable of pumping 300 to 400 percent more blood per minute than the body requires during rest—that is, a normal person has a “cardiac reserve” of 300 to 400 per- cent. Even when the cardiac reserve is reduced to as little as 100 percent, the person can still perform most nor- mal daily activities but not strenuous exercise that would overload the heart.
Pain in Coronary Heart Disease
Normally, a person cannot “feel” his or her heart, but ischemic cardiac muscle often does cause pain sensation, sometimes severe. Exactly what causes this pain is not known, but it is believed that ischemia causes the muscle to release acidic substances, such as lactic acid, or other pain-promoting products, such as histamine, kinins, or cellular proteolytic enzymes, that are not removed rap- idly enough by the slowly moving coronary blood flow. The high concentrations of these abnormal products then stimulate pain nerve endings in the cardiac muscle, send- ing pain impulses through sensory afferent nerve fibers into the central nervous system.
Angina Pectoris
In most people who develop progressive constriction of their coronary arteries, cardiac pain, called angina pectoris, begins to appear whenever the load on the heart becomes too great in relation to the available coronary blood flow. This pain is usually felt beneath the upper sternum over the heart, and in addition it is often referred to distant surface areas of the body, most commonly to the left arm
and left shoulder but also frequently to the neck and even to the side of the face. The reason for this distribution of pain is that the heart originates during embryonic life in the neck, as do the arms. Therefore, both the heart and these surface areas of the body receive pain nerve fibers from the same spinal cord segments.
Most people who have chronic angina pecto- ris feel pain when they exercise or when they experi- ence emotions that increase metabolism of the heart or temporarily constrict the coronary vessels because of sympathetic vasoconstrictor nerve signals. Anginal pain is also exacerbated by cold temperatures or by hav- ing a full stomach, both of which increase the workload of the heart. The pain usually lasts for only a few min- utes. However, some patients have such severe and last- ing ischemia that the pain is present all the time. The pain is frequently described as hot, pressing, and con- stricting; it is of such quality that it usually makes the patient stop all unnecessary body activity and come to a complete state of rest.
Treatment with Drugs. Several vasodilator drugs, when administered during an acute anginal attack, can often give immediate relief from the pain. Commonly used short-acting vasodilators are nitroglycerin and other nitrate drugs. Other vasodilators, such as angiotensin con- verting enzyme inhibitors, angiotensin receptor blockers, calcium channel blockers, and ranolazine, may be benefi- cial in treating chronic stable angina pectoris.
Another class of drugs used for prolonged treatment of angina pectoris is the beta blockers, such as propranolol. These drugs block sympathetic beta-adrenergic recep- tors, which prevents sympathetic enhancement of heart rate and cardiac metabolism during exercise or emo- tional episodes. Therefore, therapy with a beta blocker decreases the need of the heart for extra metabolic oxy- gen during stressful conditions. For obvious reasons, this can also reduce the number of anginal attacks, as well as their severity.
Surgical Treatment of Coronary Artery Disease
Aortic-Coronary Bypass Surgery. In many patients with coronary ischemia, the constricted areas of the cor- onary arteries are located at only a few discrete points blocked by atherosclerotic disease and the coronary ves- sels elsewhere are normal or almost normal. A surgical procedure was developed in the 1960s, called aortic-cor- onary bypass, for removing a section of a subcutaneous vein from an arm or leg and then grafting this vein from the root of the aorta to the side of a peripheral coronary artery beyond the atherosclerotic blockage point. One to five such grafts are usually performed, each of which sup- plies a peripheral coronary artery beyond a block.
Anginal pain is relieved in most patients. Also, in patients whose hearts have not become too severely dam- aged before the operation, the coronary bypass procedure may provide the patient with normal survival expectation. If the heart has already been severely damaged, however, the bypass procedure is likely to be of little value.
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Coronary Angioplasty. Since the 1980s, a proce- dure has been used to open partially blocked coronary vessels before they become totally occluded. This pro- cedure, called coronary artery angioplasty, is the follow- ing: A small balloon-tipped catheter, about 1 millimeter in diameter, is passed under radiographic guidance into the coronary system and pushed through the partially occluded artery until the balloon portion of the catheter straddles the partially occluded point. Then the balloon is inflated with high pressure, which markedly stretches the diseased artery. After this procedure is performed, the blood flow through the vessel often increases three- fold to fourfold, and more than 75 percent of the patients who undergo the procedure are relieved of the coronary ischemic symptoms for at least several years, although many of the patients still eventually require coronary bypass surgery.
Small stainless steel mesh tubes called “stents” are some- times placed inside a coronary artery dilated by angioplasty to hold the artery open, thus preventing its restenosis. Within a few weeks after the stent is placed in the coro- nary artery, the endothelium usually grows over the metal surface of the stent, allowing blood to flow smoothly through the stent. However, reclosure (restenosis) of the blocked coronary artery occurs in about 25 to 40 percent of patients treated with angioplasty, often within 6 months of the initial procedure. This is usually due to excessive for- mation of scar tissue that develops underneath the healthy new endothelium that has grown over the stent. Stents that slowly release drugs (drug-eluting stents) may help to pre- vent the excessive growth of scar tissue.
Newer procedures for opening atherosclerotic coro- nary arteries are constantly in experimental develop- ment. One of these employs a laser beam from the tip of a coronary artery catheter aimed at the atherosclerotic
lesion. The laser literally dissolves the lesion without sub- stantially damaging the rest of the arterial wall.
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chapter 22
Cardiac Failure
One of the most important ailments that must be treated by the physician is cardiac failure (“heart failure”). This can result from any heart condition that reduces the ability of the heart to pump
blood. The cause is usually decreased contractility of the myocardium resulting from diminished coronary blood flow. However, failure can also be caused by damaged heart valves, external pressure around the heart, vitamin B deficiency, primary cardiac muscle disease, or any other abnormality that makes the heart a hypoeffective pump.
In this chapter, we discuss mainly cardiac failure caused by ischemic heart disease resulting from partial blockage of the coronary blood vessels, the most common cause of heart failure. In Chapter 23, we discuss valvular and con- genital heart disease.
Definition of Cardiac Failure. The term “cardiac fail- ure” means simply failure of the heart to pump enough blood to satisfy the needs of the body.
Circulatory Dynamics in Cardiac Failure
Acute Effects of Moderate Cardiac Failure
If a heart suddenly becomes severely damaged, such as by myocardial infarction, the pumping ability of the heart is immediately depressed. As a result, two main effects occur: (1) reduced cardiac output and (2) damming of blood in the veins, resulting in increased venous pressure.
The progressive changes in heart pumping effective- ness at different times after an acute myocardial infarc- tion are shown graphically in Figure 22-1. The top curve of this figure shows a normal cardiac output curve. Point A on this curve is the normal operating point, showing a normal cardiac output under resting conditions of 5 L/ min and a right atrial pressure of 0 mm Hg.
Immediately after the heart becomes damaged, the cardiac output curve becomes greatly lowered, falling to the lowest curve at the bottom of the graph. Within a few seconds, a new circulatory state is established at point B,
illustrating that the cardiac output has fallen to 2 L/min, about two-fifths normal, whereas the right atrial pressure has risen to +4 mm Hg because venous blood returning to the heart from the body is dammed up in the right atrium. This low cardiac output is still sufficient to sustain life for perhaps a few hours, but it is likely to be associated with fainting. Fortunately, this acute stage usually lasts for only a few seconds because sympathetic nervous reflexes occur almost immediately and compensate, to a great extent, for the damaged heart, as follows.
Compensation for Acute Cardiac Failure by Sympathetic Nervous Reflexes. When the cardiac out- put falls precariously low, many of the circulatory reflexes discussed in Chapter 18 are rapidly activated. The best known of these is the baroreceptor reflex, which is acti- vated by diminished arterial pressure. The chemorecep- tor reflex, the central nervous system ischemic response, and even reflexes that originate in the damaged heart also likely contribute to activating the sympathetic ner- vous system. The sympathetics therefore become strongly stimulated within a few seconds, and the parasympathetic nervous signals to the heart become reciprocally inhib- ited at the same time.
Strong sympathetic stimulation has major effects on the heart itself and on the peripheral vasculature. If all the ventricular musculature is diffusely damaged but is still functional, sympathetic stimulation strengthens this dam- aged musculature. If part of the muscle is nonfunctional and part of it is still normal, the normal muscle is strongly stimulated by sympathetic stimulation, in this way par- tially compensating for the nonfunctional muscle. Thus, the heart becomes a stronger pump as a result of sympa- thetic stimulation. This effect is illustrated in Figure 22-1, showing after sympathetic compensation about twofold elevation of the very low cardiac output curve.
Sympathetic stimulation also increases venous return because it increases the tone of most of the blood vessels of the circulation, especially the veins, raising the mean systemic filling pressure to 12 to 14 mm Hg, almost 100 percent above normal. As discussed in Chapter 20, this increased filling pressure greatly increases the tendency for blood to flow from the veins back into the heart.
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Therefore, the damaged heart becomes primed with more inflowing blood than usual, and the right atrial pressure rises still further, which helps the heart to pump still larger quantities of blood. Thus, in Figure 22-1, the new circula- tory state is depicted by point C, showing a cardiac output of 4.2 L/min and a right atrial pressure of 5 mm Hg.
The sympathetic reflexes become maximally devel- oped in about 30 seconds. Therefore, a person who has a sudden, moderate heart attack might experience noth- ing more than cardiac pain and a few seconds of fainting. Shortly thereafter, with the aid of the sympathetic reflex compensations, the cardiac output may return to a level adequate to sustain the person if he or she remains quiet, although the pain might persist.
Chronic Stage of Failure—Fluid Retention and Compensated Cardiac Output
After the first few minutes of an acute heart attack, a pro- longed semichronic state begins, characterized mainly by two events: (1) retention of fluid by the kidneys and (2) varying degrees of recovery of the heart itself over a period of weeks to months, as illustrated by the light green curve in Figure 22-1; this was also discussed in Chapter 21.
Renal Retention of Fluid and Increase in Blood Volume Occur for Hours to Days
A low cardiac output has a profound effect on renal function, sometimes causing anuria when the cardiac output falls to 50 to 60 percent of normal. In general, the urine output remains below normal as long as the cardiac output and arterial pressure remain significantly less than normal; urine output usually does not return all the way to normal after an acute heart attack until the cardiac output and arterial pressure rise almost to normal levels.
Moderate Fluid Retention in Cardiac Failure Can Be Beneficial. Many cardiologists have considered fluid retention always to have a detrimental effect in cardiac failure. But it is now known that a moderate increase in body fluid and blood volume is an important factor in helping to compensate for the diminished pumping ability of the heart by increasing the venous return. The increased blood volume increases venous return in two ways: First, it increases the mean systemic filling pres- sure, which increases the pressure gradient for causing venous flow of blood toward the heart. Second, it distends the veins, which reduces the venous resistance and allows even more ease of flow of blood to the heart.
If the heart is not too greatly damaged, this increased venous return can often fully compensate for the heart’s diminished pumping ability—enough that even when the heart’s pumping ability is reduced to as low as 40 to 50 percent of normal, the increased venous return can often cause entirely nearly normal cardiac output as long as the person remains in a quiet resting state.
When the heart’s pumping capability is reduced fur- ther, blood flow to the kidneys finally becomes too low for the kidneys to excrete enough salt and water to equal salt and water intake. Therefore, fluid retention begins and continues indefinitely, unless major therapeutic pro- cedures are used to prevent this. Furthermore, because the heart is already pumping at its maximum pumping capacity, this excess fluid no longer has a beneficial effect on the circulation. Instead, the fluid retention increases the workload on the already damaged heart and severe edema develops throughout the body, which can be very detrimental in itself and can lead to death.
Detrimental Effects of Excess Fluid Retention in Severe Cardiac Failure. In contrast to the beneficial effects of moderate fluid retention in cardiac failure, in severe failure extreme excesses of fluid can have serious physiological consequences. They include (1) increasing the workload on the damaged heart, (2) overstretching of the heart, thus weakening the heart still more; (3) filtra- tion of fluid into the lungs, causing pulmonary edema and consequent deoxygenation of the blood; and (4) develop- ment of extensive edema in most parts of the body. These detrimental effects of excessive fluid are discussed in later sections of this chapter.
Recovery of the Myocardium After Myocardial Infarction
After a heart becomes suddenly damaged as a result of myocardial infarction, the natural reparative processes of the body begin to help restore normal cardiac func- tion. For instance, a new collateral blood supply begins to penetrate the peripheral portions of the infarcted area of the heart, often causing much of the heart muscle in the fringe areas to become functional again. Also, the undam- aged portion of the heart musculature hypertrophies, in this way offsetting much of the cardiac damage.
The degree of recovery depends on the type of car- diac damage, and it varies from no recovery to almost
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Figure 22-1 Progressive changes in the cardiac output curve after acute myocardial infarction. Both the cardiac output and right atrial pressure change progressively from point A to point D (illustrated by the black line) over a period of seconds, minutes, days, and weeks.
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complete recovery. After acute myocardial infarction, the heart ordinarily recovers rapidly during the first few days and weeks and achieves most of its final state of recovery within 5 to 7 weeks, although mild degrees of additional recovery can continue for months.
Cardiac Output Curve After Partial Recovery. Figure 22-1 shows function of the partially recovered heart a week or so after acute myocardial infarction. By this time, con- siderable fluid has been retained in the body and the ten- dency for venous return has increased markedly as well; therefore, the right atrial pressure has risen even more. As a result, the state of the circulation is now changed from point C to point D, which shows a normal cardiac output of 5 L/min but right atrial pressure increased to 6 mm Hg.
Because the cardiac output has returned to normal, renal output of fluid also returns to normal and no further fluid retention occurs, except that the retention of fluid that has already occurred continues to maintain moder- ate excesses of fluid. Therefore, except for the high right atrial pressure represented by point D in this figure, the person now has essentially normal cardiovascular dynam- ics as long as he or she remains at rest.
If the heart recovers to a significant extent and if ade- quate fluid volume has been retained, the sympathetic stimulation gradually abates toward normal for the fol- lowing reasons: The partial recovery of the heart can ele- vate the cardiac output curve the same as sympathetic stimulation can. Therefore, as the heart recovers even slightly, the fast pulse rate, cold skin, and pallor resulting from sympathetic stimulation in the acute stage of cardiac failure gradually disappear.
Summary of the Changes That Occur After Acute Cardiac Failure—“Compensated Heart Failure”
To summarize the events discussed in the past few sec- tions describing the dynamics of circulatory changes after an acute, moderate heart attack, we can divide the stages into (1) the instantaneous effect of the cardiac damage; (2) compensation by the sympathetic nervous system, which occurs mainly within the first 30 seconds to 1 minute; and (3) chronic compensations resulting from partial heart recovery and renal retention of fluid. All these changes are shown graphically by the black line in Figure 22-1. The progression of this line shows the normal state of the cir- culation (point A), the state a few seconds after the heart attack but before sympathetic reflexes have occurred (point B), the rise in cardiac output toward normal caused by sympathetic stimulation (point C), and final return of the cardiac output to almost normal after several days to several weeks of partial cardiac recovery and fluid reten- tion (point D). This final state is called compensated heart failure.
Compensated Heart Failure. Note especially in Figure 22-1 that the maximum pumping ability of the partly recovered heart, as depicted by the plateau level
of the light green curve, is still depressed to less than one-half normal. This demonstrates that an increase in right atrial pressure can maintain the cardiac output at a normal level despite continued weakness of the heart. Thus, many people, especially older people, have nor- mal resting cardiac outputs but mildly to moderately ele- vated right atrial pressures because of various degrees of “compensated heart failure.” These persons may not know that they have cardiac damage because the dam- age often has occurred a little at a time, and the compen- sation has occurred concurrently with the progressive stages of damage.
When a person is in compensated heart failure, any attempt to perform heavy exercise usually causes imme- diate return of the symptoms of acute failure because the heart is not able to increase its pumping capacity to the levels required for the exercise. Therefore, it is said that the cardiac reserve is reduced in compensated heart fail- ure. This concept of cardiac reserve is discussed more fully later in the chapter.
Dynamics of Severe Cardiac Failure— Decompensated Heart Failure
If the heart becomes severely damaged, no amount of compensation, either by sympathetic nervous reflexes or by fluid retention, can make the excessively weakened heart pump a normal cardiac output. As a consequence, the cardiac output cannot rise high enough to make the kidneys excrete normal quantities of fluid. Therefore, fluid continues to be retained, the person develops more and more edema, and this state of events eventually leads to death. This is called decompensated heart failure. Thus, the main cause of decompensated heart failure is failure of the heart to pump sufficient blood to make the kidneys excrete daily the necessary amounts of fluid.
Graphical Analysis of Decompensated Heart Failure. Figure 22-2 shows greatly depressed cardiac out- put at different times (points A to F) after the heart has become severely weakened. Point A on this curve rep- resents the approximate state of the circulation before any compensation has occurred, and point B, the state a few minutes later after sympathetic stimulation has
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Figure 22-2 Greatly depressed cardiac output that indicates de compensated heart disease. Progressive fluid retention raises the right atrial pressure over a period of days, and the cardiac out- put progresses from point A to point F, until death occurs.
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compensated as much as it can but before fluid retention has begun. At this time, the cardiac output has risen to 4 L/min and the right atrial pressure has risen to 5 mm Hg. The person appears to be in reasonably good condi- tion, but this state will not remain stable because the car- diac output has not risen high enough to cause adequate kidney excretion of fluid; therefore, fluid retention contin- ues and can eventually be the cause of death. These events can be explained quantitatively in the following way.
Note the straight line in Figure 22-2, at a cardiac out- put level of 5 L/min. This is approximately the critical cardiac output level that is required in the normal adult person to make the kidneys re-establish normal fluid balance—that is, for the output of salt and water to be as great as the intake of these. At any cardiac output below this level, all the fluid-retaining mechanisms discussed in the earlier section remain in play and the body fluid vol- ume increases progressively. And because of this progres- sive increase in fluid volume, the mean systemic filling pressure of the circulation continues to rise; this forces progressively increasing quantities of blood from the per- son’s peripheral veins into the right atrium, thus increas- ing the right atrial pressure. After 1 day or so, the state of the circulation changes in Figure 22-2 from point B to point C—the right atrial pressure rising to 7 mm Hg and the cardiac output to 4.2 L/min. Note again that the car- diac output is still not high enough to cause normal renal output of fluid; therefore, fluid continues to be retained. After another day or so, the right atrial pressure rises to 9 mm Hg, and the circulatory state becomes that depicted by point D. Still, the cardiac output is not enough to estab- lish normal fluid balance.
After another few days of fluid retention, the right atrial pressure has risen still further, but by now, cardiac function is beginning to decline toward a lower level. This decline is caused by overstretch of the heart, edema of the heart muscle, and other factors that diminish the heart’s pumping performance. It is now clear that further reten- tion of fluid will be more detrimental than beneficial to the circulation. Yet the cardiac output still is not high enough to bring about normal renal function, so fluid retention not only continues but accelerates because of the fall- ing cardiac output (and falling arterial pressure that also occurs). Consequently, within a few days, the state of the circulation has reached point F on the curve, with the car- diac output now less than 2.5 L/min and the right atrial pressure 16 mm Hg. This state has approached or reached incompatibility with life, and the patient dies unless this chain of events can be reversed. This state of heart failure in which the failure continues to worsen is called decom- pensated heart failure.
Thus, one can see from this analysis that failure of the cardiac output (and arterial pressure) to rise to the critical level required for normal renal function results in (1) pro- gressive retention of more and more fluid, which causes (2) progressive elevation of the mean systemic filling pressure, and (3) progressive elevation of the right atrial pressure until finally the heart is so overstretched or so
edematous that it cannot pump even moderate quantities of blood and, therefore, fails completely. Clinically, one detects this serious condition of decompensation princi- pally by the progressing edema, especially edema of the lungs, which leads to bubbling rales (a crackling sound) in the lungs and to dyspnea (air hunger). Lack of appropri- ate therapy when this state of events occurs rapidly leads to death.
Treatment of Decompensation. The decompen- sation process can often be stopped by (1) strengthen- ing the heart in any one of several ways, especially by administration of a cardiotonic drug, such as digitalis, so that the heart becomes strong enough to pump ade- quate quantities of blood required to make the kidneys function normally again, or (2) administering diuretic drugs to increase kidney excretion while at the same time reducing water and salt intake, which brings about a bal- ance between fluid intake and output despite low cardiac output.
Both methods stop the decompensation process by re- establishing normal fluid balance so that at least as much fluid leaves the body as enters it.
Mechanism of Action of the Cardiotonic Drugs Such as Digitalis. Cardiotonic drugs, such as digitalis, when administered to a person with a healthy heart, have little effect on increasing the contractile strength of the cardiac muscle. However, when administered to a person with a chronically failing heart, the same drugs can sometimes increase the strength of the failing myo- cardium as much as 50 to 100 percent. Therefore, they are one of the mainstays of therapy in chronic heart failure.
Digitalis and other cardiotonic glycosides are believed to strengthen heart contractions by increasing the quantity of calcium ions in muscle fibers. This effect is likely due to inhibition of sodium-potassium ATPase in cardiac cell membranes. Inhibition of the sodium- potassium pump increases intracellular sodium concen- tration and slows the sodium-calcium exchange pump, which extrudes calcium from the cell in exchange for sodium. Because the sodium-calcium exchange pump relies on a high sodium gradient across the cell mem- brane, accumulation of sodium inside the cell reduces its activity.
In the failing heart muscle, the sarcoplasmic reticu- lum fails to accumulate normal quantities of calcium and, therefore, cannot release enough calcium ions into the free-fluid compartment of the muscle fibers to cause full contraction of the muscle. The effect of digitalis to depress the sodium-calcium exchange pump and raise calcium ion concentration in cardiac muscle provides the extra calcium needed to increase the muscle contractile force. Therefore, it is usually beneficial to depress the cal- cium pumping mechanism a moderate amount using dig- italis, allowing the muscle fiber intracellular calcium level to rise slightly.
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Unilateral Left Heart Failure
In the discussions thus far in this chapter, we have consid- ered failure of the heart as a whole. Yet, in a large number of patients, especially those with early acute failure, left- sided failure predominates over right-sided failure, and, in rare instances, the right side fails without significant failure of the left side. Therefore, we need to discuss the special features of unilateral heart failure.
When the left side of the heart fails without concomi- tant failure of the right side, blood continues to be pumped into the lungs with usual right heart vigor, whereas it is not pumped adequately out of the lungs by the left heart into the systemic circulation. As a result, the mean pulmonary filling pressure rises because of shift of large volumes of blood from the systemic circulation into the pulmonary circulation.
As the volume of blood in the lungs increases, the pulmonary capillary pressure increases, and if this rises above a value approximately equal to the colloid osmotic pressure of the plasma, about 28 mm Hg, fluid begins to filter out of the capillaries into the lung interstitial spaces and alveoli, resulting in pulmonary edema.
Thus, among the most important problems of left heart failure are pulmonary vascular congestion and pul- monary edema. In severe, acute left heart failure, pul- monary edema occasionally occurs so rapidly that it can cause death by suffocation in 20 to 30 minutes, which we discuss later in the chapter.
Low-Output Cardiac Failure- Cardiogenic Shock
In many instances after acute heart attacks and often after prolonged periods of slow progressive cardiac deteriora- tion, the heart becomes incapable of pumping even the minimal amount of blood flow required to keep the body alive. Consequently, the body tissues begin to suffer and even to deteriorate, often leading to death within a few hours to a few days. The picture then is one of circulatory shock, as explained in Chapter 24. Even the cardiovascu- lar system suffers from lack of nutrition, and it, too (along with the remainder of the body), deteriorates, thus has- tening death. This circulatory shock syndrome caused by inadequate cardiac pumping is called cardiogenic shock or simply cardiac shock. Once a person develops cardiogenic shock, the survival rate is often less than 30 percent even with appropriate medical care.
Vicious Circle of Cardiac Deterioration in Cardiogenic Shock. The discussion of circulatory shock in Chapter 24 emphasizes the tendency for the heart to become progressively more damaged when its coronary blood supply is reduced during the course of the shock. That is, the low arterial pressure that occurs during shock reduces the coronary blood supply even more. This makes
the heart still weaker, which makes the arterial pressure fall still more, which makes the shock progressively worse, the process eventually becoming a vicious circle of car- diac deterioration. In cardiogenic shock caused by myo- cardial infarction, this problem is greatly compounded by already existing coronary vessel blockage. For instance, in a healthy heart, the arterial pressure usually must be reduced below about 45 mm Hg before cardiac deteriora- tion sets in. However, in a heart that already has a blocked major coronary vessel, deterioration begins when the coronary arterial pressure falls below 80 to 90 mm Hg. In other words, even a small decrease in arterial pres- sure can now set off a vicious circle of cardiac deteriora- tion. For this reason, in treating myocardial infarction, it is extremely important to prevent even short periods of hypotension.
Physiology of Treatment. Often a patient dies of cardiogenic shock before the various compensatory processes can return the cardiac output (and arterial pres- sure) to a life-sustaining level. Therefore, treatment of this condition is one of the most important problems in the management of acute heart attacks.
Immediate administration of digitalis is often used for strengthening the heart if the ventricular muscle shows signs of deterioration. Also, infusion of whole blood, plasma, or a blood pressure–raising drug is used to sus- tain the arterial pressure. If the arterial pressure can be elevated high enough, the coronary blood flow often will increase enough to prevent the vicious circle of deterio- ration. And this allows enough time for appropriate com- pensatory mechanisms in circulatory system to correct the shock.
Some success has also been achieved in saving the lives of patients in cardiogenic shock by using one of the follow- ing procedures: (1) surgically removing the clot in the cor- onary artery, often in combination with coronary bypass graft, or (2) catheterizing the blocked coronary artery and infusing either streptokinase or tissue-type plasminogen activator enzymes that cause dissolution of the clot. The results are occasionally astounding when one of these procedures is instituted within the first hour of cardio- genic shock but of little, if any, benefit after 3 hours.
Edema in Patients with Cardiac Failure
Inability of Acute Cardiac Failure to Cause Peripheral Edema. Acute left heart failure can cause rapid congestion of the lungs, with development of pul- monary edema and even death within minutes to hours.
However, either left or right heart failure is very slow to cause peripheral edema. This can best be explained by referring to Figure 22-3. When a previously healthy heart acutely fails as a pump, the aortic pressure falls and the right atrial pressure rises. As the cardiac out- put approaches zero, these two pressures approach each other at an equilibrium value of about 13 mm Hg.
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Capillary pressure also falls from its normal value of 17 mm Hg to the new equilibrium pressure of 13 mm Hg. Thus, severe acute cardiac failure often causes a fall in peripheral capillary pressure rather than a rise. Therefore, animal experiments, as well as experience in humans, show that acute cardiac failure almost never causes immediate development of peripheral edema.
Long-Term Fluid Retention by the Kidneys— the Cause of Peripheral Edema in Persisting Heart Failure
After the first day or so of overall heart failure or of right- ventricular heart failure, peripheral edema does begin to occur principally because of fluid retention by the kid- neys. The retention of fluid increases the mean systemic filling pressure, resulting in increased tendency for blood to return to the heart. This elevates the right atrial pres- sure to a still higher value and returns the arterial pressure back toward normal. Therefore, the capillary pressure now also rises markedly, thus causing loss of fluid into the tissues and development of severe edema.
There are several known causes of the reduced renal output of urine during cardiac failure.
1. Decreased glomerular filtration rate. A decrease in cardiac output has a tendency to reduce the glo- merular pressure in the kidneys because of (1) reduced arterial pressure and (2) intense sympathetic constric- tion of the afferent arterioles of the kidney. As a conse- quence, except in the mildest degrees of heart failure, the glomerular filtration rate becomes less than nor- mal. It is clear from the discussion of kidney function in Chapters 26 through 29 that even a slight decrease in glomerular filtration often markedly decreases urine output. When the cardiac output falls to about one- half normal, this can result in almost complete anuria.
2. Activation of the renin-angiotensin system and increased reabsorption of water and salt by the renal tubules. The reduced blood flow to the kidneys
causes marked increase in renin secretion by the kid- neys, and this in turn increases the formation of angio- tensin II, as described in Chapter 19. The angiotensin in turn has a direct effect on the arterioles of the kid- neys to decrease further the blood flow through the kidneys, which reduces the pressure in the peritubu- lar capillaries surrounding the renal tubules, promot- ing greatly increased reabsorption of both water and salt from the tubules. Angiotensin also acts directly on the renal tubular epithelial cells to stimulate reabsorp- tion of salt and water. Therefore, loss of water and salt into the urine decreases greatly, and large quantities of salt and water accumulate in the blood and interstitial fluids everywhere in the body.
3. Increased aldosterone secretion. In the chronic stage of heart failure, large quantities of aldosterone are secreted by the adrenal cortex. This results mainly from the effect of angiotensin to stimulate aldoster- one secretion by the adrenal cortex. But some of the increase in aldosterone secretion often results from increased plasma potassium. Excess potassium is one of the most powerful stimuli known for aldos- terone secretion, and the potassium concentration rises in response to reduced renal function in cardiac failure. The elevated aldosterone level further increases the reabsorption of sodium from the renal tubules. This in turn leads to a secondary increase in water reab- sorption for two reasons: First, as the sodium is reab- sorbed, it reduces the osmotic pressure in the tubules but increases the osmotic pressure in the renal inter- stitial fluids; these changes promote osmosis of water into the blood. Second, the absorbed sodium and anions that go with the sodium, mainly chloride ions, increase the osmotic concentration of the extracellular fluid everywhere in the body. This elicits antidiuretic hormone secretion by the hypothalamic-posterior pituitary gland system (discussed in Chapter 29). The antidiuretic hormone in turn promotes still greater increase in tubular reabsorption of water.
4. Activation of the sympathetic nervous system. As discussed previously, heart failure causes marked activation of the sympathetic nervous system, which in turn has several effects that lead to salt and water retention by the kidneys: (1) constriction of renal affer- ent arterioles, which reduces glomerular filtration rate; (2) stimulation of renal tubular reabsorption of salt and water by activation of alpha-adrenergic receptors on tubular epithelial cells; (3) stimulation of renin release and angiotensin II formation, which increases renal tubular reabsorption; and (4) stimulation of antidi- uretic hormone release from the posterior pituitary, which then increases water reabsorption by the renal tubules. These effects of sympathetic stimulation are discussed in more detail in Chapters 26 and 27.
Role of Atrial Natriuretic Peptide to Delay Onset of Cardiac Decompensation. Atrial natriuretic peptide
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Figure 22-3 Progressive changes in mean aortic pressure, periph- eral tissue capillary pressure, and right atrial pressure as the cardiac output falls from normal to zero.
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(ANP) is a hormone released by the atrial walls of the heart when they become stretched. Because heart fail- ure almost always increases both the right and left atrial pressures that stretch the atrial walls, the circulating levels of ANP in the blood may increase 5- to 10-fold in severe heart failure. The ANP in turn has a direct effect on the kidneys to increase greatly their excretion of salt and water. Therefore, ANP plays a natural role to help prevent extreme congestive symptoms during cardiac failure. The renal effects of ANP are discussed in Chapter 29.
Acute Pulmonary Edema in Late-Stage Heart Failure—Another Lethal Vicious Circle
A frequent cause of death in heart failure is acute pulmo- nary edema occurring in patients who have already had chronic heart failure for a long time. When this occurs in a person without new cardiac damage, it usually is set off by some temporary overload of the heart, such as might result from a bout of heavy exercise, some emotional expe- rience, or even a severe cold. The acute pulmonary edema is believed to result from the following vicious circle:
1. A temporarily increased load on the already weak left ventricle initiates the vicious circle. Because of lim- ited pumping capacity of the left heart, blood begins to dam up in the lungs.
2. The increased blood in the lungs elevates the pulmo- nary capillary pressure, and a small amount of fluid begins to transude into the lung tissues and alveoli.
3. The increased fluid in the lungs diminishes the degree of oxygenation of the blood.
4. The decreased oxygen in the blood further weakens the heart and also weakens the arterioles everywhere in the body, thus causing peripheral vasodilation.
5. The peripheral vasodilation increases venous return of blood from the peripheral circulation still more.
6. The increased venous return further increases the damming of the blood in the lungs, leading to still more transudation of fluid, more arterial oxygen desatura- tion, more venous return, and so forth. Thus, a vicious circle has been established.
Once this vicious circle has proceeded beyond a cer- tain critical point, it will continue until death of the patient unless heroic therapeutic measures are used within min- utes. The types of heroic therapeutic measures that can reverse the process and save the patient’s life include the following:
1. Putting tourniquets on both arms and legs to sequester much of the blood in the veins and, therefore, decrease the workload on the left side of the heart
2. Giving a rapidly acting diuretic, such as furosemide, to cause rapid loss of fluid from the body
3. Giving the patient pure oxygen to breathe to reverse the blood oxygen desaturation, the heart deterioration, and the peripheral vasodilation
4. Giving the patient a rapidly acting cardiotonic drug, such as digitalis, to strengthen the heart
This vicious circle of acute pulmonary edema can pro- ceed so rapidly that death can occur in 20 minutes to 1 hour. Therefore, any procedure that is to be successful must be instituted immediately.
Cardiac Reserve
The maximum percentage that the cardiac output can increase above normal is called the cardiac reserve. Thus, in the healthy young adult, the cardiac reserve is 300 to 400 percent. In athletically trained persons, it is 500 to 600 percent or more. But in heart failure, there is no cardiac reserve. As an example of normal reserve, during severe exercise the cardiac output of a healthy young adult can rise to about five times normal; this is an increase above normal of 400 percent—that is, a cardiac reserve of 400 percent.
Any factor that prevents the heart from pumping blood satisfactorily will decrease the cardiac reserve. This can result from ischemic heart disease, primary myocar- dial disease, vitamin deficiency that affects cardiac mus- cle, physical damage to the myocardium, valvular heart disease, and many other factors, some of which are shown in Figure 22-4.
Diagnosis of Low Cardiac Reserve—Exercise Test. As long as persons with low cardiac reserve remain in a state of rest, they usually will not experience major symptoms of heart disease. However, a diagnosis of low cardiac reserve usually can be easily made by requiring the person to exercise either on a treadmill or by walk- ing up and down steps, either of which requires greatly increased cardiac output. The increased load on the heart rapidly uses up the small amount of reserve that is avail- able, and the cardiac output soon fails to rise high enough to sustain the body’s new level of activity. The acute effects are as follows:
1. Immediate and sometimes extreme shortness of breath (dyspnea) resulting from failure of the heart to pump
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Figure 22-4 Cardiac reserve in different conditions, showing less than zero reserve for two of the conditions.
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sufficient blood to the tissues, thereby causing tissue ischemia and creating a sensation of air hunger
2. Extreme muscle fatigue resulting from muscle is- chemia, thus limiting the person’s ability to continue with the exercise
3. Excessive increase in heart rate because the nervous reflexes to the heart overreact in an attempt to over- come the inadequate cardiac output
Exercise tests are part of the armamentarium of the cardiologist. These tests take the place of cardiac output measurements that cannot be made with ease in most clinical settings.
Quantitative Graphical Method for Analysis of Cardiac Failure
Although it is possible to understand most general prin- ciples of cardiac failure using mainly qualitative logic, as we have done thus far in this chapter, one can grasp the importance of the different factors in cardiac failure with far greater depth by using more quantitative approaches. One such approach is the graphical method for analysis of cardiac output regulation introduced in Chapter 20. In the remaining sections of this chapter, we analyze several aspects of cardiac failure, using this graphical technique.
Graphical Analysis of Acute Heart Failure and Chronic Compensation
Figure 22-5 shows cardiac output and venous return curves for different states of the heart and peripheral cir- culation. The two curves passing through Point A are (1) the normal cardiac output curve and (2) the normal venous return curve. As pointed out in Chapter 20, there is only one point on each of these two curves at which the circulatory system can operate—point A where the two curves cross. Therefore, the normal state of the circula- tion is a cardiac output and venous return of 5 L/min and a right atrial pressure of 0 mm Hg.
Effect of Acute Heart Attack. During the first few sec- onds after a moderately severe heart attack, the cardiac output curve falls to the lowermost curve. During these few seconds, the venous return curve still has not changed because the peripheral circulatory system is still operat- ing normally. Therefore, the new state of the circulation is depicted by point B, where the new cardiac output curve
crosses the normal venous return curve. Thus, the right atrial pressure rises immediately to 4 mm Hg, whereas the cardiac output falls to 2 L/min.
Effect of Sympathetic Reflexes. Within the next 30 seconds, the sympathetic reflexes become very active. They raise both the cardiac output and the venous return curves. Sympathetic stimulation can increase the plateau level of the cardiac output curve as much as 30 to 100 per- cent. It can also increase the mean systemic filling pres- sure (depicted by the point where the venous return curve crosses the zero venous return axis) by several millimeters of mercury—in this figure, from a normal value of 7 mm Hg up to 10 mm Hg. This increase in mean systemic filling pressure shifts the entire venous return curve to the right and upward. The new cardiac output and venous return curves now equilibrate at point C, that is, at a right atrial pressure of +5 mm Hg and a cardiac output of 4 L/min.
Compensation During the Next Few Days. During the ensuing week, the cardiac output and venous return curves rise further because of (1) some recovery of the heart and (2) renal retention of salt and water, which raises the mean systemic filling pressure still further—this time up to +12 mm Hg. The two new curves now equilibrate at point D. Thus, the cardiac output has now returned to normal. The right atrial pressure, however, has risen still further to +6 mm Hg. Because the cardiac output is now normal, renal output is also normal, so a new state of equilibrated fluid balance has been achieved. The cir- culatory system will continue to function at point D and remain stable, with a normal cardiac output and an ele- vated right atrial pressure, until some additional extrin- sic factor changes either the cardiac output curve or the venous return curve.
Using this technique for analysis, one can see espe- cially the importance of moderate fluid retention and how it eventually leads to a new stable state of the circulation in mild to moderate heart failure. And one can also see the interrelation between mean systemic filling pressure and cardiac pumping at various degrees of heart failure.
Note that the events described in Figure 22-5 are the same as those presented in Figure 22-1, but in Figure 22-5, they are presented in a more quantitative manner.
Graphical Analysis of “Decompensated” Cardiac Failure
The black cardiac output curve in Figure 22-6 is the same as the curve shown in Figure 22-2, a greatly depressed curve that has already reached a degree of recovery as great as this heart can achieve. In this figure, we have added venous return curves that occur during successive days after the acute fall of the cardiac output curve to this low level. At point A, the curve at time zero equates with the normal venous return curve to give a cardiac output of about 3 L/min. However, stimulation of the sympa- thetic nervous system, caused by this low cardiac out- put, increases the mean systemic filling pressure within 30 seconds from 7 to 10.5 mm Hg. This shifts the venous
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return curve upward and to the right to produce the curve labeled “autonomic compensation.” Thus, the new venous return curve equates with the cardiac output curve at point B. The cardiac output has been improved to a level of 4 L/min but at the expense of an additional rise in right atrial pressure to 5 mm Hg.
The cardiac output of 4 L/min is still too low to cause the kidneys to function normally. Therefore, fluid con- tinues to be retained, and the mean systemic filling pres- sure rises from 10.5 to almost 13 mm Hg. Now the venous return curve becomes that labeled “2nd day” and equili- brates with the cardiac output curve at point C. The car- diac output rises to 4.2 L/min and the right atrial pressure to 7 mm Hg.
During the succeeding days, the cardiac output never rises quite high enough to re-establish normal renal func- tion. Fluid continues to be retained, the mean systemic filling pressure continues to rise, the venous return curve continues to shift to the right, and the equilibrium point between the venous return curve and the cardiac output curve also shifts progressively to point D, to point E, and, finally, to point F. The equilibration process is now on the down slope of the cardiac output curve, so further reten- tion of fluid causes even more severe cardiac edema and a detrimental effect on cardiac output. The condition accel- erates downhill until death occurs.
Thus, “decompensation” results from the fact that the cardiac output curve never rises to the critical level of 5 L/min needed to re-establish normal kidney excretion of fluid that would be required to cause balance between fluid input and output.
Treatment of Decompensated Heart Disease with Digitalis. Let us assume that the stage of decompensa- tion has already reached point E in Figure 22-6, and let us proceed to the same point E in Figure 22-7. At this time, digitalis is given to strengthen the heart. This raises the cardiac output curve to the level shown in Figure 22-7, but there is not an immediate change in the venous return curve. Therefore, the new cardiac output curve equates
with the venous return curve at point G. The cardiac out- put is now 5.7 L/min, a value greater than the critical level of 5 liters required to make the kidneys excrete normal amounts of urine. Therefore, the kidneys eliminate much more fluid than normally, causing diuresis, a well-known therapeutic effect of digitalis.
The progressive loss of fluid over a period of several days reduces the mean systemic filling pressure back down to 11.5 mm Hg, and the new venous return curve becomes the curve labeled “Several days later.” This curve equates with the cardiac output curve of the digitalized heart at point H, at an output of 5 L/min and a right atrial pressure of 4.6 mm Hg. This cardiac output is pre- cisely that required for normal fluid balance. Therefore, no additional fluid will be lost and none will be gained. Consequently, the circulatory system has now stabilized, or in other words, the decompensation of the heart fail- ure has been “compensated.” And to state this another way, the final steady-state condition of the circulation is defined by the crossing point of three curves: the cardiac output curve, the venous return curve, and the critical level for normal fluid balance. The compensatory mech- anisms automatically stabilize the circulation when all three curves cross at the same point.
Graphical Analysis of High-Output Cardiac Failure
Figure 22-8 gives an analysis of two types of high-output cardiac failure. One of these is caused by an arteriovenous fistula that overloads the heart because of excessive venous return, even though the pumping capability of the heart is not depressed. The other is caused by beriberi, in which the venous return is greatly increased because of diminished systemic vascular resistance, but at the same time, the pumping capability of the heart is depressed.
Arteriovenous Fistula. The “normal” curves of Figure 22-8 depict the normal cardiac output and normal venous return curves. These equate with each other at point A, which depicts a normal cardiac output of 5 L/min and a normal right atrial pressure of 0 mm Hg.
Now let us assume that the systemic vascular resistance (the total peripheral vascular resistance) becomes greatly decreased because of opening a large arteriovenous fistula (a direct opening between a large artery and a large vein). The venous return curve rotates upward to give the curve
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Figure 22-6 Graphical analysis of decompensated heart disease showing progressive shift of the venous return curve to the right as a result of continued fluid retention.
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Figure 22-7 Treatment of decompensated heart disease showing the effect of digitalis in elevating the cardiac output curve, this in turn causing increased urine output and progressive shift of the venous return curve to the left.
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labeled “AV fistula.” This venous return curve equates with the normal cardiac output curve at point B, with a cardiac output of 12.5 L/min and a right atrial pressure of 3 mm Hg. Thus, the cardiac output has become greatly elevated, the right atrial pressure is slightly elevated, and there are mild signs of peripheral congestion. If the per- son attempts to exercise, he or she will have little cardiac reserve because the heart is already at near-maximum capacity to pump the extra blood through the arterio- venous fistula. This condition resembles a failure condi- tion and is called “high-output failure,” but in reality, the heart is overloaded by excess venous return.
Beriberi. Figure 22-8 shows the approximate changes in the cardiac output and venous return curves caused by beriberi. The decreased level of the cardiac output curve is caused by weakening of the heart because of the avi- taminosis (mainly lack of thiamine) that causes the beri- beri syndrome. The weakening of the heart has decreased the blood flow to the kidneys. Therefore, the kidneys have retained a large amount of extra body fluid, which in turn has increased the mean systemic filling pressure (repre- sented by the point where the venous return curve now intersects the zero cardiac output level) from the nor- mal value of 7 mm Hg up to 11 mm Hg. This has shifted the venous return curve to the right. Finally, the venous return curve has rotated upward from the normal curve because the avitaminosis has dilated the peripheral blood vessels, as explained in Chapter 17.
The two blue curves (cardiac output curve and venous return curve) intersect with each other at point C, which describes the circulatory condition in beriberi, with a right atrial pressure in this instance of 9 mm Hg and a cardiac output about 65 percent above normal; this high cardiac output occurs despite the weak heart, as demonstrated by the depressed plateau level of the cardiac output curve.
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nisms, therapeutic opportunities, Acta Physiol Scand 177:391, 2003. Guyton AC, Jones CE, Coleman TG: Circulatory physiology: cardiac output
and its regulation, Philadelphia, 1973, WB Saunders. Haddad F, Doyle R, Murphy DJ, et al: Right ventricular function in cardiovas-
cular disease, part II: pathophysiology, clinical importance, and manage- ment of right ventricular failure, Circulation 117:1717, 2008.
Ikeda Y, Hoshijima M, Chien KR: Toward biologically targeted therapy of cal- cium cycling defects in heart failure, Physiology (Bethesda) 23:6, 2008.
Lohmeier TE: Neurohumoral regulation of arterial pressure in hemorrhage and heart failure, Am J Physiol Regul Integr Comp Physiol 283:R810, 2002.
Mehra MR, Gheorghiade M, Bonow RO: Mitral regurgitation in chronic heart failure: more questions than answers? Curr Cardiol Rep 6:96, 2004.
McMurray J, Pfeffer MA: New therapeutic options in congestive heart fail- ure: Part I, Circulation 105:2099, 2002.
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Morita H, Seidman J, Seidman CE: Genetic causes of human heart failure, J Clin Invest 115:518, 2005.
Pfisterer M: Right ventricular involvement in myocardial infarction and car- diogenic shock, Lancet 362:392, 2003.
Pitt B: Aldosterone blockade in patients with chronic heart failure, Cardiol Clin 26:15, 2008.
Reynolds HR, Hochman JS: Cardiogenic shock: Current concepts and improving outcomes, Circulation 117:686, 2008.
Spodick DH: Acute cardiac tamponade, N Engl J Med 349:684, 2003. Zile MR, Brutsaert DL: New concepts in diastolic dysfunction and diastolic
heart failure: Part I: diagnosis, prognosis, and measurements of diastolic function, Circulation 105:1387, 2002.
Zucker IH: Novel mechanisms of sympathetic regulation in chronic heart failure, Hypertension 48:1005, 2006.
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Figure 22-8 Graphical analysis of two types of conditions that can cause high-output cardiac failure: (1) arteriovenous (AV) fis- tula and (2) beriberi heart disease.
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chapter 23
Heart Valves and Heart Sounds; Valvular and Congenital Heart Defects
Function of the heart valves was discussed in Chapter 9, where it was pointed out that closing of the valves causes audi- ble sounds. Ordinarily, no audible sounds occur when
the valves open. In this chapter, we first discuss the fac- tors that cause the sounds in the heart under normal and abnormal conditions. Then we discuss the overall circula- tory changes that occur when valvular or congenital heart defects are present.
Heart Sounds
Normal Heart Sounds
Listening with a stethoscope to a normal heart, one hears a sound usually described as “lub, dub, lub, dub.” The “lub” is associated with closure of the atrioventricular (A-V) valves at the beginning of systole, and the “dub” is associ- ated with closure of the semilunar (aortic and pulmonary) valves at the end of systole. The “lub” sound is called the first heart sound, and the “dub” is called the second heart sound, because the normal pumping cycle of the heart is considered to start when the A-V valves close at the onset of ventricular systole.
Causes of the First and Second Heart Sounds. The earliest explanation for the cause of the heart sounds was that the “slapping” together of the valve leaflets sets up vibrations. However, this has been shown to cause little, if any, of the sound, because the blood between the leaf- lets cushions the slapping effect and prevents significant sound. Instead, the cause is vibration of the taut valves immediately after closure, along with vibration of the adjacent walls of the heart and major vessels around the heart. That is, in generating the first heart sound, con- traction of the ventricles first causes sudden backflow of blood against the A-V valves (the tricuspid and mitral valves), causing them to close and bulge toward the atria until the chordae tendineae abruptly stop the back bulg- ing. The elastic tautness of the chordae tendineae and of
the valves then causes the back-surging blood to bounce forward again into each respective ventricle. This causes the blood and the ventricular walls, as well as the taut valves, to vibrate and causes vibrating turbulence in the blood. The vibrations travel through the adjacent tissues to the chest wall, where they can be heard as sound by using the stethoscope.
The second heart sound results from sudden closure of the semilunar valves at the end of systole. When the semi- lunar valves close, they bulge backward toward the ven- tricles and their elastic stretch recoils the blood back into the arteries, which causes a short period of reverberation of blood back and forth between the walls of the arteries and the semilunar valves, as well as between these valves and the ventricular walls. The vibrations occurring in the arterial walls are then transmitted mainly along the arter- ies. When the vibrations of the vessels or ventricles come into contact with a “sounding board,” such as the chest wall, they create sound that can be heard.
Duration and Pitch of the First and Second Heart Sounds. The duration of each of the heart sounds is slightly more than 0.10 second—the first sound about 0.14 second, and the second about 0.11 second. The rea- son for the shorter second sound is that the semilunar valves are more taut than the A-V valves, so they vibrate for a shorter time than do the A-V valves.
The audible range of frequency (pitch) in the first and second heart sounds, as shown in Figure 23-1, begins at the lowest frequency the ear can detect, about 40 cycles/ sec, and goes up above 500 cycles/sec. When special elec- tronic apparatus is used to record these sounds, by far a larger proportion of the recorded sound is at frequencies and sound levels below the audible range, going down to 3 to 4 cycles/sec and peaking at about 20 cycles/sec, as illustrated by the lower shaded area in Figure 23-1. For this reason, major portions of the heart sounds can be recorded electronically in phonocardiograms even though they cannot be heard with a stethoscope.
The second heart sound normally has a higher frequency than the first heart sound for two reasons: (1) the tautness of the semilunar valves in comparison with the much less taut A-V valves, and (2) the greater elas-
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tic coefficient of the taut arterial walls that provide the principal vibrating chambers for the second sound, in comparison with the much looser, less elastic ventricu- lar chambers that provide the vibrating system for the first heart sound. The clinician uses these differences to distinguish special characteristics of the two respective sounds.
Third Heart Sound. Occasionally a weak, rumbling third heart sound is heard at the beginning of the middle third of diastole. A logical but unproved explanation of this sound is oscillation of blood back and forth between the walls of the ventricles initiated by inrushing blood from the atria. This is analogous to running water from a faucet into a paper sack, the inrushing water reverberating back and forth between the walls of the sack to cause vibra- tions in its walls. The reason the third heart sound does not occur until the middle third of diastole is believed to be that in the early part of diastole, the ventricles are not filled sufficiently to create even the small amount of elas- tic tension necessary for reverberation. The frequency of this sound is usually so low that the ear cannot hear it, yet it can often be recorded in the phonocardiogram.
Atrial Heart Sound (Fourth Heart Sound). An atrial heart sound can sometimes be recorded in the pho- nocardiogram, but it can almost never be heard with a stethoscope because of its weakness and very low fre- quency—usually 20 cycles/sec or less. This sound occurs when the atria contract, and presumably, it is caused by the inrush of blood into the ventricles, which initiates vibrations similar to those of the third heart sound.
Chest Surface Areas for Auscultation of Normal Heart Sounds Listening to the sounds of the body, usually with the aid of a stethoscope, is called auscultation. Figure 23-2 shows the areas of the chest wall from which the different heart valvular sounds can best be distinguished. Although the
sounds from all the valves can be heard from all these areas, the cardiologist distinguishes the sounds from the different valves by a process of elimination. That is, he or she moves the stethoscope from one area to another, not- ing the loudness of the sounds in different areas and grad- ually picking out the sound components from each valve.
The areas for listening to the different heart sounds are not directly over the valves themselves. The aortic area is upward along the aorta because of sound transmis- sion up the aorta, and the pulmonic area is upward along the pulmonary artery. The tricuspid area is over the right ventricle, and the mitral area is over the apex of the left ventricle, which is the portion of the heart nearest the sur- face of the chest; the heart is rotated so that the remainder of the left ventricle lies more posteriorly.
Phonocardiogram
If a microphone specially designed to detect low-fre- quency sound is placed on the chest, the heart sounds can be amplified and recorded by a high-speed record- ing apparatus. The recording is called a phonocardiogram, and the heart sounds appear as waves, as shown schemat- ically in Figure 23-3. Recording A is an example of nor- mal heart sounds, showing the vibrations of the first, second, and third heart sounds and even the very weak atrial sound. Note specifically that the third and atrial heart sounds are each a very low rumble. The third heart sound can be recorded in only one third to one half of all people, and the atrial heart sound can be recorded in perhaps one fourth of all people.
Valvular Lesions
Rheumatic Valvular Lesions
By far the greatest number of valvular lesions results from rheumatic fever. Rheumatic fever is an autoimmune dis- ease in which the heart valves are likely to be damaged or
Mitral areaTricuspid area
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Figure 23-2 Chest areas from which sound from each valve is best heard.
Inaudible Heart sounds and murmurs
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Figure 23-1 Amplitude of different-frequency vibrations in the heart sounds and heart murmurs in relation to the threshold of audibility, showing that the range of sounds that can be heard is between 40 and 520 cycles/sec. (Modified from Butterworth JS, Chassin JL, McGrath JJ: Cardiac Auscultation, 2nd ed, New York: Grune & Stratton, 1960.)
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destroyed. It is usually initiated by streptococcal toxin in the following manner.
The sequence of events almost always begins with a preliminary streptococcal infection caused specifically by group A hemolytic streptococci. These bacteria initially cause a sore throat, scarlet fever, or middle ear infection. But the streptococci also release several different proteins against which the person’s reticuloendothelial system pro- duces antibodies. The antibodies react not only with the streptococcal protein but also with other protein tissues of the body, often causing severe immunologic damage. These reactions continue to take place as long as the antibodies persist in the blood—1 year or more.
Rheumatic fever causes damage especially in certain susceptible areas, such as the heart valves. The degree of heart valve damage is directly correlated with the concen- tration and persistence of the antibodies. The principles of immunity that relate to this type of reaction are dis- cussed in Chapter 34, and it is noted in Chapter 31 that acute glomerular nephritis of the kidneys has a similar immunologic basis.
In rheumatic fever, large hemorrhagic, fibrinous, bul- bous lesions grow along the inflamed edges of the heart valves. Because the mitral valve receives more trauma dur- ing valvular action than any of the other valves, it is the one most often seriously damaged, and the aortic valve is the second most frequently damaged. The right heart valves, the tricuspid and pulmonary valves, are usually affected much less severely, probably because the low-pressure stresses that act on these valves are slight compared with the high-pressure stresses that act on the left heart valves.
Scarring of the Valves. The lesions of acute rheumatic fever frequently occur on adjacent valve leaflets simulta- neously, so the edges of the leaflets become stuck together. Then, weeks, months, or years later, the lesions become scar tissue, permanently fusing portions of adjacent valve
leaflets. Also, the free edges of the leaflets, which are nor- mally filmy and free-flapping, often become solid, scarred masses.
A valve in which the leaflets adhere to one another so extensively that blood cannot flow through it normally is said to be stenosed. Conversely, when the valve edges are so destroyed by scar tissue that they cannot close as the ventricles contract, regurgitation (backflow) of blood occurs when the valve should be closed. Stenosis usually does not occur without the coexistence of at least some degree of regurgitation, and vice versa.
Other Causes of Valvular Lesions. Stenosis or lack of one or more leaflets of a valve also occurs occasionally as a congenital defect. Complete lack of leaflets is rare; con- genital stenosis is more common, as is discussed later in this chapter.
Heart Murmurs Caused by Valvular Lesions
As shown by the phonocardiograms in Figure 23-3, many abnormal heart sounds, known as “heart murmurs,” occur when there are abnormalities of the valves, as follows.
Systolic Murmur of Aortic Stenosis. In aortic stenosis, blood is ejected from the left ventricle through only a small fibrous opening of the aortic valve. Because of the resis- tance to ejection, sometimes the blood pressure in the left ventricle rises as high as 300 mm Hg, while the pressure in the aorta is still normal. Thus, a nozzle effect is created during systole, with blood jetting at tremendous veloc- ity through the small opening of the valve. This causes severe turbulence of the blood in the root of the aorta. The turbulent blood impinging against the aortic walls causes intense vibration, and a loud murmur (see record- ing B, Figure 23-3) occurs during systole and is transmit- ted throughout the superior thoracic aorta and even into the large arteries of the neck. This sound is harsh and in severe stenosis may be so loud that it can be heard several feet away from the patient. Also, the sound vibrations can often be felt with the hand on the upper chest and lower neck, a phenomenon known as a “thrill.”
Diastolic Murmur of Aortic Regurgitation. In aortic regurgitation, no abnormal sound is heard during systole, but during diastole, blood flows backward from the high- pressure aorta into the left ventricle, causing a “blowing” murmur of relatively high pitch with a swishing quality heard maximally over the left ventricle (see recording D, Figure 23-3). This murmur results from turbulence of blood jetting backward into the blood already in the low- pressure diastolic left ventricle.
Systolic Murmur of Mitral Regurgitation. In mitral regurgitation, blood flows backward through the mitral valve into the left atrium during systole. This also causes a high-frequency “blowing,” swishing sound (see recording C, Figure 23-3) similar to that of aortic regurgitation but occurring during systole rather than diastole. It is trans- mitted most strongly into the left atrium. However, the left atrium is so deep within the chest that it is difficult to hear this sound directly over the atrium. As a result, the sound
Aortic regurgitation
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Figure 23-3 Phonocardiograms from normal and abnormal hearts.
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of mitral regurgitation is transmitted to the chest wall mainly through the left ventricle to the apex of the heart.
Diastolic Murmur of Mitral Stenosis. In mitral steno- sis, blood passes with difficulty through the stenosed mitral valve from the left atrium into the left ventricle, and because the pressure in the left atrium seldom rises above 30 mm Hg, a large pressure differential forcing blood from the left atrium into the left ventricle does not develop. Consequently, the abnormal sounds heard in mitral stenosis (see recording E, Figure 23-3) are usually weak and of very low frequency, so most of the sound spectrum is below the low-frequency end of human hearing.
During the early part of diastole, a left ventricle with a stenotic mitral valve has so little blood in it and its walls are so flabby that blood does not reverberate back and forth between the walls of the ventricle. For this reason, even in severe mitral stenosis, no murmur may be heard during the first third of diastole. Then, after partial filling, the ventricle has stretched enough for blood to reverber- ate and a low rumbling murmur begins.
Phonocardiograms of Valvular Murmurs. Phono- cardiograms B, C, D, and E of Figure 23-3 show, respec- tively, idealized records obtained from patients with aortic stenosis, mitral regurgitation, aortic regurgitation, and mitral stenosis. It is obvious from these phonocardio- grams that the aortic stenotic lesion causes the loudest murmur, and the mitral stenotic lesion causes the weak- est. The phonocardiograms show how the intensity of the murmurs varies during different portions of systole and diastole, and the relative timing of each murmur is also evident. Note especially that the murmurs of aortic stenosis and mitral regurgitation occur only during sys- tole, whereas the murmurs of aortic regurgitation and mitral stenosis occur only during diastole. If the reader does not understand this timing, extra review should be undertaken until it is understood.
Abnormal Circulatory Dynamics in Valvular Heart Disease
Dynamics of the Circulation in Aortic Stenosis and Aortic Regurgitation
In aortic stenosis, the contracting left ventricle fails to empty adequately, whereas in aortic regurgitation, blood flows backward into the ventricle from the aorta after the ventricle has just pumped the blood into the aorta. Therefore, in either case, the net stroke volume output of the heart is reduced.
Several important compensations take place that can ameliorate the severity of the circulatory defects. Some of these compensations are the following.
Hypertrophy of the Left Ventricle. In both aortic stenosis and aortic regurgitation, the left ventricular mus- culature hypertrophies because of the increased ventricu- lar workload.
In regurgitation, the left ventricular chamber also enlarges to hold all the regurgitant blood from the aorta. Sometimes the left ventricular muscle mass increases fourfold to fivefold, creating a tremendously large left side of the heart.
When the aortic valve is seriously stenosed, the hyper- trophied muscle allows the left ventricle to develop as much as 400 mm Hg intraventricular pressure at systolic peak.
In severe aortic regurgitation, sometimes the hyper- trophied muscle allows the left ventricle to pump a stroke volume output as great as 250 ml, although as much as three fourths of this blood returns to the ventricle during diastole, and only one fourth flows through the aorta to the body.
Increase in Blood Volume. Another effect that helps compensate for the diminished net pumping by the left ventricle is increased blood volume. This results from (1) an initial slight decrease in arterial pressure, plus (2) peripheral circulatory reflexes that the decrease in pres- sure induces. These together diminish renal output of urine, causing the blood volume to increase and the mean arterial pressure to return to normal. Also, red cell mass eventually increases because of a slight degree of tissue hypoxia.
The increase in blood volume tends to increase venous return to the heart. This, in turn, causes the left ventricle to pump with the extra power required to overcome the abnormal pumping dynamics.
Eventual Failure of the Left Ventricle and Development of Pulmonary Edema
In the early stages of aortic stenosis or aortic regurgitation, the intrinsic ability of the left ventricle to adapt to increas- ing loads prevents significant abnormalities in circulatory function in the person during rest, other than increased work output required of the left ventricle. Therefore, con- siderable degrees of aortic stenosis or aortic regurgitation often occur before the person knows that he or she has serious heart disease (such as a resting left ventricular sys- tolic pressure as high as 200 mm Hg in aortic stenosis or a left ventricular stroke volume output as high as double normal in aortic regurgitation).
Beyond a critical stage in these aortic valve lesions, the left ventricle finally cannot keep up with the work demand. As a consequence, the left ventricle dilates and cardiac output begins to fall; blood simultaneously dams up in the left atrium and in the lungs behind the failing left ventri- cle. The left atrial pressure rises progressively, and at mean left atrial pressures above 25 to 40 mm Hg, serious edema appears in the lungs, as discussed in detail in Chapter 38.
Dynamics of Mitral Stenosis and Mitral Regurgitation
In mitral stenosis, blood flow from the left atrium into the left ventricle is impeded, and in mitral regurgitation, much of the blood that has flowed into the left ventricle
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during diastole leaks back into the left atrium during sys- tole rather than being pumped into the aorta. Therefore, either of these conditions reduces net movement of blood from the left atrium into the left ventricle.
Pulmonary Edema in Mitral Valvular Disease. The buildup of blood in the left atrium causes progressive increase in left atrial pressure, and this eventually results in development of serious pulmonary edema. Ordinarily, lethal edema does not occur until the mean left atrial pressure rises above 25 mm Hg and sometimes as high as 40 mm Hg, because the lung lymphatic vessels enlarge manyfold and can rapidly carry fluid away from the lung tissues.
Enlarged Left Atrium and Atrial Fibrillation. The high left atrial pressure in mitral valvular disease also causes progressive enlargement of the left atrium, which increases the distance that the cardiac electrical excitatory impulse must travel in the atrial wall. This pathway may eventually become so long that it predisposes to develop- ment of excitatory signal circus movements, as discussed in Chapter 13. Therefore, in late stages of mitral valvu- lar disease, especially in mitral stenosis, atrial fibrillation usually occurs. This further reduces the pumping effec- tiveness of the heart and causes further cardiac debility.
Compensation in Early Mitral Valvular Disease. As also occurs in aortic valvular disease and in many types of congenital heart disease, the blood volume increases in mitral valvular disease principally because of dimin- ished excretion of water and salt by the kidneys. This increased blood volume increases venous return to the heart, thereby helping to overcome the effect of the car- diac debility. Therefore, after compensation, cardiac out- put may fall only minimally until the late stages of mitral valvular disease, even though the left atrial pressure is rising.
As the left atrial pressure rises, blood begins to dam up in the lungs, eventually all the way back to the pul- monary artery. In addition, incipient edema of the lungs causes pulmonary arteriolar constriction. These two effects together increase systolic pulmonary arterial pressure and also right ventricular pressure, sometimes to as high as 60 mm Hg, which is more than double nor- mal. This, in turn, causes hypertrophy of the right side of the heart, which partially compensates for its increased workload.
Circulatory Dynamics During Exercise in Patients with Valvular Lesions
During exercise, large quantities of venous blood are returned to the heart from the peripheral circulation. Therefore, all the dynamic abnormalities that occur in the different types of valvular heart disease become tremendously exacerbated. Even in mild valvular heart disease, in which the symptoms may be unrecognizable at
rest, severe symptoms often develop during heavy exer- cise. For instance, in patients with aortic valvular lesions, exercise can cause acute left ventricular failure followed by acute pulmonary edema. Also, in patients with mitral disease, exercise can cause so much damming of blood in the lungs that serious or even lethal pulmonary edema may ensue in as little as 10 minutes.
Even in mild to moderate cases of valvular disease, the patient’s cardiac reserve diminishes in proportion to the severity of the valvular dysfunction. That is, the car- diac output does not increase as much as it should dur- ing exercise. Therefore, the muscles of the body fatigue rapidly because of too little increase in muscle blood flow.
Abnormal Circulatory Dynamics in Congenital Heart Defects
Occasionally, the heart or its associated blood vessels are malformed during fetal life; the defect is called a congen- ital anomaly. There are three major types of congenital anomalies of the heart and its associated vessels: (1) steno- sis of the channel of blood flow at some point in the heart or in a closely allied major blood vessel; (2) an anomaly that allows blood to flow backward from the left side of the heart or aorta to the right side of the heart or pul- monary artery, thus failing to flow through the systemic circulation-called a left-to-right shunt; and (3) an anomaly that allows blood to flow directly from the right side of the heart into the left side of the heart, thus failing to flow through the lungs—called a right-to-left shunt.
The effects of the different stenotic lesions are easily understood. For instance, congenital aortic valve stenosis results in the same dynamic effects as aortic valve steno- sis caused by other valvular lesions, namely, a tendency to develop serious pulmonary edema and a reduced cardiac output.
Another type of congenital stenosis is coarctation of the aorta, often occurring near the level of the diaphragm. This causes the arterial pressure in the upper part of the body (above the level of the coarctation) to be much greater than the pressure in the lower body because of the great resistance to blood flow through the coarctation to the lower body; part of the blood must go around the coarctation through small collateral arteries, as discussed in Chapter 19.
Patent Ductus Arteriosus—a Left-to-Right Shunt
During fetal life, the lungs are collapsed, and the elas- tic compression of the lungs that keeps the alveoli col- lapsed keeps most of the lung blood vessels collapsed as well. Therefore, resistance to blood flow through the lungs is so great that the pulmonary arterial pressure is high in the fetus. Also, because of low resistance to blood flow from the aorta through the large vessels of the pla- centa, the pressure in the aorta of the fetus is lower than
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normal—in fact, lower than in the pulmonary artery. This causes almost all the pulmonary arterial blood to flow through a special artery present in the fetus that con- nects the pulmonary artery with the aorta (Figure 23-4), called the ductus arteriosus, thus bypassing the lungs. This allows immediate recirculation of the blood through the systemic arteries of the fetus without the blood going through the lungs. This lack of blood flow through the lungs is not detrimental to the fetus because the blood is oxygenated by the placenta.
Closure of the Ductus Arteriosus After Birth. As soon as a baby is born and begins to breathe, the lungs inflate; not only do the alveoli fill with air, but also the resistance to blood flow through the pulmonary vascu- lar tree decreases tremendously, allowing the pulmonary arterial pressure to fall. Simultaneously, the aortic pres- sure rises because of sudden cessation of blood flow from the aorta through the placenta. Thus, the pressure in the pulmonary artery falls, while that in the aorta rises. As a result, forward blood flow through the ductus arterio- sus ceases suddenly at birth, and in fact, blood begins to flow backward through the ductus from the aorta into the pulmonary artery. This new state of backward blood flow causes the ductus arteriosus to become occluded within a few hours to a few days in most babies, so blood flow through the ductus does not persist. The duc- tus is believed to close because the oxygen concentration of the aortic blood now flowing through it is about twice as high as that of the blood flowing from the pulmonary artery into the ductus during fetal life. The oxygen pre- sumably constricts the muscle in the ductus wall. This is discussed further in Chapter 83.
Unfortunately, in about 1 of every 5500 babies, the duc- tus does not close, causing the condition known as patent ductus arteriosus, which is shown in Figure 23-4.
Dynamics of the Circulation with a Persistent Patent Ductus. During the early months of an infant’s life, a patent ductus usually does not cause severely abnor- mal function. But as the child grows older, the differen- tial between the high pressure in the aorta and the lower pressure in the pulmonary artery progressively increases, with corresponding increase in backward flow of blood from the aorta into the pulmonary artery. Also, the high aortic blood pressure usually causes the diameter of the partially open ductus to increase with time, making the condition even worse.
Recirculation Through the Lungs. In an older child with a patent ductus, one half to two thirds of the aortic blood flows backward through the ductus into the pul- monary artery, then through the lungs, and finally back into the left ventricle and aorta, passing through the lungs and left side of the heart two or more times for every one time that it passes through the systemic circu- lation. These people do not show cyanosis until later in life, when the heart fails or the lungs become congested. Indeed, early in life, the arterial blood is often better oxy- genated than normal because of the extra times it passes through the lungs.
Diminished Cardiac and Respiratory Reserve. The major effects of patent ductus arteriosus on the patient are decreased cardiac and respiratory reserve. The left ventricle is pumping about two or more times the nor- mal cardiac output, and the maximum that it can pump after hypertrophy of the heart has occurred is about four to seven times normal. Therefore, during exercise, the net blood flow through the remainder of the body can never increase to the levels required for strenuous activity. With even moderately strenuous exercise, the person is likely to become weak and may even faint from momentary heart failure.
The high pressures in the pulmonary vessels caused by excess flow through the lungs often lead to pulmo- nary congestion and pulmonary edema. As a result of the excessive load on the heart, and especially because the pulmonary congestion becomes progressively more severe with age, most patients with uncorrected patent ductus die from heart disease between ages 20 and 40 years.
Heart Sounds: Machinery Murmur. In a newborn infant with patent ductus arteriosus, occasionally no abnormal heart sounds are heard because the quantity of reverse blood flow through the ductus may be insuf- ficient to cause a heart murmur. But as the baby grows older, reaching age 1 to 3 years, a harsh, blowing murmur begins to be heard in the pulmonary artery area of the chest, as shown in recording F, Figure 23-3. This sound is much more intense during systole when the aortic pres- sure is high and much less intense during diastole when the aortic pressure falls low, so that the murmur waxes and wanes with each beat of the heart, creating the so- called machinery murmur.
Head and upper extremities
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Left lung
Pulmonary artery
Aorta Ductus
arteriosus
Left pulmonary
artery
Figure 23-4 Patent ductus arteriosus, showing by the blue color that venous blood changes into oxygenated blood at different points in the circulation. The right-hand diagram shows back- flow of blood from the aorta into the pulmonary artery and then through the lungs for a second time.
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Surgical Treatment. Surgical treatment of patent ductus arteriosus is extremely simple; one need only ligate the patent ductus or divide it and then close the two ends. In fact, this was one of the first successful heart surgeries ever performed.
Tetralogy of Fallot—a Right-to-Left Shunt
Tetralogy of Fallot is shown in Figure 23-5; it is the most common cause of “blue baby.” Most of the blood bypasses the lungs, so the aortic blood is mainly unoxygenated venous blood. In this condition, four abnormalities of the heart occur simultaneously:
1. The aorta originates from the right ventricle rather than the left, or it overrides a hole in the septum, as shown in Figure 23-5, receiving blood from both ventricles.
2. The pulmonary artery is stenosed, so much lower than normal amounts of blood pass from the right ventri- cle into the lungs; instead, most of the blood passes directly into the aorta, thus bypassing the lungs.
3. Blood from the left ventricle flows either through a ventricular septal hole into the right ventricle and then into the aorta or directly into the aorta that overrides this hole.
4. Because the right side of the heart must pump large quantities of blood against the high pressure in the aorta, its musculature is highly developed, causing an enlarged right ventricle.
Abnormal Circulatory Dynamics. It is readily appar- ent that the major physiological difficulty caused by
tetralogy of Fallot is the shunting of blood past the lungs without its becoming oxygenated. As much as 75 percent of the venous blood returning to the heart passes directly from the right ventricle into the aorta without becoming oxygenated.
A diagnosis of tetralogy of Fallot is usually based on (1) the fact that the baby’s skin is cyanotic (blue); (2) mea- surement of high systolic pressure in the right ventricle, recorded through a catheter; (3) characteristic changes in the radiological silhouette of the heart, showing an enlarged right ventricle; and (4) angiograms (x-ray pic- tures) showing abnormal blood flow through the inter- ventricular septal hole and into the overriding aorta, but much less flow through the stenosed pulmonary artery.
Surgical Treatment. Tetralogy of Fallot can usually be treated successfully by surgery. The usual operation is to open the pulmonary stenosis, close the septal defect, and reconstruct the flow pathway into the aorta. When surgery is successful, the average life expectancy increases from only 3 to 4 years to 50 or more years.
Causes of Congenital Anomalies
Congenital heart disease is not uncommon, occurring in about 8 of every 1000 live births. One of the most com- mon causes of congenital heart defects is a viral infec- tion in the mother during the first trimester of pregnancy when the fetal heart is being formed. Defects are particu- larly prone to develop when the expectant mother con- tracts German measles; thus, obstetricians may advise termination of pregnancy if German measles occurs in the first trimester.
Some congenital defects of the heart are hereditary because the same defect has been known to occur in iden- tical twins, as well as in succeeding generations. Children of patients surgically treated for congenital heart disease have about a 10 times greater chance of having congenital heart disease than other children do. Congenital defects of the heart are also frequently associated with other congenital defects of the baby’s body.
Use of Extracorporeal Circulation During Cardiac Surgery
It is almost impossible to repair intracardiac defects sur- gically while the heart is still pumping. Therefore, many types of artificial heart-lung machines have been devel- oped to take the place of the heart and lungs during the course of operation. Such a system is called extra- corporeal circulation. The system consists principally of a pump and an oxygenating device. Almost any type of pump that does not cause hemolysis of the blood seems to be suitable.
Methods used for oxygenating blood include (1) bub- bling oxygen through the blood and removing the bubbles from the blood before passing it back into the patient,
Head and upper extremities
Trunk and lower extremities
Right lung
Left lung
Figure 23-5 Tetralogy of Fallot, showing by the blue color that most of the venous blood is shunted from the right ventricle into the aorta without passing through the lungs.
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(2) dripping the blood downward over the surfaces of plastic sheets in the presence of oxygen, (3) passing the blood over surfaces of rotating discs, or (4) passing the blood between thin membranes or through thin tubes that are permeable to oxygen and carbon dioxide.
The different systems have all been fraught with dif- ficulties, including hemolysis of the blood, development of small clots in the blood, likelihood of small bubbles of oxygen or small emboli of antifoam agent passing into the arteries of the patient, necessity for large quantities of blood to prime the entire system, failure to exchange ade- quate quantities of oxygen, and necessity to use heparin to prevent blood coagulation in the extracorporeal sys- tem. Heparin also interferes with adequate hemostasis during the surgical procedure. Yet despite these difficul- ties, in the hands of experts, patients can be kept alive on artificial heart-lung machines for many hours while oper- ations are performed on the inside of the heart.
Hypertrophy of the Heart in Valvular and Congenital Heart Disease
Hypertrophy of cardiac muscle is one of the most impor- tant mechanisms by which the heart adapts to increased workloads, whether these loads are caused by increased pressure against which the heart muscle must contract or by increased cardiac output that must be pumped. Some physicians believe that the increased strength of contrac- tion of the heart muscle causes the hypertrophy; others believe that the increased metabolic rate of the muscle is the primary stimulus. Regardless of which of these is correct, one can calculate approximately how much hypertrophy will occur in each chamber of the heart by multiplying ventricular output by the pressure against which the ventricle must work, with emphasis on pres- sure. Thus, hypertrophy occurs in most types of valvular and congenital disease, sometimes causing heart weights as great as 800 grams instead of the normal 300 grams.
Detrimental Effects of Late Stages of Cardiac Hypertrophy. Although the most common cause of car- diac hypertrophy is hypertension, almost all forms of car- diac diseases including valvular and congenital disease can stimulate enlargement of the heart.
“Physiological” cardiac hypertrophy is generally con- sidered to be a compensatory response of the heart to increased workload and is usually beneficial for maintaining cardiac output in the face of abnormalities that impair
the heart’s effectiveness as a pump. However, extreme degrees of hypertrophy can lead to heart failure. One of the reasons for this is that the coronary vasculature typi- cally does not increase to the same extent as the mass of cardiac muscle increases. The second reason is that fibro- sis often develops in the muscle, especially in the suben- docardial muscle where the coronary blood flow is poor, with fibrous tissue replacing degenerating muscle fibers. Because of the disproportionate increase in muscle mass relative to coronary blood flow, relative ischemia may develop as the cardiac muscle hypertrophies and coro- nary blood flow insufficiency may ensue. Anginal pain is therefore a frequent accompaniment of cardiac hyper- trophy associated with valvular and congenital heart dis- eases. Enlargement of the heart is also associated with greater risk for developing arrhythmias, which in turn can lead to further impairment of cardiac function and sud- den death because of fibrillation.
Bibliography
Braunwald E, Seidman CE, Sigwart U: Contemporary evaluation and man- agement of hypertrophic cardiomyopathy, Circulation 106:1312, 2002.
Carabello BA: The current therapy for mitral regurgitation, J Am Coll Cardiol 52:319, 2008.
Dal-Bianco JP, Khandheria BK, Mookadam F, et al: Management of asymp- tomatic severe aortic stenosis, J Am Coll Cardiol 52:1279, 2008.
Dorn GW 2nd: The fuzzy logic of physiological cardiac hypertrophy, Hypertension 49:962, 2007.
Hoffman JI, Kaplan S: The incidence of congenital heart disease, J Am Coll Cardiol 39:1890, 2002.
Jenkins KJ, Correa A, Feinstein JA, et al: Noninherited risk factors and con- genital cardiovascular defects: current knowledge: a scientific state- ment from the American Heart Association Council on Cardiovascular Disease in the Young: endorsed by the American Academy of Pediatrics, Circulation 115:2995, 2007.
Maron BJ: Hypertrophic cardiomyopathy: a systematic review, JAMA 287:1308, 2002.
McDonald M, Currie BJ, Carapetis JR: Acute rheumatic fever: a chink in the chain that links the heart to the throat? Lancet Infect Dis 4:240, 2004.
Nishimura RA, Holmes DR Jr: Clinical practice: hypertrophic obstructive cardiomyopathy, N Engl J Med 350:1320, 2004.
Reimold SC, Rutherford JD: Clinical practice: valvular heart disease in preg- nancy, N Engl J Med 349:52, 2003.
Rhodes JF, Hijazi ZM, Sommer RJ: Pathophysiology of congenital heart disease in the adult, part II. Simple obstructive lesions, Circulation 117:1228, 2008.
Schoen FJ: Evolving concepts of cardiac valve dynamics: the continuum of development, functional structure, pathobiology, and tissue engineer- ing, Circulation 118:1864, 2008.
Sommer RJ, Hijazi ZM, Rhodes JF Jr: Pathophysiology of congenital heart disease in the adult: part I: shunt lesions, Circulation 117:1090, 2008.
Sommer RJ, Hijazi ZM, Rhodes JF: Pathophysiology of congenital heart dis- ease in the adult: part III: complex congenital heart disease, Circulation 117:1340, 2008.
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chapter 24
Circulatory Shock and Its Treatment
Circulatory shock means generalized inadequate blood flow through the body, to the extent that the body tissues are damaged, especially because of too lit- tle oxygen and other nutri-
ents delivered to the tissue cells. Even the cardiovascular system itself—the heart musculature, walls of the blood vessels, vasomotor system, and other circulatory parts— begins to deteriorate, so the shock, once begun, is prone to become progressively worse.
Physiologic Causes of Shock
Circulatory Shock Caused by Decreased Cardiac Output
Shock usually results from inadequate cardiac out- put. Therefore, any condition that reduces the cardiac output far below normal will likely lead to circulatory shock. Two types of factors can severely reduce cardiac output:
1. Cardiac abnormalities that decrease the ability of the heart to pump blood. These include especially myo- cardial infarction but also toxic states of the heart, severe heart valve dysfunction, heart arrhythmias, and other conditions. The circulatory shock that results from diminished cardiac pumping ability is called cardiogenic shock. This is discussed in detail in Chapter 22 where it is pointed out that as many as 70 percent of people who develop cardiogenic shock do not survive.
2. Factors that decrease venous return also decrease cardiac output because the heart cannot pump blood that does not flow into it. The most common cause of decreased venous return is diminished blood volume, but venous return can also be reduced as a result of decreased vas- cular tone, especially of the venous blood reservoirs, or obstruction to blood flow at some point in the circu- lation, especially in the venous return pathway to the heart.
Circulatory Shock That Occurs Without Diminished Cardiac Output
Occasionally, cardiac output is normal or even greater than normal, yet the person is in circulatory shock. This can result from (1) excessive metabolic rate, so even a normal cardiac output is inadequate, or (2) abnormal tissue perfusion patterns, so most of the cardiac output is passing through blood vessels besides those that supply the local tissues with nutrition.
The specific causes of shock are discussed later in the chapter. For the present, it is important to note that all of them lead to inadequate delivery of nutrients to criti- cal tissues and critical organs and also cause inadequate removal of cellular waste products from the tissues.
What Happens to the Arterial Pressure in Circulatory Shock?
In the minds of many physicians, the arterial pressure level is the principal measure of adequacy of circulatory function. However, the arterial pressure can often be seri- ously misleading. At times, a person may be in severe shock and still have an almost normal arterial pressure because of powerful nervous reflexes that keep the pres- sure from falling. At other times, the arterial pressure can fall to half of normal, but the person still has normal tissue perfusion and is not in shock.
In most types of shock, especially shock caused by severe blood loss, the arterial blood pressure decreases at the same time the cardiac output decreases, although usually not as much.
Tissue Deterioration Is the End Result of Circulatory Shock
Once circulatory shock reaches a critical state of severity, regardless of its initiating cause, the shock itself leads to more shock. That is, the inadequate blood flow causes the body tissues to begin deteriorating, including the heart and circulatory system itself. This causes an even greater decrease in cardiac output, and a vicious circle ensues, with progressively increasing circulatory shock, less ade- quate tissue perfusion, more shock, and so forth until death. It is with this late stage of circulatory shock that we
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Unit IV The Circulation
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Figure 24-1 Effect of hemorrhage on cardiac output and arterial pressure.
are especially concerned, because appropriate physiologic treatment can often reverse the rapid slide to death.
Stages of Shock
Because the characteristics of circulatory shock change with different degrees of severity, shock is divided into the following three major stages:
1. A nonprogressive stage (sometimes called the com- pensated stage), in which the normal circulatory com- pensatory mechanisms eventually cause full recovery without help from outside therapy.
2. A progressive stage, in which, without therapy, the shock becomes steadily worse until death.
3. An irreversible stage, in which the shock has progressed to such an extent that all forms of known therapy are inadequate to save the person’s life, even though, for the moment, the person is still alive. Now, let us discuss the stages of circulatory shock
caused by decreased blood volume, which illustrate the basic principles. Then we will consider special character- istics of shock initiated by other causes.
Shock Caused by Hypovolemia— Hemorrhagic Shock
Hypovolemia means diminished blood volume. Hemorrhage is the most common cause of hypovolemic shock. Hemorrhage decreases the filling pressure of the circulation and, as a consequence, decreases venous return. As a result, the cardiac output falls below normal and shock may ensue.
Relationship of Bleeding Volume to Cardiac Output and Arterial Pressure
Figure 24-1 shows the approximate effects on both car- diac output and arterial pressure of removing blood from the circulatory system over a period of about 30 min- utes. About 10 percent of the total blood volume can be removed with almost no effect on either arterial pressure or cardiac output, but greater blood loss usually dimin-
ishes the cardiac output first and later the arterial pressure, both of which fall to zero when about 40 to 45 percent of the total blood volume has been removed.
Sympathetic Reflex Compensations in Shock— Their Special Value to Maintain Arterial Pressure. The decrease in arterial pressure after hemorrhage, as well as decreases in pressures in the pulmonary arteries and veins in the thorax, causes powerful sympathetic reflexes (initi- ated mainly by the arterial baroreceptors and other vascular stretch receptors, as explained in Chapter 18). These reflexes stimulate the sympathetic vasoconstrictor system in most tissues of the body, resulting in three important effects: (1) The arterioles constrict in most parts of the systemic cir- culation, thereby increasing the total peripheral resistance. (2) The veins and venous reservoirs constrict, thereby help- ing to maintain adequate venous return despite diminished blood volume. (3) Heart activity increases markedly, some- times increasing the heart rate from the normal value of 72 beats/min to as high as 160 to 180 beats/min.
Value of the Sympathetic Nervous Reflexes. In the absence of the sympathetic reflexes, only 15 to 20 percent of the blood volume can be removed over a period of 30 minutes before a person dies; this is in contrast to a 30 to 40 percent loss of blood volume that a person can sustain when the reflexes are intact. Therefore, the reflexes extend the amount of blood loss that can occur without causing death to about twice that which is possible in their absence.
Greater Effect of the Sympathetic Nervous Reflexes in Maintaining Arterial Pressure than in Maintaining Cardiac Output. Referring again to Figure 24-1, note that the arterial pressure is maintained at or near normal lev- els in the hemorrhaging person longer than is the cardiac output. The reason for this is that the sympathetic reflexes are geared more for maintaining arterial pressure than for maintaining cardiac output. They increase the arterial pres- sure mainly by increasing the total peripheral resistance, which has no beneficial effect on cardiac output; however, the sympathetic constriction of the veins is important to keep venous return and cardiac output from falling too much, in addition to their role in maintaining arterial pressure.
Especially interesting is the second plateau occur- ring at about 50 mm Hg in the arterial pressure curve of Figure 24-1. This results from activation of the cen- tral nervous system ischemic response, which causes extreme stimulation of the sympathetic nervous system when the brain begins to suffer from lack of oxygen or from excess buildup of carbon dioxide, as discussed in Chapter 18. This effect of the central nervous system ischemic response can be called the “last-ditch stand” of the sympathetic reflexes in their attempt to keep the arte- rial pressure from falling too low.
Protection of Coronary and Cerebral Blood Flow by the Reflexes. A special value of the maintenance of normal arterial pressure even in the presence of decreasing
Chapter 24 Circulatory Shock and Its Treatment
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cardiac output is protection of blood flow through the coronary and cerebral circulatory systems. The sympa- thetic stimulation does not cause significant constriction of either the cerebral or the cardiac vessels. In addition, in both vascular beds, local blood flow autoregulation is excellent, which prevents moderate decreases in arterial pressure from significantly decreasing their blood flows. Therefore, blood flow through the heart and brain is maintained essentially at normal levels as long as the arte- rial pressure does not fall below about 70 mm Hg, despite the fact that blood flow in some other areas of the body might be decreased to as little as one third to one quarter normal by this time because of vasoconstriction.
Progressive and Nonprogressive Hemorrhagic Shock
Figure 24-2 shows an experiment that demonstrates the effects of different degrees of sudden acute hemorrhage on the subsequent course of arterial pressure. The animals were anesthetized and bled rapidly until their arterial pres- sures fell to different levels. Those animals whose pressures fell immediately to no lower than 45 mm Hg (groups I, II, and III) all eventually recovered; the recovery occurred rap- idly if the pressure fell only slightly (group I) but occurred slowly if it fell almost to the 45 mm Hg level (group III). When the arterial pressure fell below 45 mm Hg (groups IV, V, and VI), all the animals died, although many of them hovered between life and death for hours before the circu- latory system deteriorated to the stage of death.
This experiment demonstrates that the circulatory sys- tem can recover as long as the degree of hemorrhage is no greater than a certain critical amount. Crossing this criti- cal threshold by even a few milliliters of blood loss makes the eventual difference between life and death. Thus, hemorrhage beyond a certain critical level causes shock to become progressive. That is, the shock itself causes still more shock, and the condition becomes a vicious circle that eventually leads to deterioration of the circulation and to death.
Nonprogressive Shock—Compensated Shock
If shock is not severe enough to cause its own progres- sion, the person eventually recovers. Therefore, shock
of this lesser degree is called nonprogressive shock, or compensated shock, meaning that the sympathetic reflexes and other factors compensate enough to pre- vent further deterioration of the circulation.
The factors that cause a person to recover from mod- erate degrees of shock are all the negative feedback con- trol mechanisms of the circulation that attempt to return cardiac output and arterial pressure back to normal levels. They include the following:
1. Baroreceptor reflexes, which elicit powerful sympa- thetic stimulation of the circulation.
2. Central nervous system ischemic response, which elicits even more powerful sympathetic stimulation through- out the body but is not activated significantly until the arterial pressure falls below 50 mm Hg.
3. Reverse stress-relaxation of the circulatory system, which causes the blood vessels to contract around the diminished blood volume so that the blood volume that is available more adequately fills the circulation.
4. Increased secretion of renin by the kidneys and forma- tion of angiotensin II, which constricts the peripheral arteries and also causes decreased output of water and salt by the kidneys, both of which help prevent pro- gression of shock.
5. Increased secretion by the posterior pituitary gland of vasopressin (antidiuretic hormone), which constricts the peripheral arteries and veins and greatly increases water retention by the kidneys.
6. Increased secretion by the adrenal medullae of epineph- rine and norepinephrine, which constricts the peripheral arteries and veins and increases the heart rate.
7. Compensatory mechanisms that return the blood vol- ume back toward normal, including absorption of large quantities of fluid from the intestinal tract, absorption of fluid into the blood capillaries from the interstitial spaces of the body, conservation of water and salt by the kidneys, and increased thirst and increased appe- tite for salt, which make the person drink water and eat salty foods if able.
The sympathetic reflexes and increased secretion of catecholamines by the adrenal medullae provide rapid help toward bringing about recovery because they become maximally activated within 30 seconds to a few minutes after hemorrhage.
The angiotensin and vasopressin mechanisms, as well as the reverse stress-relaxation that causes contraction of the blood vessels and venous reservoirs, all require 10 minutes to 1 hour to respond completely, but they aid greatly in increasing the arterial pressure or increasing the circulatory filling pressure and thereby increasing the return of blood to the heart.
Finally, readjustment of blood volume by absorption of fluid from the interstitial spaces and intestinal tract, as well as oral ingestion and absorption of additional quan- tities of water and salt, may require from 1 to 48 hours,
A rt
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I II
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Figure 24-2 Time course of arterial pressure in dogs after differ- ent degrees of acute hemorrhage. Each curve represents average results from six dogs.
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but recovery eventually takes place, provided the shock does not become severe enough to enter the progressive stage.
“Progressive Shock” Is Caused by a Vicious Circle of Cardiovascular Deterioration
Figure 24-3 shows some of the positive feedbacks that further depress cardiac output in shock, thus causing the shock to become progressive. Some of the more impor- tant feedbacks are the following.
Cardiac Depression. When the arterial pressure falls low enough, coronary blood flow decreases below that required for adequate nutrition of the myocardium. This weakens the heart muscle and thereby decreases the car- diac output more. Thus, a positive feedback cycle has devel- oped, whereby the shock becomes more and more severe.
Figure 24-4 shows cardiac output curves extrapolated to the human heart from studies in experimental animals, demonstrating progressive deterioration of the heart at different times after the onset of shock. An anesthetized dog was bled until the arterial pressure fell to 30 mm Hg, and the pressure was held at this level by further bleed- ing or retransfusion of blood as required. Note from the second curve in the figure that there was little deteriora- tion of the heart during the first 2 hours, but by 4 hours, the heart had deteriorated about 40 percent; then, rap- idly, during the last hour of the experiment (after 4 hours of low coronary blood pressure), the heart deteriorated completely.
Thus, one of the important features of progressive shock, whether it is hemorrhagic in origin or caused in another way, is eventual progressive deterioration of the heart. In the early stages of shock, this plays very little role in the condition of the person, partly because dete- rioration of the heart is not severe during the first hour or so of shock, but mainly because the heart has tremen- dous reserve capability that normally allows it to pump 300 to 400 percent more blood than is required by the body for adequate tissue nutrition. In the latest stages of shock, however, deterioration of the heart is probably the
Decreased systemic blood flow
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permeability
Decreased nutrition of tissuesDecreased cardiac nutrition
Decreased nutrition of brain
Decreased venous return
Decreased blood volume
Tissue ischemia
Decreased cardiac output
Decreased arterial pressure
Decreased vasomotor activity
Cardiac depression
Vascular dilation
Venous pooling of blood
Decreased nutrition of vascular system
Release of toxins
Intravascular clotting
Figure 24-3 Different types of “positive feedback” that can lead to progression of shock.
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41/2 hours
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Figure 24-4 Cardiac output curves of the heart at different times after hemorrhagic shock begins. (These curves are extrapolated to the human heart from data obtained in dog experiments by Dr. J. W. Crowell.)
Chapter 24 Circulatory Shock and Its Treatment
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most important factor in the final lethal progression of the shock.
Vasomotor Failure. In the early stages of shock, vari- ous circulatory reflexes cause intense activity of the sym- pathetic nervous system. This, as discussed earlier, helps delay depression of the cardiac output and especially helps prevent decreased arterial pressure. However, there comes a point when diminished blood flow to the brain’s vaso- motor center depresses the center so much that it, too, becomes progressively less active and finally totally inac- tive. For instance, complete circulatory arrest to the brain causes, during the first 4 to 8 minutes, the most intense of all sympathetic discharges, but by the end of 10 to 15 minutes, the vasomotor center becomes so depressed that no further evidence of sympathetic discharge can be dem- onstrated. Fortunately, the vasomotor center usually does not fail in the early stages of shock if the arterial pressure remains above 30 mm Hg.
Blockage of Very Small Vessels—“Sludged Blood.” In time, blockage occurs in many of the very small blood vessels in the circulatory system and this also causes the shock to progress. The initiating cause of this blockage is sluggish blood flow in the microvessels. Because tissue metabolism continues despite the low flow, large amounts of acid, both carbonic acid and lactic acid, continue to empty into the local blood vessels and greatly increase the local acidity of the blood. This acid, plus other dete- rioration products from the ischemic tissues, causes local blood agglutination, resulting in minute blood clots, lead- ing to very small plugs in the small vessels. Even if the vessels do not become plugged, an increased tendency for the blood cells to stick to one another makes it more dif- ficult for blood to flow through the microvasculature, giv- ing rise to the term sludged blood.
Increased Capillary Permeability. After many hours of capillary hypoxia and lack of other nutrients, the per- meability of the capillaries gradually increases, and large quantities of fluid begin to transude into the tis- sues. This decreases the blood volume even more, with a resultant further decrease in cardiac output, making the shock still more severe. Capillary hypoxia does not cause increased capillary permeability until the late stages of prolonged shock.
Release of Toxins by Ischemic Tissue. Throughout the history of research in the field of shock, it has been suggested that shock causes tissues to release toxic sub- stances, such as histamine, serotonin, and tissue enzymes, that cause further deterioration of the circulatory system. Experimental studies have proved the significance of at least one toxin, endotoxin, in some types of shock.
Cardiac Depression Caused by Endotoxin. Endotoxin is released from the bodies of dead gram-negative bacte- ria in the intestines. Diminished blood flow to the intes- tines often causes enhanced formation and absorption of this toxic substance. The circulating toxin then causes increased cellular metabolism despite inadequate nutri- tion of the cells; this has a specific effect on the heart muscle, causing cardiac depression. Endotoxin can play
a major role in some types of shock, especially “septic shock,” discussed later in the chapter.
Generalized Cellular Deterioration. As shock becomes severe, many signs of generalized cellular dete- rioration occur throughout the body. One organ espe- cially affected is the liver, as illustrated in Figure 24-5. This occurs mainly because of lack of enough nutrients to sup- port the normally high rate of metabolism in liver cells, but also partly because of the exposure of the liver cells to any vascular toxin or other abnormal metabolic factor occurring in shock.
Among the damaging cellular effects that are known to occur in most body tissues are the following:
1. Active transport of sodium and potassium through the cell membrane is greatly diminished. As a result, sodium and chloride accumulate in the cells and potas- sium is lost from the cells. In addition, the cells begin to swell.
2. Mitochondrial activity in the liver cells, as well as in many other tissues of the body, becomes severely depressed.
3. Lysosomes in the cells in widespread tissue areas begin to break open, with intracellular release of hydrolases that cause further intracellular deterioration.
4. Cellular metabolism of nutrients, such as glucose, eventually becomes greatly depressed in the last stages of shock. The actions of some hormones are depressed as well, including almost 100 percent depression of the action of insulin.
All these effects contribute to further deterioration of many organs of the body, including especially (1) the liver, with depression of its many metabolic and detoxi- fication functions; (2) the lungs, with eventual develop- ment of pulmonary edema and poor ability to oxygenate the blood; and (3) the heart, thereby further depressing its contractility.
Tissue Necrosis in Severe Shock—Patchy Areas of Necrosis Occur Because of Patchy Blood Flows in Different Organs. Not all cells of the body are equally damaged by shock because some tissues have better blood supplies than others. For instance, the cells adjacent to the arterial ends of capillaries receive better nutrition than cells adjacent to the venous ends of the same capillaries. Therefore, more nutritive deficiency occurs around the venous ends of capillaries than elsewhere. For instance, Figure 24-5 shows necrosis in the center of a liver lobule, the portion of the lobule that is last to be exposed to the blood as it passes through the liver sinusoids.
Similar punctate lesions occur in heart muscle, although here a definite repetitive pattern, such as occurs in the liver, cannot be demonstrated. Nevertheless, the cardiac lesions play an important role in leading to the final irreversible stage of shock. Deteriorative lesions also occur in the kidneys, especially in the epithelium of the kidney tubules, leading to kidney failure and occasionally uremic death several days later. Deterioration of the lungs
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also often leads to respiratory distress and death several days later—called the shock lung syndrome.
Acidosis in Shock. Most metabolic derangements that occur in shocked tissue can lead to acidosis all through the body. This results from poor delivery of oxygen to the tissues, which greatly diminishes oxidative metabolism of the foodstuffs. When this occurs, the cells obtain most of their energy by the anaerobic process of glycolysis, which leads to tremendous quantities of excess lactic acid in the blood. In addition, poor blood flow through tissues prevents normal removal of carbon dioxide. The carbon dioxide reacts locally in the cells with water to form high concentrations of intracellular carbonic acid; this, in turn, reacts with various tissue chemicals to form still other intracellular acidic substances. Thus, another deteriora- tive effect of shock is both generalized and local tissue aci- dosis, leading to further progression of the shock itself.
Positive Feedback Deterioration of Tissues in Shock and the Vicious Circle of Progressive Shock
All the factors just discussed that can lead to further pro- gression of shock are types of positive feedback. That is, each increase in the degree of shock causes a further increase in the shock.
However, positive feedback does not necessarily lead to a vicious circle. Whether a vicious circle develops depends on the intensity of the positive feedback. In mild degrees of shock, the negative feedback mechanisms of the cir- culation—sympathetic reflexes, reverse stress-relaxation mechanism of the blood reservoirs, absorption of fluid into the blood from the interstitial spaces, and others— can easily overcome the positive feedback influences and, therefore, cause recovery. But in severe degrees of shock, the deteriorative feedback mechanisms become more and more powerful, leading to such rapid deterioration of the
circulation that all the normal negative feedback systems of circulatory control acting together cannot return the cardiac output to normal.
Considering once again the principles of positive feed- back and vicious circle discussed in Chapter 1, one can readily understand why there is a critical cardiac output level above which a person in shock recovers and below which a person enters a vicious circle of circulatory dete- rioration that proceeds until death.
Irreversible Shock
After shock has progressed to a certain stage, transfusion or any other type of therapy becomes incapable of saving the person’s life. The person is then said to be in the irre- versible stage of shock. Ironically, even in this irreversible stage, therapy can, on rare occasions, return the arterial pressure and even the cardiac output to normal or near normal for short periods, but the circulatory system nev- ertheless continues to deteriorate, and death ensues in another few minutes to few hours.
Figure 24-6 demonstrates this effect, showing that transfusion during the irreversible stage can sometimes cause the cardiac output (as well as the arterial pressure) to return to nearly normal. However, the cardiac output soon begins to fall again, and subsequent transfusions have less and less effect. By this time, multiple deteriora- tive changes have occurred in the muscle cells of the heart that may not necessarily affect the heart’s immediate abil- ity to pump blood but, over a long period, depress heart pumping enough to cause death. Beyond a certain point, so much tissue damage has occurred, so many destructive enzymes have been released into the body fluids, so much acidosis has developed, and so many other destructive factors are now in progress that even a normal cardiac output for a few minutes cannot reverse the continuing deterioration. Therefore, in severe shock, a stage is even- tually reached at which the person will die even though vigorous therapy might still return the cardiac output to normal for short periods.
Depletion of Cellular High-Energy Phosphate Re - ser ves in Irreversible Shock. The high-energy phosphate reserves in the tissues of the body, especially in the liver and the heart, are greatly diminished in severe degrees
C ar
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Figure 24-6 Failure of transfusion to prevent death in irrevers- ible shock.
Figure 24-5 Necrosis of the central portion of a liver lobule in severe circulatory shock. (Courtesy Dr. J. W. Crowell.)
Chapter 24 Circulatory Shock and Its Treatment
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of shock. Essentially all the creatine phosphate has been degraded, and almost all the adenosine triphosphate has downgraded to adenosine diphosphate, adenosine monophosphate, and, eventually, adenosine. Then much of this adenosine diffuses out of the cells into the circulating blood and is converted into uric acid, a substance that cannot re-enter the cells to reconstitute the adenosine phosphate system. New adenosine can be synthesized at a rate of only about 2 percent of the normal cellular amount an hour, meaning that once the high-energy phosphate stores of the cells are depleted, they are difficult to replenish.
Thus, one of the most devastating end results of dete- rioration in shock, and the one that is perhaps most significant for development of the final state of irre- versibility, is this cellular depletion of these high-energy compounds.
Hypovolemic Shock Caused by Plasma Loss
Loss of plasma from the circulatory system, even without loss of red blood cells, can sometimes be severe enough to reduce the total blood volume markedly, causing typi- cal hypovolemic shock similar in almost all details to that caused by hemorrhage. Severe plasma loss occurs in the following conditions:
1. Intestinal obstruction may cause severely reduced plasma volume. Distention of the intestine in intes- tinal obstruction partly blocks venous blood flow in the intestinal walls, which increases intestinal capil- lary pressure. This in turn causes fluid to leak from the capillaries into the intestinal walls and also into the intestinal lumen. Because the lost fluid has high pro- tein content, the result is reduced total blood plasma protein, as well as reduced plasma volume.
2. In almost all patients who have severe burns or other denuding conditions of the skin, so much plasma is lost through the denuded skin areas that the plasma vol- ume becomes markedly reduced.
The hypovolemic shock that results from plasma loss has almost the same characteristics as the shock caused by hemorrhage, except for one additional complicating factor: the blood viscosity increases greatly as a result of increased red blood cell concentration in the remain- ing blood, and this exacerbates the sluggishness of blood flow.
Loss of fluid from all fluid compartments of the body is called dehydration; this, too, can reduce the blood vol- ume and cause hypovolemic shock similar to that result- ing from hemorrhage. Some of the causes of this type of shock are (1) excessive sweating, (2) fluid loss in severe diarrhea or vomiting, (3) excess loss of fluid by the kid- neys, (4) inadequate intake of fluid and electrolytes, or (5) destruction of the adrenal cortices, with loss of aldos- terone secretion and consequent failure of the kidneys to reabsorb sodium, chloride, and water, which occurs in the absence of the adrenocortical hormone aldosterone.
Hypovolemic Shock Caused by Trauma
One of the most common causes of circulatory shock is trauma to the body. Often the shock results simply from hemorrhage caused by the trauma, but it can also occur even without hemorrhage, because extensive contusion of the body can damage the capillaries sufficiently to allow excessive loss of plasma into the tissues. This results in greatly reduced plasma volume, with resultant hypo- volemic shock.
Various attempts have been made to implicate toxic factors released by the traumatized tissues as one of the causes of shock after trauma. However, cross-transfusion experiments into normal animals have failed to show sig- nificant toxic elements.
In summary, traumatic shock seems to result mainly from hypovolemia, although there might also be a mod- erate degree of concomitant neurogenic shock caused by loss of vasomotor tone, as discussed next.
Neurogenic Shock—Increased Vascular Capacity
Shock occasionally results without any loss of blood vol- ume. Instead, the vascular capacity increases so much that even the normal amount of blood becomes incapa- ble of filling the circulatory system adequately. One of the major causes of this is sudden loss of vasomotor tone throughout the body, resulting especially in massive dila- tion of the veins. The resulting condition is known as neu- rogenic shock.
The role of vascular capacity in helping to regulate cir- culatory function was discussed in Chapter 15, where it was pointed out that either an increase in vascular capac- ity or a decrease in blood volume reduces the mean sys- temic filling pressure, which reduces venous return to the heart. Diminished venous return caused by vascular dila- tion is called venous pooling of blood.
Causes of Neurogenic Shock. Some neurogenic factors that can cause loss of vasomotor tone include the following:
1. Deep general anesthesia often depresses the vasomo- tor center enough to cause vasomotor paralysis, with resulting neurogenic shock.
2. Spinal anesthesia, especially when this extends all the way up the spinal cord, blocks the sympathetic nervous outflow from the nervous system and can be a potent cause of neurogenic shock.
3. Brain damage is often a cause of vasomotor paralysis. Many patients who have had brain concussion or contu- sion of the basal regions of the brain develop profound neurogenic shock. Also, even though brain ischemia for a few minutes almost always causes extreme vaso- motor stimulation, prolonged ischemia (lasting longer than 5 to 10 minutes) can cause the opposite effect—
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total inactivation of the vasomotor neurons in the brain stem, with consequent development of severe neurogenic shock.
Anaphylactic Shock and Histamine Shock
Anaphylaxis is an allergic condition in which the cardiac output and arterial pressure often decrease drastically. This is discussed in Chapter 34. It results primarily from an antigen-antibody reaction that rapidly occurs after an antigen to which the person is sensitive enters the circula- tion. One of the principal effects is to cause the basophils in the blood and mast cells in the pericapillary tissues to release histamine or a histamine-like substance. The his- tamine causes (1) an increase in vascular capacity because of venous dilation, thus causing a marked decrease in venous return; (2) dilation of the arterioles, resulting in greatly reduced arterial pressure; and (3) greatly increased capillary permeability, with rapid loss of fluid and protein into the tissue spaces. The net effect is a great reduction in venous return and sometimes such serious shock that the person dies within minutes.
Intravenous injection of large amounts of histamine causes “histamine shock,” which has characteristics almost identical to those of anaphylactic shock.
Septic Shock
A condition that was formerly known by the popular name “blood poisoning” is now called septic shock by most clini- cians. This refers to a bacterial infection widely dissemi- nated to many areas of the body, with the infection being borne through the blood from one tissue to another and causing extensive damage. There are many varieties of septic shock because of the many types of bacterial infec- tions that can cause it and because infection in different parts of the body produces different effects.
Septic shock is extremely important to the clinician because other than cardiogenic shock, septic shock is the most frequent cause of shock-related death in the modern hospital.
Some of the typical causes of septic shock include the following:
1. Peritonitis caused by spread of infection from the uterus and fallopian tubes, sometimes resulting from instru- mental abortion performed under unsterile conditions.
2. Peritonitis resulting from rupture of the gastrointesti- nal system, sometimes caused by intestinal disease and sometimes by wounds.
3. Generalized bodily infection resulting from spread of a skin infection such as streptococcal or staphylococcal infection.
4. Generalized gangrenous infection resulting specifi- cally from gas gangrene bacilli, spreading first through
peripheral tissues and finally by way of the blood to the internal organs, especially the liver.
5. Infection spreading into the blood from the kidney or urinary tract, often caused by colon bacilli.
Special Features of Septic Shock. Because of the multiple types of septic shock, it is difficult to categorize this condition. Some features often observed are:
1. High fever. 2. Often marked vasodilation throughout the body, espe-
cially in the infected tissues. 3. High cardiac output in perhaps half of patients, caused
by arteriolar dilation in the infected tissues and by high metabolic rate and vasodilation elsewhere in the body, resulting from bacterial toxin stimulation of cel- lular metabolism and from high body temperature.
4. Sludging of the blood, caused by red cell agglutination in response to degenerating tissues.
5. Development of micro-blood clots in widespread areas of the body, a condition called disseminated intravascular coagulation. Also, this causes the blood clotting factors to be used up, so hemorrhaging occurs in many tissues, especially in the gut wall of the intes- tinal tract.
In early stages of septic shock, the patient usually does not have signs of circulatory collapse but only signs of the bacterial infection. As the infection becomes more severe, the circulatory system usually becomes involved either because of direct extension of the infection or secondarily as a result of toxins from the bacteria, with resultant loss of plasma into the infected tissues through deteriorating blood capillary walls. There finally comes a point at which deterioration of the circulation becomes progressive in the same way that progression occurs in all other types of shock. The end stages of septic shock are not greatly different from the end stages of hemorrhagic shock, even though the initiating factors are markedly different in the two conditions.
Physiology of Treatment in Shock
Replacement Therapy
Blood and Plasma Transfusion. If a person is in shock caused by hemorrhage, the best possible ther- apy is usually transfusion of whole blood. If the shock is caused by plasma loss, the best therapy is administra- tion of plasma; when dehydration is the cause, adminis- tration of an appropriate electrolyte solution can correct the shock.
Whole blood is not always available, such as under battlefield conditions. Plasma can usually substitute ade- quately for whole blood because it increases the blood vol- ume and restores normal hemodynamics. Plasma cannot
Chapter 24 Circulatory Shock and Its Treatment
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restore a normal hematocrit, but the human body can usually stand a decrease in hematocrit to about half of normal before serious consequences result, if cardiac out- put is adequate. Therefore, in emergency conditions, it is reasonable to use plasma in place of whole blood for treat- ment of hemorrhagic or most other types of hypovolemic shock.
Sometimes plasma is unavailable. In these instances, various plasma substitutes have been developed that per- form almost exactly the same hemodynamic functions as plasma. One of these is dextran solution.
Dextran Solution as a Plasma Substitute. The principal requirement of a truly effective plasma substi- tute is that it remain in the circulatory system—that is, does not filter through the capillary pores into the tis- sue spaces. In addition, the solution must be nontoxic and must contain appropriate electrolytes to prevent derangement of the body’s extracellular fluid electro- lytes on administration.
To remain in the circulation, the plasma substitute must contain some substance that has a large enough molecu- lar size to exert colloid osmotic pressure. One substance developed for this purpose is dextran, a large polysaccha- ride polymer of glucose. Certain bacteria secrete dextran as a by-product of their growth, and commercial dex- tran can be manufactured using a bacterial culture pro- cedure. By varying the growth conditions of the bacteria, the molecular weight of the dextran can be controlled to the desired value. Dextrans of appropriate molecular size do not pass through the capillary pores and, therefore, can replace plasma proteins as colloid osmotic agents.
Few toxic reactions have been observed when using purified dextran to provide colloid osmotic pressure; therefore, solutions containing this substance have proved to be a satisfactory substitute for plasma in most fluid replacement therapy.
Treatment of Shock with Sympathomimetic Drugs—Sometimes Useful, Sometimes Not
A sympathomimetic drug is a drug that mimics sympa- thetic stimulation. These drugs include norepinephrine, epinephrine, and a large number of long-acting drugs that have the same effect as epinephrine and norepinephrine.
In two types of shock, sympathomimetic drugs have proved to be especially beneficial. The first of these is neurogenic shock, in which the sympathetic nervous sys- tem is severely depressed. Administering a sympathomi- metic drug takes the place of the diminished sympathetic actions and can often restore full circulatory function.
The second type of shock in which sympathomimetic drugs are valuable is anaphylactic shock, in which excess histamine plays a prominent role. The sympathomimetic drugs have a vasoconstrictor effect that opposes the vaso- dilating effect of histamine. Therefore, epinephrine, nor- epinephrine, or other sympathomimetic drugs are often lifesaving.
Sympathomimetic drugs have not proved to be very valuable in hemorrhagic shock. The reason is that in this type of shock, the sympathetic nervous system is almost always maximally activated by the circulatory reflexes already; so much norepinephrine and epineph- rine are already circulating in the blood that sympath- omimetic drugs have essentially no additional beneficial effect.
Other Therapy
Treatment by the Head-Down Position. When the pressure falls too low in most types of shock, espe- cially in hemorrhagic and neurogenic shock, placing the patient with the head at least 12 inches lower than the feet helps in promoting venous return, thereby also increasing cardiac output. This head-down position is the first essential step in the treatment of many types of shock.
Oxygen Therapy. Because the major deleterious effect of most types of shock is too little delivery of oxy- gen to the tissues, giving the patient oxygen to breathe can be of benefit in some instances. However, this frequently is far less beneficial than one might expect, because the problem in most types of shock is not inadequate oxygen- ation of the blood by the lungs but inadequate transport of the blood after it is oxygenated.
Treatment with Glucocorticoids (Adrenal Cortex Hormones That Control Glucose Meta- bolism). Glucocorticoids are frequently given to patients in severe shock for several reasons: (1) experi- ments have shown empirically that glucocorticoids fre- quently increase the strength of the heart in the late stages of shock; (2) glucocorticoids stabilize lysosomes in tissue cells and thereby prevent release of lysosomal enzymes into the cytoplasm of the cells, thus preventing deterioration from this source; and (3) glucocorticoids might aid in the metabolism of glucose by the severely damaged cells.
Circulatory Arrest
A condition closely allied to circulatory shock is circu- latory arrest, in which all blood flow stops. This occurs frequently on the surgical operating table as a result of cardiac arrest or ventricular fibrillation.
Ventricular fibrillation can usually be stopped by strong electroshock of the heart, the basic principles of which are described in Chapter 13.
Cardiac arrest may result from too little oxygen in the anesthetic gaseous mixture or from a depressant effect of the anesthesia itself. A normal cardiac rhythm can usu- ally be restored by removing the anesthetic and immedi- ately applying cardiopulmonary resuscitation procedures, while at the same time supplying the patient’s lungs with adequate quantities of ventilatory oxygen.
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Effect of Circulatory Arrest on the Brain
A special problem in circulatory arrest is to prevent detri- mental effects in the brain as a result of the arrest. In gen- eral, more than 5 to 8 minutes of total circulatory arrest can cause at least some degree of permanent brain dam- age in more than half of patients. Circulatory arrest for as long as 10 to 15 minutes almost always permanently destroys significant amounts of mental power.
For many years, it was taught that this detrimen- tal effect on the brain was caused by the acute cerebral hypoxia that occurs during circulatory arrest. However, experiments have shown that if blood clots are prevented from occurring in the blood vessels of the brain, this will also prevent much of the early deterioration of the brain during circulatory arrest. For instance, in animal experi- ments, all the blood was removed from the animal’s blood vessels at the beginning of circulatory arrest and then replaced at the end of circulatory arrest so that no intra- vascular blood clotting could occur. In this experiment, the brain was usually able to withstand up to 30 minutes of circulatory arrest without permanent brain damage. Also, administration of heparin or streptokinase (to pre- vent blood coagulation) before cardiac arrest was shown to increase the survivability of the brain up to two to four times longer than usual.
It is likely that the severe brain damage that occurs from circulatory arrest is caused mainly by permanent blockage of many small blood vessels by blood clots, thus leading to prolonged ischemia and eventual death of the neurons.
Bibliography
Annane D, Sebille V, Charpentier C, et al: Effect of treatment with low doses of hydrocortisone and fludrocortisone on mortality in patients with septic shock, JAMA 288:862, 2002.
Burry LD, Wax RS: Role of corticosteroids in septic shock, Ann Pharmacother 38:464, 2004.
Crowell JW, Smith EE: Oxygen deficit and irreversible hemorrhagic shock, Am J Physiol 206:313, 1964.
Flierl MA, Rittirsch D, Huber-Lang MS, et al: Molecular events in the cardio- myopathy of sepsis, Mol Med 14:327, 2008.
Galli SJ, Tsai M, Piliponsky AM: The development of allergic inflammation, Nature 454:445, 2008.
Goodnough LT, Shander A: Evolution in alternatives to blood transfusion, Hematol J 4:87, 2003.
Guyton AC, Jones CE, Coleman TG: Circulatory physiology: cardiac output and its regulation, Philadelphia, 1973, WB Saunders.
Kemp SF, Lockey RF, Simons FE: Epinephrine: the drug of choice for anaphy- laxis. A statement of the World Allergy Organization, Allergy 63:1061, 2008.
Martin GS, Mannino DM, Eaton S, et al: The epidemiology of sepsis in the United States from 1979 through 2000, N Engl J Med 348:1546, 2003.
Reynolds HR, Hochman J: Cardiogenic shock: current concepts and improv- ing outcomes, Circulation 117:686, 2008.
Rushing GD, Britt LD: Reperfusion injury after hemorrhage: a collective review, Ann Surg 247:929, 2008.
Toh CH, Dennis M: Disseminated intravascular coagulation: old disease, new hope, BMJ 327:974, 2003.
Wheeler AP: Recent developments in the diagnosis and management of severe sepsis, Chest 132:1967, 2007.
Wilson M, Davis DP, Coimbra R: Diagnosis and monitoring of hemor- rhagic shock during the initial resuscitation of multiple trauma patients: a review, J Emerg Med 24:413, 2003.
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The Body Fluids and Kidneys 25. The Body Fluid Compartments:
Extracellular and Intracellular Fluids; Edema
26. Urine Formation by the Kidneys: I. Glomerular Filtration, Renal Blood Flow, and Their Control
27. Urine Formation by the Kidneys: II. Tubular Reabsorption and Secretion
28. Urine Concentration and Dilution; Regulation of Extracellular Fluid Osmolarity and Sodium Concentration
29. Renal Regulation of Potassium, Calcium, Phosphate, and Magnesium; Integration of Renal Mechanisms for Control of Blood Volume and Extracellular Fluid Volume
30. Acid-Base Regulation
31. Diuretics, Kidney Diseases
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chapter 25
The Body Fluid Compartments: Extracellular and Intracellular Fluids; Edema
The maintenance of a rela- tively constant volume and a stable composition of the body fluids is essential for homeostasis, as discussed in Chapter 1. Some of the most common and important
problems in clinical medicine arise because of abnormali- ties in the control systems that maintain this constancy of the body fluids. In this chapter and in the following chap- ters on the kidneys, we discuss the overall regulation of body fluid volume, constituents of the extracellular fluid, acid-base balance, and control of fluid exchange between extracellular and intracellular compartments.
Fluid Intake and Output Are Balanced During Steady-State Conditions
The relative constancy of the body fluids is remarkable because there is continuous exchange of fluid and solutes with the external environment, as well as within the dif- ferent compartments of the body. For example, there is a highly variable fluid intake that must be carefully matched by equal output of water from the body to prevent body fluid volumes from increasing or decreasing.
Daily Intake of Water
Water is added to the body by two major sources: (1) It is ingested in the form of liquids or water in the food, which together normally add about 2100 ml/day to the body flu- ids, and (2) it is synthesized in the body as a result of oxi- dation of carbohydrates, adding about 200 ml/day. This provides a total water intake of about 2300 ml/day (Table 25-1). Intake of water, however, is highly variable among different people and even within the same person on dif- ferent days, depending on climate, habits, and level of physical activity.
Daily Loss of Body Water
Insensible Water Loss. Some of the water losses cannot be precisely regulated. For example, there is a continuous loss of water by evaporation from the
respiratory tract and diffusion through the skin, which together account for about 700 ml/day of water loss under normal conditions. This is termed insensible water loss because we are not consciously aware of it, even though it occurs continually in all living humans.
The insensible water loss through the skin occurs inde- pendently of sweating and is present even in people who are born without sweat glands; the average water loss by diffusion through the skin is about 300 to 400 ml/day. This loss is minimized by the cholesterol-filled cornified layer of the skin, which provides a barrier against excessive loss by diffusion. When the cornified layer becomes denuded, as occurs with extensive burns, the rate of evaporation can increase as much as 10-fold, to 3 to 5 L/day. For this reason, burn victims must be given large amounts of fluid, usually intravenously, to balance fluid loss.
Insensible water loss through the respiratory tract aver- ages about 300 to 400 ml/day. As air enters the respira- tory tract, it becomes saturated with moisture, to a vapor pressure of about 47 mm Hg, before it is expelled. Because the vapor pressure of the inspired air is usually less than 47 mm Hg, water is continuously lost through the lungs with respiration. In cold weather, the atmospheric vapor pressure decreases to nearly 0, causing an even greater loss of water from the lungs as the temperature decreases. This explains the dry feeling in the respiratory passages in cold weather.
Fluid Loss in Sweat. The amount of water lost by sweating is highly variable, depending on physical activity and environmental temperature. The volume of sweat normally is about 100 ml/day, but in very hot weather or during heavy exercise water loss in sweat occasionally increases to 1 to 2 L/hour. This would rapidly deplete the body fluids if intake were not also increased by activating the thirst mechanism discussed in Chapter 29.
Water Loss in Feces. Only a small amount of water (100 ml/day) normally is lost in the feces. This can increase to several liters a day in people with severe diarrhea. For this reason, severe diarrhea can be life threatening if not corrected within a few days.
Unit V The Body Fluids and Kidneys
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Water Loss by the Kidneys. The remaining water loss from the body occurs in the urine excreted by the kidneys. There are multiple mechanisms that control the rate of urine excretion. In fact, the most important means by which the body maintains a balance between water intake and output, as well as a balance between intake and output of most electrolytes in the body, is by controlling the rates at which the kidneys excrete these substances. For example, urine volume can be as low as 0.5 L/day in a dehydrated person or as high as 20 L/day in a person who has been drinking tremendous amounts of water.
This variability of intake is also true for most of the electrolytes of the body, such as sodium, chloride, and potassium. In some people, sodium intake may be as low as 20 mEq/day, whereas in others, sodium intake may be as high as 300 to 500 mEq/day. The kidneys are faced with the task of adjusting the excretion rate of water and electrolytes to match precisely the intake of these substances, as well as compensating for exces- sive losses of fluids and electrolytes that occur in certain disease states. In Chapters 26 through 30, we discuss the mechanisms that allow the kidneys to perform these remarkable tasks.
Body Fluid Compartments
The total body fluid is distributed mainly between two compartments: the extracellular fluid and the intracel- lular fluid (Figure 25-1). The extracellular fluid is divided into the interstitial fluid and the blood plasma.
There is another small compartment of fluid that is referred to as transcellular fluid. This compartment includes fluid in the synovial, peritoneal, pericardial, and intraocular spaces, as well as the cerebrospinal fluid; it is usually considered to be a specialized type of extracellu- lar fluid, although in some cases its composition may differ
markedly from that of the plasma or interstitial fluid. All the transcellular fluids together constitute about 1 to 2 liters.
In the average 70-kilogram adult man, the total body water is about 60 percent of the body weight, or about 42 liters. This percentage can change, depending on age, gen- der, and degree of obesity. As a person grows older, the per- centage of total body weight that is fluid gradually decreases. This is due in part to the fact that aging is usually associated with an increased percentage of the body weight being fat, which decreases the percentage of water in the body.
Because women normally have more body fat than men, their total body water averages about 50 percent of the body weight. In premature and newborn babies, the total body water ranges from 70 to 75 percent of body weight. Therefore, when discussing the “average” body fluid com- partments, we should realize that variations exist, depend- ing on age, gender, and percentage of body fat.
Intracellular Fluid Compartment
About 28 of the 42 liters of fluid in the body are inside the 100 trillion cells and are collectively called the intracellu- lar fluid. Thus, the intracellular fluid constitutes about 40 percent of the total body weight in an “average” person.
The fluid of each cell contains its individual mixture of different constituents, but the concentrations of these substances are similar from one cell to another. In fact, the composition of cell fluids is remarkably similar even in different animals, ranging from the most primitive micro- organisms to humans. For this reason, the intracellular fluid of all the different cells together is considered to be one large fluid compartment.
Plasma 3.0 L
Interstitial fluid
11.0 L
INTAKE
Intracellular fluid
28.0 L
Capillary membrane
Ly m
ph at
ic s
E xt
ra ce
llu la
r flu
id (
14 .0
L )
Cell membrane
OUTPUT •Kidneys •Lungs •Feces •Sweat •Skin
Figure 25-1 Summary of body fluid regulation, including the major body fluid compartments and the membranes that sepa- rate these compartments. The values shown are for an average 70-kilogram person.
Normal Prolonged, Heavy Exercise
Intake
Fluids ingested 2100 ?
From metabolism 200 200
Total intake 2300 ?
Output
Insensible—skin 350 350
Insensible—lungs 350 650
Sweat 100 5000
Feces 100 100
Urine 1400 500
Total output 2300 6600
Table 25-1 Daily Intake and Output of Water (ml/day)
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Extracellular Fluid Compartment
All the fluids outside the cells are collectively called the extracellular fluid. Together these fluids account for about 20 percent of the body weight, or about 14 liters in a normal 70-kilogram man. The two largest compartments of the extracellular fluid are the interstitial fluid, which makes up more than three fourths (11 liters) of the extra- cellular fluid, and the plasma, which makes up almost one fourth of the extracellular fluid, or about 3 liters. The plasma is the noncellular part of the blood; it exchanges substances continuously with the interstitial fluid through the pores of the capillary membranes. These pores are highly permeable to almost all solutes in the extracellu- lar fluid except the proteins. Therefore, the extracellular fluids are constantly mixing, so the plasma and interstitial fluids have about the same composition except for pro- teins, which have a higher concentration in the plasma.
Blood Volume
Blood contains both extracellular fluid (the fluid in plasma) and intracellular fluid (the fluid in the red blood cells). However, blood is considered to be a separate fluid com- partment because it is contained in a chamber of its own, the circulatory system. The blood volume is especially important in the control of cardiovascular dynamics.
The average blood volume of adults is about 7 percent of body weight, or about 5 liters. About 60 percent of the blood is plasma and 40 percent is red blood cells, but these percentages can vary considerably in different peo- ple, depending on gender, weight, and other factors.
Hematocrit (Packed Red Cell Volume). The hematocrit is the fraction of the blood composed of red blood cells, as determined by centrifuging blood in a “hematocrit tube” until the cells become tightly packed in the bottom of the tube. It is impossible to completely pack the red cells together; therefore, about 3 to 4 percent of the plasma remains entrapped among the cells, and the true hematocrit is only about 96 percent of the measured hematocrit.
In men, the measured hematocrit is normally about 0.40, and in women, it is about 0.36. In severe anemia, the hematocrit may fall as low as 0.10, a value that is barely sufficient to sustain life. Conversely, there are some con- ditions in which there is excessive production of red blood cells, resulting in polycythemia. In these conditions, the hematocrit can rise to 0.65.
Constituents of Extracellular and Intracellular Fluids
Comparisons of the composition of the extracellular fluid, including the plasma and interstitial fluid, and the intra- cellular fluid are shown in Figures 25-2 and 25-3 and in Table 25-2.
Ionic Composition of Plasma and Interstitial Fluid Is Similar
Because the plasma and interstitial fluid are separated only by highly permeable capillary membranes, their ionic composition is similar. The most important differ- ence between these two compartments is the higher con- centration of protein in the plasma; because the capillaries have a low permeability to the plasma proteins, only small amounts of proteins are leaked into the interstitial spaces in most tissues.
Because of the Donnan effect, the concentration of pos- itively charged ions (cations) is slightly greater (≈2 percent) in the plasma than in the interstitial fluid. The plasma proteins have a net negative charge and, therefore, tend to bind cations, such as sodium and potassium ions, thus holding extra amounts of these cations in the plasma along with the plasma proteins. Conversely, negatively charged ions (anions) tend to have a slightly higher concentration in the interstitial fluid compared with the plasma, because the negative charges of the plasma proteins repel the neg- atively charged anions. For practical purposes, however, the concentration of ions in the interstitial fluid and in the plasma is considered to be about equal.
Referring again to Figure 25-2, one can see that the extracellular fluid, including the plasma and the intersti- tial fluid, contains large amounts of sodium and chloride ions, reasonably large amounts of bicarbonate ions, but only small quantities of potassium, calcium, magnesium, phosphate, and organic acid ions.
The composition of extracellular fluid is carefully regu- lated by various mechanisms, but especially by the kidneys, as discussed later. This allows the cells to remain continu- ally bathed in a fluid that contains the proper concentra- tion of electrolytes and nutrients for optimal cell function.
Intracellular Fluid Constituents
The intracellular fluid is separated from the extracellular fluid by a cell membrane that is highly permeable to water but not to most of the electrolytes in the body.
In contrast to the extracellular fluid, the intracellular fluid contains only small quantities of sodium and chlo- ride ions and almost no calcium ions. Instead, it con- tains large amounts of potassium and phosphate ions plus moderate quantities of magnesium and sulfate ions, all of which have low concentrations in the extracellular fluid. Also, cells contain large amounts of protein, almost four times as much as in the plasma.
Measurement of Fluid Volumes in the Different Body Fluid Compartments—the Indicator-Dilution Principle
The volume of a fluid compartment in the body can be measured by placing an indicator substance in the compart- ment, allowing it to disperse evenly throughout the com- partment’s fluid, and then analyzing the extent to which the
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E X
T R
A C
E L
L U
L A
R
Cations Anions
m E
q /L
IN T
R A
C E
L L
U L
A R
150
150
100
100
50
50
0 Ca++Na+
H C
O 3−
P O
4 a
nd o
rg an
ic a
ni on
s
P ro
te inMg++K+
Cl−
– –– –
Figure 25-2 Major cations and anions of the intracellular and extracellular fluids. The concentrations of Ca++ and Mg++ represent the sum of these two ions. The concentrations shown represent the total of free ions and complexed ions.
Phospholipids – 280 mg/dl
Cholesterol – 150 mg/dl
Glucose – 100 mg/dl
Urea – 15 mg/dl Lactic acid – 10 mg/dl Uric acid – 3 mg/dl Creatinine – 1.5 mg/dl Bilirubin – 0.5 mg/dl
Bile salts – trace
Neutral fat – 125 mg/dl
Figure 25-3 Nonelectrolytes of the plasma.
Plasma (mOsm/L H 2 O) Interstitial (mOsm/L H
2 O) Intracellular (mOsm/L H
2 O)
Na+ 142 139 14
K+ 4.2 4.0 140
Ca++ 1.3 1.2 0
Mg++ 0.8 0.7 20
Cl− 108 108 4
HCO 3
24 28.3 10
HPO 4 =, H
2 PO
4 2 2 11
SO 4 = 0.5 0.5 1
Phosphocreatine 45
Carnosine 14
Amino acids 2 2 8
Creatine 0.2 0.2 9
Lactate 1.2 1.2 1.5
Adenosine triphosphate 5
Hexose monophosphate 3.7
Glucose 5.6 5.6
Protein 1.2 0.2 4
Urea 4 4 4
Others 4.8 3.9 10
Total mOsm/L 301.8 300.8 301.2
Corrected osmolar activity (mOsm/L) 282.0 281.0 281.0
Total osmotic pressure at 37 °C (mm Hg) 5443 5423 5423
Table 25-2 Osmolar Substances in Extracellular and Intracellular Fluids
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substance becomes diluted. Figure 25-4 shows this “indi- cator-dilution” method of measuring the volume of a fluid compartment. This method is based on the conservation of mass principle, which means that the total mass of a sub- stance after dispersion in the fluid compartment will be the same as the total mass injected into the compartment.
In the example shown in Figure 25-4, a small amount of dye or other substance contained in the syringe is injected into a chamber and the substance is allowed to disperse throughout the chamber until it becomes mixed in equal concentrations in all areas. Then a sample of fluid con- taining the dispersed substance is removed and the con- centration is analyzed chemically, photoelectrically, or by other means. If none of the substance leaks out of the com- partment, the total mass of substance in the compartment (Volume B × Concentration B) will equal the total mass of the substance injected (Volume A × Concentration A). By simple rearrangement of the equation, one can calculate the unknown volume of chamber B as
Note that all one needs to know for this calculation is (1) the total amount of substance injected into the chamber (the numerator of the equation) and (2) the concentration of the fluid in the chamber after the substance has been dispersed (the denominator).
For example, if 1 milliliter of a solution containing 10 mg/ml of dye is dispersed into chamber B and the final concentration in the chamber is 0.01 milligram for each milliliter of fluid, the unknown volume of the chamber can be calculated as follows:
This method can be used to measure the volume of virtually any compartment in the body as long as (1) the indicator disperses evenly throughout the compart- ment, (2) the indicator disperses only in the compart- ment that is being measured, and (3) the indicator is not metabolized or excreted. Several substances can be used to measure the volume of each of the different body fluids.
Determination of Volumes of Specific Body Fluid Compartments
Measurement of Total Body Water. Radioactive water (tritium, 3H2O) or heavy water (deuterium, 2H2O) can be used to measure total body water. These forms of water mix with the total body water within a few hours after being injected into the blood, and the dilution principle can be used to calculate total body water (Table 25-3). Another substance that has been used to measure total body water is antipyrine, which is very lipid soluble and can rapidly penetrate cell membranes and distribute itself uniformly throughout the intracellular and extracellular compartments.
Measurement of Extracellular Fluid Volume. The volume of extracellular fluid can be estimated using any of several substances that disperse in the plasma and interstitial fluid but do not readily permeate the cell membrane. They include radioactive sodium, radioactive chloride, radioactive iothalamate, thiosulfate ion, and inulin. When any one of these substances is injected into the blood, it usually disperses almost completely throughout the extracellular fluid within 30 to 60 minutes. Some of these substances, however, such as radioactive sodium, may diffuse into the cells in small amounts. Therefore, one frequently speaks of the sodium space or the inulin space, instead of calling the measurement the true extracellular fluid volume.
Indicator Mass A = Volume A x Concentration A
Indicator Mass B = Volume B x Concentration B
Volume B = Indicator Mass B / Concentration B
Indicator Mass A = Indicator Mass B
Figure 25-4 Indicator-dilution method for measuring fluid volumes.
Volume Indicators
Total body water 3H 2 O, 2H
2 O, antipyrine
Extracellular fluid 22Na, 125I-iothalamate, thiosulfate, inulin
Intracellular fluid (Calculated as total body water − Extracellular fluid volume)
Plasma volume 125I-albumin, Evans blue dye (T-1824)
Blood volume
51Cr-labeled red blood cells, or calculated as blood volume = Plasma volume/(1 − Hematocrit)
Interstitial fluid (Calculated as extracellular fluid volume − Plasma volume)
Table 25-3 Measurement of Body Fluid VolumesVolume B = Volume A Concentration A
Concentration B
Volume B = 0.01 mg/ml
= 1000 ml 1 ml 10 mg/ml
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Calculation of Intracellular Volume. The intra- cellular volume cannot be measured directly. However, it can be calculated as
Intracellular volume = Total body water – Extracellular volume
Measurement of Plasma Volume. To measure plasma volume, a substance must be used that does not readily penetrate capillary membranes but remains in the vascular system after injection. One of the most commonly used substances for measuring plasma volume is serum albumin labeled with radioactive iodine (125I-albumin). Also, dyes that avidly bind to the plasma proteins, such as Evans blue dye (also called T-1824), can be used to measure plasma volume.
Calculation of Interstitial Fluid Volume. Intersti- tial fluid volume cannot be measured directly, but it can be calculated as
Interstitial fluid volume = Extracellular fluid volume – Plasma volume
Measurement of Blood Volume. If one measures plasma volume using the methods described earlier, blood volume can also be calculated if one knows the hematocrit (the fraction of the total blood volume composed of cells), using the following equation:
For example, if plasma volume is 3 liters and hemat- ocrit is 0.40, total blood volume would be calculated as
Another way to measure blood volume is to inject into the circulation red blood cells that have been labeled with radioactive material. After these mix in the circulation, the radioactivity of a mixed blood sample can be measured and the total blood volume can be calculated using the indicator-dilution principle. A substance frequently used to label the red blood cells is radioactive chromium (51Cr), which binds tightly with the red blood cells.
Regulation of Fluid Exchange and Osmotic Equilibrium Between Intracellular and Extracellular Fluid
A frequent problem in treating seriously ill patients is maintaining adequate fluids in one or both of the intra- cellular and extracellular compartments. As discussed in Chapter 16 and later in this chapter, the relative amounts
of extracellular fluid distributed between the plasma and interstitial spaces are determined mainly by the balance of hydrostatic and colloid osmotic forces across the capil- lary membranes.
The distribution of fluid between intracellular and extracellular compartments, in contrast, is determined mainly by the osmotic effect of the smaller solutes— especially sodium, chloride, and other electrolytes— acting across the cell membrane. The reason for this is that the cell membranes are highly permeable to water but relatively impermeable to even small ions such as sodium and chloride. Therefore, water moves across the cell membrane rapidly and the intracellular fluid remains isotonic with the extracellular fluid.
In the next section, we discuss the interrelations between intracellular and extracellular fluid volumes and the osmotic factors that can cause shifts of fluid between these two compartments.
Basic Principles of Osmosis and Osmotic Pressure
The basic principles of osmosis and osmotic pressure were presented in Chapter 4. Therefore, we review here only the most important aspects of these principles as they apply to volume regulation.
Osmosis is the net diffusion of water across a selectively permeable membrane from a region of high water concen- tration to one that has a lower water concentration. When a solute is added to pure water, this reduces the concen- tration of water in the mixture. Thus, the higher the solute concentration in a solution, the lower the water concen- tration. Further, water diffuses from a region of low solute concentration (high water concentration) to one with a high solute concentration (low water concentration).
Because cell membranes are relatively impermeable to most solutes but highly permeable to water (i.e., selectively permeable), whenever there is a higher concentration of solute on one side of the cell membrane, water diffuses across the membrane toward the region of higher solute concentration. Thus, if a solute such as sodium chloride is added to the extracellular fluid, water rapidly diffuses from the cells through the cell membranes into the extra- cellular fluid until the water concentration on both sides of the membrane becomes equal. Conversely, if a solute such as sodium chloride is removed from the extracellular fluid, water diffuses from the extracellular fluid through the cell membranes and into the cells. The rate of diffu- sion of water is called the rate of osmosis.
Relation Between Moles and Osmoles. Because the water concentration of a solution depends on the number of solute particles in the solution, a concentration term is necessary to describe the total concentration of solute particles, regardless of their exact composition. The total number of particles in a solution is measured in osmoles. One osmole (osm) is equal to 1 mole (mol) (6.02 × 1023)
Total blood volume = Plasma volume
1 – Hematocrit
3 liters
1 - 0.4 = 5 liters
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of solute particles. Therefore, a solution containing 1 mole of glucose in each liter has a concentration of 1 osm/L. If a molecule dissociates into two ions (giving two particles), such as sodium chloride ionizing to give chloride and sodium ions, then a solution containing 1 mol/L will have an osmolar concentration of 2 osm/L. Likewise, a solution that contains 1 mole of a molecule that dissociates into three ions, such as sodium sulfate (Na2SO4), will contain 3 osm/L. Thus, the term osmole refers to the number of osmotically active particles in a solution rather than to the molar concentration.
In general, the osmole is too large a unit for expressing osmotic activity of solutes in the body fluids.
The term milliosmole (mOsm), which equals 1/1000 osmole, is commonly used.
Osmolality and Osmolarity. The osmolal con- centrat ion of a solution is called osmolality when the con centration is expressed as osmoles per kilogram of water; it is called osmolarity when it is expressed as osmoles per liter of solution. In dilute solutions such as the body fluids, these two terms can be used almost synonymously because the differences are small. In most cases, it is easier to express body fluid quantities in liters of fluid rather than in kilograms of water. Therefore, most of the calculations used clinically and the calculations expressed in the next several chapters are based on osmolarities rather than osmolalities.
Calculation of the Osmolarity and Osmotic Pressure of a Solution. Using van’t Hoff ’s law, one can calculate the potential osmotic pressure of a solution, assuming that the cell membrane is impermeable to the solute.
For example, the osmotic pressure of a 0.9 percent sodium chloride solution is calculated as follows: A 0.9 percent solution means that there is 0.9 gram of sodium chloride per 100 milliliters of solution, or 9 g/L. Because the molecular weight of sodium chloride is 58.5 g/mol, the molarity of the solution is 9 g/L divided by 58.5 g/ mol, or about 0.154 mol/L. Because each molecule of sodium chloride is equal to 2 osmoles, the osmolarity of the solution is 0.154 × 2, or 0.308 osm/L. Therefore, the osmolarity of this solution is 308 mOsm/L. The potential osmotic pressure of this solution would therefore be 308 mOsm/L × 19.3 mm Hg/mOsm/L, or 5944 mm Hg.
This calculation is only an approximation because sodium and chloride ions do not behave entirely inde- pendently in solution because of interionic attraction between them. One can correct for these deviations from the predictions of van’t Hoff ’s law by using a correction factor called the osmotic coefficient. For sodium chlo- ride, the osmotic coefficient is about 0.93. Therefore, the actual osmolarity of a 0.9 percent sodium chloride solu- tion is 308 × 0.93, or about 286 mOsm/L. For practical reasons, the osmotic coefficients of different solutes are
sometimes neglected in determining the osmolarity and osmotic pressures of physiologic solutions.
Osmolarity of the Body Fluids. Turning back to Table 25-2, note the approximate osmolarity of the various osmotically active substances in plasma, interstitial fluid, and intracellular fluid. Note that about 80 percent of the total osmolarity of the interstitial fluid and plasma is due to sodium and chloride ions, whereas for intracellular fluid, almost half the osmolarity is due to potassium ions and the remainder is divided among many other intracellular substances.
As shown in Table 25-2, the total osmolarity of each of the three compartments is about 300 mOsm/L, with the plasma being about 1 mOsm/L greater than that of the interstitial and intracellular fluids. The slight differ- ence between plasma and interstitial fluid is caused by the osmotic effects of the plasma proteins, which main- tain about 20 mm Hg greater pressure in the capillaries than in the surrounding interstitial spaces, as discussed in Chapter 16.
Corrected Osmolar Activity of the Body Fluids. At the bottom of Table 25-2 are shown corrected osmolar activities of plasma, interstitial fluid, and intracellular fluid. The reason for these corrections is that cations and anions exert interionic attraction, which can cause a slight decrease in the osmotic “activity” of the dissolved substance.
Osmotic Equilibrium Is Maintained Between Intracellular and Extracellular Fluids
Large osmotic pressures can develop across the cell membrane with relatively small changes in the concen- trations of solutes in the extracellular fluid. As discussed earlier, for each milliosmole concentration gradient of an impermeant solute (one that will not permeate the cell membrane), about 19.3 mm Hg osmotic pressure is exerted across the cell membrane. If the cell membrane is exposed to pure water and the osmolarity of intracel- lular fluid is 282 mOsm/L, the potential osmotic pres- sure that can develop across the cell membrane is more than 5400 mm Hg. This demonstrates the large force that can move water across the cell membrane when the intracellular and extracellular fluids are not in osmotic equilibrium. As a result of these forces, relatively small changes in the concentration of impermeant solutes in the extracellular fluid can cause large changes in cell volume.
Isotonic, Hypotonic, and Hypertonic Fluids. The effects of different concentrations of impermeant solutes in the extracellular fluid on cell volume are shown in Figure 25-5. If a cell is placed in a solution of impermeant solutes having an osmolarity of 282 mOsm/L, the cells
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will not shrink or swell because the water concentration in the intracellular and extracellular fluids is equal and the solutes cannot enter or leave the cell. Such a solution is said to be isotonic because it neither shrinks nor swells the cells. Examples of isotonic solutions include a 0.9 percent solution of sodium chloride or a 5 percent glucose solution. These solutions are important in clinical medicine because they can be infused into the blood without the danger of upsetting osmotic equilibrium between the intracellular and extracellular fluids.
If a cell is placed into a hypotonic solution that has a lower concentration of impermeant solutes (<282 mOsm/L), water will diffuse into the cell, caus- ing it to swell; water will continue to diffuse into the cell, diluting the intracellular fluid while also concentrating the extracellular fluid until both solutions have about the same osmolarity. Solutions of sodium chloride with a concentration of less than 0.9 percent are hypotonic and cause cells to swell.
If a cell is placed in a hypertonic solution having a higher concentration of impermeant solutes, water will flow out of the cell into the extracellular fluid, concen- trating the intracellular fluid and diluting the extracellular fluid. In this case, the cell will shrink until the two con- centrations become equal. Sodium chloride solutions of greater than 0.9 percent are hypertonic.
Isosmotic, Hyperosmotic, and Hypo-osmotic Fluids. The terms isotonic, hypotonic, and hypertonic refer to whether solutions will cause a change in cell volume. The tonicity of solutions depends on the concentration of impermeant solutes. Some solutes, however, can permeate the cell membrane. Solutions with an osmolarity the same as the cell are called isosmotic, regardless of whether the solute can penetrate the cell membrane.
The terms hyperosmotic and hypo-osmotic refer to solutions that have a higher or lower osmolarity, respec- tively, compared with the normal extracellular fluid, without regard for whether the solute permeates the cell membrane. Highly permeating substances, such as urea, can cause transient shifts in fluid volume between the intracellular and extracellular fluids, but given enough time, the concentrations of these substances eventu- ally become equal in the two compartments and have little effect on intracellular volume under steady-state conditions.
Osmotic Equilibrium Between Intracellular and Extracellular Fluids Is Rapidly Attained. The transfer of fluid across the cell membrane occurs so rapidly that any differences in osmolarities between these two compartments are usually corrected within seconds or, at the most, minutes. This rapid movement of water across the cell membrane does not mean that complete equilibrium occurs between the intracellular and extracellular compartments throughout the whole body within the same short period. The reason for this is that fluid usually enters the body through the gut and must be transported by the blood to all tissues before complete osmotic equilibrium can occur. It usually takes about 30 minutes to achieve osmotic equilibrium everywhere in the body after drinking water.
Volume and Osmolality of Extracellular and Intracellular Fluids in Abnormal States
Some of the different factors that can cause extracel- lular and intracellular volumes to change markedly are ingestion of water, dehydration, intravenous infusion of different types of solutions, loss of large amounts of fluid from the gastrointestinal tract, and loss of abnormal amounts of fluid by sweating or through the kidneys.
One can calculate both the changes in intracellular and extracellular fluid volumes and the types of therapy that should be instituted if the following basic principles are kept in mind:
1. Water moves rapidly across cell membranes; there- fore, the osmolarities of intracellular and extracellu- lar fluids remain almost exactly equal to each other except for a few minutes after a change in one of the compartments.
2. Cell membranes are almost completely impermeable to many solutes; therefore, the number of osmoles in the extracellular or intracellular fluid generally remains constant unless solutes are added to or lost from the extracellular compartment.
With these basic principles in mind, we can analyze the effects of different abnormal fluid conditions on extracel- lular and intracellular fluid volumes and osmolarities.
HYPOTONIC Cell swells
200 mOsm/L
HYPERTONIC Cell shrinks
360 mOsm/L
ISOTONIC No change
280 mOsm/L B
C
A
Figure 25-5 Effects of isotonic (A), hypertonic (B), and hypotonic (C) solutions on cell volume.
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Effect of Adding Saline Solution to the Extracellular Fluid
If an isotonic saline solution is added to the extracellu- lar fluid compartment, the osmolarity of the extracellu- lar fluid does not change; therefore, no osmosis occurs through the cell membranes. The only effect is an increase in extracellular fluid volume (Figure 25-6A). The sodium and chloride largely remain in the extracellular fluid because the cell membrane behaves as though it were vir- tually impermeable to the sodium chloride.
If a hypertonic solution is added to the extracellular fluid, the extracellular osmolarity increases and causes osmosis of water out of the cells into the extracellular compartment (see Figure 25-6B). Again, almost all the added sodium chloride remains in the extracellular com- partment and fluid diffuses from the cells into the extra- cellular space to achieve osmotic equilibrium. The net effect is an increase in extracellular volume (greater than the volume of fluid added), a decrease in intracellular vol- ume, and a rise in osmolarity in both compartments.
If a hypotonic solution is added to the extracellular fluid, the osmolarity of the extracellular fluid decreases and some of the extracellular water diffuses into the cells until the intracellular and extracellular compartments have the same osmolarity (see Figure 25-6C). Both the intracellular and the extracellular volumes are increased by the addition of hypotonic fluid, although the intracel- lular volume increases to a greater extent.
Calculation of Fluid Shifts and Osmolarities After Infusion of Hypertonic Saline. We can calculate the sequential effects of infusing different solutions on extracellular and intracellular fluid volumes and
osmolarities. For example, if 2 liters of a hypertonic 3.0 percent sodium chloride solution are infused into the extracellular fluid compartment of a 70-kilogram patient whose initial plasma osmolarity is 280 mOsm/L, what would be the intracellular and extracellular fluid volumes and osmolarities after osmotic equilibrium?
The first step is to calculate the initial conditions, including the volume, concentration, and total mil- liosmoles in each compartment. Assuming that extra- cellular fluid volume is 20 percent of body weight and intracellular fluid volume is 40 percent of body weight, the following volumes and concentrations can be calculated.
Step 1. Initial Conditions
Volume (Liters)
Concentration (mOsm/L)
Total (mOsm)
Extracellular fluid 14 280 3,920 Intracellular fluid 28 280 7,840 Total body fluid 42 280 11,760
Next, we calculate the total milliosmoles added to the extracellular fluid in 2 liters of 3.0 percent sodium chloride. A 3.0 percent solution means that there are 3.0 g/100 ml, or 30 grams of sodium chloride per liter. Because the molecular weight of sodium chloride is about 58.5 g/mol, this means that there is about 0.513 mole of sodium chloride per liter of solution. For 2 liters of solu- tion, this would be 1.026 mole of sodium chloride. Because 1 mole of sodium chloride is about equal to 2 osmoles (sodium chloride has two osmotically active particles per
O sm
o la
ri ty
20
Normal State A. Add Isotonic NaCl
C. Add Hypotonic NaCl B. Add Hypertonic NaCl
30 4010
Intracellular fluid
300
200
100
0
Volume (liters)Volume (liters)
Extracellular fluid
Figure 25-6 Effect of adding isotonic, hypertonic, and hypotonic solutions to the extracellular fluid after osmotic equilibrium. The normal state is indicated by the solid lines, and the shifts from normal are shown by the shaded areas. The volumes of intracellular and extracellular fluid compartments are shown in the abscissa of each diagram, and the osmolarities of these compartments are shown on the ordinates.
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mole), the net effect of adding 2 liters of this solution is to add 2051 milliosmoles of sodium chloride to the extracel- lular fluid.
In Step 2, we calculate the instantaneous effect of add- ing 2051 milliosmoles of sodium chloride to the extra- cellular fluid along with 2 liters of volume. There would be no change in the intracellular fluid concentration or volume, and there would be no osmotic equilibrium. In the extracellular fluid, however, there would be an addi- tional 2051 milliosmoles of total solute, yielding a total of 5791 milliosmoles. Because the extracellular compart- ment now has 16 liters of volume, the concentration can be calculated by dividing 5791 milliosmoles by 16 liters to yield a concentration of 373 mOsm/L. Thus, the following values would occur instantly after adding the solution.
Step 2. Instantaneous Effect of Adding 2 Liters of 3.0 Percent Sodium Chloride
Volume (Liters)
Concentration (mOsm/L)
Total (mOsm)
Extracellular fluid 16 373 5,971 Intracellular fluid 28 280 7,840 Total body fluid 44 No equilibrium 13,811
In the third step, we calculate the volumes and con- centrations that would occur within a few minutes after osmotic equilibrium develops. In this case, the concentra- tions in the intracellular and extracellular fluid compart- ments would be equal and can be calculated by dividing the total milliosmoles in the body, 13,811, by the total vol- ume, which is now 44 liters. This yields a concentration of 313.9 mOsm/L. Therefore, all the body fluid compart- ments will have this same concentration after osmotic equilibrium. Assuming that no solute or water has been lost from the body and that there is no movement of sodium chloride into or out of the cells, we then calculate the vol- umes of the intracellular and extracellular compartments. The intracellular fluid volume is calculated by dividing the total milliosmoles in the intracellular fluid (7840) by the concentration (313.9 mOsm/L), to yield a volume of 24.98 liters. Extracellular fluid volume is calculated by dividing the total milliosmoles in extracellular fluid (5971) by the concentration (313.9 mOsm/L), to yield a volume of 19.02 liters. Again, these calculations are based on the assump- tion that the sodium chloride added to the extracellular fluid remains there and does not move into the cells.
Step 3. Effect of Adding 2 Liters of 3.0 Percent Sodium Chloride After Osmotic Equilibrium
Volume (Liters)
Concentration (mOsm/L)
Total (mOsm)
Extracellular fluid 19.02 313.9 5,971 Intracellular fluid 24.98 313.9 7,840 Total body fluid 44.0 313.9 13,811
Thus, one can see from this example that adding 2 liters of a hypertonic sodium chloride solution causes more than a 5-liter increase in extracellular fluid vol- ume while decreasing intracellular fluid volume by almost 3 liters.
This method of calculating changes in intracellular and extracellular fluid volumes and osmolarities can be applied to virtually any clinical problem of fluid volume regulation. The reader should be familiar with such cal- culations because an understanding of the mathematical aspects of osmotic equilibrium between intracellular and extracellular fluid compartments is essential for under- standing almost all fluid abnormalities of the body and their treatment.
Glucose and Other Solutions Administered for Nutritive Purposes
Many types of solutions are administered intravenously to provide nutrition to people who cannot otherwise take adequate amounts of nutrition. Glucose solutions are widely used, and amino acid and homogenized fat solu- tions are used to a lesser extent. When these solutions are administered, their concentrations of osmotically active substances are usually adjusted nearly to isotonicity, or they are given slowly enough that they do not upset the osmotic equilibrium of the body fluids. After the glucose or other nutrients are metabolized, an excess of water often remains, especially if additional fluid is ingested. Ordinarily, the kidneys excrete this in the form of a very dilute urine. The net result, therefore, is the addition of only nutrients to the body.
Clinical Abnormalities of Fluid Volume Regulation: Hyponatremia and Hypernatremia
The primary measurement that is readily available to the clinician for evaluating a patient’s fluid status is the plasma sodium concentration. Plasma osmolarity is not routinely measured, but because sodium and its associated anions (mainly chloride) account for more than 90 percent of the solute in the extracellular fluid, plasma sodium concentra- tion is a reasonable indicator of plasma osmolarity under many conditions. When plasma sodium concentration is reduced more than a few milliequivalents below nor- mal (about 142 mEq/L), a person is said to have hypona- tremia. When plasma sodium concentration is elevated above normal, a person is said to have hypernatremia.
Causes of Hyponatremia: Excess Water or Loss of Sodium
Decreased plasma sodium concentration can result from loss of sodium chloride from the extracellular fluid or addi- tion of excess water to the extracellular fluid (Table 25-4).
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A primary loss of sodium chloride usually results in hyponatremia—dehydration and is associated with decreased extracellular fluid volume. Conditions that can cause hyponatremia owing to loss of sodium chloride include diarrhea and vomiting. Overuse of diuretics that inhibit the ability of the kidneys to conserve sodium and certain types of sodium-wasting kidney diseases can also cause modest degrees of hyponatremia. Finally, Addison’s disease, which results from decreased secretion of the hormone aldosterone, impairs the ability of the kidneys to reabsorb sodium and can cause a modest degree of hyponatremia.
Hyponatremia can also be associated with excess water retention, which dilutes the sodium in the extracellular fluid, a condition that is referred to as hyponatremia—overhydration. For example, excessive secretion of antidiuretic hormone, which causes the kid- ney tubules to reabsorb more water, can lead to hypona- tremia and overhydration.
Consequences of Hyponatremia: Cell Swelling
Rapid changes in cell volume as a result of hypona- tremia can have profound effects on tissue and organ function, especially the brain. A rapid reduction in plasma sodium concentration, for example, can cause brain cell edema and neurological symptoms, includ- ing headache, nausea, lethargy, and disorientation. If plasma sodium concentration rapidly falls below 115 to 120 mmol/L, brain swelling may lead to seizures, coma, permanent brain damage, and death. Because the skull is rigid, the brain cannot increase its volume by more than about 10 percent without it being forced down the neck (herniation), which can lead to permanent brain injury and death.
When hyponatremia evolves more slowly over several days, the brain and other tissues respond by transporting sodium, chloride, potassium, and organic solutes, such as glutamate, from the cells into the extracellular compart- ment. This attenuates osmotic flow of water into the cells and swelling of the tissues (Figure 25-7).
Transport of solutes from the cells during slowly devel- oping hyponatremia, however, can make the brain vulner- able to injury if the hyponatremia is corrected too rapidly. When hypertonic solutions are added too rapidly to cor- rect hyponatremia, this can outpace the brain’s ability to recapture the solutes lost from the cells and may lead to osmotic injury of the neurons that is associated with demyelination, a loss of the myelin sheath from nerves. This osmotic-mediated demyelination of neurons can be avoided by limiting the correction of chronic hypona- tremia to less than 10 to 12 mmol/L in 24 hours and to less than 18 mmol/L in 48 hours. This slow rate of cor- rection permits the brain to recover the lost osmoles that have occurred as a result of adaptation to chronic hyponatremia.
Hyponatremia is the most common electrolyte disor- der encountered in clinical practice and may occur in up to 15% to 25% of hospitalized patients.
Causes of Hypernatremia: Water Loss or Excess Sodium
Increased plasma sodium concentration, which also causes increased osmolarity, can be due to either loss of water from the extracellular fluid, which concentrates the sodium ions, or excess sodium in the extracellular fluid. When there is primary loss of water from the extracellular fluid, this results in hypernatremia—dehydration. This condition can occur from an inability to secrete antidi- uretic hormone, which is needed for the kidneys to con- serve water. As a result of lack of antidiuretic hormone, the kidneys excrete large amounts of dilute urine (a dis- order referred to as diabetes insipidus), causing dehydra- tion and increased concentration of sodium chloride in the extracellular fluid. In certain types of renal diseases, the kidneys cannot respond to antidiuretic hormone, also causing a type of nephrogenic diabetes insipidus. A more common cause of hypernatremia associated with decreased extracellular fluid volume is dehydration caused by water intake that is less than water loss, as can occur with sweating during prolonged, heavy exercise.
Abnormality Cause Plasma Na+ Concentration
Extracellular Fluid Volume
Intracellular Fluid Volume
Hyponatremia—dehydration Adrenal insufficiency; overuse of diuretics
↓ ↓ ↑
Hyponatremia—overhydration Excess ADH (SIADH); bronchogenic tumors
↓ ↑ ↑
Hypernatremia—dehydration Diabetes insipidus; excessive sweating
↑ ↓ ↓
Hypernatremia—overhydration Cushing’s disease; primary aldosteronism
↑ ↑ ↓
Table 25-4 Abnormalities of Body Fluid Volume Regulation: Hyponatremia and Hypernatremia
ADH, antidiuretic hormone; SIADH, syndrome of inappropriate ADH.
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Hypernatremia can also occur as a result of excessive sodium chloride added to the extracellular fluid. This often results in hypernatremia—overhydration because excess extracellular sodium chloride is usually associated with at least some degree of water retention by the kidneys as well. For example, excessive secretion of the sodium- retaining hormone aldosterone can cause a mild degree of hypernatremia and overhydration. The reason that the hypernatremia is not more severe is that increased aldos- terone secretion causes the kidneys to reabsorb greater amounts of water, as well as sodium.
Thus, in analyzing abnormalities of plasma sodium concentration and deciding on proper therapy, one should first determine whether the abnormality is caused by a primary loss or gain of sodium or a primary loss or gain of water.
Consequences of Hypernatremia: Cell Shrinkage
Hypernatremia is much less common than hyponatremia and severe symptoms usually occur only with rapid and large increases in plasma sodium concentration above 158 to 160 mmol/L. One reason for this is that hyper- natremia promotes intense thirst that protects against a large increase in plasma and extracellular fluid sodium, as discussed in Chapter 28. However, severe hyperna- tremia can occur in patients with hypothalamic lesions that impair their sense of thirst, in infants who may not have ready access to water, or elderly patients with altered mental status.
Correction of hypernatremia can be achieved by administering hypo-osmotic sodium chloride or dextrose solutions. However, it is prudent to correct the hyperna- tremia slowly in patients who have had chronic increases in plasma sodium concentration. The reason for this is that hypernatremia also activates defense mechanisms that protect the cell from changes in volume. These defense mechanisms are opposite to those that occur for hypona- tremia and consist of mechanisms that increase the intra- cellular concentration of sodium and other solutes.
Edema: Excess Fluid in the Tissues
Edema refers to the presence of excess fluid in the body tissues. In most instances, edema occurs mainly in the extracellular fluid compartment, but it can involve intra- cellular fluid as well.
Intracellular Edema
Three conditions are especially prone to cause intracel- lular swelling: (1) hyponatremia, as discussed earlier; (2) depression of the metabolic systems of the tissues; and (3) lack of adequate nutrition to the cells. For example, when blood flow to a tissue is decreased, the delivery of oxygen and nutrients is reduced. If the blood flow becomes too low to maintain normal tissue metabolism, the cell mem- brane ionic pumps become depressed. When this occurs, sodium ions that normally leak into the interior of the cell can no longer be pumped out of the cells and the excess intracellular sodium ions cause osmosis of water into the cells. Sometimes this can increase intracellular volume of a tissue area—even of an entire ischemic leg, for example— to two to three times normal. When this occurs, it is usu- ally a prelude to death of the tissue.
Intracellular edema can also occur in inflamed tissues. Inflammation usually increases cell membrane permeability, allowing sodium and other ions to diffuse into the interior of the cell, with subsequent osmosis of water into the cells.
Normonatremia
Acute hyponatremia
Chronic hyponatremia
Figure 25-7 Brain cell volume regulation during hyponatremia. During acute hyponatremia, caused by loss of Na+ or excess H
2 O,
there is diffusion of H 2 O into the cells (1) and swelling of the brain
tissue. This stimulates transport of Na+, K+, and organic solutes out of the cells (2), which then cause water diffusion out of the cells (3). With chronic hyponatremia, the brain swelling is attenuated by the transport of solutes from the cells.
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Extracellular Edema
Extracellular fluid edema occurs when there is excess fluid accumulation in the extracellular spaces. There are two general causes of extracellular edema: (1) abnormal leakage of fluid from the plasma to the interstitial spaces across the capillaries, and (2) failure of the lymphatics to return fluid from the interstitium back into the blood, often called lymphedema. The most common clinical cause of interstitial fluid accumulation is excessive capil- lary fluid filtration.
Factors That Can Increase Capillary Filtration
To understand the causes of excessive capillary filtration, it is useful to review the determinants of capillary filtra- tion discussed in Chapter 16. Mathematically, capillary filtration rate can be expressed as
Filtration = K f × (P
c − P
if − π
c + π
if ),
where Kf is the capillary filtration coefficient (the product of the permeability and surface area of the capillaries), Pc is the capillary hydrostatic pressure, Pif is the interstitial fluid hydrostatic pressure, πc is the capillary plasma col- loid osmotic pressure, and πif is the interstitial fluid col- loid osmotic pressure. From this equation, one can see that any one of the following changes can increase the capillary filtration rate:
• Increased capillary filtration coefficient. • Increased capillary hydrostatic pressure. • Decreased plasma colloid osmotic pressure.
Lymphedema—Failure of the Lymph Vessels to Return Fluid and Protein to the Blood
When lymph vessel function is greatly impaired, due to blockage or loss of the lymph vessels, edema can become especially severe because plasma proteins that leak into the interstitium have no other way to be removed. The rise in protein concentration raises the colloid osmotic pressure of the interstitial fluid, which draws even more fluid out of the capillaries.
Blockage of lymph flow can be especially severe with infections of the lymph nodes, such as occurs with infec- tion by filaria nematodes (Wuchereria bancrofti), which are microscopic, threadlike worms. The adult worms live in the human lymph system and are spread from person to person by mosquitoes. People with filarial infections can suffer from severe lymphedema and elephantiasis and in men, swelling of the scrotum, called hydrocele. Lymphatic filariasis affects over 120 million people in 80 countries throughout the tropics and subtropics of Asia, Africa, the Western Pacific, and parts of the Caribbean and South America.
Lymphedema can also occur in certain types of can- cer or after surgery in which lymph vessels are removed or obstructed. For example, large numbers of lymph ves- sels are removed during radical mastectomy, impair- ing removal of fluid from the breast and arm areas and
causing edema and swelling of the tissue spaces. A few lymph vessels eventually regrow after this type of surgery, so the interstitial edema is usually temporary.
Summary of Causes of Extracellular Edema
A large number of conditions can cause fluid accumu- lation in the interstitial spaces by abnormal leaking of fluid from the capillaries or by preventing the lymphat- ics from returning fluid from the interstitium back to the circulation. The following is a partial list of conditions that can cause extracellular edema by these two types of abnormalities: I. Increased capillary pressure
A. Excessive kidney retention of salt and water 1. Acute or chronic kidney failure 2. Mineralocorticoid excess
B. High venous pressure and venous constriction 1. Heart failure 2. Venous obstruction 3. Failure of venous pumps
(a) Paralysis of muscles (b) Immobilization of parts of the body (c) Failure of venous valves
C. Decreased arteriolar resistance 1. Excessive body heat 2. Insufficiency of sympathetic nervous system 3. Vasodilator drugs
II. Decreased plasma proteins
A. Loss of proteins in urine (nephrotic syndrome) B. Loss of protein from denuded skin areas
1. Burns 2. Wounds
C. Failure to produce proteins 1. Liver disease (e.g., cirrhosis) 2. Serious protein or caloric malnutrition
III. Increased capillary permeability
A. Immune reactions that cause release of histamine and other immune products
B. Toxins C. Bacterial infections D. Vitamin deficiency, especially vitamin C E. Prolonged ischemia F. Burns
IV. Blockage of lymph return
A. Cancer B. Infections (e.g., filaria nematodes) C. Surgery D. Congenital absence or abnormality of lymphatic
vessels
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Edema Caused by Heart Failure. One of the most serious and most common causes of edema is heart failure. In heart failure, the heart fails to pump blood normally from the veins into the arteries; this raises venous pressure and capillary pressure, causing increased capillary filtration. In addition, the arterial pressure tends to fall, causing decreased excretion of salt and water by the kidneys, which increases blood volume and further raises capillary hydrostatic pressure to cause still more edema. Also, blood flow to the kidneys is reduced in heart failure and this stim ulates secretion of renin, causing increased formation of angiotensin II and increased secretion of aldosterone, both of which cause additional salt and water retention by the kidneys. Thus, in untreated heart failure, all these factors acting together cause serious generalized extracellular edema.
In patients with left-sided heart failure but without sig- nificant failure of the right side of the heart, blood is pumped into the lungs normally by the right side of the heart but can- not escape easily from the pulmonary veins to the left side of the heart because this part of the heart has been greatly weakened. Consequently, all the pulmonary vascular pres- sures, including pulmonary capillary pressure, rise far above normal, causing serious and life-threatening pulmonary edema. When untreated, fluid accumulation in the lungs can rapidly progress, causing death within a few hours.
Edema Caused by Decreased Kidney Excretion of Salt and Water. As discussed earlier, most sodium chloride added to the blood remains in the extracellular compartment, and only small amounts enter the cells. Therefore, in kidney diseases that compromise urinary excretion of salt and water, large amounts of sodium chloride and water are added to the extracellular fluid. Most of this salt and water leaks from the blood into the inter stitial spaces, but some remains in the blood. The main effects of this are to cause (1) widespread increases in interstitial fluid volume (extracellular edema) and (2) hypertension because of the increase in blood volume, as explained in Chapter 19. As an example, children who develop acute glomerulonephritis, in which the renal glomeruli are injured by inflammation and therefore fail to filter adequate amounts of fluid, also develop serious extracellular fluid edema in the entire body; along with the edema, these children usually develop severe hypertension.
Edema Caused by Decreased Plasma Proteins. A reduction in plasma concentration of proteins because of either failure to produce normal amounts of proteins or leakage of proteins from the plasma causes the plasma colloid osmotic pressure to fall. This leads to increased cap illary filtration throughout the body and extracellular edema.
One of the most important causes of decreased plasma protein concentration is loss of proteins in the urine in cer- tain kidney diseases, a condition referred to as nephrotic syndrome. Multiple types of renal diseases can damage the membranes of the renal glomeruli, causing the mem-
branes to become leaky to the plasma proteins and often allowing large quantities of these proteins to pass into the urine. When this loss exceeds the ability of the body to synthesize proteins, a reduction in plasma protein concen- tration occurs. Serious generalized edema occurs when the plasma protein concentration falls below 2.5 g/100 ml.
Cirrhosis of the liver is another condition that causes a reduction in plasma protein concentration. Cirrhosis means development of large amounts of fibrous tissue among the liver parenchymal cells. One result is failure of these cells to produce sufficient plasma proteins, lead- ing to decreased plasma colloid osmotic pressure and the generalized edema that goes with this condition.
Another way liver cirrhosis causes edema is that the liver fibrosis sometimes compresses the abdominal por- tal venous drainage vessels as they pass through the liver before emptying back into the general circulation. Blockage of this portal venous outflow raises capillary hydrostatic pressure throughout the gastrointestinal area and fur- ther increases filtration of fluid out of the plasma into the intra-abdominal areas. When this occurs, the combined effects of decreased plasma protein concentration and high portal capillary pressures cause transudation of large amounts of fluid and protein into the abdominal cavity, a condition referred to as ascites.
Safety Factors That Normally Prevent Edema
Even though many disturbances can cause edema, usu- ally the abnormality must be severe before serious edema develops. The reason for this is that three major safety factors prevent excessive fluid accumulation in the inter- stitial spaces: (1) low compliance of the interstitium when interstitial fluid pressure is in the negative pressure range, (2) the ability of lymph flow to increase 10- to 50-fold, and (3) washdown of interstitial fluid protein concentration, which reduces interstitial fluid colloid osmotic pressure as capillary filtration increases.
Safety Factor Caused by Low Compliance of the Interstitium in the Negative Pressure Range
In Chapter 16, we noted that interstitial fluid hydrostatic pressure in most loose subcutaneous tissues of the body is slightly less than atmospheric pressure, averaging about −3 mm Hg. This slight suction in the tissues helps hold the tissues together. Figure 25-8 shows the approximate rela- tions between different levels of interstitial fluid pressure and interstitial fluid volume, as extrapolated to the human being from animal studies. Note in Figure 25-8 that as long as the interstitial fluid pressure is in the negative range, small changes in interstitial fluid volume are associated with relatively large changes in interstitial fluid hydrostatic pressure. Therefore, in the negative pressure range, the compliance of the tissues, defined as the change in volume per millimeter of mercury pressure change, is low.
How does the low compliance of the tissues in the nega- tive pressure range act as a safety factor against edema? To answer this question, recall the determinants of capillary
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filtration discussed previously. When interstitial fluid hydro- static pressure increases, this increased pressure tends to oppose further capillary filtration. Therefore, as long as the interstitial fluid hydrostatic pressure is in the negative pres- sure range, small increases in interstitial fluid volume cause relatively large increases in interstitial fluid hydrostatic pres- sure, opposing further filtration of fluid into the tissues.
Because the normal interstitial fluid hydrostatic pres- sure is −3 mm Hg, the interstitial fluid hydrostatic pressure must increase by about 3 mm Hg before large amounts of fluid will begin to accumulate in the tissues. Therefore, the safety factor against edema is a change of interstitial fluid pressure of about 3 mm Hg.
Once interstitial fluid pressure rises above 0 mm Hg, the compliance of the tissues increases markedly, allowing large amounts of fluid to accumulate in the tissues with relatively small additional increases in interstitial fluid hydrostatic pressure. Thus, in the positive tissue pressure range, this safety factor against edema is lost because of the large increase in compliance of the tissues.
Importance of Interstitial Gel in Preventing Fluid Accumulation in the Interstitium. Note in Figure 25-8 that in normal tissues with negative interstitial fluid pres- sure, virtually all the fluid in the interstitium is in gel form. That is, the fluid is bound in a proteoglycan meshwork so that there are virtually no “free” fluid spaces larger than a few hundredths of a micrometer in diameter. The impor- tance of the gel is that it prevents fluid from flowing eas-
ily through the tissues because of impediment from the “brush pile” of trillions of proteoglycan filaments. Also, when the interstitial fluid pressure falls to very negative values, the gel does not contract greatly because the mesh- work of proteoglycan filaments offers an elastic resistance to compression. In the negative fluid pressure range, the interstitial fluid volume does not change greatly, regard- less of whether the degree of suction is only a few milli- meters of mercury negative pressure or 10 to 20 mm Hg negative pressure. In other words, the compliance of the tissues is very low in the negative pressure range.
By contrast, when interstitial fluid pressure rises to the positive pressure range, there is a tremendous accumu- lation of free fluid in the tissues. In this pressure range, the tissues are compliant, allowing large amounts of fluid to accumulate with relatively small additional increases in interstitial fluid hydrostatic pressure. Most of the extra fluid that accumulates is “free fluid” because it pushes the brush pile of proteoglycan filaments apart. Therefore, the fluid can flow freely through the tissue spaces because it is not in gel form. When this occurs, the edema is said to be pitting edema because one can press the thumb against the tissue area and push the fluid out of the area. When the thumb is removed, a pit is left in the skin for a few sec- onds until the fluid flows back from the surrounding tis- sues. This type of edema is distinguished from nonpitting edema, which occurs when the tissue cells swell instead of the interstitium or when the fluid in the interstitium becomes clotted with fibrinogen so that it cannot move freely within the tissue spaces.
Importance of the Proteoglycan Filaments as a “Spacer” for the Cells and in Preventing Rapid Flow of Fluid in the Tissues. The proteoglycan filaments, along with much larger collagen fibrils in the interstitial spaces, act as a “spacer” between the cells. Nutrients and ions do not diffuse readily through cell membranes; therefore, without adequate spacing between the cells, these nutri- ents, electrolytes, and cell waste products could not be rapidly exchanged between the blood capillaries and cells located at a distance from one another.
The proteoglycan filaments also prevent fluid from flowing too easily through the tissue spaces. If it were not for the proteoglycan filaments, the simple act of a person standing up would cause large amounts of interstitial fluid to flow from the upper body to the lower body. When too much fluid accumulates in the interstitium, as occurs in edema, this extra fluid creates large channels that allow the fluid to flow readily through the interstitium. Therefore, when severe edema occurs in the legs, the edema fluid often can be decreased by simply elevating the legs.
Even though fluid does not flow easily through the tis- sues in the presence of the compacted proteoglycan filaments, different substances within the fluid can diffuse through the tissues at least 95 percent as easily as they normally diffuse. Therefore, the usual diffusion of nutrients to the cells and the removal of waste products from the cells are not compromised by the proteoglycan filaments of the interstitium.
In te
rs ti
ti al
f lu
id v
o lu
m e
(l it
er s)
−10 −8 −6 −4 −2 0
(Low compliance)
To ta
l i nt
er st
iti al
fl ui
d
(High compliance)
Normal
2 4 6
60
56
52
48
44
40
36
32
28
24
20
16
12
8
4
0
Interstitial free fluid pressure (mm Hg)
Interstitial free fluid pressure (mm Hg)
Free fluid Gel fluid
Figure 25-8 Relation between interstitial fluid hydrostatic pres- sure and interstitial fluid volumes, including total volume, free fluid volume, and gel fluid volume, for loose tissues such as skin. Note that significant amounts of free fluid occur only when the interstitial fluid pressure becomes positive. (Modified from Guyton AC, Granger HJ, Taylor AE: Interstitial fluid pressure. Physiol Rev 51:527, 1971.)
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Increased Lymph Flow as a Safety Factor Against Edema
A major function of the lymphatic system is to return to the circulation the fluid and proteins filtered from the capillaries into the interstitium. Without this continuous return of the filtered proteins and fluid to the blood, the plasma volume would be rapidly depleted, and interstitial edema would occur.
The lymphatics act as a safety factor against edema because lymph flow can increase 10- to 50-fold when fluid begins to accumulate in the tissues. This allows the lym- phatics to carry away large amounts of fluid and proteins in response to increased capillary filtration, preventing the interstitial pressure from rising into the positive pres- sure range. The safety factor caused by increased lymph flow has been calculated to be about 7 mm Hg.
“Washdown” of the Interstitial Fluid Protein as a Safety Factor Against Edema
As increased amounts of fluid are filtered into the inter- stitium, the interstitial fluid pressure increases, causing increased lymph flow. In most tissues the protein concentra- tion of the interstitium decreases as lymph flow is increased, because larger amounts of protein are carried away than can be filtered out of the capillaries; the reason for this is that the capillaries are relatively impermeable to proteins, compared with the lymph vessels. Therefore, the proteins are “washed out” of the interstitial fluid as lymph flow increases.
Because the interstitial fluid colloid osmotic pressure caused by the proteins tends to draw fluid out of the cap- illaries, decreasing the interstitial fluid proteins lowers the net filtration force across the capillaries and tends to pre- vent further accumulation of fluid. The safety factor from this effect has been calculated to be about 7 mm Hg.
Summary of Safety Factors That Prevent Edema
Putting together all the safety factors against edema, we find the following:
1. The safety factor caused by low tissue compliance in the negative pressure range is about 3 mm Hg.
2. The safety factor caused by increased lymph flow is about 7 mm Hg.
3. The safety factor caused by washdown of proteins from the interstitial spaces is about 7 mm Hg. Therefore, the total safety factor against edema is
about 17 mm Hg. This means that the capillary pressure in a peripheral tissue could theoretically rise by 17 mm Hg, or approximately double the normal value, before marked edema would occur.
Fluids in the “Potential Spaces” of the Body
Some examples of “potential spaces” are pleural cavity, pericardial cavity, peritoneal cavity, and synovial cavi- ties, including both the joint cavities and the bursae.
Virtually all these potential spaces have surfaces that almost touch each other, with only a thin layer of fluid in between, and the surfaces slide over each other. To facilitate the sliding, a viscous proteinaceous fluid lubri- cates the surfaces.
Fluid Is Exchanged Between the Capillaries and the Potential Spaces. The surface membrane of a potential space usually does not offer significant resistance to the passage of fluids, electrolytes, or even proteins, which all move back and forth between the space and the interstitial fluid in the surrounding tissue with relative ease. Therefore, each potential space is in reality a large tissue space. Consequently, fluid in the capillaries adjacent to the potential space diffuses not only into the interstitial fluid but also into the potential space.
Lymphatic Vessels Drain Protein from the Potential Spaces. Proteins collect in the potential spaces because of leakage out of the capillaries, similar to the collection of protein in the interstitial spaces throughout the body. The protein must be removed through lymphatics or other channels and returned to the circulation. Each potential space is either directly or indirectly connected with lymph vessels. In some cases, such as the pleural cavity and peritoneal cavity, large lymph vessels arise directly from the cavity itself.
Edema Fluid in the Potential Spaces Is Called “Effusion.” When edema occurs in the subcutaneous tissues adjacent to the potential space, edema fluid usually collects in the potential space as well and this fluid is called effusion. Thus, lymph blockage or any of the multiple abnormalities that can cause excessive capillary filtration can cause effusion in the same way that interstitial edema is caused. The abdominal cavity is especially prone to collect effusion fluid, and in this instance, the effusion is called ascites. In serious cases, 20 liters or more of ascitic fluid can accumulate.
The other potential spaces, such as the pleural cav- ity, pericardial cavity, and joint spaces, can become seri- ously swollen when there is generalized edema. Also, injury or local infection in any one of the cavities often blocks the lymph drainage, causing isolated swelling in the cavity.
The dynamics of fluid exchange in the pleural cavity are discussed in detail in Chapter 38. These dynamics are mainly representative of all the other potential spaces as well. It is especially interesting that the normal fluid pres- sure in most or all of the potential spaces in the nonedem- atous state is negative in the same way that this pressure is negative (subatmospheric) in loose subcutaneous tissue. For instance, the interstitial fluid hydrostatic pressure is normally about −7 to −8 mm Hg in the pleural cavity, −3 to −5 mm Hg in the joint spaces, and −5 to −6 mm Hg in the pericardial cavity.
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chapter 26
Urine Formation by the Kidneys: I. Glomerular Filtration, Renal Blood Flow, and Their Control
Multiple Functions of the Kidneys
Most people are familiar with one important function of the kidneys—to rid the body
of waste materials that are either ingested or produced by metabolism. A second function that is especially criti- cal is to control the volume and composition of the body fluids. For water and virtually all electrolytes in the body, the balance between intake (due to ingestion or metabolic production) and output (due to excretion or metabolic consumption) is maintained largely by the kidneys. This regulatory function of the kidneys maintains the stable internal environment necessary for the cells to perform their various activities.
The kidneys perform their most important functions by filtering the plasma and removing substances from the filtrate at variable rates, depending on the needs of the body. Ultimately, the kidneys “clear” unwanted substances from the filtrate (and therefore from the blood) by excret- ing them in the urine while returning substances that are needed back to the blood.
Although this chapter and the next few chapters focus mainly on the control of renal excretion of water, electro- lytes, and metabolic waste products, the kidneys serve many important homeostatic functions, including the following:
• Excretion of metabolic waste products and foreign chemicals
• Regulation of water and electrolyte balances • Regulation of body fluid osmolality and electrolyte
concentrations • Regulation of arterial pressure • Regulation of acid-base balance • Secretion, metabolism, and excretion of hormones • Gluconeogenesis
Excretion of Metabolic Waste Products, Foreign Chemicals, Drugs, and Hormone Metabolites. The kidneys are the primary means for eliminating waste products of metabolism that are no longer needed by
the body. These products include urea (from the metabo- lism of amino acids), creatinine (from muscle creatine), uric acid (from nucleic acids), end products of hemoglobin breakdown (such as bilirubin), and metabolites of various hormones. These waste products must be eliminated from the body as rapidly as they are produced. The kidneys also eliminate most toxins and other foreign substances that are either produced by the body or ingested, such as pes- ticides, drugs, and food additives.
Regulation of Water and Electrolyte Balances. For maintenance of homeostasis, excretion of water and electrolytes must precisely match intake. If intake exceeds excretion, the amount of that substance in the body will increase. If intake is less than excretion, the amount of that substance in the body will decrease.
Intake of water and many electrolytes is governed mainly by a person’s eating and drinking habits, requir- ing the kidneys to adjust their excretion rates to match the intakes of various substances. Figure 26-1 shows the response of the kidneys to a sudden 10-fold increase in sodium intake from a low level of 30 mEq/day to a high level of 300 mEq/day. Within 2 to 3 days after raising the sodium intake, renal excretion also increases to about 300 mEq/day so that a balance between intake and out- put is re-established. However, during the 2 to 3 days of renal adaptation to the high sodium intake, there is a modest accumulation of sodium that raises extracellular fluid volume slightly and triggers hormonal changes and other compensatory responses that signal the kidneys to increase their sodium excretion.
The capacity of the kidneys to alter sodium excretion in response to changes in sodium intake is enormous. Experimental studies have shown that in many people, sodium intake can be increased to 1500 mEq/day (more than 10 times normal) or decreased to 10 mEq/day (less than one-tenth normal) with relatively small changes in extracellular fluid volume or plasma sodium concentration. This is also true for water and for most other electrolytes, such as chloride, potassium, calcium, hydrogen, magne- sium, and phosphate ions. In the next few chapters, we dis- cuss the specific mechanisms that permit the kidneys to perform these amazing feats of homeostasis.
Unit V The Body Fluids and Kidneys
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Regulation of Arterial Pressure. As discussed in Chapter 19, the kidneys play a dominant role in long- term regulation of arterial pressure by excreting variable amounts of sodium and water. The kidneys also contrib- ute to short-term arterial pressure regulation by secret- ing hormones and vasoactive factors or substances (e.g., renin) that lead to the formation of vasoactive products (e.g., angiotensin II).
Regulation of Acid-Base Balance. The kidneys contribute to acid-base regulation, along with the lungs and body fluid buffers, by excreting acids and by regulat- ing the body fluid buffer stores. The kidneys are the only means of eliminating from the body certain types of acids, such as sulfuric acid and phosphoric acid, generated by the metabolism of proteins.
Regulation of Erythrocyte Production. The kid- neys secrete erythropoietin, which stimulates the produc- tion of red blood cells by hematopoietic stem cells in the bone marrow, as discussed in Chapter 32. One impor- tant stimulus for erythropoietin secretion by the kidneys is hypoxia. The kidneys normally account for almost all the erythropoietin secreted into the circulation. In people with severe kidney disease or who have had their kidneys removed and have been placed on hemodialysis, severe anemia develops as a result of decreased erythropoietin production.
Regulation of 1,25-Dihydroxyvitamin D 3 Pro -
d uction. The kidneys produce the active form of vitamin D, 1,25-dihydroxyvitamin D3 (calcitriol), by hydroxylating this vitamin at the “number 1” position. Calcitriol is essential for normal calcium deposition in bone and calcium reabsorption by the gastrointestinal tract. As
discussed in Chapter 79, calcitriol plays an important role in calcium and phosphate regulation.
Glucose Synthesis. The kidneys synthesize glucose from amino acids and other precursors during prolonged fasting, a process referred to as gluconeogenesis. The kid- neys’ capacity to add glucose to the blood during pro- longed periods of fasting rivals that of the liver.
With chronic kidney disease or acute failure of the kid- neys, these homeostatic functions are disrupted and severe abnormalities of body fluid volumes and composition rap- idly occur. With complete renal failure, enough potassium, acids, fluid, and other substances accumulate in the body to cause death within a few days, unless clinical interven- tions such as hemodialysis are initiated to restore, at least partially, the body fluid and electrolyte balances.
Physiologic Anatomy of the Kidneys
General Organization of the Kidneys and Urinary Tract
The two kidneys lie on the posterior wall of the abdomen, outside the peritoneal cavity (Figure 26-2). Each kidney of the adult human weighs about 150 grams and is about the size of a clenched fist. The medial side of each kidney con- tains an indented region called the hilum through which pass the renal artery and vein, lymphatics, nerve supply, and ureter, which carries the final urine from the kidney to the bladder, where it is stored until emptied. The kid- ney is surrounded by a tough, fibrous capsule that pro- tects its delicate inner structures.
If the kidney is bisected from top to bottom, the two major regions that can be visualized are the outer cortex and the inner medulla regions. The medulla is divided into 8 to 10 cone-shaped masses of tissue called renal pyr- amids. The base of each pyramid originates at the bor- der between the cortex and medulla and terminates in the papilla, which projects into the space of the renal pelvis, a funnel-shaped continuation of the upper end of the ureter. The outer border of the pelvis is divided into open-ended pouches called major calyces that extend downward and divide into minor calyces, which collect urine from the tubules of each papilla. The walls of the calyces, pelvis, and ureter contain contractile elements that propel the urine toward the bladder, where urine is stored until it is emptied by micturition, discussed later in this chapter.
Renal Blood Supply
Blood flow to the two kidneys is normally about 22 per- cent of the cardiac output, or 1100 ml/min. The renal artery enters the kidney through the hilum and then branches progressively to form the interlobar arteries, arcuate arteries, interlobular arteries (also called radial arteries) and afferent arterioles, which lead to the glomer- ular capillaries, where large amounts of fluid and solutes (except the plasma proteins) are filtered to begin urine
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Sodium retention
Intake
Excretion Sodium loss
200
100
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15
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Figure 26-1 Effect of increasing sodium intake 10-fold (from 30 to 300 mEq/day) on urinary sodium excretion and extracellular fluid volume. The shaded areas represent the net sodium retention or the net sodium loss, determined from the difference between sodium intake and sodium excretion.
Chapter 26 Urine Formation by the Kidneys: I. Glomerular Filtration, Renal Blood Flow, and Their Control
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formation (Figure 26-3). The distal ends of the capillaries of each glomerulus coalesce to form the efferent arteriole, which leads to a second capillary network, the peritubular capillaries, that surrounds the renal tubules.
The renal circulation is unique in having two capillary beds, the glomerular and peritubular capillaries, which are arranged in series and separated by the efferent arteri- oles, which help regulate the hydrostatic pressure in both sets of capillaries. High hydrostatic pressure in the glo-
merular capillaries (about 60 mm Hg) causes rapid fluid filtration, whereas a much lower hydrostatic pressure in the peritubular capillaries (about 13 mm Hg) permits rapid fluid reabsorption. By adjusting the resistance of the afferent and efferent arterioles, the kidneys can regulate the hydrostatic pressure in both the glomerular and the peritubular capillaries, thereby changing the rate of glom- erular filtration, tubular reabsorption, or both in response to body homeostatic demands.
The peritubular capillaries empty into the vessels of the venous system, which run parallel to the arteriolar vessels. The blood vessels of the venous system progres- sively form the interlobular vein, arcuate vein, interlobar vein, and renal vein, which leaves the kidney beside the renal artery and ureter.
The Nephron Is the Functional Unit of the Kidney
Each kidney in the human contains about 800,000 to 1,000,000 nephrons, each capable of forming urine. The kidney cannot regenerate new nephrons. Therefore, with renal injury, disease, or normal aging, there is a gradual decrease in nephron number. After age 40, the number of functioning nephrons usually decreases about 10 percent every 10 years; thus, at age 80, many people have 40 per- cent fewer functioning nephrons than they did at age 40. This loss is not life threatening because adaptive changes in the remaining nephrons allow them to excrete the proper amounts of water, electrolytes, and waste prod- ucts, as discussed in Chapter 31.
Each nephron contains (1) a tuft of glomerular capil- laries called the glomerulus, through which large amounts of fluid are filtered from the blood, and (2) a long tubule in which the filtered fluid is converted into urine on its way to the pelvis of the kidney (see Figure 26-3).
The glomerulus contains a network of branching and anastomosing glomerular capillaries that, compared with other capillaries, have high hydrostatic pressure (about
Bladder
Urethra
Ureter
Kidney
Nephron (enlarged)
Minor calyx
Major calyx
Papilla
Renal cortex
Renal medulla
Renal pelvis
Renal pyramid
Capsule of kidney Ureter
Figure 26-2 General organization of the kidneys and the urinary system.
Renal artery
Juxtaglomerular apparatus
Efferent arteriole
Bowman's capsuleGlomerulus
Interlobar arteries
Arcuate arteries
Cortical collecting tubule
Collecting duct
Loop of Henle
Proximal tubule
Interlobular arterioles
Segmental arteries
Afferent arteriole Arcuate
artery
Distal tubule
Arcuate vein
Peritubular capillaries
Renal vein
Figure 26-3 Section of the human kidney showing the major vessels that supply the blood flow to the kidney and schematic of the microcirculation of each nephron.
Unit V The Body Fluids and Kidneys
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60 mm Hg). The glomerular capillaries are covered by epithelial cells, and the total glomerulus is encased in Bowman’s capsule.
Fluid filtered from the glomerular capillaries flows into Bowman’s capsule and then into the proximal tubule, which lies in the cortex of the kidney (Figure 26-4). From the proximal tubule, fluid flows into the loop of Henle, which dips into the renal medulla. Each loop consists of a descending and an ascending limb. The walls of the descending limb and the lower end of the ascending limb
are very thin and therefore are called the thin segment of the loop of Henle. After the ascending limb of the loop returns partway back to the cortex, its wall becomes much thicker, and it is referred to as the thick segment of the ascending limb.
At the end of the thick ascending limb is a short seg- ment that has in its wall a plaque of specialized epithe- lial cells, known as the macula densa. As discussed later, the macula densa plays an important role in controlling nephron function. Beyond the macula densa, fluid enters the distal tubule, which, like the proximal tubule, lies in the renal cortex. This is followed by the connecting tubule and the cortical collecting tubule, which lead to the corti- cal collecting duct. The initial parts of 8 to 10 cortical col- lecting ducts join to form a single larger collecting duct that runs downward into the medulla and becomes the medullary collecting duct. The collecting ducts merge to form progressively larger ducts that eventually empty into the renal pelvis through the tips of the renal papillae. In each kidney, there are about 250 of the very large collect- ing ducts, each of which collects urine from about 4000 nephrons.
Regional Differences in Nephron Structure: Cortical and Juxtamedullary Nephrons. Although each nephron has all the components described earlier, there are some differences, depending on how deep the nephron lies within the kidney mass. Those nephrons that have glomeruli located in the outer cortex are called cor- tical nephrons; they have short loops of Henle that pene- trate only a short distance into the medulla (Figure 26-5).
Cortical collecting tubule
Macula densa
Loop of Henle: Thick segment of ascending limb
Thin segment of ascending limb Descending limb
Distal tubule
Proximal tubule
Collecting duct
Medullary collecting tubule
Medulla
Cortex
Cortical collecting tubule
Bowman's capsule
Connecting tubule
Figure 26-4 Basic tubular segments of the nephron. The relative lengths of the different tubular segments are not drawn to scale.
C or
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Efferent arteriole
Afferent arteriole
Collecting duct
Thick loop of Henle
Juxtamedullary nephron
Cortical nephron
Duct of Bellini
Vasa recta
Thin loop of Henle
Interlobular artery
vein
Arcuate artery
vein
Interlobar artery vein
Figure 26-5 Schematic of relations between blood vessels and tubular structures and differences between cortical and juxtamedullary nephrons.
Chapter 26 Urine Formation by the Kidneys: I. Glomerular Filtration, Renal Blood Flow, and Their Control
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About 20 to 30 percent of the nephrons have glomer- uli that lie deep in the renal cortex near the medulla and are called juxtamedullary nephrons. These nephrons have long loops of Henle that dip deeply into the medulla, in some cases all the way to the tips of the renal papillae.
The vascular structures supplying the juxtamedullary nephrons also differ from those supplying the cortical nephrons. For the cortical nephrons, the entire tubular system is surrounded by an extensive network of peritu- bular capillaries. For the juxtamedullary nephrons, long efferent arterioles extend from the glomeruli down into the outer medulla and then divide into specialized peri- tubular capillaries called vasa recta that extend down- ward into the medulla, lying side by side with the loops of Henle. Like the loops of Henle, the vasa recta return toward the cortex and empty into the cortical veins. This specialized network of capillaries in the medulla plays an essential role in the formation of a concentrated urine and is discussed in Chapter 28.
Micturition
Micturition is the process by which the urinary bladder empties when it becomes filled. This involves two main steps: First, the bladder fills progressively until the tension
in its walls rises above a threshold level; this elicits the second step, which is a nervous reflex called the micturi- tion reflex that empties the bladder or, if this fails, at least causes a conscious desire to urinate. Although the mictu- rition reflex is an autonomic spinal cord reflex, it can also be inhibited or facilitated by centers in the cerebral cortex or brain stem.
Physiologic Anatomy of the Bladder
The urinary bladder, shown in Figure 26-6, is a smooth muscle chamber composed of two main parts: (1) the body, which is the major part of the bladder in which urine collects, and (2) the neck, which is a funnel- shaped extension of the body, passing inferiorly and anteriorly into the urogenital triangle and connecting with the urethra. The lower part of the bladder neck is also called the posterior urethra because of its relation to the urethra.
The smooth muscle of the bladder is called the detru- sor muscle. Its muscle fibers extend in all directions and, when contracted, can increase the pressure in the bladder to 40 to 60 mm Hg. Thus, contraction of the detrusor mus- cle is a major step in emptying the bladder. Smooth muscle cells of the detrusor muscle fuse with one another so that low- resistance electrical pathways exist from one muscle
Ureters Detrusor muscle
Ureteric openings
Female Male
Internal sphincter
Urethra
Prostate gland
Bulbourethral gland
Trigone
External urethral opening
Urogenital diaphragm (including external sphincter)
Figure 26-6 Anatomy of the urinary bladder in males and females.
Unit V The Body Fluids and Kidneys
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cell to the other. Therefore, an action potential can spread throughout the detrusor muscle, from one muscle cell to the next, to cause contraction of the entire bladder at once.
On the posterior wall of the bladder, lying immediately above the bladder neck, is a small triangular area called the trigone. At the lowermost apex of the trigone, the bladder neck opens into the posterior urethra and the two ureters enter the bladder at the uppermost angles of the trigone. The trigone can be identified by the fact that its mucosa, the inner lining of the bladder, is smooth, in con- trast to the remaining bladder mucosa, which is folded to form rugae.
Each ureter, as it enters the bladder, courses obliquely through the detrusor muscle and then passes another 1 to 2 centimeters beneath the bladder mucosa before empty- ing into the bladder.
The bladder neck (posterior urethra) is 2 to 3 centi- meters long, and its wall is composed of detrusor muscle interlaced with a large amount of elastic tissue. The mus- cle in this area is called the internal sphincter. Its natu- ral tone normally keeps the bladder neck and posterior urethra empty of urine and, therefore, prevents emptying of the bladder until the pressure in the main part of the bladder rises above a critical threshold.
Beyond the posterior urethra, the urethra passes through the urogenital diaphragm, which contains a layer of muscle called the external sphincter of the bladder. This muscle is a voluntary skeletal muscle, in contrast to the muscle of the bladder body and bladder neck, which is entirely smooth muscle. The external sphincter muscle is under voluntary control of the nervous system and can be used to consciously prevent urination even when involun- tary controls are attempting to empty the bladder.
Innervation of the Bladder
The principal nerve supply of the bladder is by way of the pelvic nerves, which connect with the spinal cord through
the sacral plexus, mainly connecting with cord seg- ments S2 and S3 (Figure 26-7). Coursing through the pel- vic nerves are both sensory nerve fibers and motor nerve fibers. The sensory fibers detect the degree of stretch in the bladder wall. Stretch signals from the posterior ure- thra are especially strong and are mainly responsible for initiating the reflexes that cause bladder emptying.
The motor nerves transmitted in the pelvic nerves are parasympathetic fibers. These terminate on ganglion cells located in the wall of the bladder. Short postganglionic nerves then innervate the detrusor muscle.
In addition to the pelvic nerves, two other types of innervation are important in bladder function. Most important are the skeletal motor fibers transmitted through the pudendal nerve to the external bladder sphincter. These are somatic nerve fibers that innervate and control the voluntary skeletal muscle of the sphincter. Also, the bladder receives sympathetic innervation from the sym- pathetic chain through the hypogastric nerves, connect- ing mainly with the L2 segment of the spinal cord. These sympathetic fibers stimulate mainly the blood vessels and have little to do with bladder contraction. Some sensory nerve fibers also pass by way of the sympathetic nerves and may be important in the sensation of fullness and, in some instances, pain.
Transport of Urine from the Kidney Through the Ureters and into the Bladder
Urine that is expelled from the bladder has essentially the same composition as fluid flowing out of the collecting ducts; there are no significant changes in the composition of urine as it flows through the renal calyces and ureters to the bladder.
Urine flowing from the collecting ducts into the renal calyces stretches the calyces and increases their inherent
L1 L2 L3 L4 L5 S1 S2 S3 S4
Ureter
Body
Pudendal
Sympathetics
Parasympathetics Trigone
Bladder neck (posterior urethra)
External sphincter
Figure 26-7 Innervation of the urinary bladder.
Chapter 26 Urine Formation by the Kidneys: I. Glomerular Filtration, Renal Blood Flow, and Their Control
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pacemaker activity, which in turn initiates peristaltic con- tractions that spread to the renal pelvis and then downward along the length of the ureter, thereby forcing urine from the renal pelvis toward the bladder. In adults, the ureters are normally 25 to 35 centimeters (10 to 14 inches) long.
The walls of the ureters contain smooth muscle and are innervated by both sympathetic and parasympathetic nerves, as well as by an intramural plexus of neurons and nerve fibers that extends along the entire length of the ureters. As with other visceral smooth muscle, per- istaltic contractions in the ureter are enhanced by para- sympathetic stimulation and inhibited by sympathetic stimulation.
The ureters enter the bladder through the detrusor mus- cle in the trigone region of the bladder, as shown in Figure 26-6. Normally, the ureters course obliquely for several centimeters through the bladder wall. The normal tone of the detrusor muscle in the bladder wall tends to compress the ureter, thereby preventing backflow (reflux) of urine from the bladder when pressure builds up in the bladder during micturition or bladder compression. Each peristal- tic wave along the ureter increases the pressure within the ureter so that the region passing through the bladder wall opens and allows urine to flow into the bladder.
In some people, the distance that the ureter courses through the bladder wall is less than normal, so contrac- tion of the bladder during micturition does not always lead to complete occlusion of the ureter. As a result, some of the urine in the bladder is propelled backward into the ureter, a condition called vesicoureteral reflux. Such reflux can lead to enlargement of the ureters and, if severe, can increase the pressure in the renal calyces and structures of the renal medulla, causing damage to these regions.
Pain Sensation in the Ureters, and the Ureterorenal Reflex. The ureters are well supplied with pain nerve fibers. When a ureter becomes blocked (e.g., by a ureteral stone), intense reflex constriction occurs, associated with severe pain. Also, the pain impulses cause a sympathetic reflex back to the kidney to constrict the renal arterioles, thereby decreasing urine output from the kidney. This effect is called the ureterorenal reflex and is important for preventing excessive flow of fluid into the pelvis of a kidney with a blocked ureter.
Filling of the Bladder and Bladder Wall Tone; the Cystometrogram
Figure 26-8 shows the approximate changes in intravesicu- lar pressure as the bladder fills with urine. When there is no urine in the bladder, the intravesicular pressure is about 0, but by the time 30 to 50 milliliters of urine have collected, the pressure rises to 5 to 10 centimeters of water. Additional urine—200 to 300 milliliters—can collect with only a small additional rise in pressure; this constant level of pressure is caused by intrinsic tone of the bladder wall itself. Beyond 300 to 400 milliliters, collection of more urine in the bladder causes the pressure to rise rapidly.
Superimposed on the tonic pressure changes during fill- ing of the bladder are periodic acute increases in pressure that last from a few seconds to more than a minute. The pressure peaks may rise only a few centimeters of water or may rise to more than 100 centimeters of water. These pres- sure peaks are called micturition waves in the cystometro- gram and are caused by the micturition reflex.
Micturition Reflex
Referring again to Figure 26-8, one can see that as the bladder fills, many superimposed micturition contractions begin to appear, as shown by the dashed spikes. They are the result of a stretch reflex initiated by sensory stretch receptors in the bladder wall, especially by the receptors in the posterior urethra when this area begins to fill with urine at the higher bladder pressures. Sensory signals from the bladder stretch receptors are conducted to the sacral segments of the cord through the pelvic nerves and then reflexively back again to the bladder through the para- sympathetic nerve fibers by way of these same nerves.
When the bladder is only partially filled, these micturi- tion contractions usually relax spontaneously after a frac- tion of a minute, the detrusor muscles stop contracting, and pressure falls back to the baseline. As the bladder continues to fill, the micturition reflexes become more frequent and cause greater contractions of the detrusor muscle.
Once a micturition reflex begins, it is “self- regenerative.” That is, initial contraction of the bladder activates the stretch receptors to cause a greater increase in sensory impulses from the bladder and posterior urethra, which causes a further increase in reflex contraction of the bladder; thus, the cycle is repeated again and again until the bladder has reached a strong degree of contraction. Then, after a few seconds to more than a minute, the self-regenerative reflex begins to fatigue and the regenerative cycle of the micturi- tion reflex ceases, permitting the bladder to relax.
Thus, the micturition reflex is a single complete cycle of (1) progressive and rapid increase of pressure, (2) a
Micturition contractions
Basal cystometrogramIn tr
av es
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p re
ss u
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en ti
m et
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Figure 26-8 Normal cystometrogram, showing also acute pressure waves (dashed spikes) caused by micturition reflexes.
Unit V The Body Fluids and Kidneys
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period of sustained pressure, and (3) return of the pres- sure to the basal tone of the bladder. Once a micturition reflex has occurred but has not succeeded in emptying the bladder, the nervous elements of this reflex usually remain in an inhibited state for a few minutes to 1 hour or more before another micturition reflex occurs. As the bladder becomes more and more filled, micturition reflexes occur more and more often and more and more powerfully.
Once the micturition reflex becomes powerful enough, it causes another reflex, which passes through the pudendal nerves to the external sphincter to inhibit it. If this inhibition is more potent in the brain than the voluntary constrictor signals to the external sphincter, urination will occur. If not, urination will not occur until the bladder fills still further and the micturition reflex becomes more powerful.
Facilitation or Inhibition of Micturition by the Brain
The micturition reflex is an autonomic spinal cord reflex, but it can be inhibited or facilitated by centers in the brain. These centers include (1) strong facilitative and inhibitory centers in the brain stem, located mainly in the pons, and (2) several centers located in the cerebral cortex that are mainly inhibitory but can become excitatory.
The micturition reflex is the basic cause of micturition, but the higher centers normally exert final control of mic- turition as follows:
1. The higher centers keep the micturition reflex partially inhibited, except when micturition is desired.
2. The higher centers can prevent micturition, even if the micturition reflex occurs, by tonic contraction of the external bladder sphincter until a convenient time presents itself.
3. When it is time to urinate, the cortical centers can facilitate the sacral micturition centers to help initiate a micturition reflex and at the same time inhibit the external urinary sphincter so that urination can occur.
Voluntary urination is usually initiated in the following way: First, a person voluntarily contracts his or her abdom- inal muscles, which increases the pressure in the bladder and allows extra urine to enter the bladder neck and pos- terior urethra under pressure, thus stretching their walls. This stimulates the stretch receptors, which excites the micturition reflex and simultaneously inhibits the external urethral sphincter. Ordinarily, all the urine will be emptied, with rarely more than 5 to 10 milliliters left in the bladder.
Abnormalities of Micturition
Atonic Bladder and Incontinence Caused by Destruction of Sensory Nerve Fibers. Micturition reflex contraction cannot occur if the sensory nerve fibers from the bladder to the spinal cord are destroyed, thereby preventing transmission of stretch signals from the bladder. When this happens, a person loses bladder control, despite intact
efferent fibers from the cord to the bladder and despite intact neurogenic connections within the brain. Instead of emptying periodically, the bladder fills to capacity and overflows a few drops at a time through the urethra. This is called overflow incontinence.
A common cause of atonic bladder is crush injury to the sacral region of the spinal cord. Certain diseases can also cause damage to the dorsal root nerve fibers that enter the spinal cord. For example, syphilis can cause constrictive fibrosis around the dorsal root nerve fibers, destroying them. This condition is called tabes dorsalis, and the resulting blad- der condition is called tabetic bladder.
Automatic Bladder Caused by Spinal Cord Damage Above the Sacral Region. If the spinal cord is damaged above the sacral region but the sacral cord segments are still intact, typical micturition reflexes can still occur. However, they are no longer controlled by the brain. During the first few days to several weeks after the damage to the cord has occurred, the micturition reflexes are suppressed because of the state of “spinal shock” caused by the sudden loss of facilitative impulses from the brain stem and cerebrum. However, if the bladder is emptied periodically by catheterization to prevent bladder injury caused by overstretching of the bladder, the excitability of the micturition reflex gradually increases until typical micturition reflexes return; then, periodic (but unannounced) bladder emptying occurs.
Some patients can still control urination in this condition by stimulating the skin (scratching or tickling) in the genital region, which sometimes elicits a micturition reflex.
Uninhibited Neurogenic Bladder Caused by Lack of Inhibitory Signals from the Brain. Another abnormality of micturition is the so-called uninhibited neurogenic bladder, which results in frequent and relatively uncontrolled micturition. This condition derives from partial damage in the spinal cord or the brain stem that interrupts most of the inhibitory signals. Therefore, facilitative impulses passing continually down the cord keep the sacral centers so excitable that even a small quantity of urine elicits an uncontrollable micturition reflex, thereby promoting frequent urination.
Urine Formation Results from Glomerular Filtration, Tubular Reabsorption, and Tubular Secretion
The rates at which different substances are excreted in the urine represent the sum of three renal processes, shown in Figure 26-9: (1) glomerular filtration, (2) reabsorption of substances from the renal tubules into the blood, and (3) secretion of substances from the blood into the renal tubules. Expressed mathematically,
Urinary excretion rate = Filtration rate - Reabsorption rate + Secretion rate
Urine formation begins when a large amount of fluid that is virtually free of protein is filtered from the glomerular capillaries into Bowman’s capsule. Most sub- stances in the plasma, except for proteins, are freely fil- tered, so their concentration in the glomerular filtrate in Bowman’s capsule is almost the same as in the plasma. As filtered fluid leaves Bowman’s capsule and passes through
Chapter 26 Urine Formation by the Kidneys: I. Glomerular Filtration, Renal Blood Flow, and Their Control
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the tubules, it is modified by reabsorption of water and specific solutes back into the blood or by secretion of other substances from the peritubular capillaries into the tubules.
Figure 26-10 shows the renal handling of four hypo- thetical substances. The substance shown in panel A is freely filtered by the glomerular capillaries but is neither reabsorbed nor secreted. Therefore, its excretion rate is equal to the rate at which it was filtered. Certain waste products in the body, such as creatinine, are handled by the kidneys in this manner, allowing excretion of essen- tially all that is filtered.
In panel B, the substance is freely filtered but is also partly reabsorbed from the tubules back into the blood. Therefore, the rate of urinary excretion is less than the rate of filtration at the glomerular capillaries. In this case, the excretion rate is calculated as the filtration rate minus the reabsorption rate. This is typical for many of the elec- trolytes of the body such as sodium and chloride ions.
In panel C, the substance is freely filtered at the glomer- ular capillaries but is not excreted into the urine because all the filtered substance is reabsorbed from the tubules back into the blood. This pattern occurs for some of the nutri- tional substances in the blood, such as amino acids and glu- cose, allowing them to be conserved in the body fluids.
The substance in panel D is freely filtered at the glo- merular capillaries and is not reabsorbed, but additional quantities of this substance are secreted from the peritu- bular capillary blood into the renal tubules. This pattern often occurs for organic acids and bases, permitting them
to be rapidly cleared from the blood and excreted in large amounts in the urine. The excretion rate in this case is calculated as filtration rate plus tubular secretion rate.
For each substance in the plasma, a particular combi- nation of filtration, reabsorption, and secretion occurs. The rate at which the substance is excreted in the urine depends on the relative rates of these three basic renal processes.
Filtration, Reabsorption, and Secretion of Different Substances
In general, tubular reabsorption is quantitatively more important than tubular secretion in the formation of urine, but secretion plays an important role in determin- ing the amounts of potassium and hydrogen ions and a few other substances that are excreted in the urine. Most substances that must be cleared from the blood, especially the end products of metabolism such as urea, creatinine, uric acid, and urates, are poorly reabsorbed and are therefore excreted in large amounts in the urine. Certain foreign substances and drugs are also poorly reabsorbed but, in addition, are secreted from the blood into the tubules, so their excretion rates are high.
1. Filtration 2. Reabsorption 3. Secretion 4. Excretion
Peritubular capillaries
1
2
3
4 Renal vein
Urinary excretion Excretion = Filtration – Reabsorption + Secretion
Glomerular capillaries
Bowman's capsule
Afferent arteriole
Efferent arteriole
Figure 26-9 Basic kidney processes that determine the composi- tion of the urine. Urinary excretion rate of a substance is equal to the rate at which the substance is filtered minus its reabsorption rate plus the rate at which it is secreted from the peritubular capil- lary blood into the tubules.
A
C
B
D
Urine
Filtration only
Substance BSubstance A
Urine
Filtration, partial reabsorption
Substance C
Urine
Filtration, complete reabsorption
Substance D
Urine
Filtration, secretion
Figure 26-10 Renal handling of four hypothetical substances. A, The substance is freely filtered but not reabsorbed. B, The sub- stance is freely filtered, but part of the filtered load is reabsorbed back in the blood. C, The substance is freely filtered but is not excreted in the urine because all the filtered substance is reab- sorbed from the tubules into the blood. D, The substance is freely filtered and is not reabsorbed but is secreted from the peritubular capillary blood into the renal tubules.
Unit V The Body Fluids and Kidneys
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Conversely, electrolytes, such as sodium ions, chloride ions, and bicarbonate ions, are highly reabsorbed, so only small amounts appear in the urine. Certain nutri- tional substances, such as amino acids and glucose, are completely reabsorbed from the tubules and do not appear in the urine even though large amounts are fil- tered by the glomerular capillaries.
Each of the processes—glomerular filtration, tubular reabsorption, and tubular secretion—is regulated accord- ing to the needs of the body. For example, when there is excess sodium in the body, the rate at which sodium is filtered increases and a smaller fraction of the filtered sodium is reabsorbed, causing increased urinary excre- tion of sodium.
For most substances, the rates of filtration and reab- sorption are extremely large relative to the rates of excretion. Therefore, subtle adjustments of filtration or reabsorption can lead to relatively large changes in renal excretion. For example, an increase in glomerular filtra- tion rate (GFR) of only 10 percent (from 180 to 198 L/ day) would raise urine volume 13-fold (from 1.5 to 19.5 L/ day) if tubular reabsorption remained constant. In reality, changes in glomerular filtration and tubular reabsorption usually act in a coordinated manner to produce the neces- sary changes in renal excretion.
Why Are Large Amounts of Solutes Filtered and Then Reabsorbed by the Kidneys? One might question the wisdom of filtering such large amounts of water and solutes and then reabsorbing most of these substances. One advantage of a high GFR is that it allows the kid- neys to rapidly remove waste products from the body that depend mainly on glomerular filtration for their excre- tion. Most waste products are poorly reabsorbed by the tubules and, therefore, depend on a high GFR for effective removal from the body.
A second advantage of a high GFR is that it allows all the body fluids to be filtered and processed by the kidneys many times each day. Because the entire plasma volume is only about 3 liters, whereas the GFR is about 180 L/day, the entire plasma can be filtered and processed about 60 times each day. This high GFR allows the kidneys to pre- cisely and rapidly control the volume and composition of the body fluids.
Glomerular Filtration—the First Step in Urine Formation
Composition of the Glomerular Filtrate
Urine formation begins with filtration of large amounts of fluid through the glomerular capillaries into Bowman’s capsule. Like most capillaries, the glomerular capillaries are relatively impermeable to proteins, so the filtered fluid (called the glomerular filtrate) is essentially protein free and devoid of cellular elements, including red blood cells.
The concentrations of other constituents of the glo- merular filtrate, including most salts and organic
molecules, are similar to the concentrations in the plasma. Exceptions to this generalization include a few low- molecular-weight substances, such as calcium and fatty acids, that are not freely filtered because they are partially bound to the plasma proteins. For example, almost one half of the plasma calcium and most of the plasma fatty acids are bound to proteins and these bound portions are not filtered through the glomerular capillaries.
GFR Is About 20 Percent of the Renal Plasma Flow
As in other capillaries, the GFR is determined by (1) the balance of hydrostatic and colloid osmotic forces acting across the capillary membrane and (2) the capillary filtra- tion coefficient (Kf), the product of the permeability and filtering surface area of the capillaries. The glomerular capillaries have a much higher rate of filtration than most other capillaries because of a high glomerular hydrostatic pressure and a large Kf. In the average adult human, the GFR is about 125 ml/min, or 180 L/day. The fraction of the renal plasma flow that is filtered (the filtration frac- tion) averages about 0.2; this means that about 20 per- cent of the plasma flowing through the kidney is filtered through the glomerular capillaries. The filtration fraction is calculated as follows:
Filtration fraction = GFR/Renal plasma flow
Glomerular Capillary Membrane
The glomerular capillary membrane is similar to that of other capillaries, except that it has three (instead of the usual two) major layers: (1) the endothelium of the capil- lary, (2) a basement membrane, and (3) a layer of epithelial cells (podocytes) surrounding the outer surface of the cap- illary basement membrane (Figure 26-11). Together, these layers make up the filtration barrier, which, despite the three layers, filters several hundred times as much water and solutes as the usual capillary membrane. Even with this high rate of filtration, the glomerular capillary mem- brane normally prevents filtration of plasma proteins.
The high filtration rate across the glomerular capillary membrane is due partly to its special characteristics. The capillary endothelium is perforated by thousands of small holes called fenestrae, similar to the fenestrated capillaries found in the liver. Although the fenestrations are relatively large, endothelial cells are richly endowed with fixed neg- ative charges that hinder the passage of plasma proteins.
Surrounding the endothelium is the basement membrane, which consists of a meshwork of collagen and proteoglycan fibrillae that have large spaces through which large amounts of water and small solutes can filter. The basement membrane effectively prevents filtration of plasma proteins, in part because of strong negative electrical charges associated with the proteoglycans.
The final part of the glomerular membrane is a layer of epithelial cells that line the outer surface of the glomerulus. These cells are not continuous but have long footlike processes (podocytes) that encircle the outer surface
Chapter 26 Urine Formation by the Kidneys: I. Glomerular Filtration, Renal Blood Flow, and Their Control
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U n
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of the capillaries (see Figure 26-11). The foot processes are separated by gaps called slit pores through which the glomerular filtrate moves. The epithelial cells, which also have negative charges, provide additional restriction to fil- tration of plasma proteins. Thus, all layers of the glomeru- lar capillary wall provide a barrier to filtration of plasma proteins.
Filterability of Solutes Is Inversely Related to Their Size. The glomerular capillary membrane is thicker than most other capillaries, but it is also much more porous and therefore filters fluid at a high rate. Despite the high filtration rate, the glomerular filtration barrier is selective in determining which molecules will filter, based on their size and electrical charge.
Table 26-1 lists the effect of molecular size on filter- ability of different molecules. A filterability of 1.0 means that the substance is filtered as freely as water; a filter- ability of 0.75 means that the substance is filtered only 75 percent as rapidly as water. Note that electrolytes such as sodium and small organic compounds such as glucose are freely filtered. As the molecular weight of the mol- ecule approaches that of albumin, the filterability rapidly decreases, approaching zero.
Negatively Charged Large Molecules Are Filtered Less Easily Than Positively Charged Molecules of Equal Molecular Size. The molecular diameter of the plasma protein albumin is only about 6 nanometers, whereas the pores of the glomerular membrane are thought to be about
8 nanometers (80 angstroms). Albumin is restricted from filtration, however, because of its negative charge and the electrostatic repulsion exerted by negative charges of the glomerular capillary wall proteoglycans.
Figure 26-12 shows how electrical charge affects the filtration of different molecular weight dextrans by the glomerulus. Dextrans are polysaccharides that can be manufactured as neutral molecules or with negative or positive charges. Note that for any given molecular radius, positively charged molecules are filtered much more readily than negatively charged molecules. Neutral dex- trans are also filtered more readily than negatively charged dextrans of equal molecular weight. The reason for these differences in filterability is that the negative charges of the basement membrane and the podocytes provide an important means for restricting large negatively charged molecules, including the plasma proteins.
In certain kidney diseases, the negative charges on the basement membrane are lost even before there are notice- able changes in kidney histology, a condition referred to as minimal change nephropathy. As a result of this loss of negative charges on the basement membranes, some
Efferent arterioleBowman's capsule
Bowman's space
Capillary loops
Afferent arteriole
Slit pores
Epithelium
Basement membrane
Endothelium
Fenestrations
Proximal tubule
Podocytes
A
B
Figure 26-11 A, Basic ultrastructure of the glomerular capillaries. B, Cross section of the glomerular capillary membrane and its major components: capillary endothelium, basement membrane, and epithelium (podocytes).
Substance Molecular Weight Filterability
Water 18 1.0
Sodium 23 1.0
Glucose 180 1.0
Inulin 5,500 1.0
Myoglobin 17,000 0.75
Albumin 69,000 0.005
Table 26-1 Filterability of Substances by Glomerular Capillaries Based on Molecular Weight
Effective molecular radius (Å)Effective molecular radius (Å)
R el
at iv
e fi
lt er
ab ili
ty
1.0
0.8
0.6
0.4
0.2
0 18 22 26 30
Polycationic dextran
Polyanionic dextran
Neutral dextran
34 38 42
Figure 26-12 Effect of molecular radius and electrical charge of dextran on its filterability by the glomerular capillaries. A value of 1.0 indicates that the substance is filtered as freely as water, whereas a value of 0 indicates that it is not filtered. Dextrans are polysaccha- rides that can be manufactured as neutral molecules or with nega- tive or positive charges and with varying molecular weights.
Unit V The Body Fluids and Kidneys
314
of the lower-molecular-weight proteins, especially albu- min, are filtered and appear in the urine, a condition known as proteinuria or albuminuria.
Determinants of the GFR
The GFR is determined by (1) the sum of the hydrostatic and colloid osmotic forces across the glomerular mem- brane, which gives the net filtration pressure, and (2) the glomerular capillary filtration coefficient, Kf. Expressed mathematically, the GFR equals the product of Kf and the net filtration pressure:
GFR = K f ¥ Net filtration pressure
The net filtration pressure represents the sum of the hydrostatic and colloid osmotic forces that either favor or oppose filtration across the glomerular capillaries (Figure 26-13). These forces include (1) hydrostatic pres- sure inside the glomerular capillaries (glomerular hydro- static pressure, PG), which promotes filtration; (2) the hydrostatic pressure in Bowman’s capsule (PB) outside the capillaries, which opposes filtration; (3) the colloid osmotic pressure of the glomerular capillary plasma pro- teins (πG), which opposes filtration; and (4) the colloid osmotic pressure of the proteins in Bowman’s capsule (πB), which promotes filtration. (Under normal condi- tions, the concentration of protein in the glomerular fil- trate is so low that the colloid osmotic pressure of the Bowman’s capsule fluid is considered to be zero.)
The GFR can therefore be expressed as
GFR = K f ¥ (P
G - P
B - π
G + π
B )
Although the normal values for the determinants of GFR have not been measured directly in humans, they have been estimated in animals such as dogs and rats. Based on the results in animals, the approximate normal
forces favoring and opposing glomerular filtration in humans are believed to be as follows (see Figure 26-13):
Forces Favoring Filtration (mm Hg)
Glomerular hydrostatic pressure 60 Bowman’s capsule colloid osmotic pressure 0
Forces Opposing Filtration (mm Hg)
Bowman’s capsule hydrostatic pressure 18 Glomerular capillary colloid osmotic pressure 32
Net filtration pressure = 60 - 18 - 32 = +10 mm Hg
Some of these values can change markedly under dif- ferent physiologic conditions, whereas others are altered mainly in disease states, as discussed later.
Increased Glomerular Capillary Filtration Coefficient Increases GFR
The Kf is a measure of the product of the hydraulic con- ductivity and surface area of the glomerular capillaries. The Kf cannot be measured directly, but it is estimated experimentally by dividing the rate of glomerular filtra- tion by net filtration pressure:
K f = GFR/Net filtration pressure
Because total GFR for both kidneys is about 125 ml/ min and the net filtration pressure is 10 mm Hg, the normal Kf is calculated to be about 12.5 ml/min/mm Hg of filtration pressure. When Kf is expressed per 100 grams of kidney weight, it averages about 4.2 ml/min/mm Hg, a value about 400 times as high as the Kf of most other cap- illary systems of the body; the average Kf of many other tissues in the body is only about 0.01 ml/min/mm Hg per 100 grams. This high Kf for the glomerular capillar- ies contributes tremendously to their rapid rate of fluid filtration.
Although increased Kf raises GFR and decreased Kf reduces GFR, changes in Kf probably do not provide a pri- mary mechanism for the normal day-to-day regulation of GFR. Some diseases, however, lower Kf by reducing the number of functional glomerular capillaries (thereby reducing the surface area for filtration) or by increasing the thickness of the glomerular capillary membrane and reducing its hydraulic conductivity. For example, chronic, uncontrolled hypertension and diabetes mellitus grad- ually reduce Kf by increasing the thickness of the glo- merular capillary basement membrane and, eventually, by damaging the capillaries so severely that there is loss of capillary function.
Increased Bowman’s Capsule Hydrostatic Pressure Decreases GFR
Direct measurements, using micropipettes, of hydrostatic pressure in Bowman’s capsule and at different points in the proximal tubule in experimental animals suggest that a reasonable estimate for Bowman’s capsule pressure in humans is about 18 mm Hg under normal conditions. Increasing the hydrostatic pressure in Bowman’s capsule
Glomerular hydrostatic pressure (60 mm Hg)
Net filtration pressure (10 mm Hg)
= − − Glomerular oncotic pressure (32 mm Hg)
Bowman's capsule pressure (18 mm Hg)
Afferent arteriole
Efferent arteriole
Bowman's capsule pressure
(18 mm Hg)
Bowman's capsule pressure
(18 mm Hg)
Glomerular hydrostatic
pressure (60 mm Hg)
Glomerular hydrostatic
pressure (60 mm Hg)
Glomerular colloid osmotic
pressure (32 mm Hg)
Glomerular colloid osmotic
pressure (32 mm Hg)
Figure 26-13 Summary of forces causing filtration by the glo- merular capillaries. The values shown are estimates for healthy humans.
Chapter 26 Urine Formation by the Kidneys: I. Glomerular Filtration, Renal Blood Flow, and Their Control
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reduces GFR, whereas decreasing this pressure raises GFR. However, changes in Bowman’s capsule pressure normally do not serve as a primary means for regulating GFR.
In certain pathological states associated with obstruc- tion of the urinary tract, Bowman’s capsule pressure can increase markedly, causing serious reduction of GFR. For example, precipitation of calcium or of uric acid may lead to “stones” that lodge in the urinary tract, often in the ureter, thereby obstructing outflow of the urinary tract and raising Bowman’s capsule pressure. This reduces GFR and eventually can cause hydronephrosis (distention and dilation of the renal pelvis and calyces) and can dam- age or even destroy the kidney unless the obstruction is relieved.
Increased Glomerular Capillary Colloid Osmotic Pressure Decreases GFR
As blood passes from the afferent arteriole through the glomerular capillaries to the efferent arterioles, the plasma protein concentration increases about 20 percent (Figure 26-14). The reason for this is that about one fifth of the fluid in the capillaries filters into Bowman’s capsule, thereby concentrating the glomerular plasma proteins that are not filtered. Assuming that the normal colloid osmotic pressure of plasma entering the glomerular capillaries is 28 mm Hg, this value usually rises to about 36 mm Hg by the time the blood reaches the efferent end of the capillar- ies. Therefore, the average colloid osmotic pressure of the glomerular capillary plasma proteins is midway between 28 and 36 mm Hg, or about 32 mm Hg.
Thus, two factors that influence the glomerular capil- lary colloid osmotic pressure are (1) the arterial plasma col- loid osmotic pressure and (2) the fraction of plasma filtered by the glomerular capillaries (filtration fraction). Increasing the arterial plasma colloid osmotic pressure raises the glomerular capillary colloid osmotic pressure, which in turn decreases GFR.
Increasing the filtration fraction also concentrates the plasma proteins and raises the glomerular colloid osmotic pressure (see Figure 26-14). Because the filtration fraction is defined as GFR/renal plasma flow, the filtration frac- tion can be increased either by raising GFR or by reduc- ing renal plasma flow. For example, a reduction in renal plasma flow with no initial change in GFR would tend to increase the filtration fraction, which would raise the glomerular capillary colloid osmotic pressure and tend to reduce GFR. For this reason, changes in renal blood flow can influence GFR independently of changes in glomeru- lar hydrostatic pressure.
With increasing renal blood flow, a lower fraction of the plasma is initially filtered out of the glomerular capil- laries, causing a slower rise in the glomerular capillary col- loid osmotic pressure and less inhibitory effect on GFR. Consequently, even with a constant glomerular hydrostatic pressure, a greater rate of blood flow into the glomerulus tends to increase GFR and a lower rate of blood flow into the glomerulus tends to decrease GFR.
Increased Glomerular Capillary Hydrostatic Pressure Increases GFR
The glomerular capillary hydrostatic pressure has been estimated to be about 60 mm Hg under normal conditions. Changes in glomerular hydrostatic pressure serve as the primary means for physiologic regulation of GFR. Increases in glomerular hydrostatic pressure raise GFR, whereas decreases in glomerular hydrostatic pressure reduce GFR.
Glomerular hydrostatic pressure is determined by three variables, each of which is under physiologic con- trol: (1) arterial pressure, (2) afferent arteriolar resistance, and (3) efferent arteriolar resistance.
Increased arterial pressure tends to raise glomeru- lar hydrostatic pressure and, therefore, to increase GFR. (However, as discussed later, this effect is buffered by auto- regulatory mechanisms that maintain a relatively constant glomerular pressure as blood pressure fluctuates.)
Increased resistance of afferent arterioles reduces glo- merular hydrostatic pressure and decreases GFR. Conversely, dilation of the afferent arterioles increases both glomeru- lar hydrostatic pressure and GFR (Figure 26-15).
Constriction of the efferent arterioles increases the resistance to outflow from the glomerular capillaries. This raises the glomerular hydrostatic pressure, and as long as the increase in efferent resistance does not reduce renal blood flow too much, GFR increases slightly (see Figure 26-15). However, because efferent arteriolar constriction also reduces renal blood flow, the filtration fraction and glomerular colloid osmotic pres- sure increase as efferent arteriolar resistance increases. Therefore, if the constriction of efferent arterioles is severe (more than about a threefold increase in efferent arteriolar resistance), the rise in colloid osmotic pres- sure exceeds the increase in glomerular capillary hydro- static pressure caused by efferent arteriolar constriction. When this occurs, the net force for filtration actually decreases, causing a reduction in GFR.
Distance along glomerular capillary
Distance along glomerular capillary
G lo
m er
u la
r co
llo id
o sm
o ti
c p
re ss
u re
(m m
H g
)
40
38
36
34
32
30
28
Afferent end
Efferent end
Filtration fraction
Filtration fraction
Normal
Figure 26-14 Increase in colloid osmotic pressure in plasma flow- ing through the glomerular capillary. Normally, about one fifth of the fluid in the glomerular capillaries filters into Bowman’s capsule, thereby concentrating the plasma proteins that are not filtered. Increases in the filtration fraction (glomerular filtration rate/ renal plasma flow) increase the rate at which the plasma colloid osmotic pressure rises along the glomerular capillary; decreases in the filtration fraction have the opposite effect.
Unit V The Body Fluids and Kidneys
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Thus, efferent arteriolar constriction has a biphasic effect on GFR. At moderate levels of constriction, there is a slight increase in GFR, but with severe constriction, there is a decrease in GFR. The primary cause of the eventual decrease in GFR is as follows: As efferent con- striction becomes severe and as plasma protein concen- tration increases, there is a rapid, nonlinear increase in colloid osmotic pressure caused by the Donnan effect; the higher the protein concentration, the more rapidly the colloid osmotic pressure rises because of the interaction of ions bound to the plasma proteins, which also exert an osmotic effect, as discussed in Chapter 16.
To summarize, constriction of afferent arterioles reduces GFR. However, the effect of efferent arteriolar constriction depends on the severity of the constriction; modest efferent constriction raises GFR, but severe effer- ent constriction (more than a threefold increase in resis- tance) tends to reduce GFR.
Table 26-2 summarizes the factors that can decrease GFR.
Renal Blood Flow
In an average 70-kilogram man, the combined blood flow through both kidneys is about 1100 ml/min, or about 22 percent of the cardiac output. Considering that the two kidneys constitute only about 0.4 percent of the total body weight, one can readily see that they receive an extremely high blood flow compared with other organs.
As with other tissues, blood flow supplies the kidneys with nutrients and removes waste products. However, the high flow to the kidneys greatly exceeds this need. The purpose of this additional flow is to supply enough plasma for the high rates of glomerular filtration that are nec- essary for precise regulation of body fluid volumes and solute concentrations. As might be expected, the mech- anisms that regulate renal blood flow are closely linked to the control of GFR and the excretory functions of the kidneys.
Renal Blood Flow and Oxygen Consumption
On a per-gram-weight basis, the kidneys normally con- sume oxygen at twice the rate of the brain but have almost seven times the blood flow of the brain. Thus, the oxygen delivered to the kidneys far exceeds their metabolic needs, and the arterial-venous extraction of oxygen is relatively low compared with that of most other tissues.
A large fraction of the oxygen consumed by the kidneys is related to the high rate of active sodium reabsorption by the renal tubules. If renal blood flow and GFR are reduced and less sodium is filtered, less sodium is reabsorbed and less oxygen is consumed. Therefore, renal oxygen con- sumption varies in proportion to renal tubular sodium reabsorption, which in turn is closely related to GFR and the rate of sodium filtered (Figure 26-16). If glomerular filtration completely ceases, renal sodium reabsorption also ceases and oxygen consumption decreases to about one-fourth normal. This residual oxygen consumption reflects the basic metabolic needs of the renal cells.
Efferent arteriolar resistance (X normal)
Efferent arteriolar resistance (X normal)
G lo
m er
u la
r fi
lt ra
ti o
n ra
te (
m l/m
in )
150
100
50
0
R en
al b
lo o
d f
lo w
(m l/m
in )
2000
1400
800
200 0 1
Normal
Renal blood flow
Glomerular filtration
rate
2 3 4
Afferent arteriolar resistance (X normal)
Afferent arteriolar resistance (X normal)
G lo
m er
u la
r fi
lt ra
ti o
n ra
te (
m l/m
in )
250
100
150
100
50
0
R en
al b
lo o
d f
lo w
(m l/m
in )
2000
1400
800
200 0 1
Normal
Renal blood flowGlomerular
filtration rate
2 3 4
Figure 26-15 Effect of change in afferent arteriolar resistance or efferent arteriolar resistance on glomerular filtration rate and renal blood flow.
Physical Determinants*
Physiologic/Pathophysiologic Causes
↓K f → ↓ GFR Renal disease, diabetes mellitus,
hypertension
↑ P B → ↓ GFR Urinary tract obstruction (e.g., kidney
stones)
↑ π G → ↓ GFR ↓ Renal blood flow, increased plasma
proteins
↓ P G → ↓ GFR
↓ A P → ↓ P
G ↓ Arterial pressure (has only small
effect due to autoregulation)
↓ R E → ↓ P
G ↓ Angiotensin II (drugs that block
angiotensin II formation)
↑ R A → ↓ P
G ↑ Sympathetic activity, vasoconstrictor hormones (e.g., norepinephrine, endothelin)
Table 26-2 Factors That Can Decrease the Glomerular Filtration Rate (GFR)
*Opposite changes in the determinants usually increase GFR. K
f , glomerular filtration coefficient; P
B , Bowman’s capsule hydrostatic
pressure; π G , glomerular capillary colloid osmotic pressure; P
G , glomeru-
lar capillary hydrostatic pressure; A P , systemic arterial pressure; R
E , efferent
arteriolar resistance; R A , afferent arteriolar resistance.
Chapter 26 Urine Formation by the Kidneys: I. Glomerular Filtration, Renal Blood Flow, and Their Control
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Determinants of Renal Blood Flow
Renal blood flow is determined by the pressure gradient across the renal vasculature (the difference between renal artery and renal vein hydrostatic pressures), divided by the total renal vascular resistance:
Total renal vascular resistance
(Renal artery pressure−Renal vein pressure)
Renal artery pressure is about equal to systemic arte- rial pressure, and renal vein pressure averages about 3 to 4 mm Hg under most conditions. As in other vascular beds, the total vascular resistance through the kidneys is determined by the sum of the resistances in the individual vasculature segments, including the arteries, arterioles, capillaries, and veins (Table 26-3).
Most of the renal vascular resistance resides in three major segments: interlobular arteries, afferent arterioles, and efferent arterioles. Resistance of these vessels is con- trolled by the sympathetic nervous system, various hor- mones, and local internal renal control mechanisms, as discussed later. An increase in the resistance of any of the vascular segments of the kidneys tends to reduce the renal blood flow, whereas a decrease in vascular resistance increases renal blood flow if renal artery and renal vein pressures remain constant.
Although changes in arterial pressure have some influ- ence on renal blood flow, the kidneys have effective mecha- nisms for maintaining renal blood flow and GFR relatively constant over an arterial pressure range between 80 and 170 mm Hg, a process called autoregulation. This capacity for autoregulation occurs through mechanisms that are completely intrinsic to the kidneys, as discussed later in this chapter.
Blood Flow in the Vasa Recta of the Renal Medulla Is Very Low Compared with Flow in the Renal Cortex
The outer part of the kidney, the renal cortex, receives most of the kidney’s blood flow. Blood flow in the renal medulla accounts for only 1 to 2 percent of the total renal blood flow. Flow to the renal medulla is supplied by a spe- cialized portion of the peritubular capillary system called the vasa recta. These vessels descend into the medulla in parallel with the loops of Henle and then loop back along with the loops of Henle and return to the cortex before emptying into the venous system. As discussed in Chapter 28, the vasa recta play an important role in allowing the kidneys to form concentrated urine.
Physiologic Control of Glomerular Filtration and Renal Blood Flow
The determinants of GFR that are most variable and subject to physiologic control include the glomerular hydrostatic pressure and the glomerular capillary colloid osmotic pressure. These variables, in turn, are influenced by the sympathetic nervous system, hormones and auta- coids (vasoactive substances that are released in the kid- neys and act locally), and other feedback controls that are intrinsic to the kidneys.
Sympathetic Nervous System Activation Decreases GFR
Essentially all the blood vessels of the kidneys, including the afferent and the efferent arterioles, are richly innervated
O xy
g en
c o
n su
m p
ti o
n (m
l/m in
/1 00
g m
k id
n ey
w ei
g h
t)
3.0
2.5
2.0
1.5
0.5
1.0
0
Sodium reabsorption (mEq/min per 100 g kidney weight)
Sodium reabsorption (mEq/min per 100 g kidney weight)
0 5 10
Basal oxygen consumption
15 20
Figure 26-16 Relationship between oxygen consumption and sodium reabsorption in dog kidneys. (Kramer K, Deetjen P: Relation of renal oxygen consumption to blood supply and glomerular filtration during variations of blood pressure. Pflugers Arch Physiol 271:782, 1960.)
Pressure in Vessel (mm Hg)
Vessel Beginning End Percent of Total Renal Vascular
Resistance
Renal artery 100 100 ≈0
Interlobar, arcuate, and interlobular arteries
≈100
85
≈16
Afferent arteriole 85 60 ≈26
Glomerular capillaries
60 59 ≈1
Efferent arteriole 59 18 ≈43
Peritubular capillaries
18 8 ≈10
Interlobar, interlobular, and arcuate veins
8
4
≈4
Renal vein 4 ≈4 ≈0
Table 26-3 Approximate Pressures and Vascular Resistances in the Circulation of a Normal Kidney
Unit V The Body Fluids and Kidneys
318
by sympathetic nerve fibers. Strong activation of the renal sympathetic nerves can constrict the renal arterioles and decrease renal blood flow and GFR. Moderate or mild sympathetic stimulation has little influence on renal blood flow and GFR. For example, reflex activation of the sympa- thetic nervous system resulting from moderate decreases in pressure at the carotid sinus baroreceptors or cardiopul- monary receptors has little influence on renal blood flow or GFR.
The renal sympathetic nerves seem to be most impor- tant in reducing GFR during severe, acute disturbances lasting for a few minutes to a few hours, such as those elicited by the defense reaction, brain ischemia, or severe hemorrhage. In the healthy resting person, sympathetic tone appears to have little influence on renal blood flow.
Hormonal and Autacoid Control of Renal Circulation
Several hormones and autacoids can influence GFR and renal blood flow, as summarized in Table 26-4.
Norepinephrine, Epinephrine, and Endothelin Constrict Renal Blood Vessels and Decrease GFR. Hormones that constrict afferent and efferent arterioles, causing reductions in GFR and renal blood flow, include norepinephrine and epinephrine released from the adrenal medulla. In general, blood levels of these hormones par- allel the activity of the sympathetic nervous system; thus, norepinephrine and epinephrine have little influence on renal hemodynamics except under extreme conditions, such as severe hemorrhage.
Another vasoconstrictor, endothelin, is a peptide that can be released by damaged vascular endothelial cells of the kidneys, as well as by other tissues. The physio- logic role of this autacoid is not completely understood. However, endothelin may contribute to hemostasis (mini- mizing blood loss) when a blood vessel is severed, which damages the endothelium and releases this powerful vaso- constrictor. Plasma endothelin levels are also increased in certain disease states associated with vascular injury, such as toxemia of pregnancy, acute renal failure, and chronic uremia, and may contribute to renal vasoconstriction and decreased GFR in some of these pathophysiologic conditions.
Angiotensin II Preferentially Constricts Efferent Arterioles in Most Physiologic Conditions. A power- ful renal vasoconstrictor, angiotensin II, can be considered a circulating hormone, as well as a locally produced auta- coid because it is formed in the kidneys and in the sys- temic circulation. Receptors for angiotensin II are present in virtually all blood vessels of the kidneys. However, the preglomerular blood vessels, especially the afferent arte- rioles, appear to be relatively protected from angiotensin II–mediated constriction in most physiologic conditions associated with activation of the renin-angiotensin sys- tem such as during a low-sodium diet or reduced renal perfusion pressure due to renal artery stenosis. This pro- tection is due to release of vasodilators, especially nitric oxide and prostaglandins, which counteract the vasocon- strictor effects of angiotensin II in these blood vessels.
The efferent arterioles, however, are highly sensitive to angiotensin II. Because angiotensin II preferentially con- stricts efferent arterioles in most physiologic conditions, increased angiotensin II levels raise glomerular hydrostatic pressure while reducing renal blood flow. It should be kept in mind that increased angiotensin II formation usually occurs in circumstances associated with decreased arte- rial pressure or volume depletion, which tend to decrease GFR. In these circumstances, the increased level of angio- tensin II, by constricting efferent arterioles, helps prevent decreases in glomerular hydrostatic pressure and GFR; at the same time, though, the reduction in renal blood flow caused by efferent arteriolar constriction contributes to decreased flow through the peritubular capillaries, which in turn increases reabsorption of sodium and water, as discussed in Chapter 27.
Thus, increased angiotensin II levels that occur with a low-sodium diet or volume depletion help maintain GFR and normal excretion of metabolic waste products such as urea and creatinine that depend on glomerular filtra- tion for their excretion; at the same time, the angiotensin II-induced constriction of efferent arterioles increases tubular reabsorption of sodium and water, which helps restore blood volume and blood pressure. This effect of angiotensin II in helping to “autoregulate” GFR is dis- cussed in more detail later in this chapter.
Endothelial-Derived Nitric Oxide Decreases Renal Vascular Resistance and Increases GFR. An autacoid that decreases renal vascular resistance and is released by the vascular endothelium throughout the body is endothe- lial-derived nitric oxide. A basal level of nitric oxide produc- tion appears to be important for maintaining vasodilation of the kidneys. This allows the kidneys to excrete normal amounts of sodium and water. Therefore, administration of drugs that inhibit formation of nitric oxide increases renal vascular resistance and decreases GFR and urinary sodium excretion, eventually causing high blood pressure. In some hypertensive patients or in patients with athero- sclerosis, damage of the vascular endothelium and impaired nitric oxide production may contribute to increased renal vasoconstriction and elevated blood pressure.
Hormone or Autacoid Effect on GFR
Norepinephrine ↓
Epinephrine ↓
Endothelin ↓
Angiotensin II ↔ (prevents ↓)
Endothelial-derived nitric oxide ↑
Prostaglandins ↑
Table 26-4 Hormones and Autacoids That Influence Glomerular Filtration Rate (GFR)
Chapter 26 Urine Formation by the Kidneys: I. Glomerular Filtration, Renal Blood Flow, and Their Control
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Prostaglandins and Bradykinin Tend to Increase GFR. Hormones and autacoids that cause vasodilation and increased renal blood flow and GFR include the prostaglandins (PGE2 and PGI2) and bradykinin. These substances are discussed in Chapter 17. Although these vasodilators do not appear to be of major importance in regulating renal blood flow or GFR in normal conditions, they may dampen the renal vasoconstrictor effects of the sympathetic nerves or angiotensin II, especially their effects to constrict the afferent arterioles.
By opposing vasoconstriction of afferent arterioles, the prostaglandins may help prevent excessive reductions in GFR and renal blood flow. Under stressful conditions, such as volume depletion or after surgery, the adminis- tration of nonsteroidal anti-inflammatory agents, such as aspirin, that inhibit prostaglandin synthesis may cause significant reductions in GFR.
Autoregulation of GFR and Renal Blood Flow
Feedback mechanisms intrinsic to the kidneys normally keep the renal blood flow and GFR relatively constant, despite marked changes in arterial blood pressure. These mechanisms still function in blood-perfused kidneys that have been removed from the body, independent of sys- temic influences. This relative constancy of GFR and renal blood flow is referred to as autoregulation (Figure 26-17).
The primary function of blood flow autoregulation in most tissues other than the kidneys is to maintain the delivery of oxygen and nutrients at a normal level and to remove the waste products of metabolism, despite changes in the arterial pressure. In the kidneys, the normal blood flow is much higher than that required for these functions. The major function of autoregulation in the kidneys is to
maintain a relatively constant GFR and to allow precise control of renal excretion of water and solutes.
The GFR normally remains autoregulated (that is, remains relatively constant), despite considerable arterial pressure fluctuations that occur during a person’s usual activities. For instance, a decrease in arterial pressure to as low as 75 mm Hg or an increase to as high as 160 mm Hg usually changes GFR less than 10 percent. In general, renal blood flow is autoregulated in parallel with GFR, but GFR is more efficiently autoregulated under certain conditions.
Importance of GFR Autoregulation in Preventing Extreme Changes in Renal Excretion
Although the renal autoregulatory mechanisms are not perfect, they do prevent potentially large changes in GFR and renal excretion of water and solutes that would oth- erwise occur with changes in blood pressure. One can understand the quantitative importance of autoregulation by considering the relative magnitudes of glomerular fil- tration, tubular reabsorption, and renal excretion and the changes in renal excretion that would occur without auto- regulatory mechanisms.
Normally, GFR is about 180 L/day and tubular reabsorp- tion is 178.5 L/day, leaving 1.5 L/day of fluid to be excreted in the urine. In the absence of autoregulation, a relatively small increase in blood pressure (from 100 to 125 mm Hg) would cause a similar 25 percent increase in GFR (from about 180 to 225 L/day). If tubular reabsorption remained constant at 178.5 L/day, this would increase the urine flow to 46.5 L/day (the difference between GFR and tubular reabsorption)— a total increase in urine of more than 30-fold. Because the total plasma volume is only about 3 liters, such a change would quickly deplete the blood volume.
In reality, changes in arterial pressure usually exert much less of an effect on urine volume for two reasons: (1) renal autoregulation prevents large changes in GFR that would otherwise occur, and (2) there are additional adaptive mechanisms in the renal tubules that cause them to increase their reabsorption rate when GFR rises, a phe- nomenon referred to as glomerulotubular balance (dis- cussed in Chapter 27). Even with these special control mechanisms, changes in arterial pressure still have signifi- cant effects on renal excretion of water and sodium; this is referred to as pressure diuresis or pressure natriuresis, and it is crucial in the regulation of body fluid volumes and arterial pressure, as discussed in Chapters 19 and 29.
Tubuloglomerular Feedback and Autoregulation of GFR
To perform the function of autoregulation, the kidneys have a feedback mechanism that links changes in sodium chloride concentration at the macula densa with the control of renal arteriolar resistance. This feedback helps ensure a relatively constant delivery of sodium chloride to the distal tubule and helps prevent spurious fluctuations in renal excretion that would otherwise occur. In many circumstances, this feedback autoregulates renal blood
R en
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lo w
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in )
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1200
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0 G lo
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50 100 150 200
U ri
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o u
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)
8
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Figure 26-17 Autoregulation of renal blood flow and glomerular filtration rate but lack of autoregulation of urine flow during changes in renal arterial pressure.
Unit V The Body Fluids and Kidneys
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flow and GFR in parallel. However, because this mech- anism is specifically directed toward stabilizing sodium chloride delivery to the distal tubule, there are instances when GFR is autoregulated at the expense of changes in renal blood flow, as discussed later.
The tubuloglomerular feedback mechanism has two components that act together to control GFR: (1) an affer- ent arteriolar feedback mechanism and (2) an efferent arteriolar feedback mechanism. These feedback mecha- nisms depend on special anatomical arrangements of the juxtaglomerular complex (Figure 26-18).
The juxtaglomerular complex consists of macula densa cells in the initial portion of the distal tubule and juxta- glomerular cells in the walls of the afferent and efferent arterioles. The macula densa is a specialized group of epi- thelial cells in the distal tubules that comes in close con- tact with the afferent and efferent arterioles. The macula densa cells contain Golgi apparatus, which are intracel- lular secretory organelles directed toward the arterioles, suggesting that these cells may be secreting a substance toward the arterioles.
Decreased Macula Densa Sodium Chloride Causes Dilation of Afferent Arterioles and Increased Renin Release. The macula densa cells sense changes in volume delivery to the distal tubule by way of signals that are not completely understood. Experimental studies suggest that decreased GFR slows the flow rate in the loop of Henle, causing increased reabsorption of sodium and chloride ions in the ascending loop of Henle, thereby reducing the
concentration of sodium chloride at the macula densa cells. This decrease in sodium chloride concentration ini- tiates a signal from the macula densa that has two effects (Figure 26-19): (1) It decreases resistance to blood flow in the afferent arterioles, which raises glomerular hydro- static pressure and helps return GFR toward normal, and (2) it increases renin release from the juxtaglomerular cells of the afferent and efferent arterioles, which are the major storage sites for renin. Renin released from these cells then functions as an enzyme to increase the forma- tion of angiotensin I, which is converted to angiotensin II. Finally, the angiotensin II constricts the efferent arteri- oles, thereby increasing glomerular hydrostatic pressure and helping to return GFR toward normal.
These two components of the tubuloglomerular feed- back mechanism, operating together by way of the special anatomical structure of the juxtaglomerular apparatus, provide feedback signals to both the afferent and the effer- ent arterioles for efficient autoregulation of GFR during changes in arterial pressure. When both of these mecha- nisms are functioning together, the GFR changes only a few percentage points, even with large fluctuations in arterial pressure between the limits of 75 and 160 mm Hg.
Blockade of Angiotensin II Formation Further Reduces GFR During Renal Hypoperfusion. As discussed earlier, a preferential constrictor action of angiotensin II on efferent arterioles helps prevent serious reductions in glomerular hydrostatic pressure and GFR when renal perfusion pressure falls below normal. The administration of drugs that block
− − Glomerular hydrostatic
pressure
GFR
Renin
Angiotensin II
Proximal NaCl
reabsorption Macula densa
NaCl
Efferent arteriolar
resistance
Afferent arteriolar
resistance
Arterial pressure
Figure 26-19 Macula densa feedback mechanism for autoregula- tion of glomerular hydrostatic pressure and glomerular filtration rate (GFR) during decreased renal arterial pressure.
Juxtaglomerular cells Afferent
arteriole
Internal elastic lamina
Efferent arteriole
Smooth muscle
fiber Distal tubule
Glomerular epithelium
Basement membrane
Macula densa
Figure 26-18 Structure of the juxtaglomerular apparatus, demonstrating its possible feedback role in the control of nephron function.
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the formation of angiotensin II (angiotensin-converting enzyme inhibitors) or that block the action of angiotensin II (angiotensin II receptor antagonists) causes greater reductions in GFR than usual when the renal arterial pressure falls below normal. Therefore, an important complication of using these drugs to treat patients who have hypertension because of renal artery stenosis (partial blockage of the renal artery) is a severe decrease in GFR that can, in some cases, cause acute renal failure. Nevertheless, angiotensin II–blocking drugs can be useful therapeutic agents in many patients with hypertension, congestive heart failure, and other conditions, as long as they are monitored to ensure that severe decreases in GFR do not occur.
Myogenic Autoregulation of Renal Blood Flow and GFR
Another mechanism that contributes to the maintenance of a relatively constant renal blood flow and GFR is the ability of individual blood vessels to resist stretching dur- ing increased arterial pressure, a phenomenon referred to as the myogenic mechanism. Studies of individual blood vessels (especially small arterioles) throughout the body have shown that they respond to increased wall tension or wall stretch by contraction of the vascular smooth muscle. Stretch of the vascular wall allows increased movement of calcium ions from the extracellular fluid into the cells, causing them to contract through the mechanisms dis- cussed in Chapter 8. This contraction prevents excessive stretch of the vessel and at the same time, by raising vas- cular resistance, helps prevent excessive increases in renal blood flow and GFR when arterial pressure increases.
Although the myogenic mechanism probably operates in most arterioles throughout the body, its importance in renal blood flow and GFR autoregulation has been questioned by some physiologists because this pressure- sensitive mechanism has no means of directly detecting changes in renal blood flow or GFR per se. On the other hand, this mechanism may be more important in pro- tecting the kidney from hypertension-induced injury. In response to sudden increases in blood pressure, the myo- genic constrictor response in afferent arterioles occurs within seconds and therefore attenuates transmission of increased arterial pressure to the glomerular capillaries.
Other Factors That Increase Renal Blood Flow and GFR: High Protein Intake and Increased Blood Glucose Although renal blood flow and GFR are relatively stable under most conditions, there are circumstances in which these variables change significantly. For example, a high protein intake is known to increase both renal blood flow and GFR. With a chronic high-protein diet, such as one that con- tains large amounts of meat, the increases in GFR and renal blood flow are due partly to growth of the kidneys. However, GFR and renal blood flow increase 20 to 30 percent within 1 or 2 hours after a person eats a high-protein meal.
One likely explanation for the increased GFR is the follow- ing: A high-protein meal increases the release of amino acids into the blood, which are reabsorbed in the proximal tubule. Because amino acids and sodium are reabsorbed together by the proximal tubules, increased amino acid reabsorption also stimulates sodium reabsorption in the proximal tubules. This
decreases sodium delivery to the macula densa (see Figure 26-19), which elicits a tubuloglomerular feedback– mediated decrease in resistance of the afferent arterioles, as discussed earlier. The decreased afferent arteriolar resistance then raises renal blood flow and GFR. This increased GFR allows sodium excretion to be maintained at a nearly normal level while increasing the excretion of the waste products of protein metabolism, such as urea.
A similar mechanism may also explain the marked increases in renal blood flow and GFR that occur with large increases in blood glucose levels in uncontrolled diabetes melli- tus. Because glucose, like some of the amino acids, is also reab- sorbed along with sodium in the proximal tubule, increased glucose delivery to the tubules causes them to reabsorb excess sodium along with glucose. This, in turn, decreases delivery of sodium chloride to the macula densa, activating a tubu- loglomerular feedback-mediated dilation of the afferent arte- rioles and subsequent increases in renal blood flow and GFR.
These examples demonstrate that renal blood flow and GFR per se are not the primary variables controlled by the tubuloglomerular feedback mechanism. The main purpose of this feedback is to ensure a constant delivery of sodium chloride to the distal tubule, where final processing of the urine takes place. Thus, disturbances that tend to increase reabsorption of sodium chloride at tubular sites before the macula densa tend to elicit increased renal blood flow and GFR, which helps return distal sodium chloride delivery toward normal so that normal rates of sodium and water excretion can be maintained (see Figure 26-19).
An opposite sequence of events occurs when proximal tubular reabsorption is reduced. For example, when the proxi- mal tubules are damaged (which can occur as a result of poi- soning by heavy metals, such as mercury, or large doses of drugs, such as tetracyclines), their ability to reabsorb sodium chloride is decreased. As a consequence, large amounts of sodium chloride are delivered to the distal tubule and, without appropriate compensations, would quickly cause excessive vol- ume depletion. One of the important compensatory responses appears to be a tubuloglomerular feedback–mediated renal vasoconstriction that occurs in response to the increased sodium chloride delivery to the macula densa in these circum- stances. These examples again demonstrate the importance of this feedback mechanism in ensuring that the distal tubule receives the proper rate of delivery of sodium chloride, other tubular fluid solutes, and tubular fluid volume so that appro- priate amounts of these substances are excreted in the urine.
Bibliography
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Bell PD, Lapointe JY, Peti-Peterdi J: Macula densa cell signaling, Annu Rev Physiol 65:481, 2003.
Cowley AW Jr, Mori T, Mattson D, et al: Role of renal NO production in the regulation of medullary blood flow, Am J Physiol Regul Integr Comp Physiol 284:R1355, 2003.
Cupples WA, Braam B: Assessment of renal autoregulation, Am J Physiol Renal Physiol 292:F1105, 2007.
Deen WN: What determines glomerular capillary permeability? J Clin Invest 114:1412, 2004.
DiBona GF: Physiology in perspective: The Wisdom of the Body. Neural control of the kidney, Am J Physiol Regul Integr Comp Physiol 289:R633, 2005.
Drummond HA, Grifoni SC, Jernigan NL: A new trick for an old dogma: ENaC proteins as mechanotransducers in vascular smooth muscle, Physiology (Bethesda) 23:23, 2008.
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Fowler CJ, Griffiths D, de Groat WC: The neural control of micturition, Nat Rev Neurosci 9:453, 2008.
Hall JE: Angiotensin II and long-term arterial pressure regulation: the overrid- ing dominance of the kidney, J Am Soc Nephrol 10:(Suppl 12):s258, 1999.
Hall JE, Brands MW: The renin-angiotensin-aldosterone system: renal mechanisms and circulatory homeostasis. In Seldin DW, Giebisch G, eds: The Kidney—Physiology and Pathophysiology, ed 3, New York, 2000, Raven Press, pp 1009–1046.
Hall JE, Henegar JR, Dwyer TM, et al: Is obesity a major cause of chronic kidney disease? Adv Ren Replace Ther 11:41, 2004.
Haraldsson B, Sörensson J: Why do we not all have proteinuria? An update of our current understanding of the glomerular barrier, News Physiol Sci 19:7, 2004.
Kriz W, Kaissling B: Structural organization of the mammalian kid- ney. In Seldin DW, Giebisch G, eds: The Kidney—Physiology and Pathophysiology, ed 3, New York, 2000, Raven Press, pp 587–654.
Loutzenhiser R, Griffin K, Williamson G, et al: Renal autoregulation: new perspectives regarding the protective and regulatory roles of the under- lying mechanisms, Am J Physiol Regul Integr Comp Physiol 290:R1153, 2006.
Pallone TL, Zhang Z, Rhinehart K: Physiology of the renal medullary micro- circulation, Am J Physiol Renal Physiol 284:F253, 2003.
Roman RJ: P-450 metabolites of arachidonic acid in the control of cardio- vascular function, Physiol Rev 82:131, 2002.
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chapter 27
Urine Formation by the Kidneys: II. Tubular Reabsorption and Secretion
Renal Tubular Reabsorption and Secretion
As the glomerular filtrate enters the renal tubules, it
flows sequentially through the successive parts of the tubule—the proximal tubule, the loop of Henle, the distal tubule, the collecting tubule, and, finally, the collecting duct—before it is excreted as urine. Along this course, some substances are selectively reabsorbed from the tubules back into the blood, whereas others are secreted from the blood into the tubular lumen. Eventually, the urine that is formed and all the substances in the urine represent the sum of three basic renal processes—glomerular filtration, tubular reabsorption, and tubular secretion:
Urinary excretion = Glomerular filtration – Tubular reabsorption + Tubular secretion
For many substances, tubular reabsorption plays a much more important role than secretion in determining the final urinary excretion rate. However, tubular secre- tion accounts for significant amounts of potassium ions, hydrogen ions, and a few other substances that appear in the urine.
Tubular Reabsorption Is Quantitatively Large and Highly Selective
Table 27-1 shows the renal handling of several substances that are all freely filtered in the kidneys and reabsorbed at variable rates. The rate at which each of these substances is filtered is calculated as
Filtration = Glomerular filtration rate × Plasma concentration
This calculation assumes that the substance is freely filtered and not bound to plasma proteins. For example, if plasma glucose concentration is 1 g/L, the amount of glucose filtered each day is about 180 L/day × 1 g/L, or 180 g/day. Because virtually none of the filtered glucose is normally excreted, the rate of glucose reabsorption is also 180 g/day.
From Table 27-1, two things are immediately appar- ent. First, the processes of glomerular filtration and tubular reabsorption are quantitatively large relative to urinary excretion for many substances. This means that a small change in glomerular filtration or tubular reab- sorption can potentially cause a relatively large change in urinary excretion. For example, a 10 percent decrease in tubular reabsorption, from 178.5 to 160.7 L/day, would increase urine volume from 1.5 to 19.3 L/day (almost a 13-fold increase) if the glomerular filtration rate (GFR) remained constant. In reality, however, changes in tubu- lar reabsorption and glomerular filtration are closely coordinated so that large fluctuations in urinary excre- tion are avoided.
Second, unlike glomerular filtration, which is relatively nonselective (essentially all solutes in the plasma are fil- tered except the plasma proteins or substances bound to them), tubular reabsorption is highly selective. Some substances, such as glucose and amino acids, are almost completely reabsorbed from the tubules, so the urinary excretion rate is essentially zero. Many of the ions in the plasma, such as sodium, chloride, and bicarbonate, are also highly reabsorbed, but their rates of reabsorption and urinary excretion are variable, depending on the needs of the body. Waste products, such as urea and creatinine, conversely, are poorly reabsorbed from the tubules and excreted in relatively large amounts.
Therefore, by controlling the rate at which they reab- sorb different substances, the kidneys regulate the excre- tion of solutes independently of one another, a capability that is essential for precise control of the body fluid composition. In this chapter, we discuss the mechanisms that allow the kidneys to selectively reabsorb or secrete different substances at variable rates.
Tubular Reabsorption Includes Passive and Active Mechanisms
For a substance to be reabsorbed, it must first be trans- ported (1) across the tubular epithelial membranes into the renal interstitial fluid and then (2) through the peritubu- lar capillary membrane back into the blood (Figure 27-1).
Unit V The Body Fluids and Kidneys
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Thus, reabsorption of water and solutes includes a series of transport steps. Reabsorption across the tubular epi- thelium into the interstitial fluid includes active or pas- sive transport by the same basic mechanisms discussed in Chapter 4 for transport across other membranes of the body. For instance, water and solutes can be transported through the cell membranes themselves (transcellular route) or through the spaces between the cell junctions (paracellular route). Then, after absorption across the tubular epithelial cells into the interstitial fluid, water and solutes are transported through the peritubular capillary walls into the blood by ultrafiltration (bulk flow) that is mediated by hydrostatic and colloid osmotic forces. The peritubular capillaries behave like the venous ends of most other capillaries because there is a net reabsorptive force that moves the fluid and solutes from the interstitium into the blood.
Active Transport
Active transport can move a solute against an electro- chemical gradient and requires energy derived from metabolism. Transport that is coupled directly to an energy source, such as the hydrolysis of adenosine triphosphate (ATP), is termed primary active transport. A good example of this is the sodium-potassium ATPase pump that functions throughout most parts of the renal tubule. Transport that is coupled indirectly to an energy source, such as that due to an ion gradient, is referred to as secondary active transport. Reabsorption of glucose by the renal tubule is an example of secondary active trans- port. Although solutes can be reabsorbed by active and/ or passive mechanisms by the tubule, water is always reabsorbed by a passive (nonactive) physical mecha- nism called osmosis, which means water diffusion from a region of low solute concentration (high water concen- tration) to one of high solute concentration (low water concentration).
Solutes Can Be Transported Through Epithelial Cells or Between Cells. Renal tubular cells, like other epithelial cells, are held together by tight junctions. Lateral intercellular spaces lie behind the tight junctions and separate the epithelial cells of the tubule. Solutes can be reabsorbed or secreted across the cells through the trans cellular pathway or between the cells by moving across the tight junctions and intercellular spaces by way of the paracellular pathway. Sodium is a substance that moves through both routes, although most of the sodium is transported through the transcellular pathway. In some nephron segments, especially the proximal tubule, water is also reabsorbed across the paracellular pathway, and substances dissolved in the water, especially potassium, magnesium, and chloride ions, are carried with the reab- sorbed fluid between the cells.
Primary Active Transport Through the Tubular Membrane Is Linked to Hydrolysis of ATP. The special importance of primary active transport is that it can move solutes against an electrochemical gradient. The energy for this active transport comes from the hydrolysis of ATP by way of membrane-bound ATPase; the ATPase is also
Peritubular capillary
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Solutes
H2O
FILTRATION
EXCRETIONREABSORPTION
Bulk flow
Blood
Active
Passive (diffusion)
Osmosis
ATP
Figure 27-1 Reabsorption of filtered water and solutes from the tubular lumen across the tubular epithelial cells, through the renal interstitium, and back into the blood. Solutes are transported through the cells (transcellular path) by passive diffusion or active transport, or between the cells (paracellular path) by diffusion. Water is transported through the cells and between the tubular cells by osmosis. Transport of water and solutes from the inter- stitial fluid into the peritubular capillaries occurs by ultrafiltration (bulk flow).
Amount Filtered Amount Reabsorbed Amount Excreted % of Filtered Load Reabsorbed
Glucose (g/day) 180 180 0 100
Bicarbonate (mEq/day) 4,320 4,318 2 >99.9
Sodium (mEq/day) 25,560 25,410 150 99.4
Chloride (mEq/day) 19,440 19,260 180 99.1
Potassium (mEq/day) 756 664 92 87.8
Urea (g/day) 46.8 23.4 23.4 50
Creatinine (g/day) 1.8 0 1.8 0
Table 27-1 Filtration, Reabsorption, and Excretion Rates of Different Substances by the Kidneys
Chapter 27 Urine Formation by the Kidneys: II. Tubular Reabsorption and Secretion
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a component of the carrier mechanism that binds and moves solutes across the cell membranes. The primary active transporters in the kidneys that are known include sodiumpotassium ATPase, hydrogen ATPase, hydrogen potassium ATPase, and calcium ATPase.
A good example of a primary active transport sys- tem is the reabsorption of sodium ions across the prox- imal tubular membrane, as shown in Figure 27-2. On the basolateral sides of the tubular epithelial cell, the cell membrane has an extensive sodium-potassium ATPase system that hydrolyzes ATP and uses the released energy to transport sodium ions out of the cell into the intersti- tium. At the same time, potassium is transported from the interstitium to the inside of the cell. The operation of this ion pump maintains low intracellular sodium and high intracellular potassium concentrations and creates a net negative charge of about −70 millivolts within the cell. This active pumping of sodium out of the cell across the basolateral membrane of the cell favors passive diffusion of sodium across the luminal membrane of the cell, from the tubular lumen into the cell, for two reasons: (1) There is a concentration gradient favoring sodium diffusion into the cell because intracellular sodium concentration is low (12 mEq/L) and tubular fluid sodium concentration is high (140 mEq/L) and (2) the negative, −70-millivolt, intracellular potential attracts the positive sodium ions from the tubular lumen into the cell.
Active reabsorption of sodium by sodium-potassium ATPase occurs in most parts of the tubule. In certain parts of the nephron, there are also additional provi- sions for moving large amounts of sodium into the cell. In the proximal tubule, there is an extensive brush bor- der on the luminal side of the membrane (the side that faces the tubular lumen) that multiplies the surface area
about 20-fold. There are also carrier proteins that bind sodium ions on the luminal surface of the membrane and release them inside the cell, providing facilitated diffu sion of sodium through the membrane into the cell. These sodium carrier proteins are also important for secondary active transport of other substances, such as glucose and amino acids, as discussed later.
Thus, the net reabsorption of sodium ions from the tubular lumen back into the blood involves at least three steps:
1. Sodium diffuses across the luminal membrane (also called the apical membrane) into the cell down an electrochemical gradient established by the sodium- potassium ATPase pump on the basolateral side of the membrane.
2. Sodium is transported across the basolateral mem- brane against an electrochemical gradient by the sodium-potassium ATPase pump.
3. Sodium, water, and other substances are reabsorbed from the interstitial fluid into the peritubular cap- illaries by ultrafiltration, a passive process driven by the hydrostatic and colloid osmotic pressure gradients.
Secondary Active Reabsorption Through the Tubular Membrane. In secondary active transport, two or more substances interact with a specific mem- brane protein (a carrier molecule) and are transported together across the membrane. As one of the substances (for instance, sodium) diffuses down its electrochemi- cal gradient, the energy released is used to drive another substance (for instance, glucose) against its electrochemi- cal gradient. Thus, secondary active transport does not require energy directly from ATP or from other high- energy phosphate sources. Rather, the direct source of the energy is that liberated by the simultaneous facilitated diffusion of another transported substance down its own electrochemical gradient.
Figure 27-3 shows secondary active transport of glu- cose and amino acids in the proximal tubule. In both instances, specific carrier proteins in the brush border combine with a sodium ion and an amino acid or a glu- cose molecule at the same time. These transport mech- anisms are so efficient that they remove virtually all the glucose and amino acids from the tubular lumen. After entry into the cell, glucose and amino acids exit across the basolateral membranes by diffusion, driven by the high glucose and amino acid concentrations in the cell facili- tated by specific transport proteins.
Sodium glucose cotransporters (SGLT2 and SGLT1) are located on the brush border of proximal tubular cells and carry glucose into the cell cytoplasm against a concentra- tion gradient, as described previously. Approximately 90 percent of the filtered glucose is reabsorbed by SGLT2 in the early part of the proximal tubule (S1 segment) and the residual 10 percent is transported by SGLT1 in the lat- ter segments of the proximal tubule. On the basolateral
Peritubular capillary
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Na+
K+ K+
Figure 27-2 Basic mechanism for active transport of sodium through the tubular epithelial cell. The sodium-potassium pump transports sodium from the interior of the cell across the basolat- eral membrane, creating a low intracellular sodium concentration and a negative intracellular electrical potential. The low intracel- lular sodium concentration and the negative electrical potential cause sodium ions to diffuse from the tubular lumen into the cell through the brush border.
Unit V The Body Fluids and Kidneys
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side of the membrane, glucose diffuses out of the cell into the interstitial spaces with the help of glucose transporters GLUT2, in the S1 segment and GLUT1 in the latter part (S3 segment) of the proximal tubule.
Although transport of glucose against a chemical gradi- ent does not directly use ATP, the reabsorption of glucose depends on energy expended by the primary active sodium- potassium ATPase pump in the basolateral membrane. Because of the activity of this pump, an electrochemical gra- dient for facilitated diffusion of sodium across the luminal membrane is maintained, and it is this downhill diffusion of sodium to the interior of the cell that provides the energy for the simultaneous uphill transport of glucose across the luminal membrane. Thus, this reabsorption of glucose is referred to as “secondary active transport” because glucose itself is reabsorbed uphill against a chemical gradient, but it is “secondary” to primary active transport of sodium.
Another important point is that a substance is said to undergo “active” transport when at least one of the steps in the reabsorption involves primary or secondary active transport, even though other steps in the reabsorption process may be passive. For glucose reabsorption, sec- ondary active transport occurs at the luminal membrane, but passive facilitated diffusion occurs at the basolateral membrane, and passive uptake by bulk flow occurs at the peritubular capillaries.
Secondary Active Secretion into the Tubules. Some substances are secreted into the tubules by secondary active transport. This often involves countertransport of the substance with sodium ions. In counter-trans- port, the energy liberated from the downhill movement of one of the substances (e.g., sodium ions) enables uphill movement of a second substance in the opposite direction.
One example of counter-transport, shown in Figure 27-3, is the active secretion of hydrogen ions coupled to sodium reabsorption in the luminal membrane of the proximal tubule. In this case, sodium entry into the cell is coupled with hydrogen extrusion from the cell by sodium- hydrogen counter-transport. This transport is mediated by a specific protein (sodiumhydrogen exchanger) in the brush border of the luminal membrane. As sodium is car- ried to the interior of the cell, hydrogen ions are forced outward in the opposite direction into the tubular lumen. The basic principles of primary and secondary active transport are discussed in additional detail in Chapter 4.
Pinocytosis—An Active Transport Mechanism for Reabsorption of Proteins. Some parts of the tubule, especially the proximal tubule, reabsorb large molecules such as proteins by pinocytosis. In this process the pro- tein attaches to the brush border of the luminal mem- brane, and this portion of the membrane then invaginates to the interior of the cell until it is completely pinched off and a vesicle is formed containing the protein. Once inside the cell, the protein is digested into its constituent amino acids, which are reabsorbed through the basolat- eral membrane into the interstitial fluid. Because pino- cytosis requires energy, it is considered a form of active transport.
Transport Maximum for Substances That Are Actively Reabsorbed. For most substances that are actively reabsorbed or secreted, there is a limit to the rate at which the solute can be transported, often referred to as the transport maximum. This limit is due to satu- ration of the specific transport systems involved when the amount of solute delivered to the tubule (referred to as tubular load) exceeds the capacity of the carrier proteins and specific enzymes involved in the transport process.
The glucose transport system in the proximal tubule is a good example. Normally, measurable glucose does not appear in the urine because essentially all the filtered glu- cose is reabsorbed in the proximal tubule. However, when the filtered load exceeds the capability of the tubules to reabsorb glucose, urinary excretion of glucose does occur.
In the adult human, the transport maximum for glucose averages about 375 mg/min, whereas the filtered load of glucose is only about 125 mg/min (GFR × plasma glucose = 125 ml/min × 1 mg/ml). With large increases in GFR and/or plasma glucose concentration that increase the
Tubular cells
−70 mV
Co-transport
Interstitial fluid
Tubular lumen
Na+Na+
Na+Na+ATP
GLUT SGLT
Na+Na+
K+K+
−70 mV
Counter-transport
ATP NHE Na+Na+
K+K+
Na+Na+
H+H+
Amino acids Amino acids
Glucose Glucose
Figure 27-3 Mechanisms of secondary active transport. The upper cell shows the co-transport of glucose and amino acids along with sodium ions through the apical side of the tubular epi- thelial cells, followed by facilitated diffusion through the baso- lateral membranes. The lower cell shows the counter-transport of hydrogen ions from the interior of the cell across the apical membrane and into the tubular lumen; movement of sodium ions into the cell, down an electrochemical gradient established by the sodium-potassium pump on the basolateral membrane, provides the energy for transport of the hydrogen ions from inside the cell into the tubular lumen. GLUT, glucose transporter; NHE, sodium- hydrogen exchanger; SGLT, sodium-glucose co-transporter.
Chapter 27 Urine Formation by the Kidneys: II. Tubular Reabsorption and Secretion
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filtered load of glucose above 375 mg/min, the excess glu- cose filtered is not reabsorbed and passes into the urine.
Figure 27-4 shows the relation between plasma con- centration of glucose, filtered load of glucose, tubular transport maximum for glucose, and rate of glucose loss in the urine. Note that when the plasma glucose concen- tration is 100 mg/100 ml and the filtered load is at its normal level, 125 mg/min, there is no loss of glucose in the urine. However, when the plasma concentration of glucose rises above about 200 mg/100 ml, increasing the filtered load to about 250 mg/min, a small amount of glu- cose begins to appear in the urine. This point is termed the threshold for glucose. Note that this appearance of glucose in the urine (at the threshold) occurs before the transport maximum is reached. One reason for the dif- ference between threshold and transport maximum is that not all nephrons have the same transport maximum for glucose, and some of the nephrons therefore begin to excrete glucose before others have reached their trans- port maximum. The overall transport maximum for the kidneys, which is normally about 375 mg/min, is reached when all nephrons have reached their maximal capacity to reabsorb glucose.
The plasma glucose of a healthy person almost never becomes high enough to cause glucose excretion in the urine, even after eating a meal. However, in uncontrolled diabetes mellitus, plasma glucose may rise to high lev- els, causing the filtered load of glucose to exceed the transport maximum and resulting in urinary glucose excretion. Some of the important transport maximums for substances actively reabsorbed by the tubules are as follows:
Substance Transport Maximum Glucose 375 mg/min Phosphate 0.10 mM/min Sulfate 0.06 mM/min Amino acids 1.5 mM/min Urate 15 mg/min Lactate 75 mg/min Plasma protein 30 mg/min
Transport Maximums for Substances That Are Actively Secreted. Substances that are actively secreted also exhibit transport maximums as follows:
Substance Transport Maximum Creatinine 16 mg/min Para-aminohippuric acid 80 mg/min
Substances That Are Actively Transported but Do Not Exhibit a Transport Maximum. The reason that actively transported solutes often exhibit a transport maximum is that the transport carrier system becomes saturated as the tubular load increases. Some substances that are passively reabsorbed do not demonstrate a trans port maximum because their rate of transport is deter- mined by other factors, such as (1) the electrochemical gradient for diffusion of the substance across the mem- brane, (2) the permeability of the membrane for the sub- stance, and (3) the time that the fluid containing the substance remains within the tubule. Transport of this type is referred to as gradienttime transport because the rate of transport depends on the electrochemical gradient and the time that the substance is in the tubule, which in turn depends on the tubular flow rate.
Some actively transported substances also have char acteristics of gradienttime transport. An example is sodium reabsorption in the proximal tubule. The main reason that sodium transport in the proximal tubule does not exhibit a transport maximum is that other fac- tors limit the reabsorption rate besides the maximum rate of active transport. For example, in the proximal tubules, the maximum transport capacity of the baso- lateral sodium-potassium ATPase pump is usually far greater than the actual rate of net sodium reabsorption. One of the reasons for this is that a significant amount of sodium transported out of the cell leaks back into the tubular lumen through the epithelial tight junc- tions. The rate at which this backleak occurs depends on several factors, including (1) the permeability of the tight junctions and (2) the interstitial physical forces, which determine the rate of bulk flow reabsorption from the interstitial fluid into the peritubular capillar- ies. Therefore, sodium transport in the proximal tubules obeys mainly gradient-time transport principles rather than tubular maximum transport characteristics. This means that the greater the concentration of sodium in the proximal tubules, the greater its reabsorption rate.
G lu
co se
f ilt
er ed
lo ad
, r ea
b so
rp ti
o n
o r
ex cr
et io
n (
m g
/m in
)
Plasma glucose concentration (mg/100 ml)
0 800700600500400300200100
Filtered load
Filtered load
900
800
700
600
500
400
300
200
100
Normal
Threshold
Transport maximum Reabsorption
Excretion
0
Figure 27-4 Relations among the filtered load of glucose, the rate of glucose reabsorption by the renal tubules, and the rate of glu- cose excretion in the urine. The transport maximum is the maxi- mum rate at which glucose can be reabsorbed from the tubules. The threshold for glucose refers to the filtered load of glucose at which glucose first begins to be excreted in the urine.
Unit V The Body Fluids and Kidneys
328
Also, the slower the flow rate of tubular fluid, the greater the percentage of sodium that can be reabsorbed from the proximal tubules.
In the more distal parts of the nephron, the epithe- lial cells have much tighter junctions and transport much smaller amounts of sodium. In these segments, sodium reabsorption exhibits a transport maximum similar to that for other actively transported substances. Furthermore, this transport maximum can be increased by certain hor- mones, such as aldosterone.
Passive Water Reabsorption by Osmosis Is Coupled Mainly to Sodium Reabsorption
When solutes are transported out of the tubule by either primary or secondary active transport, their concentra- tions tend to decrease inside the tubule while increasing in the renal interstitium. This creates a concentration dif- ference that causes osmosis of water in the same direction that the solutes are transported, from the tubular lumen to the renal interstitium. Some parts of the renal tubule, especially the proximal tubule, are highly permeable to water, and water reabsorption occurs so rapidly that there is only a small concentration gradient for solutes across the tubular membrane.
A large part of the osmotic flow of water in the proxi- mal tubules occurs through the so-called tight junctions between the epithelial cells, as well as through the cells themselves. The reason for this, as already discussed, is that the junctions between the cells are not as tight as their name would imply and permit significant diffu- sion of water and small ions. This is especially true in the proximal tubules, which have a high permeability for water and a smaller but significant permeability to most ions, such as sodium, chloride, potassium, calcium, and magnesium.
As water moves across the tight junctions by osmo- sis, it can also carry with it some of the solutes, a process referred to as solvent drag. And because the reabsorption of water, organic solutes, and ions is coupled to sodium reabsorption, changes in sodium reabsorption signifi- cantly influence the reabsorption of water and many other solutes.
In the more distal parts of the nephron, beginning in the loop of Henle and extending through the collecting tubule, the tight junctions become far less permeable to water and solutes and the epithelial cells also have a greatly decreased membrane surface area. Therefore, water can- not move easily across the tight junctions of the tubular membrane by osmosis. However, antidiuretic hormone (ADH) greatly increases the water permeability in the dis- tal and collecting tubules, as discussed later.
Thus, water movement across the tubular epithelium can occur only if the membrane is permeable to water, no matter how large the osmotic gradient. In the proximal tubule, the water permeability is always high and water is reabsorbed as rapidly as the solutes. In the ascending loop of Henle, water permeability is always low, so almost no water is reabsorbed despite a large osmotic gradient.
Water permeability in the last parts of the tubules—the distal tubules, collecting tubules, and collecting ducts— can be high or low, depending on the presence or absence of ADH.
Reabsorption of Chloride, Urea, and Other Solutes by Passive Diffusion
When sodium is reabsorbed through the tubular epithe- lial cell, negative ions such as chloride are transported along with sodium because of electrical potentials. That is, transport of positively charged sodium ions out of the lumen leaves the inside of the lumen negatively charged, compared with the interstitial fluid. This causes chloride ions to diffuse passively through the paracellular pathway. Additional reabsorption of chloride ions occurs because of a chloride concentration gradient that develops when water is reabsorbed from the tubule by osmosis, thereby concentrating the chloride ions in the tubular lumen (Figure 27-5). Thus, the active reabsorption of sodium is closely coupled to the passive reabsorption of chloride by way of an electrical potential and a chloride concentra- tion gradient.
Chloride ions can also be reabsorbed by secondary active transport. The most important of the second- ary active transport processes for chloride reabsorption involves co-transport of chloride with sodium across the luminal membrane.
Urea is also passively reabsorbed from the tubule, but to a much lesser extent than chloride ions. As water is reab- sorbed from the tubules (by osmosis coupled to sodium reabsorption), urea concentration in the tubular lumen increases (see Figure 27-5). This creates a concentration gradient favoring the reabsorption of urea. However, urea does not permeate the tubule as readily as water. In some parts of the nephron, especially the inner medullary col- lecting duct, passive urea reabsorption is facilitated by specific urea transporters. Yet, only about one half of the urea that is filtered by the glomerular capillaries is reabsorbed from the tubules. The remainder of the urea passes into the urine, allowing the kidneys to excrete large
Passive Cl– reabsorption
Passive urea reabsorption
Na+ reabsorption
H2O reabsorption
Lumen negative potential
Lumen negative potential
Luminal Cl– concentration Luminal Cl– concentration
Luminal urea concentration
Luminal urea concentration
Figure 27-5 Mechanisms by which water, chloride, and urea reab- sorption are coupled with sodium reabsorption.
Chapter 27 Urine Formation by the Kidneys: II. Tubular Reabsorption and Secretion
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amounts of this waste product of metabolism. In mam- mals, greater than 90 percent of waste nitrogen, mainly generated in the liver as a product of protein metabolism, is normally excreted by the kidneys as urea.
Another waste product of metabolism, creatinine, is an even larger molecule than urea and is essentially imper- meant to the tubular membrane. Therefore, almost none of the creatinine that is filtered is reabsorbed, so that virtually all the creatinine filtered by the glomerulus is excreted in the urine.
Reabsorption and Secretion Along Different Parts of the Nephron
In the previous sections, we discussed the basic princi- ples by which water and solutes are transported across the tubular membrane. With these generalizations in mind, we can now discuss the different characteristics of the individual tubular segments that enable them to per- form their specific functions. Only the tubular transport functions that are quantitatively most important are dis- cussed, especially as they relate to the reabsorption of sodium, chloride, and water. In subsequent chapters, we discuss the reabsorption and secretion of other specific substances in different parts of the tubular system.
Proximal Tubular Reabsorption
Normally, about 65 percent of the filtered load of sodium and water and a slightly lower percentage of filtered chlo- ride are reabsorbed by the proximal tubule before the fil- trate reaches the loops of Henle. These percentages can be increased or decreased in different physiologic condi- tions, as discussed later.
Proximal Tubules Have a High Capacity for Active and Passive Reabsorption. The high capacity of the proximal tubule for reabsorption results from its spe- cial cellular characteristics, as shown in Figure 27-6. The proximal tubule epithelial cells are highly metabolic and have large numbers of mitochondria to support power- ful active transport processes. In addition, the proximal tubular cells have an extensive brush border on the lumi- nal (apical) side of the membrane, as well as an extensive labyrinth of intercellular and basal channels, all of which together provide an extensive membrane surface area on the luminal and basolateral sides of the epithelium for rapid transport of sodium ions and other substances.
The extensive membrane surface of the epithelial brush border is also loaded with protein carrier molecules that transport a large fraction of the sodium ions across the luminal membrane linked by way of the cotransport mechanism with multiple organic nutrients such as amino acids and glucose. Additional sodium is transported from the tubular lumen into the cell by countertransport mechanisms that reabsorb sodium while secreting other substances into the tubular lumen, especially hydrogen
ions. As discussed in Chapter 30, the secretion of hydro- gen ions into the tubular lumen is an important step in the removal of bicarbonate ions from the tubule (by combin- ing H+ with the HCO3
− to form H2CO3, which then disso- ciates into H2O and CO2).
Although the sodium-potassium ATPase pump pro- vides the major force for reabsorption of sodium, chloride, and water throughout the proximal tubule, there are some differences in the mechanisms by which sodium and chlo- ride are transported through the luminal side of the early and late portions of the proximal tubular membrane.
In the first half of the proximal tubule, sodium is reab- sorbed by co-transport along with glucose, amino acids, and other solutes. But in the second half of the proximal tubule, little glucose and amino acids remain to be reab- sorbed. Instead, sodium is now reabsorbed mainly with chloride ions. The second half of the proximal tubule has a relatively high concentration of chloride (around 140 mEq/L) compared with the early proximal tubule (about 105 mEq/L) because when sodium is reabsorbed, it preferentially carries with it glucose, bicarbonate, and organic ions in the early proximal tubule, leaving behind a solution that has a higher concentration of chloride. In the second half of the proximal tubule, the higher chlo- ride concentration favors the diffusion of this ion from the tubule lumen through the intercellular junctions into the renal interstitial fluid. Smaller amounts of chloride may also be reabsorbed through specific chloride chan- nels in the proximal tubular cell membrane.
Concentrations of Solutes Along the Proximal Tubule. Figure 27-7 summarizes the changes in con- centrations of various solutes along the proximal tubule. Although the amount of sodium in the tubular fluid decreases markedly along the proximal tubule, the con centration of sodium (and the total osmolarity) remains relatively constant because water permeability of the
Proximal tubule
65%
Isosmotic
H+, organic acids, bases
Na+, Cl–, HCO3 –, K+,
H2O, glucose, amino acids
Figure 27-6 Cellular ultrastructure and primary transport char- acteristics of the proximal tubule. The proximal tubules reabsorb about 65 percent of the filtered sodium, chloride, bicarbonate, and potassium and essentially all the filtered glucose and amino acids. The proximal tubules also secrete organic acids, bases, and hydro- gen ions into the tubular lumen.
Unit V The Body Fluids and Kidneys
330
proximal tubules is so great that water reabsorption keeps pace with sodium reabsorption. Certain organic solutes, such as glucose, amino acids, and bicarbonate, are much more avidly reabsorbed than water, so their concentra- tions decrease markedly along the length of the proximal tubule. Other organic solutes that are less permeant and not actively reabsorbed, such as creatinine, increase their concentration along the proximal tubule. The total solute concentration, as reflected by osmolarity, remains essen- tially the same all along the proximal tubule because of the extremely high permeability of this part of the nephron to water.
Secretion of Organic Acids and Bases by the Proximal Tubule. The proximal tubule is also an impor- tant site for secretion of organic acids and bases such as bile salts, oxalate, urate, and catecholamines. Many of these substances are the end products of metabolism and must be rapidly removed from the body. The secre tion of these substances into the proximal tubule plus filtration into the proximal tubule by the glomerular capillaries and the almost total lack of reabsorption by the tubules, all combined, contribute to rapid excretion in the urine.
In addition to the waste products of metabolism, the kidneys secrete many potentially harmful drugs or toxins directly through the tubular cells into the tubules and rap- idly clear these substances from the blood. In the case of certain drugs, such as penicillin and salicylates, the rapid clearance by the kidneys creates a problem in maintaining a therapeutically effective drug concentration.
Another compound that is rapidly secreted by the proximal tubule is para-aminohippuric acid (PAH). PAH is secreted so rapidly that the average person can clear about 90 percent of the PAH from the plasma flowing through the kidneys and excrete it in the urine. For this reason, the rate of PAH clearance can be used to estimate the renal plasma flow, as discussed later.
Solute and Water Transport in the Loop of Henle
The loop of Henle consists of three functionally distinct segments: the thin descending segment, the thin ascend ing segment, and the thick ascending segment. The thin descending and thin ascending segments, as their names imply, have thin epithelial membranes with no brush bor- ders, few mitochondria, and minimal levels of metabolic activity (Figure 27-8).
The descending part of the thin segment is highly per- meable to water and moderately permeable to most sol- utes, including urea and sodium. The function of this nephron segment is mainly to allow simple diffusion of substances through its walls. About 20 percent of the fil- tered water is reabsorbed in the loop of Henle, and almost
Thick ascending loop of Henle
25%
Hypo- osmotic
H+
Na+, Cl–, K+, Ca++, HCO3
–, Mg++
Thin descending loop of Henle
H2O
Figure 27-8 Cellular ultrastructure and transport characteristics of the thin descending loop of Henle (top) and the thick ascend- ing segment of the loop of Henle (bottom). The descending part of the thin segment of the loop of Henle is highly permeable to water and moderately permeable to most solutes but has few mitochon- dria and little or no active reabsorption. The thick ascending limb of the loop of Henle reabsorbs about 25 percent of the filtered loads of sodium, chloride, and potassium, as well as large amounts of calcium, bicarbonate, and magnesium. This segment also secretes hydrogen ions into the tubular lumen.
% Total proximal tubule length
Tu bu
la r
fl u
id /p
la sm
a co
n ce
n tr
at io
n
200
Glucose
Amino acids
Osmolarity
Urea
Creatinine
HCO3 −
Cl−
Na+
40 60 80 100
5.0
2.0
1.0
0.5
0.05
0.2
0.1
0.01
Figure 27-7 Changes in concentrations of different substances in tubular fluid along the proximal convoluted tubule relative to the concentrations of these substances in the plasma and in the glo- merular filtrate. A value of 1.0 indicates that the concentration of the substance in the tubular fluid is the same as the concentra- tion in the plasma. Values below 1.0 indicate that the substance is reabsorbed more avidly than water, whereas values above 1.0 indicate that the substance is reabsorbed to a lesser extent than water or is secreted into the tubules.
Chapter 27 Urine Formation by the Kidneys: II. Tubular Reabsorption and Secretion
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all of this occurs in the thin descending limb. The ascend- ing limb, including both the thin and the thick portions, is virtually impermeable to water, a characteristic that is important for concentrating the urine.
The thick segment of the loop of Henle, which begins about halfway up the ascending limb, has thick epithe- lial cells that have high metabolic activity and are capable of active reabsorption of sodium, chloride, and potas- sium (see Figure 27-8). About 25 percent of the filtered loads of sodium, chloride, and potassium are reabsorbed in the loop of Henle, mostly in the thick ascending limb. Considerable amounts of other ions, such as calcium, bicarbonate, and magnesium, are also reabsorbed in the thick ascending loop of Henle. The thin segment of the ascending limb has a much lower reabsorptive capacity than the thick segment, and the thin descending limb does not reabsorb significant amounts of any of these solutes.
An important component of solute reabsorption in the thick ascending limb is the sodium-potassium ATPase pump in the epithelial cell basolateral membranes. As in the proximal tubule, the reabsorption of other solutes in the thick segment of the ascending loop of Henle is closely linked to the reabsorptive capability of the sodium-potas- sium ATPase pump, which maintains a low intracellular sodium concentration. The low intracellular sodium con- centration in turn provides a favorable gradient for move- ment of sodium from the tubular fluid into the cell. In the thick ascending loop, movement of sodium across the luminal membrane is mediated primarily by a 1sodium, 2chloride, 1potassium cotransporter (Figure 27-9). This co-transport protein carrier in the luminal membrane uses the potential energy released by downhill diffusion of sodium into the cell to drive the reabsorption of potas- sium into the cell against a concentration gradient.
The thick ascending limb of the loop of Henle is the site of action of the powerful “loop” diuretics furosemide, ethacrynic acid, and bumetanide, all of which inhibit the action of the sodium, 2-chloride, potassium co- transporter. These diuretics are discussed in Chapter 31.
The thick ascending limb also has a sodium-hydrogen counter-transport mechanism in its luminal cell mem- brane that mediates sodium reabsorption and hydrogen secretion in this segment (see Figure 27-9).
There is also significant paracellular reabsorption of cations, such as Mg++, Ca++, Na+, and K+, in the thick ascending limb owing to the slight positive charge of the tubular lumen relative to the interstitial fluid. Although the 1-sodium, 2-chloride, 1-potassium co-transporter moves equal amounts of cations and anions into the cell, there is a slight backleak of potassium ions into the lumen, creating a positive charge of about +8 millivolts in the tubular lumen. This positive charge forces cations such as Mg++ and Ca++ to diffuse from the tubular lumen through the paracellular space and into the interstitial fluid.
The thick segment of the ascending loop of Henle is vir tually impermeable to water. Therefore, most of the water delivered to this segment remains in the tubule despite reabsorption of large amounts of solute. The tubular fluid
in the ascending limb becomes very dilute as it flows toward the distal tubule, a feature that is important in allowing the kidneys to dilute or concentrate the urine under different conditions, as we discuss much more fully in Chapter 28.
Distal Tubule
The thick segment of the ascending limb of the loop of Henle empties into the distal tubule. The first portion of the distal tubule forms the macula densa, a group of closely packed epithelial cells that is part of the juxtaglo merular complex and provides feedback control of GFR and blood flow in this same nephron.
The next part of the distal tubule is highly convoluted and has many of the same reabsorptive characteristics of the thick segment of the ascending limb of the loop of Henle. That is, it avidly reabsorbs most of the ions, including sodium, potassium, and chloride, but is virtu- ally impermeable to water and urea. For this reason, it is referred to as the diluting segment because it also dilutes the tubular fluid.
Approximately 5 percent of the filtered load of sodium chloride is reabsorbed in the early distal tubule. The sodiumchloride cotransporter moves sodium chloride from the tubular lumen into the cell, and the
--
Paracellular
Renal interstitial
fluid
Tubular lumen
(+8 mV) Tubular
cells
diffusion Mg++, Ca++Mg++, Ca++ Na+, K+Na+, K+
ATP Na+Na+
Cl−Cl− K+K+
K+K+
Na+Na+
H+H+
Loop diuretics • Furosemide • Ethacrynic acid • Bumetanide
Na+Na+
2Cl–2Cl–
K+K+
Figure 27-9 Mechanisms of sodium, chloride, and potassium trans- port in the thick ascending loop of Henle. The sodium- potassium ATPase pump in the basolateral cell membrane maintains a low intracellular sodium concentration and a negative electrical poten- tial in the cell. The 1-sodium, 2-chloride, 1-potassium co-trans- porter in the luminal membrane transports these three ions from the tubular lumen into the cells, using the potential energy released by diffusion of sodium down an electrochemical gradient into the cells. Sodium is also transported into the tubular cell by sodium-hydrogen counter-transport. The positive charge (+8 mV) of the tubular lumen relative to the interstitial fluid forces cations such as Mg++ and Ca++ to diffuse from the lumen to the interstitial fluid via the paracellular pathway.
Unit V The Body Fluids and Kidneys
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sodium-potassium ATPase pump transports sodium out of the cell across the basolateral membrane (Figure 27-10). Chloride diffuses out of the cell into the renal interstitial fluid through chloride channels in the basolat- eral membrane.
The thiazide diuretics, which are widely used to treat disorders such as hypertension and heart failure, inhibit the sodium-chloride co-transporter.
Late Distal Tubule and Cortical Collecting Tubule
The second half of the distal tubule and the subsequent cortical collecting tubule have similar functional char- acteristics. Anatomically, they are composed of two dis- tinct cell types, the principal cells and intercalated cells (Figure 27-11). The principal cells reabsorb sodium and water from the lumen and secrete potassium ions into the lumen. The intercalated cells reabsorb potassium ions and secrete hydrogen ions into the tubular lumen.
Principal Cells Reabsorb Sodium and Secrete Potassium. Sodium reabsorption and potassium secre tion by the principal cells depend on the activity of a sodium-potassium ATPase pump in each cell’s basolateral membrane (Figure 27-12). This pump maintains a low sodium concentration inside the cell and, therefore, favors sodium diffusion into the cell through special channels. The secretion of potassium by these cells from the blood into the tubular lumen involves two steps: (1) Potassium enters the cell because of the sodium-potassium ATPase pump, which maintains a high intracellular potassium concentration, and then (2) once in the cell, potassium diffuses down its concentration gradient across the lumi- nal membrane into the tubular fluid.
The principal cells are the primary sites of action of the potassiumsparing diuretics, including spironolactone, eplerenone, amiloride, and triamterene. Spironolactone
and eplerenone are mineralocorticoid receptor antago- nists that compete with aldosterone for receptor sites in the principal cells and therefore inhibit the stimula- tory effects of aldosterone on sodium reabsorption and potassium secretion. Amiloride and triamterene are sodium channel blockers that directly inhibit the entry of sodium into the sodium channels of the luminal mem- branes and therefore reduce the amount of sodium that can be transported across the basolateral membranes by the sodium-potassium ATPase pump. This, in turn, decreases transport of potassium into the cells and ulti- mately reduces potassium secretion into the tubular fluid. For this reason the sodium channel blockers, as well as the aldosterone antagonists, decrease urinary excretion of potassium and act as potassium-sparing diuretics.
Intercalated Cells Secrete Hydrogen and Reabsorb Bicarbonate and Potassium Ions. Hydrogen ion secre- tion by the intercalated cells is mediated by a hydrogen- ATPase transporter. Hydrogen is generated in this cell by the action of carbonic anhydrase on water and carbon
−−
Renal interstitial
fluid
Tubular lumen
(−10mV) Tubular
cells
ATP Na+Na+
Cl–Cl–
K+K+
Thiazide diuretics:Thiazide diuretics:
Na+Na+
Cl–Cl–
Figure 27-10 Mechanism of sodium chloride transport in the early distal tubule. Sodium and chloride are transported from the tubular lumen into the cell by a co-transporter that is inhibited by thiazide diuretics. Sodium is pumped out of the cell by sodium- potassium ATPase and chloride diffuses into the interstitial fluid via chloride channels.
Intercalated cells
Early distal tubule
Late distal tubule and collecting tubule
Principal cells
Na+, Cl–, Ca++, Mg++
Na+, Cl–
(+ADH) H2O
HCO3 –
H+
K+
K+
Figure 27-11 Cellular ultrastructure and transport characteristics of the early distal tubule and the late distal tubule and collecting tubule. The early distal tubule has many of the same characteris- tics as the thick ascending loop of Henle and reabsorbs sodium, chloride, calcium, and magnesium but is virtually impermeable to water and urea. The late distal tubules and cortical collecting tubules are composed of two distinct cell types, the principal cells and the intercalated cells. The principal cells reabsorb sodium from the lumen and secrete potassium ions into the lumen. The inter- calated cells reabsorb potassium and bicarbonate ions from the lumen and secrete hydrogen ions into the lumen. The reabsorption of water from this tubular segment is controlled by the concentra- tion of antidiuretic hormone.
Chapter 27 Urine Formation by the Kidneys: II. Tubular Reabsorption and Secretion
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dioxide to form carbonic acid, which then dissociates into hydrogen ions and bicarbonate ions. The hydrogen ions are then secreted into the tubular lumen, and for each hydrogen ion secreted, a bicarbonate ion becomes avail- able for reabsorption across the basolateral membrane. A more detailed discussion of this mechanism is presented in Chapter 30. The intercalated cells can also reabsorb potassium ions.
The functional characteristics of the late distal tubule and cortical collecting tubule can be summarized as follows:
1. The tubular membranes of both segments are almost completely impermeable to urea, similar to the dilut- ing segment of the early distal tubule; thus, almost all the urea that enters these segments passes on through and into the collecting duct to be excreted in the urine, although some reabsorption of urea occurs in the med- ullary collecting ducts.
2. Both the late distal tubule and the cortical collecting tubule segments reabsorb sodium ions, and the rate of reabsorption is controlled by hormones, especially aldosterone. At the same time, these segments secrete potassium ions from the peritubular capillary blood into the tubular lumen, a process that is also controlled by aldosterone and by other factors such as the con- centration of potassium ions in the body fluids.
3. The intercalated cells of these nephron segments avidly secrete hydrogen ions by an active hydrogenATPase mechanism. This process is different from the second- ary active secretion of hydrogen ions by the proximal tubule because it is capable of secreting hydrogen ions
against a large concentration gradient, as much as 1000 to 1. This is in contrast to the relatively small gradient (4- to 10-fold) for hydrogen ions that can be achieved by secondary active secretion in the proximal tubule. Thus, the intercalated cells play a key role in acid-base regulation of the body fluids.
4. The permeability of the late distal tubule and cortical collecting duct to water is controlled by the concen- tration of ADH, which is also called vasopressin. With high levels of ADH, these tubular segments are perme- able to water, but in the absence of ADH, they are vir- tually impermeable to water. This special characteristic provides an important mechanism for controlling the degree of dilution or concentration of the urine.
Medullary Collecting Duct
Although the medullary collecting ducts reabsorb less than 10 percent of the filtered water and sodium, they are the final site for processing the urine and, therefore, play an extremely important role in determining the final urine output of water and solutes.
The epithelial cells of the collecting ducts are nearly cuboidal in shape with smooth surfaces and relatively few mitochondria (Figure 27-13). Special characteristics of this tubular segment are as follows:
1. The permeability of the medullary collecting duct to water is controlled by the level of ADH. With high lev- els of ADH, water is avidly reabsorbed into the med- ullary interstitium, thereby reducing the urine volume and concentrating most of the solutes in the urine.
2. Unlike the cortical collecting tubule, the medullary col- lecting duct is permeable to urea and there are special urea transporters that facilitate urea diffusion across the luminal and basolateral membranes. Therefore, some of the tubular urea is reabsorbed into the medul- lary interstitium, helping to raise the osmolality in this
----
Renal interstitial
fluid
Tubular lumen
(−50 mV) Tubular
cells
ATP Na+Na+
K+K+
Na+Na+
Cl−Cl−
K+K+
Na+ channel blockers • Amiloride • Triamterene
Aldosterone antagonists • Spironolactone • Eplerenone
Figure 27-12 Mechanism of sodium chloride reabsorption and potassium secretion in the late distal tubules and cortical collect- ing tubules. Sodium enters the cell through special channels and is transported out of the cell by the sodium-potassium ATPase pump. Aldosterone antagonists compete with aldosterone for binding sites in the cell and therefore inhibit the effects of aldos- terone to stimulate sodium reabsorption and potassium secretion. Sodium channel blockers directly inhibit the entry of sodium into the sodium channels.
Medullary collecting duct
Na+ , Cl–
Urea
HCO 3 –
(+ADH) H2O
H+
Figure 27-13 Cellular ultrastructure and transport characteristics of the medullary collecting duct. The medullary collecting ducts actively reabsorb sodium and secrete hydrogen ions and are per- meable to urea, which is reabsorbed in these tubular segments. The reabsorption of water in medullary collecting ducts is con- trolled by the concentration of antidiuretic hormone.
Unit V The Body Fluids and Kidneys
334
region of the kidneys and contributing to the kidneys’ overall ability to form concentrated urine. This is dis- cussed in Chapter 28.
3. The medullary collecting duct is capable of secreting hydrogen ions against a large concentration gradient, as also occurs in the cortical collecting tubule. Thus, the medullary collecting duct also plays a key role in regulating acid-base balance.
Summary of Concentrations of Different Solutes in the Different Tubular Segments
Whether a solute will become concentrated in the tubular fluid is determined by the relative degree of reabsorption of that solute versus the reabsorption of water. If a greater percentage of water is reabsorbed, the substance becomes more concentrated. If a greater percentage of the solute is reabsorbed, the substance becomes more diluted.
Figure 27-14 shows the degree of concentration of several substances in the different tubular segments. All the values in this figure represent the tubular fluid concentration divided by the plasma concentration of a substance. If plasma con- centration of the substance is assumed to be constant, any change in the ratio of tubular fluid/plasma concentration rate reflects changes in tubular fluid concentration.
As the filtrate moves along the tubular system, the con- centration rises to progressively greater than 1.0 if more
water is reabsorbed than solute, or if there has been a net secretion of the solute into the tubular fluid. If the concen- tration ratio becomes progressively less than 1.0, this means that relatively more solute has been reabsorbed than water.
The substances represented at the top of Figure 27-14, such as creatinine, become highly concentrated in the urine. In general, these substances are not needed by the body, and the kidneys have become adapted to reabsorb them only slightly or not at all, or even to secrete them into the tubules, thereby excreting especially great quan- tities into the urine. Conversely, the substances repre- sented toward the bottom of the figure, such as glucose and amino acids, are all strongly reabsorbed; these are all substances that the body needs to conserve, and almost none of them are lost in the urine.
Tubular Fluid/Plasma Inulin Concentration Ratio Can Be Used to Measure Water Reabsorption by the Renal Tubules. Inulin, a polysaccharide used to measure GFR, is not reabsorbed or secreted by the renal tubules. Changes in inulin concentration at different points along the renal tubule, therefore, reflect changes in the amount of water present in the tubular fluid.
For example, the tubular fluid/plasma concentration ratio for inulin rises to about 3.0 at the end of the prox- imal tubules, indicating that inulin concentration in the tubular fluid is three times greater than in the plasma and in the glomerular filtrate. Because inulin is not secreted or reabsorbed from the tubules, a tubular fluid/plasma concentration ratio of 3.0 means that only one third of the water that was filtered remains in the renal tubule and that two thirds of the filtered water has been reabsorbed as the fluid passes through the proximal tubule. At the end of the collecting ducts, the tubular fluid/plasma inulin concentration ratio rises to about 125 (see Figure 27-14), indicating that only 1/125 of the filtered water remains in the tubule and that more than 99% has been reabsorbed.
Regulation of Tubular Reabsorption
Because it is essential to maintain a precise balance between tubular reabsorption and glomerular filtration, there are multiple nervous, hormonal, and local control mechanisms that regulate tubular reabsorption, just as there are for control of glomerular filtration. An impor- tant feature of tubular reabsorption is that reabsorption of some solutes can be regulated independently of others, especially through hormonal control mechanisms.
Glomerulotubular Balance—The Ability of the Tubules to Increase Reabsorption Rate in Response to Increased Tubular Load
One of the most basic mechanisms for controlling tubu- lar reabsorption is the intrinsic ability of the tubules to increase their reabsorption rate in response to increased tubular load (increased tubular inflow). This phenomenon
Tu bu
la r
fl u
id /p
la sm
a co
n ce
n tr
at io
n
Proximal tubule
Loop of Henle
Distal tubule
Collecting tubule
PAHPAH
ClCl
ClClKK
NaNa
to 5
85 to
5 85
to 1
40 to
1 40
to 1
25 to
1 25
HCO3HCO3
K and Na
K and Na
Creatinine
Creatinine
G lucose
G lucose
P rotein
P rotein
A m
ino acids
A m
ino acids
Inulin Inulin
UreaUrea
100.0
50.0
20.0
10.0
5.0
2.0
1.0
0.50
0.20
0.10
0.05
0.02
Figure 27-14 Changes in average concentrations of different sub- stances at different points in the tubular system relative to the concentration of that substance in the plasma and in the glomeru- lar filtrate. A value of 1.0 indicates that the concentration of the substance in the tubular fluid is the same as the concentration of that substance in the plasma. Values below 1.0 indicate that the substance is reabsorbed more avidly than water, whereas values above 1.0 indicate that the substance is reabsorbed to a lesser extent than water or is secreted into the tubules.
Chapter 27 Urine Formation by the Kidneys: II. Tubular Reabsorption and Secretion
335
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is referred to as glomerulotubular balance. For exam- ple, if GFR is increased from 125 ml/min to 150 ml/min, the absolute rate of proximal tubular reabsorption also increases from about 81 ml/min (65 percent of GFR) to about 97.5 ml/min (65 percent of GFR). Thus, glomeru- lotubular balance refers to the fact that the total rate of reabsorption increases as the filtered load increases, even though the percentage of GFR reabsorbed in the proximal tubule remains relatively constant at about 65 percent.
Some degree of glomerulotubular balance also occurs in other tubular segments, especially the loop of Henle. The precise mechanisms responsible for this are not fully understood but may be due partly to changes in physical forces in the tubule and surrounding renal interstitium, as discussed later. It is clear that the mechanisms for glo- merulotubular balance can occur independently of hor- mones and can be demonstrated in completely isolated kidneys or even in completely isolated proximal tubular segments.
Glomerulotubular balance helps to prevent overload- ing of the distal tubular segments when GFR increases. Glomerulotubular balance acts as a second line of defense to buffer the effects of spontaneous changes in GFR on urine output. (The first line of defense, discussed earlier, includes the renal autoregulatory mechanisms, especially tubuloglomerular feedback, which help prevent changes in GFR.) Working together, the autoregulatory and glo- merulotubular balance mechanisms prevent large changes in fluid flow in the distal tubules when the arterial pres- sure changes or when there are other disturbances that would otherwise upset sodium and volume homeostasis.
Peritubular Capillary and Renal Interstitial Fluid Physical Forces
Hydrostatic and colloid osmotic forces govern the rate of reabsorption across the peritubular capillaries, just as they control filtration in the glomerular capillaries. Changes in peritubular capillary reabsorption can in turn influence the hydrostatic and colloid osmotic pressures of the renal interstitium and, ultimately, reabsorption of water and solutes from the renal tubules.
Normal Values for Physical Forces and Reabsorption Rate. As the glomerular filtrate passes through the renal tubules, more than 99 percent of the water and most of the solutes are normally reabsorbed. Fluid and electro- lytes are reabsorbed from the tubules into the renal inter- stitium and from there into the peritubular capillaries. The normal rate of peritubular capillary reabsorption is about 124 ml/min.
Reabsorption across the peritubular capillaries can be calculated as
Reabsorption = K f × Net reabsorptive force
The net reabsorptive force represents the sum of the hydrostatic and colloid osmotic forces that either favor or oppose reabsorption across the peritubular capillaries. These forces include (1) hydrostatic pressure inside the
peritubular capillaries (peritubular hydrostatic pressure [Pc]), which opposes reabsorption; (2) hydrostatic pres- sure in the renal interstitium (Pif) outside the capillaries, which favors reabsorption; (3) colloid osmotic pressure of the peritubular capillary plasma proteins (πc), which favors reabsorption; and (4) colloid osmotic pressure of the proteins in the renal interstitium (πif), which opposes reabsorption.
Figure 27-15 shows the approximate normal forces that favor and oppose peritubular reabsorption. Because the normal peritubular capillary pressure averages about 13 mm Hg and renal interstitial fluid hydrostatic pres- sure averages 6 mm Hg, there is a positive hydrostatic pressure gradient from the peritubular capillary to the interstitial fluid of about 7 mm Hg, which opposes fluid reabsorption. This is more than counterbalanced by the colloid osmotic pressures that favor reabsorption. The plasma colloid osmotic pressure, which favors reabsorp- tion, is about 32 mm Hg, and the colloid osmotic pressure of the interstitium, which opposes reabsorption, is 15 mm Hg, causing a net colloid osmotic force of about 17 mm Hg, favoring reabsorption. Therefore, subtracting the net hydrostatic forces that oppose reabsorption (7 mm Hg) from the net colloid osmotic forces that favor reabsorp- tion (17 mm Hg) gives a net reabsorptive force of about 10 mm Hg. This is a high value, similar to that found in the glomerular capillaries, but in the opposite direction.
The other factor that contributes to the high rate of fluid reabsorption in the peritubular capillaries is a large filtra- tion coefficient (Kf) because of the high hydraulic conduc- tivity and large surface area of the capillaries. Because the reabsorption rate is normally about 124 ml/min and net reabsorption pressure is 10 mm Hg, Kf normally is about 12.4 ml/min/mm Hg.
ATP Na+Na+
Tubular cells
Tubular cells
Tubular lumen Tubular lumen
Interstitial fluid
Interstitial fluid
Peritubular capillary
Peritubular capillary
H2O
Na+Na+ 10 mm Hg
Net reabsorption pressure
10 mm Hg Net reabsorption
pressure
Pc 13 mm Hg
Pc 13 mm Hg πif
15 mm Hg πif
15 mm Hg
Pif 6 mm Hg
Pif 6 mm Hg
H2O Bulk flow Bulk flow
πc 32 mm Hg
πc 32 mm Hg
Figure 27-15 Summary of the hydrostatic and colloid osmotic forces that determine fluid reabsorption by the peritubular cap- illaries. The numerical values shown are estimates of the normal values for humans. The net reabsorptive pressure is normally about 10 mm Hg, causing fluid and solutes to be reabsorbed into the peritubular capillaries as they are transported across the renal tubular cells. ATP, adenosine triphosphate; P
c , peritubular capillary
hydrostatic pressure; P if , interstitial fluid hydrostatic pressure; π
c ,
peritubular capillary colloid osmotic pressure; π if , interstitial fluid
colloid osmotic pressure.
Unit V The Body Fluids and Kidneys
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Regulation of Peritubular Capillary Physical Forces. The two determinants of peritubular capillary reabsorption that are directly influenced by renal hemo- dynamic changes are the hydrostatic and colloid osmotic pressures of the peritubular capillaries. The peritubular capillary hydrostatic pressure is influenced by the arte rial pressure and resistances of the afferent and efferent arterioles. (1) Increases in arterial pressure tend to raise peritubular capillary hydrostatic pressure and decrease reabsorption rate. This effect is buffered to some extent by autoregulatory mechanisms that maintain relatively con- stant renal blood flow, as well as relatively constant hydro- static pressures in the renal blood vessels. (2) Increase in resistance of either the afferent or the efferent arterioles reduces peritubular capillary hydrostatic pressure and tends to increase reabsorption rate. Although constric- tion of the efferent arterioles increases glomerular capil- lary hydrostatic pressure, it lowers peritubular capillary hydrostatic pressure.
The second major determinant of peritubular capillary reabsorption is the colloid osmotic pressure of the plasma in these capillaries; raising the colloid osmotic pressure increases peritubular capillary reabsorption. The col loid osmotic pressure of peritubular capillaries is deter mined by (1) the systemic plasma colloid osmotic pressure; increasing the plasma protein concentration of systemic blood tends to raise peritubular capillary colloid osmotic pressure, thereby increasing reabsorption; and (2) the filtration fraction; the higher the filtration fraction, the greater the fraction of plasma filtered through the glo- merulus and, consequently, the more concentrated the protein becomes in the plasma that remains behind. Thus, increasing the filtration fraction also tends to increase the peritubular capillary reabsorption rate. Because filtra- tion fraction is defined as the ratio of GFR/renal plasma flow, increased filtration fraction can occur as a result of increased GFR or decreased renal plasma flow. Some renal vasoconstrictors, such as angiotensin II, increase peritu- bular capillary reabsorption by decreasing renal plasma flow and increasing filtration fraction, as discussed later.
Changes in the peritubular capillary Kf can also influ- ence the reabsorption rate because Kf is a measure of the permeability and surface area of the capillaries. Increases in Kf raise reabsorption, whereas decreases in Kf lower peritubular capillary reabsorption. Kf remains relatively constant in most physiologic conditions. Table 27-2 sum- marizes the factors that can influence the peritubular cap- illary reabsorption rate.
Renal Interstitial Hydrostatic and Colloid Osmotic Pressures. Ultimately, changes in peritubular capil- lary physical forces influence tubular reabsorption by changing the physical forces in the renal interstitium surrounding the tubules. For example, a decrease in the reabsorptive force across the peritubular capillary mem- branes, caused by either increased peritubular capillary hydrostatic pressure or decreased peritubular capillary colloid osmotic pressure, reduces the uptake of fluid and
solutes from the interstitium into the peritubular capillar- ies. This in turn raises renal interstitial fluid hydrostatic pressure and decreases interstitial fluid colloid osmotic pressure because of dilution of the proteins in the renal interstitium. These changes then decrease the net reab- sorption of fluid from the renal tubules into the intersti- tium, especially in the proximal tubules.
The mechanisms by which changes in interstitial fluid hydrostatic and colloid osmotic pressures influence tubu- lar reabsorption can be understood by examining the pathways through which solute and water are reabsorbed (Figure 27-16). Once the solutes enter the intercellu- lar channels or renal interstitium by active transport or passive diffusion, water is drawn from the tubular lumen into the interstitium by osmosis. And once the water and solutes are in the interstitial spaces, they can either be swept up into the peritubular capillaries or diffuse back through the epithelial junctions into the tubular lumen. The so-called tight junctions between the epithelial cells of the proximal tubule are actually leaky, so considerable amounts of sodium can diffuse in both directions through these junctions. With the normal high rate of peritubu- lar capillary reabsorption, the net movement of water and solutes is into the peritubular capillaries with little back- leak into the lumen of the tubule. However, when peritu- bular capillary reabsorption is reduced, there is increased interstitial fluid hydrostatic pressure and a tendency for greater amounts of solute and water to backleak into the tubular lumen, thereby reducing the rate of net reabsorp- tion (refer again to Figure 27-16).
The opposite is true when there is increased peritu- bular capillary reabsorption above the normal level. An initial increase in reabsorption by the peritubular capillar- ies tends to reduce interstitial fluid hydrostatic pressure and raise interstitial fluid colloid osmotic pressure. Both of these forces favor movement of fluid and solutes out of the tubular lumen and into the interstitium; therefore, backleak of water and solutes into the tubular lumen is reduced, and net tubular reabsorption increases.
Thus, through changes in the hydrostatic and colloid osmotic pressures of the renal interstitium, the uptake of water and solutes by the peritubular capillaries is closely
↑ P c → ↓ Reabsorption
• ↓ R A → ↑ P
c
• ↓ R E → ↑ P
c
• ↑ Arterial Pressure → ↑ P c
↑ π c → ↑ Reabsorption
• ↑ π A → ↑ π
c
• ↑ FF → ↑ π c
↑ K f → ↑ Reabsorption
Table 27-2 Factors That Can Influence Peritubular Capillary Reabsorption
P c , peritubular capillary hydrostatic pressure; R
A and R
E , afferent and efferent
arteriolar resistances, respectively; π c , peritubular capillary colloid osmotic
pressure; π A , arterial plasma colloid osmotic pressure; FF, filtration fraction;
K f , peritubular capillary filtration coefficient.
Chapter 27 Urine Formation by the Kidneys: II. Tubular Reabsorption and Secretion
337
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matched to the net reabsorption of water and solutes from the tubular lumen into the interstitium. Therefore, in general, forces that increase peritubular capillary reab sorption also increase reabsorption from the renal tubules. Conversely, hemodynamic changes that inhibit peritubu lar capillary reabsorption also inhibit tubular reabsorp tion of water and solutes.
Effect of Arterial Pressure on Urine Output— Pressure Natriuresis and Pressure Diuresis
Even small increases in arterial pressure can cause marked increases in urinary excretion of sodium and water, phe- nomena that are referred to as pressure natriuresis and pressure diuresis. Because of the autoregulatory mecha- nisms described in Chapter 26, increasing the arterial pressure between the limits of 75 and 160 mm Hg usually has only a small effect on renal blood flow and GFR. The slight increase in GFR that does occur contributes in part to the effect of increased arterial pressure on urine output. When GFR autoregulation is impaired, as often occurs in
kidney disease, increases in arterial pressure cause much larger increases in GFR.
A second effect of increased renal arterial pressure that raises urine output is that it decreases the percent- age of the filtered load of sodium and water that is reab- sorbed by the tubules. The mechanisms responsible for this effect include a slight increase in peritubular capil- lary hydrostatic pressure, especially in the vasa recta of the renal medulla, and a subsequent increase in the renal interstitial fluid hydrostatic pressure. As discussed ear- lier, an increase in the renal interstitial fluid hydrostatic pressure enhances backleak of sodium into the tubular lumen, thereby reducing the net reabsorption of sodium and water and further increasing the rate of urine output when renal arterial pressure rises.
A third factor that contributes to the pressure natriure- sis and pressure diuresis mechanisms is reduced angio- tensin II formation. Angiotensin II itself increases sodium reabsorption by the tubules; it also stimulates aldosterone secretion, which further increases sodium reabsorption. Therefore, decreased angiotensin II formation contrib- utes to the decreased tubular sodium reabsorption that occurs when arterial pressure is increased.
Hormonal Control of Tubular Reabsorption
Precise regulation of body fluid volumes and solute con- centrations requires the kidneys to excrete different solutes and water at variable rates, sometimes independently of one another. For example, when potassium intake is increased, the kidneys must excrete more potassium while maintaining normal excretion of sodium and other electrolytes. Likewise, when sodium intake is changed, the kidneys must appropri- ately adjust urinary sodium excretion without major changes in excretion of other electrolytes. Several hormones in the body provide this specificity of tubular reabsorption for dif- ferent electrolytes and water. Table 27-3 summarizes some of the most important hormones for regulating tubular reab- sorption, their principal sites of action on the renal tubule, and their effects on solute and water excretion. Some of these hormones are discussed in more detail in Chapters 28 and 29, but we briefly review their renal tubular actions in the next few paragraphs.
Aldosterone Increases Sodium Reabsorption and Stimulates Potassium Secretion. Aldosterone, secreted by the zona glomerulosa cells of the adrenal cortex, is an important regulator of sodium reabsorption and potas- sium secretion by the renal tubules. A major renal tubu lar site of aldosterone action is on the principal cells of the cortical collecting tubule. The mechanism by which aldo- sterone increases sodium reabsorption while at the same time increasing potassium secretion is by stimulating the sodium-potassium ATPase pump on the basolateral side of the cortical collecting tubule membrane. Aldosterone also increases the sodium permeability of the luminal side of the membrane. The cellular mechanisms of aldoster- one action are discussed in Chapter 77.
Decreased reabsorptionDecreased reabsorption
NormalNormal
ATP
ATP
Tubular cells
Tubular cells
LumenLumen
BackleakBackleak
Interstitial fluid
Interstitial fluid
Peritubular capillary
Peritubular capillary
Net reabsorption
Net reabsorption
ATP
ATP Increased backleak Increased backleak
Decreased net reabsorption
Decreased net reabsorption
PcPc
πcπc
PcPc
πcπc
Figure 27-16 Proximal tubular and peritubular capillary reabsorp- tion under normal conditions (top) and during decreased peritu- bular capillary reabsorption (bottom) caused by either increasing peritubular capillary hydrostatic pressure (P
c ) or decreasing peritu-
bular capillary colloid osmotic pressure (π c ). Reduced peritubular
capillary reabsorption, in turn, decreases the net reabsorption of solutes and water by increasing the amounts of solutes and water that leak back into the tubular lumen through the tight junctions of the tubular epithelial cells, especially in the proximal tubule.
Unit V The Body Fluids and Kidneys
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The most important stimuli for aldosterone are (1) increased extracellular potassium concentration and (2) increased angiotensin II levels, which typically occur in conditions associated with sodium and volume depletion or low blood pressure. The increased secretion of aldoster- one associated with these conditions causes renal sodium and water retention, helping to increase extracellular fluid volume and restore blood pressure toward normal.
In the absence of aldosterone, as occurs with adrenal destruction or malfunction (Addison’s disease), there is marked loss of sodium from the body and accumulation of potassium. Conversely, excess aldosterone secretion, as occurs in patients with adrenal tumors (Conn’s syndrome), is associated with sodium retention and decreased plasma potassium concentration due, in part, to excessive potas- sium secretion by the kidneys. Although day-to-day reg- ulation of sodium balance can be maintained as long as minimal levels of aldosterone are present, the inability to appropriately adjust aldosterone secretion greatly impairs the regulation of renal potassium excretion and potas- sium concentration of the body fluids. Thus, aldosterone is even more important as a regulator of potassium con- centration than it is for sodium concentration.
Angiotensin II Increases Sodium and Water Reabsorption. Angiotensin II is perhaps the body’s most powerful sodium-retaining hormone. As discussed in Chapter 19, angiotensin II formation increases in circum- stances associated with low blood pressure and/or low extracellular fluid volume, such as during hemorrhage or loss of salt and water from the body fluids by excessive sweating or severe diarrhea. The increased formation of angiotensin II helps to return blood pressure and extra- cellular volume toward normal by increasing sodium and water reabsorption from the renal tubules through three main effects:
1. Angiotensin II stimulates aldosterone secretion, which in turn increases sodium reabsorption.
2. Angiotensin II constricts the efferent arterioles, which has two effects on peritubular capillary dynamics that increase sodium and water reabsorption. First, effer- ent arteriolar constriction reduces peritubular capil- lary hydrostatic pressure, which increases net tubular
reabsorption, especially from the proximal tubules. Second, efferent arteriolar constriction, by reducing renal blood flow, raises filtration fraction in the glo- merulus and increases the concentration of proteins and the colloid osmotic pressure in the peritubular capillaries; this increases the reabsorptive force at the peritubular capillaries and raises tubular reabsorption of sodium and water.
3. Angiotensin II directly stimulates sodium reabsorp tion in the proximal tubules, the loops of Henle, the dis tal tubules, and the collecting tubules. One of the direct effects of angiotensin II is to stimulate the sodium- potassium ATPase pump on the tubular epithelial cell basolateral membrane. A second effect is to stimulate sodium-hydrogen exchange in the luminal membrane, especially in the proximal tubule. A third effect of angio- tensin II is to stimulate sodium-bicarbonate co-transport in the basolateral membrane (Figure 27-17).
Thus, angiotensin II stimulates sodium transport across both the luminal and the basolateral surfaces of the epithelial cell membrane in most renal tubular segments. These multiple actions of angiotensin II cause marked sodium and water retention by the kidneys when angio- tensin II levels are increased and play a critical role in
Hormone Site of Action Effects
Aldosterone Collecting tubule and duct ↑ NaCl, H 2 O reabsorption, ↑ K+ secretion
Angiotensin II Proximal tubule, thick ascending loop of Henle/distal tubule, collecting tubule
↑ NaCl, H 2 O reabsorption, ↑ H+ secretion
Antidiuretic hormone Distal tubule/collecting tubule and duct ↑ H 2 O reabsorption
Atrial natriuretic peptide Distal tubule/collecting tubule and duct ↓ NaCl reabsorption
Parathyroid hormone Proximal tubule, thick ascending loop of Henle/distal tubule ↓ PO 4 ≡ reabsorption, ↑ Ca++ reabsorption
Table 27-3 Hormones That Regulate Tubular Reabsorption
++++
++
Renal interstitial
fluid
Tubular lumenTubular
cells
ATP Na+Na+
K+K+
Na+Na+
HCO3 -HCO3 -
NHE Na+Na+
H+H+
Ang II Ang IIAT1 AT1
Figure 27-17 Direct effects of angiotensin II (Ang II) to increase proximal tubular sodium reabsorption. Ang II stimulates sodium sodium-hydrogen exchange (NHE) on the luminal membrane and the sodium-potassium ATPase transporter as well as sodium-bicarbonate co-transport on the basolateral membrane. These same effects of Ang II likely occur in several other parts of the renal tubule, including the loop of Henle, distal tubule, and collecting tubule.
Chapter 27 Urine Formation by the Kidneys: II. Tubular Reabsorption and Secretion
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permitting the body to adapt to wide variations in sodium intake without large changes in extracellular fluid volume and blood pressure, as discussed in Chapter 29.
At the same time that angiotensin II increases renal tubular sodium reabsorption, its vasoconstrictor effect on efferent arterioles also aids in the maintenance of nor- mal excretion of metabolic waste products such as urea and creatinine that depend mainly on adequate GFR for their excretion. Thus, increased formation of angiotensin II permits the kidneys to retain sodium and water without causing retention of metabolic waste products.
ADH Increases Water Reabsorption. The most impor- tant renal action of ADH is to increase the water permeabil- ity of the distal tubule, collecting tubule, and collecting duct epithelia. This effect helps the body to conserve water in cir- cumstances such as dehydration. In the absence of ADH, the permeability of the distal tubules and collecting ducts to water is low, causing the kidneys to excrete large amounts of dilute urine. Thus, the actions of ADH play a key role in controlling the degree of dilution or concentration of the urine, as discussed further in Chapters 28 and 75.
ADH binds to specific V2 receptors in the late distal tubules, collecting tubules, and collecting ducts, increas- ing the formation of cyclic AMP and activating pro- tein kinases (Figure 27-18). This, in turn, stimulates the movement of an intracellular protein, called aquaporin2 (AQP2), to the luminal side of the cell membranes. The molecules of AQP-2 cluster together and fuse with the cell membrane by exocytosis to form water channels that per- mit rapid diffusion of water through the cells. There are other aquaporins, AQP-3 and AQP-4, in the basolateral side of the cell membrane that provide a path for water to rapidly exit the cells, although these are not believed to be regulated by ADH. Chronic increases in ADH levels also increase the formation of AQP-2 protein in the renal tubular cells by stimulating AQP-2 gene transcription. When the concentration of ADH decreases, the molecules of AQP-2 are shuttled back to the cell cytoplasm, thereby removing the water channels from the luminal membrane and reducing water permeability. These cellular actions of ADH are discussed further in Chapter 75.
Atrial Natriuretic Peptide Decreases Sodium and Water Reabsorption. Specific cells of the cardiac atria, when distended because of plasma volume expansion, secrete a peptide called atrial natriuretic peptide (ANP). Increased levels of this peptide in turn directly inhibit the reabsorption of sodium and water by the renal tubules, espe- cially in the collecting ducts. ANP also inhibits renin secre- tion and therefore angiotensin II formation, which in turn reduces renal tubular reabsorption. This decreased sodium and water reabsorption increases urinary excretion, which helps to return blood volume back toward normal.
ANP levels are greatly elevated in congestive heart fail- ure when the cardiac atria are stretched because of impaired pumping of the ventricles. The increased ANP helps to attenuate sodium and water retention in heart failure.
Parathyroid Hormone Increases Calcium Reabsorption. Parathyroid hormone is one of the most important calcium-regulating hormones in the body. Its principal action in the kidneys is to increase tubular reab- sorption of calcium, especially in the distal tubules and perhaps also in the loops of Henle. Parathyroid hormone also has other actions, including inhibition of phosphate reabsorption by the proximal tubule and stimulation of magnesium reabsorption by the loop of Henle, as dis- cussed in Chapter 29.
Sympathetic Nervous System Activation Increases Sodium Reabsorption
Activation of the sympathetic nervous system, if severe, can decrease sodium and water excretion by constricting the renal arterioles, thereby reducing GFR. Even low lev- els of sympathetic activation, however, decrease sodium and water excretion by increasing sodium reabsorption in the proximal tubule, the thick ascending limb of the loop of Henle, and perhaps in more distal parts of the renal tubule. This occurs by activation of α-adrenergic recep- tors on the renal tubular epithelial cells.
Sympathetic nervous system stimulation also increases renin release and angiotensin II formation, which adds to the overall effect to increase tubular reabsorption and decrease renal excretion of sodium.
Renal interstitial
fluid
Tubular lumenTubular
cells
H2OH2O
H2OH2O
ATPATP
cAMPcAMP
AC
Protein Kinase
A
Protein Phosphorylation
Aquaporin-2 (AQP-2)
AQP-2
AQP-3 AQP-4
AVP
AVP V2
V2
Gs
Gs
Figure 27-18 Mechanism of action of arginine vasopressin (AVP) on the epithelial cells of the late distal tubules, collecting tubules and collecting ducts. AVP binds to its V
2 receptors, which are
coupled with stimulatory G proteins (G s ) that activate adenylate
cyclase (AC) and stimulate formation of cyclic adenosine mono- phosphate (cAMP). This, in turn, activates protein kinase A and phosphorylation of intracellular proteins, causing movement of aquaporin-2 (AQP-2) to the luminal side of the cell membrane. The molecules of AQP-2 fuse together to form water channels. On the basolateral side of the cell membrane are other aquaporins, AQP- 3 and AQP-4, that permit water to flow out of the cell, although these aquaporins do not appear to be regulated by AVP.
Unit V The Body Fluids and Kidneys
340
Use of Clearance Methods to Quantify Kidney Function
The rates at which different substances are “cleared” from the plasma provide a useful way of quantitating the effec- tiveness with which the kidneys excrete various sub- stances (Table 27-4). By definition, the renal clearance of a substance is the volume of plasma that is completely cleared of the substance by the kidneys per unit time.
This concept is somewhat abstract because there is no single volume of plasma that is completely cleared of a substance. However, renal clearance provides a useful way of quantifying the excretory function of the kidneys and, as discussed later, can be used to quantify the rate at which blood flows through the kidneys, as well as the basic functions of the kidneys: glomerular filtration, tubu- lar reabsorption, and tubular secretion.
To illustrate the clearance principle, consider the following example: If the plasma passing through the kidneys contains 1 milligram of a substance in each mil- liliter and if 1 milligram of this substance is also excreted into the urine each minute, then 1 ml/min of the plasma is “cleared” of the substance. Thus, clearance refers to the volume of plasma that would be necessary to supply the amount of substance excreted in the urine per unit time. Stated mathematically,
C s × P
s = U
s × V,
where Cs is the clearance rate of a substance s, Ps is the plasma concentration of the substance, Us is the urine concentration
of that substance, and V is the urine flow rate. Rearranging this equation, clearance can be expressed as
Thus, renal clearance of a substance is calculated from the urinary excretion rate (Us × V) of that substance divided by its plasma concentration.
Inulin Clearance Can Be Used to Estimate GFR If a substance is freely filtered (filtered as freely as water) and is not reabsorbed or secreted by the renal tubules, then the rate at which that substance is excreted in the urine (Us × V) is equal to the filtration rate of the substance by the kidneys (GFR × Ps). Thus,
GFR × P s = U
s × V
The GFR, therefore, can be calculated as the clearance of the substance as follows:
GFR Us V
Ps Cs= =
A substance that fits these criteria is inulin, a polysaccha- ride molecule with a molecular weight of about 5200. Inulin, which is not produced in the body, is found in the roots of certain plants and must be administered intravenously to a patient to measure GFR.
Figure 27-19 shows the renal handling of inulin. In this example, the plasma concentration is 1 mg/ml, urine con- centration is 125 mg/ml, and urine flow rate is 1 ml/min. Therefore, 125 mg/min of inulin passes into the urine. Then, inulin clearance is calculated as the urine excretion rate of
Cs Us V
Ps =
Term Equation Units
Clearance rate (C s ) ml/min
Glomerular filtration rate (GFR)
Clearance ratio None
Effective renal plasma flow (ERPF) ml/min
Renal plasma flow (RPF)
ml/min
Renal blood flow (RBF) ml/min
Excretion rate mg/min, mmol/min, or mEq/min
Reabsorption rate mg/min, mmol/min, or mEq/min
Secretion rate Secretion rate = Excretion rate − Filtered load mg/min, mmol/min, or mEq/min
Cs = Us V
Ps
•
GFR = Uinulin V
Pinulin
•
Clearance ratio Cs
Cinulin =
ERPF = CPAH = UPAHV
PPAH
•
RPF = CPAH EPAH
(UPAH V/PPAH)
UPAH V PPAH − VPAH
= (PPAH − VPAH)/PPAH
=
•
•
RBF= RPF
1−Hematocrit
Excretion rate = Us × V •
Reabsorption rate = Filtered load − Excretion rate = (GFR × Ps ) − (Us × V)
•
Table 27-4 Use of Clearance to Quantify Kidney Function
S, a substance; U, urine concentration; • V, urine flow rate; P, plasma concentration; PAH, para-aminohippuric acid; P
PAH , renal arterial PAH concentration; E
PAH ,
PAH extraction ratio; V PAH
, renal venous PAH concentration.
Chapter 27 Urine Formation by the Kidneys: II. Tubular Reabsorption and Secretion
341
U n
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inulin divided by the plasma concentration, which yields a value of 125 ml/min. Thus, 125 milliliters of plasma flowing through the kidneys must be filtered to deliver the inulin that appears in the urine.
Inulin is not the only substance that can be used for deter- mining GFR. Other substances that have been used clini- cally to estimate GFR include radioactive iothalamate and creatinine.
Creatinine Clearance and Plasma Creatinine Concentration Can Be Used to Estimate GFR Creatinine is a by-product of muscle metabolism and is cleared from the body fluids almost entirely by glomeru- lar filtration. Therefore, the clearance of creatinine can also be used to assess GFR. Because measurement of creatinine clearance does not require intravenous infusion into the patient, this method is much more widely used than inulin clearance for estimating GFR clinically. However, creatinine clearance is not a perfect marker of GFR because a small amount of it is secreted by the tubules, so the amount of cre- atinine excreted slightly exceeds the amount filtered. There is normally a slight error in measuring plasma creatinine that leads to an overestimate of the plasma creatinine concentra- tion, and fortuitously, these two errors tend to cancel each other. Therefore, creatinine clearance provides a reasonable estimate of GFR.
In some cases, it may not be practical to collect urine in a patient for measuring creatinine clearance (CCr). An approxi- mation of changes in GFR, however, can be obtained by sim- ply measuring plasma creatinine concentration (PCr), which is inversely proportional to GFR:
GFR ≈ CCr = UCr V
PCr
•
If GFR suddenly decreases by 50%, the kidneys will tran- siently filter and excrete only half as much creatinine, causing
accumulation of creatinine in the body fluids and raising plasma concentration. Plasma concentration of creatinine will continue to rise until the filtered load of creatinine (PCr × GFR) and creatinine excretion (UCr ×
•
) return to normal and a balance between creatinine production and creatinine excretion is re-established. This will occur when plasma cre- atinine increases to approximately twice normal, as shown in Figure 27-20.
If GFR falls to one-fourth normal, plasma creatinine would increase to about four times normal and a decrease of GFR to one-eighth normal would raise plasma creatinine to eight times normal. Thus, under steady-state conditions, the creatinine excretion rate equals the rate of creatinine pro- duction, despite reductions in GFR. However, this normal rate of creatinine excretion occurs at the expense of elevated plasma creatinine concentration, as shown in Figure 27-21.
PAH Clearance Can Be Used to Estimate Renal Plasma Flow Theoretically, if a substance is completely cleared from the plasma, the clearance rate of that substance is equal to the total renal plasma flow. In other words, the amount of the sub- stance delivered to the kidneys in the blood (renal plasma flow × Ps) would be equal to the amount excreted in the urine (Us ×
•
). Thus, renal plasma flow (RPF) could be calculated as
RPF Us V
Ps Cs= =
•
Uinulin = 125 mg/ml
Pinulin = 1 mg/ml
V = 1 ml/min
Amount filtered = Amount excreted
GFR x Pinulin = Uinulin x V
Uinulin x V
Pinulin
GFR =
GFR = 125 ml/min
.
.
.
Figure 27-19 Measurement of glomerular filtration rate (GFR) from the renal clearance of inulin. Inulin is freely filtered by the glomerular capillaries but is not reabsorbed by the renal tubules. P
inulin , plasma inulin concentration; U
inulin , urine inulin concentration;
• V, urine flow rate.
Figure 27-20 Effect of reducing glomerular filtration rate (GRF) by 50 percent on serum creatinine concentration and on creati- nine excretion rate when the production rate of creatinine remains constant. P
Creatinine , plasma creatinine concentration.
C re
at in
in e
p ro
d u
ct io
n a
n d
re n
al e
xc re
ti o
n (
g /d
ay )
Days
2
1
0 0 1 2 3 4
Positive balance Production
Excretion GFR x PCreatinine
S er
u m
c re
at in
in e
co n
ce n
tr at
io n
( m
g /d
l) 2
1
0
G F
R (
m l/m
in )
100
50
0
Unit V The Body Fluids and Kidneys
342
Because the GFR is only about 20 percent of the total plasma flow, a substance that is completely cleared from the plasma must be excreted by tubular secretion, as well as glomerular filtration (Figure 27-22). There is no known sub- stance that is completely cleared by the kidneys. One sub- stance, however, PAH, is about 90 percent cleared from the plasma. Therefore, the clearance of PAH can be used as an approximation of renal plasma flow. To be more accurate, one can correct for the percentage of PAH that is still in the blood when it leaves the kidneys. The percentage of PAH removed from the blood is known as the extraction ratio of PAH and averages about 90 percent in normal kidneys. In diseased kidneys, this extraction ratio may be reduced because of inability of damaged tubules to secrete PAH into the tubular fluid.
The calculation of RPF can be demonstrated by the fol- lowing example: Assume that the plasma concentration of PAH is 0.01 mg/ml, urine concentration is 5.85 mg/ml, and urine flow rate is 1 ml/min. PAH clearance can be calculated from the rate of urinary PAH excretion (5.85 mg/ml × 1 ml/ min) divided by the plasma PAH concentration (0.01 mg/ ml). Thus, clearance of PAH calculates to be 585 ml/min.
If the extraction ratio for PAH is 90 percent, the actual renal plasma flow can be calculated by dividing 585 ml/ min by 0.9, yielding a value of 650 ml/min. Thus, total renal plasma flow can be calculated as
The extraction ratio (EPAH) is calculated as the difference between the renal arterial PAH (PPAH) and renal venous PAH (VPAH) concentrations, divided by the renal arterial PAH concentration:
EPAH PPAH − VPAH
PPAH =
One can calculate the total blood flow through the kid- neys from the total renal plasma flow and hematocrit (the percentage of red blood cells in the blood). If the hematocrit is 0.45 and the total renal plasma flow is 650 ml/min, the total blood flow through both kidneys is 650/(1 to 0.45), or 1182 ml/min.
Filtration Fraction Is Calculated from GFR Divided by Renal Plasma Flow To calculate the filtration fraction, which is the fraction of plasma that filters through the glomerular membrane, one must first know the renal plasma flow (PAH clear- ance) and the GFR (inulin clearance). If renal plasma flow is 650 ml/min and GFR is 125 ml/min, the filtration frac- tion (FF) is calculated as
FF = GFR/RPF = 125/650 = 0.19
Calculation of Tubular Reabsorption or Secretion from Renal Clearances
If the rates of glomerular filtration and renal excretion of a substance are known, one can calculate whether there is a net reabsorption or a net secretion of that substance by the renal tubules. For example, if the rate of excretion of the sub- stance (Us ×
•
) is less than the filtered load of the substance (GFR × Ps), then some of the substance must have been reab- sorbed from the renal tubules.
Conversely, if the excretion rate of the substance is greater than its filtered load, then the rate at which it appears in the urine represents the sum of the rate of glomerular filtration plus tubular secretion.
Total renal plasma flow = PAH clearance
PAH extraction ratio
P la
sm a
cr ea
ti n
in e
co n
ce n
tr at
io n
(m g
/1 00
m l)
Glomerular filtration rate (ml/min)
14
12
10
8
6
4
2
25 50
Normal
75 100 125 150
Figure 27-21 Approximate relationship between glomerular fil- tration rate (GFR) and plasma creatinine concentration under steady-state conditions. Decreasing GFR by 50 percent will increase plasma creatinine to twice normal if creatinine produc- tion by the body remains constant.
UPAH = 5.85 mg/ml
PPAH = 0.01 mg/ml
V = 1 ml/min
UPAH x V
PPAH
Renal plasma flow
=
Renal venous PAH = 0.001 mg/ml
.
Figure 27-22 Measurement of renal plasma flow from the renal clearance of para-aminohippuric acid (PAH). PAH is freely filtered by the glomerular capillaries and is also secreted from the peritu- bular capillary blood into the tubular lumen. The amount of PAH in the plasma of the renal artery is about equal to the amount of PAH excreted in the urine. Therefore, the renal plasma flow can be calculated from the clearance of PAH (C
PAH ). To be more accurate,
one can correct for the percentage of PAH that is still in the blood when it leaves the kidneys. P
PAH , arterial plasma PAH concentra-
tion; U PAH
, urine PAH concentration; • V, urine flow rate.
Chapter 27 Urine Formation by the Kidneys: II. Tubular Reabsorption and Secretion
343
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The following example demonstrates the calculation of tubular reabsorption. Assume the following laboratory val- ues for a patient were obtained:
Urine flow rate = 1 ml/min Urine concentration of sodium (UNa) = 70 mEq/L = 70 μEq/ml Plasma sodium concentration = 140 mEq/L = 140 μEq/ml GFR (inulin clearance) = 100 ml/min
In this example, the filtered sodium load is GFR × PNa, or 100 ml/min × 140 μEq/ml = 14,000 μEq/min. Urinary sodium excretion (UNa × urine flow rate) is 70 μEq/min. Therefore, tubular reabsorption of sodium is the difference between the filtered load and urinary excretion, or 14,000 μEq/min − 70 μEq/min = 13,930 μEq/min.
Comparisons of Inulin Clearance with Clearances of Different Solutes. The following generalizations can be made by comparing the clearance of a substance with the clearance of inulin, a measure of GFR: (1) If the clearance rate of the substance equals that of inulin, the substance is only filtered and not reabsorbed or secreted; (2) if the clearance rate of a substance is less than inulin clearance, the substance must have been reabsorbed by the nephron tubules; and (3) if the clearance rate of a substance is greater than that of inulin, the substance must be secreted by the nephron tubules. Listed below are the approximate clearance rates for some of the substances normally handled by the kidneys:
Substance Glucose Sodium Chloride Potassium Phosphate Inulin Creatinine
Clearance Rate (ml/min) 0 0.9 1.3 12.0 25.0 125.0 140.0
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chapter 28
Urine Concentration and Dilution; Regulation of Extracellular Fluid Osmolarity
and Sodium Concentration
chapter 28
For the cells of the body to function properly, they must be bathed in extracellular fluid with a relatively con- stant concentration of elec- trolytes and other solutes. The total concentration of
solutes in the extracellular fluid—and therefore the osmo- larity—is determined by the amount of solute divided by the volume of the extracellular fluid. Thus, to a large extent, extracellular fluid sodium concentration and osmolarity are regulated by the amount of extracellular water. The total body water is controlled by (1) fluid intake, which is regu- lated by factors that determine thirst, and (2) renal excre- tion of water, which is controlled by multiple factors that influence glomerular filtration and tubular reabsorption.
In this chapter, we discuss (1) the mechanisms that cause the kidneys to eliminate excess water by excreting a dilute urine; (2) the mechanisms that cause the kidneys to con- serve water by excreting a concentrated urine; (3) the renal feedback mechanisms that control the extracellular fluid sodium concentration and osmolarity; and (4) the thirst and salt appetite mechanisms that determine the intakes of water and salt, which also help to control extracellular fluid volume, osmolarity, and sodium concentration.
Kidneys Excrete Excess Water by Forming Dilute Urine
Normal kidneys have tremendous capability to vary the relative proportions of solutes and water in the urine in response to various challenges. When there is excess water in the body and body fluid osmolarity is reduced, the kidney can excrete urine with an osmolarity as low as 50 mOsm/L, a concentration that is only about one-sixth the osmolarity of normal extracellular fluid. Conversely, when there is a deficit of water and extracellular fluid osmolarity is high, the kidney can excrete urine with a concentration of 1200 to 1400 mOsm/L. Equally impor- tant, the kidney can excrete a large volume of dilute urine or a small volume of concentrated urine without major changes in rates of excretion of solutes such as sodium
and potassium. This ability to regulate water excretion independently of solute excretion is necessary for survival, especially when fluid intake is limited.
Antidiuretic Hormone Controls Urine Concentration
There is a powerful feedback system for regulating plasma osmolarity and sodium concentration that operates by altering renal excretion of water independently of the rate of solute excretion. A primary effector of this feedback is antidiuretic hormone (ADH), also called vasopressin.
When osmolarity of the body fluids increases above normal (i.e., the solutes in the body fluids become too concentrated), the posterior pituitary gland secretes more ADH, which increases the permeability of the dis- tal tubules and collecting ducts to water, as discussed in Chapter 27. This permits large amounts of water to be reabsorbed and decreases urine volume but does not markedly alter the rate of renal excretion of the solutes.
When there is excess water in the body and extracel- lular fluid osmolarity is reduced, the secretion of ADH by the posterior pituitary decreases, thereby reducing the permeability of the distal tubule and collecting ducts to water, which causes large amounts of dilute urine to be excreted. Thus, the rate of ADH secretion determines, to a large extent, whether the kidney excretes dilute or concentrated urine.
Renal Mechanisms for Excreting Dilute Urine
When there is a large excess of water in the body, the kid- ney can excrete as much as 20 L/day of dilute urine, with a concentration as low as 50 mOsm/L. The kidney per- forms this impressive feat by continuing to reabsorb sol- utes while failing to reabsorb large amounts of water in the distal parts of the nephron, including the late distal tubule and the collecting ducts.
Figure 28-1 shows the approximate renal responses in a human after ingestion of 1 liter of water. Note that urine volume increases to about six times normal within 45 minutes after the water has been drunk. However, the total amount of solute excreted remains relatively constant because the urine formed becomes very dilute and urine osmolarity decreases from 600 to about 100 mOsm/L.
Unit V The Body Fluids and Kidneys
346
Thus, after ingestion of excess water, the kidney rids the body of the excess water but does not excrete excess amounts of solutes.
When the glomerular filtrate is initially formed, its osmolarity is about the same as that of plasma (300 mOsm/L). To excrete excess water, it is necessary to dilute the filtrate as it passes along the tubule. This is achieved by reabsorbing solutes to a greater extent than water, as shown in Figure 28-2, but this occurs only in certain segments of the tubular system as follows.
Tubular Fluid Remains Isosmotic in the Proximal Tubule. As fluid flows through the proximal tubule, sol- utes and water are reabsorbed in equal proportions, so little change in osmolarity occurs; thus, the proximal tubule fluid remains isosmotic to the plasma, with an osmolarity of about 300 mOsm/L. As fluid passes down the descending loop of Henle, water is reabsorbed by osmosis and the tubular fluid reaches equilibrium with the surrounding interstitial fluid of the renal medulla, which is very hypertonic—about two to four times the osmolarity of the original glomerular fil- trate. Therefore, the tubular fluid becomes more concen- trated as it flows into the inner medulla.
Tubular Fluid Is Diluted in the Ascending Loop of Henle. In the ascending limb of the loop of Henle, espe- cially in the thick segment, sodium, potassium, and chloride are avidly reabsorbed. However, this portion of the tubular segment is impermeable to water, even in the presence of large amounts of ADH. Therefore, the tubular fluid becomes more dilute as it flows up the ascending loop of Henle into
the early distal tubule, with the osmolarity decreasing pro- gressively to about 100 mOsm/L by the time the fluid enters the early distal tubular segment. Thus, regardless of whether ADH is present or absent, fluid leaving the early distal tubu- lar segment is hypo-osmotic, with an osmolarity of only about one-third the osmolarity of plasma.
Tubular Fluid in Distal and Collecting Tubules Is Further Diluted in the Absence of ADH. As the dilute fluid in the early distal tubule passes into the late distal convoluted tubule, cortical collecting duct, and collecting duct, there is additional reabsorption of sodium chloride. In the absence of ADH, this portion of the tubule is also impermeable to water and the additional reabsorption of solutes causes the tubular fluid to become even more dilute, decreasing its osmolarity to as low as 50 mOsm/L. The failure to reabsorb water and the continued reabsorp- tion of solutes lead to a large volume of dilute urine.
To summarize, the mechanism for forming dilute urine is to continue reabsorbing solutes from the distal segments of the tubular system while failing to reabsorb water. In healthy kidneys, fluid leaving the ascending loop of Henle and early distal tubule is always dilute, regard- less of the level of ADH. In the absence of ADH, the urine is further diluted in the late distal tubule and collecting ducts and a large volume of dilute urine is excreted.
Kidneys Conserve Water by Excreting Concentrated Urine
The ability of the kidney to form urine that is more con- centrated than plasma is essential for survival of mammals that live on land, including humans. Water is continuously lost from the body through various routes, including the
U ri
n ar
y so
lu te
ex cr
et io
n (m
O sm
/m in
) 1.2
0.6
0 0 18012060
Time (minutes)
U ri
n e
fl o
w r
at e
(m l/m
in )
6
4
2
0
O sm
o la
ri ty
(m O
sm /L
)
800 Urine
osmolarity Plasma
osmolarity 400
0
Drink 1.0 L H2O
Figure 28-1 Water diuresis in a human after ingestion of 1 liter of water. Note that after water ingestion, urine volume increases and urine osmolarity decreases, causing the excretion of a large vol- ume of dilute urine; however, the total amount of solute excreted by the kidneys remains relatively constant. These responses of the kidneys prevent plasma osmolarity from decreasing markedly dur- ing excess water ingestion.
NaCl
NaCl
NaCl
NaCl
NaCl
300 300
400 400
M ed
ul la
C or
te x
600
400
600
H2O
H2O
600
300 100 100
70
50
Figure 28-2 Formation of dilute urine when antidiuretic hormone (ADH) levels are very low. Note that in the ascending loop of Henle, the tubular fluid becomes very dilute. In the distal tubules and collecting tubules, the tubular fluid is further diluted by the reabsorption of sodium chloride and the failure to reabsorb water when ADH levels are very low. The failure to reabsorb water and continued reabsorption of solutes lead to a large volume of dilute urine. (Numerical values are in milliosmoles per liter.)
Chapter 28 Urine Concentration and Dilution; Regulation of Extracellular Fluid Osmolarity and Sodium Concentration
347
U n
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lungs by evaporation into the expired air, the gastrointesti- nal tract by way of the feces, the skin through evaporation and perspiration, and the kidneys through the excretion of urine. Fluid intake is required to match this loss, but the ability of the kidney to form a small volume of con- centrated urine minimizes the intake of fluid required to maintain homeostasis, a function that is especially important when water is in short supply.
When there is a water deficit in the body, the kidney forms concentrated urine by continuing to excrete solutes while increasing water reabsorption and decreasing the volume of urine formed. The human kidney can produce a maximal urine concentration of 1200 to 1400 mOsm/L, four to five times the osmolarity of plasma.
Some desert animals, such as the Australian hop- ping mouse, can concentrate urine to as high as 10,000 mOsm/L. This allows the mouse to survive in the desert without drinking water; sufficient water can be obtained through the food ingested and water produced in the body by metabolism of the food. Animals adapted to fresh water environments usually have minimal urine concen- trating ability. Beavers, for example, can concentrate the urine only to about 500 mOsm/L.
Obligatory Urine Volume
The maximal concentrating ability of the kidney dictates how much urine volume must be excreted each day to rid the body of waste products of metabolism and ions that are ingested. A normal 70-kilogram human must excrete about 600 milliosmoles of solute each day. If maximal urine con- centrating ability is 1200 mOsm/L, the minimal volume of urine that must be excreted, called the obligatory urine vol- ume, can be calculated as
This minimal loss of volume in the urine contributes to dehydration, along with water loss from the skin, respiratory tract, and gastrointestinal tract, when water is not available to drink.
The limited ability of the human kidney to concentrate the urine to a maximal concentration of 1200 mOsm/L explains why severe dehydration occurs if one attempts to drink sea- water. Sodium chloride concentration in the oceans aver- ages about 3.0 to 3.5 percent, with an osmolarity between about 1000 and 1200 mOsm/L. Drinking 1 liter of seawater with a concentration of 1200 mOsm/L would provide a total sodium chloride intake of 1200 milliosmoles. If maximal urine concentrating ability is 1200 mOsm/L, the amount of urine volume needed to excrete 1200 milliosmoles would be 1200 milliosmoles divided by 1200 mOsm/L, or 1.0 liter. Why then does drinking seawater cause dehydration? The answer is that the kidney must also excrete other solutes, especially urea, which contribute about 600 mOsm/L when the urine is maximally concentrated. Therefore, the maximum concen- tration of sodium chloride that can be excreted by the kid- neys is about 600 mOsm/L. Thus, for every liter of seawater drunk, 1.5 liters of urine volume would be required to rid the body of 1200 milliosmoles of sodium chloride ingested in addition to 600 milliosmoles of other solutes such as urea.
This would result in a net fluid loss of 0.5 liter for every liter of seawater drunk, explaining the rapid dehydration that occurs in shipwreck victims who drink seawater. However, a shipwreck victim’s pet Australian hopping mouse could drink with impunity all the seawater it wanted.
Urine Specific Gravity
Urine specific gravity is often used in clinical settings to provide a rapid estimate of urine solute concentration. The more concentrated the urine, the higher the urine specific gravity. In most cases, urine specific gravity increases lin- early with increasing urine osmolarity (Figure 28-3). Urine specific gravity, however, is a measure of the weight of sol- utes in a given volume of urine and is therefore determined by the number and size of the solute molecules. This con- trasts with osmolarity, which is determined only by the number of solute molecules in a given volume.
Urine specific gravity is generally expressed in grams/ ml and, in humans, normally ranges from 1.002 to 1.028 g/ml, rising by .001 for every 35 to 40 mOsmol/L increase in urine osmolarity. This relationship between specific gravity and osmolarity is altered when there are signifi- cant amounts of large molecules in the urine, such as glu- cose, radiocontrast media used for diagnostic purposes, or some antibiotics. In these cases, urine specific grav- ity measurements may falsely suggest a very concentrated urine, despite a normal urine osmolality.
Dipsticks are available that measure approximate urine specific gravity, but most laboratories measure specific gravity with a refractometer.
Requirements for Excreting a Concentrated Urine—High ADH Levels and Hyperosmotic Renal Medulla
The basic requirements for forming a concentrated urine are (1) a high level of ADH, which increases the perme- ability of the distal tubules and collecting ducts to water,
600 mOsm/day
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Figure 28-3 Relationship between specific gravity (grams/ml) and osmolarity of the urine.
Unit V The Body Fluids and Kidneys
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thereby allowing these tubular segments to avidly reabsorb water, and (2) a high osmolarity of the renal medullary interstitial fluid, which provides the osmotic gradient necessary for water reabsorption to occur in the presence of high levels of ADH.
The renal medullary interstitium surrounding the col- lecting ducts is normally hyperosmotic, so when ADH lev- els are high, water moves through the tubular membrane by osmosis into the renal interstitium; from there it is car- ried away by the vasa recta back into the blood. Thus, the urine concentrating ability is limited by the level of ADH and by the degree of hyperosmolarity of the renal medulla. We discuss the factors that control ADH secretion later, but for now, what is the process by which renal medul- lary interstitial fluid becomes hyperosmotic? This process involves the operation of the countercurrent mechanism.
The countercurrent mechanism depends on the special anatomical arrangement of the loops of Henle and the vasa recta, the specialized peritubular capillaries of the renal medulla. In the human, about 25 percent of the nephrons are juxtamedullary nephrons, with loops of Henle and vasa recta that go deeply into the medulla before return- ing to the cortex. Some of the loops of Henle dip all the way to the tips of the renal papillae that project from the medulla into the renal pelvis. Paralleling the long loops of Henle are the vasa recta, which also loop down into the medulla before returning to the renal cortex. And finally, the collecting ducts, which carry urine through the hyper- osmotic renal medulla before it is excreted, also play a critical role in the countercurrent mechanism.
Countercurrent Mechanism Produces a Hyperosmotic Renal Medullary Interstitium
The osmolarity of interstitial fluid in almost all parts of the body is about 300 mOsm/L, which is similar to the plasma osmolarity. (As discussed in Chapter 25, the corrected osmolar activity, which accounts for intermo- lecular attraction, is about 282 mOsm/L.) The osmolarity
of the interstitial fluid in the medulla of the kidney is much higher and may increase progressively to about 1200 to 1400 mOsm/L in the pelvic tip of the medulla. This means that the renal medullary interstitium has accumulated solutes in great excess of water. Once the high solute con- centration in the medulla is achieved, it is maintained by a balanced inflow and outflow of solutes and water in the medulla.
The major factors that contribute to the buildup of sol- ute concentration into the renal medulla are as follows:
1. Active transport of sodium ions and co-transport of potassium, chloride, and other ions out of the thick portion of the ascending limb of the loop of Henle into the medullary interstitium
2. Active transport of ions from the collecting ducts into the medullary interstitium
3. Facilitated diffusion of urea from the inner medullary collecting ducts into the medullary interstitium
4. Diffusion of only small amounts of water from the medullary tubules into the medullary interstitium, far less than the reabsorption of solutes into the medullary interstitium
Special Characteristics of Loop of Henle That Cause Solutes to Be Trapped in the Renal Medulla. The trans- port characteristics of the loops of Henle are summarized in Table 28-1, along with the properties of the proximal tubules, distal tubules, cortical collecting tubules, and inner medullary collecting ducts.
The most important cause of the high medullary osmolarity is active transport of sodium and co-trans- port of potassium, chloride, and other ions from the thick ascending loop of Henle into the interstitium. This pump is capable of establishing about a 200-milliosmole con- centration gradient between the tubular lumen and the interstitial fluid. Because the thick ascending limb is vir- tually impermeable to water, the solutes pumped out are
Permeability
Active NaCl Transport H 2 O NaCl Urea
Proximal tubule ++ ++ + +
Thin descending limb 0 ++ + +
Thin ascending limb 0 0 + +
Thick ascending limb ++ 0 0 0
Distal tubule + +ADH 0 0
Cortical collecting tubule + +ADH 0 0
Inner medullary collecting duct + +ADH 0 ++ADH
Table 28-1 Summary of Tubule Characteristics—Urine Concentration
0, minimal level of active transport or permeability; +, moderate level of active transport or permeability; ++, high level of active transport or permeability; +ADH, permeability to water or urea is increased by ADH.
Chapter 28 Urine Concentration and Dilution; Regulation of Extracellular Fluid Osmolarity and Sodium Concentration
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not followed by osmotic flow of water into the intersti- tium. Thus, the active transport of sodium and other ions out of the thick ascending loop adds solutes in excess of water to the renal medullary interstitium. There is some passive reabsorption of sodium chloride from the thin ascending limb of Henle’s loop, which is also impermeable to water, adding further to the high solute concentration of the renal medullary interstitium.
The descending limb of Henle’s loop, in contrast to the ascending limb, is very permeable to water, and the tubu- lar fluid osmolarity quickly becomes equal to the renal medullary osmolarity. Therefore, water diffuses out of the descending limb of Henle’s loop into the interstitium and the tubular fluid osmolarity gradually rises as it flows toward the tip of the loop of Henle.
Steps Involved in Causing Hyperosmotic Renal Medullary Interstitium. Keeping in mind these charac- teristics of the loop of Henle, let us now discuss how the renal medulla becomes hyperosmotic. First, assume that the loop of Henle is filled with fluid with a concentration of 300 mOsm/L, the same as that leaving the proximal tubule (Figure 28-4, step 1). Next, the active ion pump of the thick ascending limb on the loop of Henle reduces the concentration inside the tubule and raises the intersti- tial concentration; this pump establishes a 200-mOsm/L concentration gradient between the tubular fluid and the interstitial fluid (step 2). The limit to the gradient is about 200 mOsm/L because paracellular diffusion of ions back into the tubule eventually counterbalances transport of ions out of the lumen when the 200-mOsm/L concentra- tion gradient is achieved.
Step 3 is that the tubular fluid in the descending limb of the loop of Henle and the interstitial fluid quickly reach osmotic equilibrium because of osmosis of water out of the descending limb. The interstitial osmolarity is
maintained at 400 mOsm/L because of continued trans- port of ions out of the thick ascending loop of Henle. Thus, by itself, the active transport of sodium chloride out of the thick ascending limb is capable of establishing only a 200-mOsm/L concentration gradient, much less than that achieved by the countercurrent system.
Step 4 is additional flow of fluid into the loop of Henle from the proximal tubule, which causes the hyperosmotic fluid previously formed in the descending limb to flow into the ascending limb. Once this fluid is in the ascending limb, additional ions are pumped into the interstitium, with water remaining in the tubular fluid, until a 200-mOsm/L osmotic gradient is established, with the interstitial fluid osmolarity rising to 500 mOsm/L (step 5). Then, once again, the fluid in the descending limb reaches equilibrium with the hyperosmotic medullary interstitial fluid (step 6), and as the hyperosmotic tubular fluid from the descending limb of the loop of Henle flows into the ascending limb, still more solute is continuously pumped out of the tubules and deposited into the medullary interstitium.
These steps are repeated over and over, with the net effect of adding more and more solute to the medulla in excess of water; with sufficient time, this process gradually traps solutes in the medulla and multiplies the concentra- tion gradient established by the active pumping of ions out of the thick ascending loop of Henle, eventually raising the interstitial fluid osmolarity to 1200 to 1400 mOsm/L as shown in step 7.
Thus, the repetitive reabsorption of sodium chlo- ride by the thick ascending loop of Henle and continued inflow of new sodium chloride from the proximal tubule into the loop of Henle is called the countercurrent multi- plier. The sodium chloride reabsorbed from the ascending loop of Henle keeps adding to the newly arrived sodium chloride, thus “multiplying” its concentration in the med- ullary interstitium.
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Figure 28-4 Countercurrent multiplier system in the loop of Henle for producing a hyperosmotic renal medulla. (Numerical values are in milliosmoles per liter.)
Unit V The Body Fluids and Kidneys
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Role of Distal Tubule and Collecting Ducts in Excreting Concentrated Urine
When the tubular fluid leaves the loop of Henle and flows into the distal convoluted tubule in the renal cor- tex, the fluid is dilute, with an osmolarity of only about 100 mOsm/L (Figure 28-5). The early distal tubule fur- ther dilutes the tubular fluid because this segment, like the ascending loop of Henle, actively transports sodium chloride out of the tubule but is relatively impermeable to water.
As fluid flows into the cortical collecting tubule, the amount of water reabsorbed is critically dependent on the plasma concentration of ADH. In the absence of ADH, this segment is almost impermeable to water and fails to reab- sorb water but continues to reabsorb solutes and further dilutes the urine. When there is a high concentration of ADH, the cortical collecting tubule becomes highly per- meable to water, so large amounts of water are now reab- sorbed from the tubule into the cortex interstitium, where it is swept away by the rapidly flowing peritubular capillaries. The fact that these large amounts of water are reabsorbed into the cortex, rather than into the renal medulla, helps to preserve the high medullary interstitial fluid osmolarity.
As the tubular fluid flows along the medullary collect- ing ducts, there is further water reabsorption from the tubular fluid into the interstitium, but the total amount of water is relatively small compared with that added to the cortex interstitium. The reabsorbed water is quickly carried away by the vasa recta into the venous blood. When high levels of ADH are present, the collecting ducts become permeable to water, so the fluid at the end of the collecting ducts has essentially the same osmolar- ity as the interstitial fluid of the renal medulla—about
1200 mOsm/L (see Figure 28-4). Thus, by reabsorbing as much water as possible, the kidneys form highly concen- trated urine, excreting normal amounts of solutes in the urine while adding water back to the extracellular fluid and compensating for deficits of body water.
Urea Contributes to Hyperosmotic Renal Medullary Interstitium and Formation of Concentrated Urine
Thus far, we have considered only the contribution of sodium chloride to the hyperosmotic renal medullary interstitium. However, urea contributes about 40 to 50 percent of the osmolarity (500 to 600 mOsm/L) of the renal medullary interstitium when the kidney is forming a maximally concentrated urine. Unlike sodium chloride, urea is passively reabsorbed from the tubule. When there is water deficit and blood concentration of ADH is high, large amounts of urea are passively reabsorbed from the inner medullary collecting ducts into the interstitium.
The mechanism for reabsorption of urea into the renal medulla is as follows: As water flows up the ascending loop of Henle and into the distal and cortical collecting tubules, little urea is reabsorbed because these segments are impermeable to urea (see Table 28-1). In the pres- ence of high concentrations of ADH, water is reabsorbed rapidly from the cortical collecting tubule and the urea concentration increases rapidly because urea is not very permeant in this part of the tubule.
As the tubular fluid flows into the inner medullary col- lecting ducts, still more water reabsorption takes place, causing an even higher concentration of urea in the fluid. This high concentration of urea in the tubular fluid of the inner medullary collecting duct causes urea to diffuse out of the tubule into the renal interstitial fluid. This diffusion is greatly facilitated by specific urea transporters, UT-A1 and UT-A3. One of these urea transporters, UT-A3, is activated by ADH, increasing transport of urea out of the inner medullary collecting duct even more when ADH lev- els are elevated. The simultaneous movement of water and urea out of the inner medullary collecting ducts maintains a high concentration of urea in the tubular fluid and, even- tually, in the urine, even though urea is being reabsorbed.
The fundamental role of urea in contributing to urine concentrating ability is evidenced by the fact that people who ingest a high-protein diet, yielding large amounts of urea as a nitrogenous “waste” product, can concentrate their urine much better than people whose protein intake and urea production are low. Malnutrition is associated with a low urea concentration in the medullary interstitium and considerable impairment of urine concentrating ability.
Recirculation of Urea from Collecting Duct to Loop of Henle Contributes to Hyperosmotic Renal Medulla. A healthy person usually excretes about 20 to 50 percent of the filtered load of urea. In general, the rate of urea excretion is determined mainly by two factors: (1) the concentration of urea in the plasma and
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Figure 28-5 Formation of a concentrated urine when antidi- uretic hormone (ADH) levels are high. Note that the fluid leaving the loop of Henle is dilute but becomes concentrated as water is absorbed from the distal tubules and collecting tubules. With high ADH levels, the osmolarity of the urine is about the same as the osmolarity of the renal medullary interstitial fluid in the papilla, which is about 1200 mOsm/L. (Numerical values are in milliosmoles per liter.)
Chapter 28 Urine Concentration and Dilution; Regulation of Extracellular Fluid Osmolarity and Sodium Concentration
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(2) the glomerular filtration rate (GFR). In patients with renal disease who have large reductions of GFR, the plasma urea concentration increases markedly, returning the filtered urea load and urea excretion rate to the nor- mal level (equal to the rate of urea production), despite the reduced GFR.
In the proximal tubule, 40 to 50 percent of the filtered urea is reabsorbed, but even so, the tubular fluid urea concentration increases because urea is not nearly as per- meant as water. The concentration of urea continues to rise as the tubular fluid flows into the thin segments of the loop of Henle, partly because of water reabsorption out of the descending loop of Henle but also because of some secretion of urea into the thin loop of Henle from the med- ullary interstitium (Figure 28-6). The passive secretion of urea into the thin loops of Henle is facilitated by the urea transporter UT-A2.
The thick limb of the loop of Henle, the distal tubule, and the cortical collecting tubule are all relatively imper- meable to urea, and very little urea reabsorption occurs in these tubular segments. When the kidney is forming concentrated urine and high levels of ADH are present, reabsorption of water from the distal tubule and cortical collecting tubule further raises the tubular fluid concen- tration of urea. As this urea flows into the inner medul- lary collecting duct, the high tubular fluid concentration of urea and specific urea transporters cause urea to dif- fuse into the medullary interstitium. A moderate share of the urea that moves into the medullary interstitium
eventually diffuses into the thin loop of Henle and then passes upward through the ascending loop of Henle, the distal tubule, the cortical collecting tubule, and back down into the medullary collecting duct again. In this way, urea can recirculate through these terminal parts of the tubu- lar system several times before it is excreted. Each time around the circuit contributes to a higher concentration of urea.
This urea recirculation provides an additional mecha- nism for forming a hyperosmotic renal medulla. Because urea is one of the most abundant waste products that must be excreted by the kidneys, this mechanism for concen- trating urea before it is excreted is essential to the econ- omy of the body fluid when water is in short supply.
When there is excess water in the body, urine flow rate is usually increased and therefore the concentration of urea in the inner medullary collecting ducts is reduced, causing less diffusion of urea into the renal medullary interstitium. ADH levels are also reduced when there is excess body water and this, in turn, decreases the per- meability of the inner medullary collecting ducts to both water and urea, and more urea is excreted in the urine.
Countercurrent Exchange in the Vasa Recta Preserves Hyperosmolarity of the Renal Medulla
Blood flow must be provided to the renal medulla to sup- ply the metabolic needs of the cells in this part of the kid- ney. Without a special medullary blood flow system, the solutes pumped into the renal medulla by the countercur- rent multiplier system would be rapidly dissipated.
There are two special features of the renal medullary blood flow that contribute to the preservation of the high solute concentrations:
1. The medullary blood flow is low, accounting for less than 5 percent of the total renal blood flow. This slug- gish blood flow is sufficient to supply the metabolic needs of the tissues but helps to minimize solute loss from the medullary interstitium.
2. The vasa recta serve as countercurrent exchangers, minimizing washout of solutes from the medullary interstitium.
The countercurrent exchange mechanism operates as follows (Figure 28-7): Blood enters and leaves the medulla by way of the vasa recta at the boundary of the cortex and renal medulla. The vasa recta, like other capillaries, are highly permeable to solutes in the blood, except for the plasma proteins. As blood descends into the medulla toward the papillae, it becomes progressively more con- centrated, partly by solute entry from the interstitium and partly by loss of water into the interstitium. By the time the blood reaches the tips of the vasa recta, it has a con- centration of about 1200 mOsm/L, the same as that of the medullary interstitium. As blood ascends back toward the cortex, it becomes progressively less concentrated as sol- utes diffuse back out into the medullary interstitium and as water moves into the vasa recta.
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Figure 28-6 Recirculation of urea absorbed from the medullary collecting duct into the interstitial fluid. This urea diffuses into the thin loop of Henle and then passes through the distal tubules, and it finally passes back into the collecting duct. The recirculation of urea helps to trap urea in the renal medulla and contributes to the hyperosmolarity of the renal medulla. The heavy tan lines, from the thick ascending loop of Henle to the medullary collect- ing ducts, indicate that these segments are not very permeable to urea. (Numerical values are in milliosmoles per liter of urea dur- ing antidiuresis, when large amounts of antidiuretic hormone are present. Percentages of the filtered load of urea that remain in the tubules are indicated in the blue boxes.)
Unit V The Body Fluids and Kidneys
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Although there are large amounts of fluid and sol- ute exchange across the vasa recta, there is little net dilution of the concentration of the interstitial fluid at each level of the renal medulla because of the U shape of the vasa recta capillaries, which act as countercur- rent exchangers. Thus, the vasa recta do not create the medullary hyperosmolarity, but they do prevent it from being dissipated.
The U-shaped structure of the vessels minimizes loss of solute from the interstitium but does not prevent the bulk flow of fluid and solutes into the blood through the usual colloid osmotic and hydrostatic pressures that favor reabsorption in these capillaries. Under steady-state con- ditions, the vasa recta carry away only as much solute and
water as is absorbed from the medullary tubules and the high concentration of solutes established by the counter- current mechanism is preserved.
Increased Medullary Blood Flow Reduces Urine Concentrating Ability. Certain vasodilators can mark- edly increase renal medullary blood flow, thereby “wash- ing out” some of the solutes from the renal medulla and reducing maximum urine concentrating ability. Large increases in arterial pressure can also increase the blood flow of the renal medulla to a greater extent than in other regions of the kidney and tend to wash out the hyperos- motic interstitium, thereby reducing urine concentrat- ing ability. As discussed earlier, maximum concentrating ability of the kidney is determined not only by the level of ADH but also by the osmolarity of the renal medulla interstitial fluid. Even with maximal levels of ADH, urine concentrating ability will be reduced if medullary blood flow increases enough to reduce the hyperosmolarity in the renal medulla.
Summary of Urine Concentrating Mechanism and Changes in Osmolarity in Different Segments of the Tubules
The changes in osmolarity and volume of the tubular fluid as it passes through the different parts of the nephron are shown in Figure 28-8.
Proximal Tubule. About 65 percent of the filtered ele ctrolytes is reabsorbed in the proximal tubule. However, the proximal tubular membranes are highly permeable to water, so that whenever solutes are reabsorbed, water also diffuses through the tubular membrane by osmosis. Therefore, the osmolarity of the fluid remains about the same as the glomerular filtrate, 300 mOsm/L.
Descending Loop of Henle. As fluid flows down the descending loop of Henle, water is absorbed into
Vasa recta mOsm/L
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Figure 28-7 Countercurrent exchange in the vasa recta. Plasma flowing down the descending limb of the vasa recta becomes more hyperosmotic because of diffusion of water out of the blood and diffusion of solutes from the renal interstitial fluid into the blood. In the ascending limb of the vasa recta, solutes diffuse back into the interstitial fluid and water diffuses back into the vasa recta. Large amounts of solutes would be lost from the renal medulla without the U shape of the vasa recta capillaries. (Numerical val- ues are in milliosmoles per liter.)
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Chapter 28 Urine Concentration and Dilution; Regulation of Extracellular Fluid Osmolarity and Sodium Concentration
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the medulla. The descending limb is highly permeable to water but much less permeable to sodium chloride and urea. Therefore, the osmolarity of the fluid flowing through the descending loop gradually increases until it is nearly equal to that of the surrounding interstitial fluid, which is about 1200 mOsm/L when the blood concentra- tion of ADH is high.
When dilute urine is being formed, owing to low ADH concentrations, the medullary interstitial osmolarity is less than 1200 mOsm/L; consequently, the descending loop tubular fluid osmolarity also becomes less concentrated. This is due partly to the fact that less urea is absorbed into the medullary interstitium from the collecting ducts when ADH levels are low and the kidney is forming a large vol- ume of dilute urine.
Thin Ascending Loop of Henle. The thin ascending limb is essentially impermeable to water but reabsorbs some sodium chloride. Because of the high concentra- tion of sodium chloride in the tubular fluid, owing to water removal from the descending loop of Henle, there is some passive diffusion of sodium chloride from the thin ascending limb into the medullary interstitium. Thus, the tubular fluid becomes more dilute as the sodium chlo- ride diffuses out of the tubule and water remains in the tubule.
Some of the urea absorbed into the medullary inter- stitium from the collecting ducts also diffuses into the ascending limb, thereby returning the urea to the tubular system and helping to prevent its washout from the renal medulla. This urea recycling is an additional mechanism that contributes to the hyperosmotic renal medulla.
Thick Ascending Loop of Henle. The thick part of the ascending loop of Henle is also virtually imperme- able to water, but large amounts of sodium, chloride, potassium, and other ions are actively transported from the tubule into the medullary interstitium. Therefore, fluid in the thick ascending limb of the loop of Henle becomes very dilute, falling to a concentration of about 100 mOsm/L.
Early Distal Tubule. The early distal tubule has prop- erties similar to those of the thick ascending loop of Henle, so further dilution of the tubular fluid to about 50 mOsm/L occurs as solutes are reabsorbed while water remains in the tubule.
Late Distal Tubule and Cortical Collecting Tubules. In the late distal tubule and cortical collect- ing tubules, the osmolarity of the fluid depends on the level of ADH. With high levels of ADH, these tubules are highly permeable to water and significant amounts of water are reabsorbed. Urea, however, is not very per- meant in this part of the nephron, resulting in increased urea concentration as water is reabsorbed. This allows most of the urea delivered to the distal tubule and collecting tubule to pass into the inner medullary
collecting ducts, from which it is eventually reabsorbed or excreted in the urine. In the absence of ADH, little water is reabsorbed in the late distal tubule and cortical collecting tubule; therefore, osmolarity decreases even further because of continued active reabsorption of ions from these segments.
Inner Medullary Collecting Ducts. The concentra- tion of fluid in the inner medullary collecting ducts also depends on (1) ADH and (2) the surrounding medul- lary interstitium osmolarity established by the counter- current mechanism. In the presence of large amounts of ADH, these ducts are highly permeable to water, and water diffuses from the tubule into the interstitial fluid until osmotic equilibrium is reached, with the tubu- lar fluid having about the same concentration as the renal medullary interstitium (1200 to 1400 mOsm/L). Thus, a small volume of concentrated urine is produced when ADH levels are high. Because water reabsorp- tion increases urea concentration in the tubular fluid and because the inner medullary collecting ducts have specific urea transporters that greatly facilitate diffu- sion, much of the highly concentrated urea in the ducts diffuses out of the tubular lumen into the medullary interstitium. This absorption of the urea into the renal medulla contributes to the high osmolarity of the med- ullary interstitium and the high concentrating ability of the kidney.
Several important points to consider may not be obvi- ous from this discussion. First, although sodium chlo- ride is one of the principal solutes that contribute to the hyperosmolarity of the medullary interstitium, the kidney can, when needed, excrete a highly concentrated urine that contains little sodium chloride. The hyperosmolarity of the urine in these circumstances is due to high concentra- tions of other solutes, especially of waste products such as urea. One condition in which this occurs is dehydra- tion accompanied by low sodium intake. As discussed in Chapter 29, low sodium intake stimulates formation of the hormones angiotensin II and aldosterone, which together cause avid sodium reabsorption from the tubules while leaving the urea and other solutes to maintain the highly concentrated urine.
Second, large quantities of dilute urine can be excreted without increasing the excretion of sodium. This is accomplished by decreasing ADH secretion, which reduces water reabsorption in the more distal tubular segments without significantly altering sodium reabsorption.
And finally, there is an obligatory urine volume that is dictated by the maximum concentrating ability of the kidney and the amount of solute that must be excreted. Therefore, if large amounts of solute must be excreted, they must be accompanied by the minimal amount of water necessary to excrete them. For example, if 600 mil- liosmoles of solute must be excreted each day, this requires at least 0.5 liter of urine if maximal urine concentrating ability is 1200 mOsm/L.
Unit V The Body Fluids and Kidneys
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Quantifying Renal Urine Concentration and Dilution: “Free Water” and Osmolar Clearances
The process of concentrating or diluting the urine requires the kidneys to excrete water and solutes somewhat indepen- dently. When the urine is dilute, water is excreted in excess of solutes. Conversely, when the urine is concentrated, solutes are excreted in excess of water.
The total clearance of solutes from the blood can be expressed as the osmolar clearance (Cosm); this is the volume of plasma cleared of solutes each minute, in the same way that clearance of a single substance is calculated:
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osm
= •
where Uosm is the urine osmolarity, V̇ is the urine flow rate, and Posm is the plasma osmolarity. For example, if plasma osmo- larity is 300 mOsm/L, urine osmolarity is 600 mOsm/L, and urine flow rate is 1 ml/min (0.001 L/min), the rate of osmolar excretion is 0.6 mOsm/min (600 mOsm/L × 0.001 L/min) and osmolar clearance is 0.6 mOsm/min divided by 300 mOsm/L, or 0.002 L/min (2.0 ml/min). This means that 2 milliliters of plasma are being cleared of solute each minute.
Relative Rates at Which Solutes and Water Are Excreted Can Be Assessed Using the Concept of “Free-Water Clearance.”
Free-water clearance (CH2O) is calculated as the difference between water excretion (urine flow rate) and osmolar clearance:
CH2O = V − Cosm = V − (Uosm V)
(Posm)
•
Thus, the rate of free-water clearance represents the rate at which solute-free water is excreted by the kidneys. When free-water clearance is positive, excess water is being excreted by the kidneys; when free-water clearance is nega- tive, excess solutes are being removed from the blood by the kidneys and water is being conserved.
Using the example discussed earlier, if urine flow rate is 1 ml/min and osmolar clearance is 2 ml/min, free-water clearance would be −1 ml/min. This means that instead of water being cleared from the kidneys in excess of solutes, the kidneys are actually returning water back to the systemic circulation, as occurs during water deficits. Thus, whenever urine osmolarity is greater than plasma osmolarity, free- water clearance is negative, indicating water conservation.
When the kidneys are forming a dilute urine (i.e., urine osmolarity is less than plasma osmolarity), free-water clear- ance will be a positive value, denoting that water is being removed from the plasma by the kidneys in excess of solutes. Thus, water free of solutes, called “free water,” is being lost from the body and the plasma is being concentrated when free-water clearance is positive.
Disorders of Urinary Concentrating Ability
Impairment in the ability of the kidneys to concentrate or dilute the urine appropriately can occur with one or more of the following abnormalities:
1. Inappropriate secretion of ADH. Either too much or too little ADH secretion results in abnormal fluid handling by the kidneys.
2. Impairment of the countercurrent mechanism. A hyper- osmotic medullary interstitium is required for maxi- mal urine concentrating ability. No matter how much ADH is present, maximal urine concentration is lim- ited by the degree of hyperosmolarity of the medullary interstitium.
3. Inability of the distal tubule, collecting tubule, and collect- ing ducts to respond to ADH.
Failure to Produce ADH: “Central” Diabetes Insipidus. An inability to produce or release ADH from the posterior pituitary can be caused by head injuries or infections, or it can be congenital. Because the distal tubular segments can- not reabsorb water in the absence of ADH, this condition, called “central” diabetes insipidus, results in the formation of a large volume of dilute urine with urine volumes that can exceed 15 L/day. The thirst mechanisms, discussed later in this chapter, are activated when excessive water is lost from the body; therefore, as long as the person drinks enough water, large decreases in body fluid water do not occur. The primary abnormality observed clinically in people with this condition is the large volume of dilute urine. However, if water intake is restricted, as can occur in a hospital setting when fluid intake is restricted or the patient is unconscious (e.g., because of a head injury), severe dehydration can rap- idly occur.
The treatment for central diabetes insipidus is adminis- tration of a synthetic analog of ADH, desmopressin, which acts selectively on V2 receptors to increase water permeabil- ity in the late distal and collecting tubules. Desmopressin can be given by injection, as a nasal spray, or orally, and it rapidly restores urine output toward normal.
Inability of the Kidneys to Respond to ADH: “Nephrogenic” Diabetes Insipidus. In some circumstances normal or elevated levels of ADH are present but the renal tubular segments cannot respond appropriately. This condi- tion is referred to as “nephrogenic” diabetes insipidus because the abnormality resides in the kidneys. This abnormality can be due to either failure of the countercurrent mechanism to form a hyperosmotic renal medullary interstitium or failure of the distal and collecting tubules and collecting ducts to respond to ADH. In either case, large volumes of dilute urine are formed, which tends to cause dehydration unless fluid intake is increased by the same amount as urine volume is increased.
Many types of renal diseases can impair the concentrating mechanism, especially those that damage the renal medulla (see Chapter 31 for further discussion). Also, impairment of the function of the loop of Henle, as occurs with diuretics that inhibit electrolyte reabsorption by this segment, such as furosemide, can compromise urine concentrating abil- ity. And certain drugs, such as lithium (used to treat manic- depressive disorders) and tetracyclines (used as antibiotics), can impair the ability of the distal nephron segments to respond to ADH.
Nephrogenic diabetes insipidus can be distinguished from central diabetes insipidus by administration of desmopressin, the synthetic analog of ADH. Lack of a prompt decrease in urine volume and an increase in urine osmolarity within
Chapter 28 Urine Concentration and Dilution; Regulation of Extracellular Fluid Osmolarity and Sodium Concentration
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2 hours after injection of desmopressin is strongly sugges- tive of nephrogenic diabetes insipidus. The treatment for nephrogenic diabetes insipidus is to correct, if possible, the underlying renal disorder. The hypernatremia can also be attenuated by a low-sodium diet and administration of a diuretic that enhances renal sodium excretion, such as a thi- azide diuretic.
Control of Extracellular Fluid Osmolarity and Sodium Concentration
Regulation of extracellular fluid osmolarity and sodium concentration are closely linked because sodium is the most abundant ion in the extracellular compartment. Plasma sodium concentration is normally regulated within close limits of 140 to 145 mEq/L, with an average concentration of about 142 mEq/L. Osmolarity averages about 300 mOsm/L (about 282 mOsm/L when corrected for interionic attraction) and seldom changes more than ±2 to 3 percent. As discussed in Chapter 25, these vari- ables must be precisely controlled because they determine the distribution of fluid between the intracellular and extracellular compartments.
Estimating Plasma Osmolarity from Plasma Sodium Concentration
In most clinical laboratories, plasma osmolarity is not routinely measured. However, because sodium and its associated anions account for about 94 percent of the sol- ute in the extracellular compartment, plasma osmolarity (Posm) can be roughly approximated as
P osm
= 2.1 × Plasma sodium concentration
For instance, with a plasma sodium concentration of 142 mEq/L, the plasma osmolarity would be estimated from this formula to be about 298 mOsm/L. To be more exact, especially in conditions associated with renal dis- ease, the contribution of two other solutes, glucose and urea, should be included. Such estimates of plasma osmo- larity are usually accurate within a few percentage points of those measured directly.
Normally, sodium ions and associated anions (pri- marily bicarbonate and chloride) represent about 94 per- cent of the extracellular osmoles, with glucose and urea contributing about 3 to 5 percent of the total osmoles. However, because urea easily permeates most cell mem- branes, it exerts little effective osmotic pressure under steady-state conditions. Therefore, the sodium ions in the extracellular fluid and associated anions are the prin- cipal determinants of fluid movement across the cell membrane. Consequently, we can discuss the control of osmolarity and control of sodium ion concentration at the same time.
Although multiple mechanisms control the amount of sodium and water excretion by the kidneys, two pri- mary systems are especially involved in regulating the concentration of sodium and osmolarity of extracellular
fluid: (1) the osmoreceptor-ADH system and (2) the thirst mechanism.
Osmoreceptor-ADH Feedback System
Figure 28-9 shows the basic components of the osmo- receptor-ADH feedback system for control of extracel- lular fluid sodium concentration and osmolarity. When osmolarity (plasma sodium concentration) increases above normal because of water deficit, for example, this feedback system operates as follows:
1. An increase in extracellular fluid osmolarity (which in practical terms means an increase in plasma sodium concentration) causes the special nerve cells called osmoreceptor cells, located in the anterior hypothala- mus near the supraoptic nuclei, to shrink.
2. Shrinkage of the osmoreceptor cells causes them to fire, sending nerve signals to additional nerve cells in the supraoptic nuclei, which then relay these signals down the stalk of the pituitary gland to the posterior pituitary.
3. These action potentials conducted to the poste- rior pituitary stimulate the release of ADH, which is stored in secretory granules (or vesicles) in the nerve endings.
− Water deficit
Extracellular osmolarity
Osmoreceptors
Plasma ADH
ADH secretion (posterior pituitary)
H2O permeability in distal tubules,
collecting ducts
H2O reabsorption
H2O excreted
Figure 28-9 Osmoreceptor-antidiuretic hormone (ADH) feed- back mechanism for regulating extracellular fluid osmolarity in response to a water deficit.
Unit V The Body Fluids and Kidneys
356
4. ADH enters the blood stream and is transported to the kidneys, where it increases the water permeability of the late distal tubules, cortical collecting tubules, and medullary collecting ducts.
5. The increased water permeability in the distal nephron segments causes increased water reabsorption and excretion of a small volume of concentrated urine.
Thus, water is conserved in the body while sodium and other solutes continue to be excreted in the urine. This causes dilution of the solutes in the extracellular fluid, thereby correcting the initial excessively concentrated extracellular fluid.
The opposite sequence of events occurs when the extracellular fluid becomes too dilute (hypo-osmotic). For example, with excess water ingestion and a decrease in extracellular fluid osmolarity, less ADH is formed, the renal tubules decrease their permeability for water, less water is reabsorbed, and a large volume of dilute urine is formed. This in turn concentrates the body fluids and returns plasma osmolarity toward normal.
ADH Synthesis in Supraoptic and Paraventricular Nuclei of the Hypothalamus and ADH Release from the Posterior Pituitary
Figure 28-10 shows the neuroanatomy of the hypo- thalamus and the pituitary gland, where ADH is syn- thesized and released. The hypothalamus contains two types of magnocellular (large) neurons that synthesize ADH in the supraoptic and paraventricular nuclei of the hypothalamus, about five sixths in the supraoptic nuclei and about one sixth in the paraventricular nuclei. Both of these nuclei have axonal extensions to the pos- terior pituitary. Once ADH is synthesized, it is trans- ported down the axons of the neurons to their tips, terminating in the posterior pituitary gland. When the supraoptic and paraventricular nuclei are stimulated by increased osmolarity or other factors, nerve impulses pass down these nerve endings, changing their mem- brane permeability and increasing calcium entry. ADH stored in the secretory granules (also called vesicles) of the nerve endings is released in response to increased calcium entry. The released ADH is then carried away in the capillary blood of the posterior pituitary into the systemic circulation.
Secretion of ADH in response to an osmotic stimulus is rapid, so plasma ADH levels can increase severalfold within minutes, thereby providing a rapid means for alter- ing renal excretion of water.
A second neuronal area important in controlling osmo- larity and ADH secretion is located along the anteroven- tral region of the third ventricle, called the AV3V region. At the upper part of this region is a structure called the subfornical organ, and at the inferior part is another struc- ture called the organum vasculosum of the lamina termi- nalis. Between these two organs is the median preoptic nucleus, which has multiple nerve connections with the two organs, as well as with the supraoptic nuclei and
the blood pressure control centers in the medulla of the brain. Lesions of the AV3V region cause multiple deficits in the control of ADH secretion, thirst, sodium appetite, and blood pressure. Electrical stimulation of this region or stimulation by angiotensin II can increase ADH secre- tion, thirst, and sodium appetite.
In the vicinity of the AV3V region and the supraoptic nuclei are neuronal cells that are excited by small increases in extracellular fluid osmolarity; hence, the term osmo- receptors has been used to describe these neurons. These cells send nerve signals to the supraoptic nuclei to control their firing and secretion of ADH. It is also likely that they induce thirst in response to increased extra cellular fluid osmolarity.
Both the subfornical organ and the organum vasculo- sum of the lamina terminalis have vascular supplies that lack the typical blood-brain barrier that impedes the dif- fusion of most ions from the blood into the brain tissue. This makes it possible for ions and other solutes to cross between the blood and the local interstitial fluid in this region. As a result, the osmoreceptors rapidly respond to changes in osmolarity of the extracellular fluid, exert- ing powerful control over the secretion of ADH and over thirst, as discussed later.
ADH
Urine: decreased flow and concentrated
Baroreceptors Cardiopulmonary receptors
Paraventricular neuron
Osmoreceptors
Pituitary
Posterior lobe
Anterior lobe
Supraoptic neuron
Figure 28-10 Neuroanatomy of the hypothalamus, where antidi- uretic hormone (ADH) is synthesized, and the posterior pituitary gland, where ADH is released.
Chapter 28 Urine Concentration and Dilution; Regulation of Extracellular Fluid Osmolarity and Sodium Concentration
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Stimulation of ADH Release by Decreased Arterial Pressure and/or Decreased Blood Volume
ADH release is also controlled by cardiovascular reflexes that respond to decreases in blood pressure and/or blood volume, including (1) the arterial baroreceptor reflexes and (2) the cardiopulmonary reflexes, both of which are discussed in Chapter 18. These reflex pathways origi- nate in high-pressure regions of the circulation, such as the aortic arch and carotid sinus, and in the low-pressure regions, especially in the cardiac atria. Afferent stimuli are carried by the vagus and glossopharyngeal nerves with synapses in the nuclei of the tractus solitarius. Projections from these nuclei relay signals to the hypothalamic nuclei that control ADH synthesis and secretion.
Thus, in addition to increased osmolarity, two other stimuli increase ADH secretion: (1) decreased arterial pressure and (2) decreased blood volume. Whenever blood pressure and blood volume are reduced, such as occurs dur- ing hemorrhage, increased ADH secretion causes increased fluid reabsorption by the kidneys, helping to restore blood pressure and blood volume toward normal.
Quantitative Importance of Osmolarity and Cardiovascular Reflexes in Stimulating ADH Secretion
As shown in Figure 28-11, either a decrease in effective blood volume or an increase in extracellular fluid osmo- larity stimulates ADH secretion. However, ADH is con- siderably more sensitive to small changes in osmolarity
than to similar percentage changes in blood volume. For example, a change in plasma osmolarity of only 1 percent is sufficient to increase ADH levels. By contrast, after blood loss, plasma ADH levels do not change appreciably until blood volume is reduced by about 10 percent. With further decreases in blood volume, ADH levels rapidly increase. Thus, with severe decreases in blood volume, the cardiovascular reflexes play a major role in stimu- lating ADH secretion. The usual day-to-day regulation of ADH secretion during simple dehydration is effected mainly by changes in plasma osmolarity. Decreased blood volume, however, greatly enhances the ADH response to increased osmolarity.
Other Stimuli for ADH Secretion
ADH secretion can also be increased or decreased by other stimuli to the central nervous system, as well as by various drugs and hormones, as shown in Table 28-2. For example, nausea is a potent stimulus for ADH release, which may increase to as much as 100 times normal after vomiting. Also, drugs such as nicotine and morphine stimulate ADH release, whereas some drugs, such as alcohol, inhibit ADH release. The marked diuresis that occurs after ingestion of alcohol is due in part to inhibition of ADH release.
Importance of Thirst in Controlling Extracellular Fluid Osmolarity and Sodium Concentration
The kidneys minimize fluid loss during water defi- cits through the osmoreceptor-ADH feedback system. Adequate fluid intake, however, is necessary to counter- balance whatever fluid loss does occur through sweat- ing and breathing and through the gastrointestinal tract. Fluid intake is regulated by the thirst mechanism, which, together with the osmoreceptor-ADH mechanism, main- tains precise control of extracellular fluid osmolarity and sodium concentration.
Many of the same factors that stimulate ADH secre- tion also increase thirst, which is defined as the conscious desire for water.
P la
sm a
A D
H (
p g
/m l)
Percent change
PAVP = 2.5 Osm + 2.0
PAVP = 1.3 e−0.17 vol.
0
5
10
15
20
25
30
35
40
50
45
0 5 10 15 20
Isotonic volume depletion Isovolemic osmotic increase
Figure 28-11 The effect of increased plasma osmolarity or decreased blood volume on the level of plasma (P) antidiuretic hormone (ADH), also called arginine vasopressin (AVP). (Redrawn from Dunn FL, Brennan TJ, Nelson AE, et al: The role of blood osmo- lality and volume in regulating vasopressin secretion in the rat. J Clin Invest 52(12):3212, 1973. By copyright permission of the American Society of Clinical Investigation.)
Increase ADH Decrease ADH
↑ Plasma osmolarity ↓ Plasma osmolarity
↓ Blood volume ↑ Blood volume
↓ Blood pressure ↑ Blood pressure
Nausea
Hypoxia
Drugs: Morphine Nicotine Cyclophosphamide
Drugs: Alcohol Clonidine (antihypertensive drug) Haloperidol (dopamine blocker)
Table 28-2 Regulation of ADH Secretion
Unit V The Body Fluids and Kidneys
358
Central Nervous System Centers for Thirst
Referring again to Figure 28-10, the same area along the anteroventral wall of the third ventricle that promotes ADH release also stimulates thirst. Located anterolat- erally in the preoptic nucleus is another small area that, when stimulated electrically, causes immediate drinking that continues as long as the stimulation lasts. All these areas together are called the thirst center.
The neurons of the thirst center respond to injec- tions of hypertonic salt solutions by stimulating drinking behavior. These cells almost certainly function as osmo- receptors to activate the thirst mechanism, in the same way that the osmoreceptors stimulate ADH release.
Increased osmolarity of the cerebrospinal fluid in the third ventricle has essentially the same effect to promote drinking. It is likely that the organum vasculosum of the lamina terminalis, which lies immediately beneath the ventricular surface at the inferior end of the AV3V region, is intimately involved in mediating this response.
Stimuli for Thirst
Table 28-3 summarizes some of the known stimuli for thirst. One of the most important is increased extracel- lular fluid osmolarity, which causes intracellular dehydra- tion in the thirst centers, thereby stimulating the sensation of thirst. The value of this response is obvious: it helps to dilute extracellular fluids and returns osmolarity toward normal.
Decreases in extracellular fluid volume and arterial pressure also stimulate thirst by a pathway that is inde- pendent of the one stimulated by increased plasma osmo- larity. Thus, blood volume loss by hemorrhage stimulates thirst even though there might be no change in plasma osmolarity. This probably occurs because of neural input from cardiopulmonary and systemic arterial barorecep- tors in the circulation.
A third important stimulus for thirst is angiotensin II. Studies in animals have shown that angiotensin II acts on the subfornical organ and on the organum vasculo- sum of the lamina terminalis. These regions are outside the blood-brain barrier, and peptides such as angiotensin II diffuse into the tissues. Because angiotensin II is also stimulated by factors associated with hypovolemia and low blood pressure, its effect on thirst helps to restore blood volume and blood pressure toward normal, along
with the other actions of angiotensin II on the kidneys to decrease fluid excretion.
Dryness of the mouth and mucous membranes of the esophagus can elicit the sensation of thirst. As a result, a thirsty person may receive relief from thirst almost imme- diately after drinking water, even though the water has not been absorbed from the gastrointestinal tract and has not yet had an effect on extracellular fluid osmolarity.
Gastrointestinal and pharyngeal stimuli influence thirst. In animals that have an esophageal opening to the exterior so that water is never absorbed into the blood, partial relief of thirst occurs after drinking, although the relief is only temporary. Also, gastrointestinal distention may partially alleviate thirst; for instance, simple inflation of a balloon in the stomach can relieve thirst. However, relief of thirst sensations through gastrointestinal or pha- ryngeal mechanisms is short-lived; the desire to drink is completely satisfied only when plasma osmolarity and/or blood volume returns to normal.
The ability of animals and humans to “meter” fluid intake is important because it prevents overhydration. After a person drinks water, 30 to 60 minutes may be required for the water to be reabsorbed and distributed throughout the body. If the thirst sensation were not tem- porarily relieved after drinking water, the person would continue to drink more and more, eventually leading to overhydration and excess dilution of the body fluids. Experimental studies have repeatedly shown that ani- mals drink almost exactly the amount necessary to return plasma osmolarity and volume to normal.
Threshold for Osmolar Stimulus of Drinking
The kidneys must continually excrete an obligatory amount of water even in a dehydrated person, to rid the body of excess solutes that are ingested or produced by metabolism. Water is also lost by evaporation from the lungs and the gastrointestinal tract and by evaporation and sweating from the skin. Therefore, there is always a tendency for dehydration, with resultant increased extra- cellular fluid sodium concentration and osmolarity.
When the sodium concentration increases only about 2 mEq/L above normal, the thirst mechanism is activated, causing a desire to drink water. This is called the thresh- old for drinking. Thus, even small increases in plasma osmolarity are normally followed by water intake, which restores extracellular fluid osmolarity and volume toward normal. In this way, the extracellular fluid osmolarity and sodium concentration are precisely controlled.
Integrated Responses of Osmoreceptor-ADH and Thirst Mechanisms in Controlling Extracellular Fluid Osmolarity and Sodium Concentration
In a healthy person, the osmoreceptor-ADH and thirst mechanisms work in parallel to precisely regulate extracel- lular fluid osmolarity and sodium concentration, despite the constant challenges of dehydration. Even with addi- tional challenges, such as high salt intake, these feedback
Increase Thirst Decrease Thirst
↑ Plasma osmolarity ↓ Plasma osmolarity
↓ Blood volume ↑ Blood volume
↓ Blood pressure ↑ Blood pressure
↑ Angiotensin II ↓ Angiotensin II
Dryness of mouth Gastric distention
Table 28-3 Control of Thirst
Chapter 28 Urine Concentration and Dilution; Regulation of Extracellular Fluid Osmolarity and Sodium Concentration
359
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systems are able to keep plasma osmolarity reasonably constant. Figure 28-12 shows that an increase in sodium intake to as high as six times normal has only a small effect on plasma sodium concentration as long as the ADH and thirst mechanisms are both functioning normally.
When either the ADH or the thirst mechanism fails, the other ordinarily can still control extracellular osmolar- ity and sodium concentration with reasonable effective- ness, as long as there is enough fluid intake to balance the daily obligatory urine volume and water losses caused by respiration, sweating, or gastrointestinal losses. However, if both the ADH and thirst mechanisms fail simultane- ously, plasma sodium concentration and osmolarity are poorly controlled; thus, when sodium intake is increased after blocking the total ADH-thirst system, relatively large changes in plasma sodium concentration occur. In the absence of the ADH-thirst mechanisms, no other feedback mechanism is capable of adequately regulating plasma sodium concentration and osmolarity.
Role of Angiotensin II and Aldosterone in Controlling Extracellular Fluid Osmolarity and Sodium Concentration
As discussed in Chapter 27, both angiotensin II and aldoster- one play an important role in regulating sodium reabsorption by the renal tubules. When sodium intake is low, increased levels of these hormones stimulate sodium reabsorption by the kidneys and, therefore, prevent large sodium losses, even though sodium intake may be reduced to as low as 10 percent of normal. Conversely, with high sodium intake, decreased formation of these hormones permits the kidneys to excrete large amounts of sodium.
Because of the importance of angiotensin II and aldoster- one in regulating sodium excretion by the kidneys, one might mistakenly infer that they also play an important role in reg- ulating extracellular fluid sodium concentration. Although these hormones increase the amount of sodium in the extra- cellular fluid, they also increase the extracellular fluid volume by increasing reabsorption of water along with the sodium. Therefore, angiotensin II and aldosterone have little effect on sodium concentration, except under extreme conditions.
This relative unimportance of aldosterone in regulat- ing extracellular fluid sodium concentration is shown by the experiment of Figure 28-13. This figure shows the effect on plasma sodium concentration of changing sodium intake more than sixfold under two conditions: (1) under normal conditions and (2) after the aldosterone feedback system was blocked by removing the adrenal glands and infusing the ani- mals with aldosterone at a constant rate so that plasma lev- els could not change upward or downward. Note that when sodium intake was increased sixfold, plasma concentration changed only about 1 to 2 percent in either case. This indi- cates that even without a functional aldosterone feedback sys- tem, plasma sodium concentration can be well regulated. The same type of experiment has been conducted after blocking angiotensin II formation, with the same result.
There are two primary reasons why changes in angio- tensin II and aldosterone do not have a major effect on plasma sodium concentration. First, as discussed earlier, angiotensin II and aldosterone increase both sodium and water reabsorp- tion by the renal tubules, leading to increases in extracellular fluid volume and sodium quantity but little change in sodium concentration. Second, as long as the ADH-thirst mechanism is functional, any tendency toward increased plasma sodium concentration is compensated for by increased water intake or increased plasma ADH secretion, which tends to dilute the extracellular fluid back toward normal. The ADH-thirst system far overshadows the angiotensin II and aldosterone systems for regulating sodium concentration under normal conditions. Even in patients with primary aldosteronism, who have extremely high levels of aldosterone, the plasma sodium concentration usually increases only about 3 to 5 mEq/L above normal.
Normal
ADH and thirst
systems blocked
P la
sm a
so d
iu m
c o
n ce
n tr
at io
n (
m E
q /L
)
Sodium intake (mEq/day)
136
152
148
144
140
0 30 60 90 120 150 180
Figure 28-12 Effect of large changes in sodium intake on extra- cellular fluid sodium concentration in dogs under normal con- ditions (red line) and after the antidiuretic hormone (ADH) and thirst feedback systems had been blocked (blue line). Note that control of extracellular fluid sodium concentration is poor in the absence of these feedback systems. (Courtesy Dr. David B. Young.)
P la
sm a
so d
iu m
c o
n ce
n tr
at io
n (m
E q
/L )
Sodium intake (mEq/L)
100
110
120
130
140
150
0 30 60
Normal
Aldosterone system blockedAldosterone system blocked
90 120 150 180 210
Figure 28-13 Effect of large changes in sodium intake on extracellu- lar fluid sodium concentration in dogs under normal conditions (red line) and after the aldosterone feedback system had been blocked (blue line). Note that sodium concentration is maintained relatively constant over this wide range of sodium intakes, with or without aldosterone feedback control. (Courtesy Dr. David B. Young.)
Unit V The Body Fluids and Kidneys
360
Under extreme conditions, caused by complete loss of aldosterone secretion because of adrenalectomy or in patients with Addison’s disease (severely impaired secre- tion or total lack of aldosterone), there is tremendous loss of sodium by the kidneys, which can lead to reductions in plasma sodium concentration. One of the reasons for this is that large losses of sodium eventually cause severe volume depletion and decreased blood pressure, which can activate the thirst mechanism through the cardiovascular reflexes. This leads to a further dilution of the plasma sodium con- centration, even though the increased water intake helps to minimize the decrease in body fluid volumes under these conditions.
Thus, there are extreme situations in which plasma sodium concentration may change significantly, even with a functional ADH-thirst mechanism. Even so, the ADH-thirst mechanism is by far the most powerful feedback system in the body for controlling extracellular fluid osmolarity and sodium concentration.
Salt-Appetite Mechanism for Controlling Extracellular Fluid Sodium Concentration and Volume
Maintenance of normal extracellular fluid volume and sodium concentration requires a balance between sodium excretion and sodium intake. In modern civilizations, sodium intake is almost always greater than necessary for homeostasis. In fact, the average sodium intake for individuals in industrial- ized cultures eating processed foods usually ranges between 100 and 200 mEq/day, even though humans can survive and function normally on 10 to 20 mEq/day. Thus, most people eat far more sodium than is necessary for homeostasis, and there is evidence that our usual high sodium intake may con- tribute to cardiovascular disorders such as hypertension.
Salt appetite is due in part to the fact that animals and humans like salt and eat it regardless of whether they are salt deficient. There is also a regulatory component to salt appe- tite in which there is a behavioral drive to obtain salt when there is sodium deficiency in the body. This is particularly important in herbivores, which naturally eat a low-sodium diet, but salt craving may also be important in humans who have extreme deficiency of sodium, such as occurs in Addison’s disease. In this instance, there is deficiency of aldosterone secretion, which causes excessive loss of sodium
in the urine and leads to decreased extracellular fluid volume and decreased sodium concentration; both of these changes elicit the desire for salt.
In general, the primary stimuli that increase salt appetite are those associated with sodium deficits and decreased blood volume or decreased blood pressure, associated with circula- tory insufficiency.
The neuronal mechanism for salt appetite is analogous to that of the thirst mechanism. Some of the same neuronal centers in the AV3V region of the brain seem to be involved because lesions in this region frequently affect both thirst and salt appetite simultaneously in animals. Also, circulatory reflexes elicited by low blood pressure or decreased blood volume affect both thirst and salt appetite at the same time.
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chapter 29
Renal Regulation of Potassium, Calcium, Phosphate, and Magnesium; Integration of Renal Mechanisms for
Control of Blood Volume and Extracellular Fluid Volume
chapter 29
Regulation of Extracellular Fluid Potassium Concentration and Potassium Excretion
Extracellular fluid potassium concentration normally is regulated precisely at about 4.2 mEq/L, seldom rising or falling more than ±0.3 mEq/L. This precise control is nec- essary because many cell functions are very sensitive to changes in extracellular fluid potassium concentration. For instance, an increase in plasma potassium concentra- tion of only 3 to 4 mEq/L can cause cardiac arrhythmias, and higher concentrations can lead to cardiac arrest or fibrillation.
A special difficulty in regulating extracellular potassium concentration is the fact that more than 98 percent of the total body potassium is contained in the cells and only 2 percent in the extracellular fluid (Figure 29-1). For a 70-kilo- gram adult, who has about 28 liters of intracellular fluid (40 percent of body weight) and 14 liters of extracellular fluid (20 percent of body weight), about 3920 mEq of potassium are inside the cells and only about 59 mEq are in the extra- cellular fluid. Also, the potassium contained in a single meal is often as high as 50 mEq, and the daily intake usu- ally ranges between 50 and 200 mEq/day; therefore, failure to rapidly rid the extracellular fluid of the ingested potas- sium could cause life-threatening hyperkalemia (increased plasma potassium concentration). Likewise, a small loss of potassium from the extracellular fluid could cause severe hypokalemia (low plasma potassium concentration) in the absence of rapid and appropriate compensatory responses.
Maintenance of balance between intake and output of potassium depends primarily on excretion by the kidneys because the amount excreted in the feces is only about 5 to 10 percent of the potassium intake. Thus, the mainte- nance of normal potassium balance requires the kidneys to adjust their potassium excretion rapidly and precisely in response to wide variations in intake, as is also true for most other electrolytes.
Control of potassium distribution between the extracel- lular and intracellular compartments also plays an impor- tant role in potassium homeostasis. Because more than
98 percent of the total body potassium is contained in the cells, they can serve as an overflow site for excess extracel- lular fluid potassium during hyperkalemia or as a source of potassium during hypokalemia. Thus, redistribution of potassium between the intracellular and extracellular fluid compartments provides a first line of defense against changes in extracellular fluid potassium concentration.
Regulation of Internal Potassium Distribution
After ingestion of a normal meal, extracellular fluid potassium concentration would rise to a lethal level if the ingested potassium did not rapidly move into the cells. For example, absorption of 40 mEq of potassium (the amount contained in a meal rich in vegetables and fruit) into an extracellular fluid volume of 14 liters would raise plasma potassium concentration by about 2.9 mEq/L if all the potassium remained in the extracellular compart- ment. Fortunately, most of the ingested potassium rapidly moves into the cells until the kidneys can eliminate the excess. Table 29-1 summarizes some of the factors that can influence the distribution of potassium between the intracellular and extracellular compartments.
Insulin Stimulates Potassium Uptake into Cells. Insulin is important for increasing cell potassium uptake after a meal. In people who have insulin deficiency owing to diabetes mellitus, the rise in plasma potassium concentration after eating a meal is much greater than normal. Injections of insulin, however, can help to correct the hyperkalemia.
Aldosterone Increases Potassium Uptake into Cells. Increased potassium intake also stimulates secretion of aldosterone, which increases cell potassium uptake. Excess aldosterone secretion (Conn’s syndrome) is almost invari- ably associated with hypokalemia, due in part to move- ment of extracellular potassium into the cells. Conversely, patients with deficient aldosterone production (Addison’s disease) often have clinically significant hyperkalemia due to accumulation of potassium in the extracellular space, as well as to renal retention of potassium.
β-Adrenergic Stimulation Increases Cellular Uptake of Potassium. Increased secretion of catecholamines,
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especially epinephrine, can cause movement of potassium from the extracellular to the intracellular fluid, mainly by activation of β2-adrenergic receptors. Conversely, treat- ment of hypertension with β-adrenergic receptor block- ers, such as propranolol, causes potassium to move out of the cells and creates a tendency toward hyperkalemia.
Acid-Base Abnormalities Can Cause Changes in Potassium Distribution. Metabolic acidosis increases extracellular potassium concentration, in part by causing loss of potassium from the cells, whereas metabolic alka- losis decreases extracellular fluid potassium concentration. Although the mechanisms responsible for the effect of hydrogen ion concentration on potassium internal distribu- tion are not completely understood, one effect of increased hydrogen ion concentration is to reduce the activity of the sodium-potassium adenosine triphosphatase (ATPase) pump. This in turn decreases cellular uptake of potassium and raises extracellular potassium concentration.
Cell Lysis Causes Increased Extracellular Potassium Concentration. As cells are destroyed, the large amounts of potassium contained in the cells are released into the
extracellular compartment. This can cause significant hyperkalemia if large amounts of tissue are destroyed, as occurs with severe muscle injury or with red blood cell lysis.
Strenuous Exercise Can Cause Hyperkalemia by Releasing Potassium from Skeletal Muscle. During prolonged exercise, potassium is released from skeletal muscle into the extracellular fluid. Usually the hyper- kalemia is mild, but it may be clinically significant after heavy exercise, especially in patients treated with β-adrenergic blockers or in individuals with insulin defi- ciency. In rare instances, hyperkalemia after exercise may be severe enough to cause cardiac arrhythmias and sudden death.
Increased Extracellular Fluid Osmolarity Causes Redistribution of Potassium from the Cells to Extracellular Fluid. Increased extracellular fluid osmo- larity causes osmotic flow of water out of the cells. The cellular dehydration increases intracellular potassium concentration, thereby promoting diffusion of potassium out of the cells and increasing extracellular fluid potas- sium concentration. Decreased extracellular fluid osmo- larity has the opposite effect.
Overview of Renal Potassium Excretion
Renal potassium excretion is determined by the sum of three processes: (1) the rate of potassium filtration (GFR multi- plied by the plasma potassium concentration), (2) the rate of potassium reabsorption by the tubules, and (3) the rate of potassium secretion by the tubules. The normal rate of potassium filtration by the glomerular capillaries is about 756 mEq/day (GFR, 180 L/day multiplied by plasma potas- sium, 4.2 mEq/L); this rate of filtration is relatively constant in healthy persons because of the autoregulatory mecha- nisms for GFR discussed previously and the precision with which plasma potassium concentration is regulated. Severe decreases in GFR in certain renal diseases, however, can cause serious potassium accumulation and hyperkalemia.
Figure 29-2 summarizes the tubular handling of potas- sium under normal conditions. About 65 percent of the filtered potassium is reabsorbed in the proximal tubule. Another 25 to 30 percent of the filtered potassium is reabsorbed in the loop of Henle, especially in the thick ascending part where potassium is actively co-transported along with sodium and chloride. In both the proximal tubule and the loop of Henle, a relatively constant frac- tion of the filtered potassium load is reabsorbed. Changes in potassium reabsorption in these segments can influ- ence potassium excretion, but most of the day-to-day variation of potassium excretion is not due to changes in reabsorption in the proximal tubule or loop of Henle.
Daily Variations in Potassium Excretion Are Caused Mainly by Changes in Potassium Secretion in Distal and Collecting Tubules. The most important sites for regulating potassium excretion are the principal cells
K+ intake 100 mEq/day
K+ output Urine 92 mEq/day Feces 8 mEq/day 100 mEq/day
Extracellular fluid K+
4.2 mEq/L x 14 L
Intracellular fluid K+
140 mEq/L x 28 L
59 mEq 3920 mEq
Figure 29-1 Normal potassium intake, distribution of potassium in the body fluids, and potassium output from the body.
Factors That Shift K+ into Cells (Decrease Extracellular [K+])
Factors That Shift K+ Out of Cells (Increase Extracellular [K+])
• Insulin • Aldosterone • β-adrenergic stimulation • Alkalosis
• Insulin deficiency (diabetes mellitus)
• Aldosterone deficiency (Addison’s disease)
• β-adrenergic blockade • Acidosis • Cell lysis • Strenuous exercise • Increased extracellular
fluid osmolarity
Table 29-1 Factors That Can Alter Potassium Distribution Between the Intracellular and Extracellular Fluid
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of the late distal tubules and cortical collecting tubules. In these tubular segments, potassium can at times be reabsorbed or at other times be secreted, depending on the needs of the body. With a normal potassium intake of 100 mEq/day, the kidneys must excrete about 92 mEq/ day (the remaining 8 mEq are lost in the feces). About 31 mEq/day of potassium are secreted into the distal and collecting tubules, accounting for about one third of the excreted potassium.
With high potassium intakes, the required extra excre- tion of potassium is achieved almost entirely by increasing the secretion of potassium into the distal and collecting tubules. In fact, with extremely high potassium diets, the rate of potassium excretion can exceed the amount of potassium in the glomerular filtrate, indicating a powerful mechanism for secreting potassium.
When potassium intake is low, the secretion rate of potassium in the distal and collecting tubules decreases, causing a reduction in urinary potassium secretion. With extreme reductions in potassium intake, there is net reabsorption of potassium in the distal seg- ments of the nephron, and potassium excretion can fall to 1 percent of the potassium in the glomerular filtrate (to <10 mEq/day). With potassium intakes below this level, severe hypokalemia can develop.
Thus, most of the day-to-day regulation of potassium excretion occurs in the late distal and cortical collecting tubules, where potassium can be either reabsorbed or secreted, depending on the needs of the body. In the next section, we consider the basic mechanisms of potassium secretion and the factors that regulate this process.
Potassium Secretion by Principal Cells of Late Distal and Cortical Collecting Tubules
The cells in the late distal and cortical collecting tubules that secrete potassium are called principal cells and make up about 90 percent of the epithelial cells in these regions. Figure 29-3 shows the basic cellular mechanisms of potas- sium secretion by the principal cells.
Secretion of potassium from the blood into the tubular lumen is a two-step process, beginning with uptake from the interstitium into the cell by the sodium-potassium ATPase pump in the basolateral cell membrane; this pump moves sodium out of the cell into the interstitium and at the same time moves potassium to the interior of the cell.
The second step of the process is passive diffusion of potassium from the interior of the cell into the tubular fluid. The sodium-potassium ATPase pump creates a high intracellular potassium concentration, which provides the driving force for passive diffusion of potassium from the cell into the tubular lumen. The luminal membrane of the principal cells is highly permeable to potassium. One rea- son for this high permeability is that special channels are specifically permeable to potassium ions, thus allowing these ions to rapidly diffuse across the membrane.
Control of Potassium Secretion by Principal Cells. The primary factors that control potassium secretion by the principal cells of the late distal and cortical collect- ing tubules are (1) the activity of the sodium-potassium ATPase pump, (2) the electrochemical gradient for potas- sium secretion from the blood to the tubular lumen,
65% (491 mEq/day) 4%
(31 mEq/day)
12% (92 mEq/day)
756 mEq/day (180 L/day x 4.2 mEq/L)
27% (204 mEq/day)
Figure 29-2 Renal tubular sites of potassium reabsorption and secretion. Potassium is reabsorbed in the proximal tubule and in the ascending loop of Henle, so only about 8 percent of the filtered load is delivered to the distal tubule. Secretion of potassium into the late distal tubules and collecting ducts adds to the amount delivered; therefore, the daily excretion is about 12 percent of the potassium filtered at the glomerular capillaries. The percentages indicate how much of the filtered load is reabsorbed or secreted into the different tubular segments.
Na+Na+
K+K+
Na+Na+
Na+Na+
K+K+
ENaCENaC
Renal interstitial
fluid
Renal interstitial
fluid
Tubular lumen Tubular lumen
Principal cells
Principal cells
0 mV0 mV −50 mV−50 mV−70 mV−70 mV
K+K+ ATP
Figure 29-3 Mechanisms of potassium secretion and sodium reabsorption by the principal cells of the late distal and collecting tubules.
Unit V The Body Fluids and Kidneys
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and (3) the permeability of the luminal membrane for potassium. These three determinants of potassium secre- tion are in turn regulated by the factors discussed later.
Intercalated Cells Can Reabsorb Potassium During Potassium Depletion. In circumstances associated with severe potassium depletion, there is a cessation of potassium secretion and actually a net reabsorption of potassium in the late distal and collecting tubules. This reabsorption occurs through the intercalated cells; although this reabsorptive process is not completely understood, one mechanism believed to contribute is a hydrogen-potassium ATPase transport mechanism located in the luminal membrane. This transporter reab- sorbs potassium in exchange for hydrogen ions secreted into the tubular lumen, and the potassium then diffuses through the basolateral membrane of the cell into the blood. This transporter is necessary to allow potassium reabsorption during extracellular fluid potassium deple- tion, but under normal conditions it plays only a small role in controlling potassium excretion.
Summary of Factors That Regulate Potassium Secretion: Plasma Potassium Concentration, Aldosterone, Tubular Flow Rate, and Hydrogen Ion Concentration
Because normal regulation of potassium excretion occurs mainly as a result of changes in potassium secretion by the principal cells of the late distal and collecting tubules, in this chapter we discuss the primary factors that influ- ence secretion by these cells. The most important factors that stimulate potassium secretion by the principal cells include (1) increased extracellular fluid potassium con- centration, (2) increased aldosterone, and (3) increased tubular flow rate.
One factor that decreases potassium secretion is increased hydrogen ion concentration (acidosis).
Increased Extracellular Fluid Potassium Concen- tration Stimulates Potassium Secretion. The rate of potassium secretion in the late distal and cortical collect- ing tubules is directly stimulated by increased extracellu- lar fluid potassium concentration, leading to increases in potassium excretion, as shown in Figure 29-4. This effect is especially pronounced when extracellular fluid potas- sium concentration rises above about 4.1 mEq/L, slightly less than the normal concentration. Increased plasma potassium concentration, therefore, serves as one of the most important mechanisms for increasing potassium secretion and regulating extracellular fluid potassium ion concentration.
Increased extracellular fluid potassium concentra- tion raises potassium secretion by three mechanisms: (1) Increased extracellular fluid potassium concentration stimulates the sodium-potassium ATPase pump, thereby increasing potassium uptake across the basolateral mem- brane. This in turn increases intracellular potassium ion
concentration, causing potassium to diffuse across the luminal membrane into the tubule. (2) Increased extra- cellular potassium concentration increases the potassium gradient from the renal interstitial fluid to the interior of the epithelial cell; this reduces back leakage of potassium ions from inside the cells through the basolateral mem- brane. (3) Increased potassium concentration stimulates aldosterone secretion by the adrenal cortex, which further stimulates potassium secretion, as discussed next.
Aldosterone Stimulates Potassium Secretion. Aldosterone stimulates active reabsorption of sodium ions by the principal cells of the late distal tubules and collecting ducts (see Chapter 27). This effect is mediated through a sodium-potassium ATPase pump that trans- ports sodium outward through the basolateral membrane of the cell and into the blood at the same time that it pumps potassium into the cell. Thus, aldosterone also has a powerful effect to control the rate at which the principal cells secrete potassium.
A second effect of aldosterone is to increase the per- meability of the luminal membrane for potassium, further adding to the effectiveness of aldosterone in stimulating potassium secretion. Therefore, aldosterone has a pow- erful effect to increase potassium excretion, as shown in Figure 29-4.
Increased Extracellular Potassium Ion Concentra- tion Stimulates Aldosterone Secretion. In negative feedback control systems, the factor that is controlled usually has a feedback effect on the controller. In the case of the aldosterone-potassium control system, the rate of aldosterone secretion by the adrenal gland is controlled
U ri
n ar
y p
o ta
ss iu
m e
xc re
ti o
n (t
im es
n o
rm al
)
4
3
2
1
0 0
11
Effect of aldosteroneEffect of aldosterone
Effect of extracellular K+ concentration Effect of extracellular K+ concentration
22 33 5544 Extracellular potassium concentration
(mEq/L) Extracellular potassium concentration
(mEq/L)
1 2 3 54 Plasma aldosterone (times normal)
Figure 29-4 Effect of plasma aldosterone concentration (red line) and extracellular potassium ion concentration (black line) on the rate of urinary potassium excretion. These factors stimulate potassium secretion by the principal cells of the cortical collecting tubules. (Drawn from data in Young DB, Paulsen AW: Interrelated effects of aldosterone and plasma potassium on potassium excre- tion. Am J Physiol 244:F28, 1983.)
Chapter 29 Renal Regulation of Potassium, Calcium, Phosphate, and Magnesium
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strongly by extracellular fluid potassium ion concentra- tion. Figure 29-5 shows that an increase in plasma potas- sium concentration of about 3 mEq/L can increase plasma aldosterone concentration from nearly 0 to as high as 60 ng/100 ml, a concentration almost 10 times normal.
The effect of potassium ion concentration to stimu- late aldosterone secretion is part of a powerful feedback system for regulating potassium excretion, as shown in Figure 29-6. In this feedback system, an increase in plasma potassium concentration stimulates aldoster- one secretion and, therefore, increases the blood level of aldosterone (block 1). The increase in blood aldosterone then causes a marked increase in potassium excretion by the kidneys (block 2). The increased potassium excretion then reduces the extracellular fluid potassium concentra- tion back toward normal (blocks 3 and 4). Thus, this feed- back mechanism acts synergistically with the direct effect of increased extracellular potassium concentration to ele- vate potassium excretion when potassium intake is raised (Figure 29-7).
Blockade of Aldosterone Feedback System Greatly Impairs Control of Potassium Concentration. In the absence of aldosterone secretion, as occurs in patients with Addison’s disease, renal secretion of potassium is impaired, thus causing extracellular fluid potassium con- centration to rise to dangerously high levels. Conversely, with excess aldosterone secretion (primary aldoster- onism), potassium secretion becomes greatly increased, causing potassium loss by the kidneys and thus leading to hypokalemia.
In addition to its stimulatory effect on renal secretion of potassium, aldosterone also increases cellular uptake of potassium, which contributes to the powerful aldosterone- potassium feedback system, as discussed previously.
The special quantitative importance of the aldosterone feedback system in controlling potassium concentration is shown in Figure 29-8. In this experiment, potassium intake was increased almost sevenfold in dogs under two conditions: (1) under normal conditions and (2) after the aldosterone feedback system had been blocked by remov- ing the adrenal glands and placing the animals on a fixed rate of aldosterone infusion so that plasma aldosterone concentration could neither increase nor decrease.
Note that in the normal animals, a sevenfold increase in potassium intake caused only a slight increase in potas- sium concentration, from 4.2 to 4.3 mEq/L. Thus, when the aldosterone feedback system is functioning normally, potassium concentration is precisely controlled, despite large changes in potassium intake.
When the aldosterone feedback system was blocked, the same increases in potassium intake caused a much larger increase in potassium concentration, from 3.8 to almost 4.7 mEq/L. Thus, control of potassium concentra- tion is greatly impaired when the aldosterone feedback system is blocked. A similar impairment of potassium regulation is observed in humans with poorly functioning aldosterone feedback systems, such as occurs in patients
A p
p ro
xi m
at e
p la
sm a
al d
o st
er o
n e
co n
ce n
tr at
io n
( n
g /1
00 m
l p la
sm a)
0
70
60
50
40
30
20
10
3.0 6.55.54.5 6.03.5 4.0 5.0
Serum potassium concentration (mEq/L)Serum potassium concentration (mEq/L)
Figure 29-5 Effect of extracellular fluid potassium ion concen- tration on plasma aldosterone concentration. Note that small changes in potassium concentration cause large changes in aldo- sterone concentration.
Ald.Ald.
11
K+K+ 44
33
Ald.Ald. 22
K+ excretionK+ excretion
K+
excretion K+
excretion
K+
concentration K+
concentration
Aldosterone concentration Aldosterone
concentration
++ −−
K+ intakeK+ intake
Figure 29-6 Basic feedback mechanism for control of extracellu- lar fluid potassium concentration by aldosterone (Ald.).
K+ intake
K+ secretion
Aldosterone
Plasma K+
concentration
K+ secretion
Cortical collecting tubules
Figure 29-7 Primary mechanisms by which high potassium intake raises potassium excretion. Note that increased plasma potassium concentration directly raises potassium secretion by the cortical collecting tubules and indirectly increases potassium secretion by raising plasma aldosterone concentration.
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with either primary aldosteronism (too much aldoster- one) or Addison’s disease (too little aldosterone).
Increased Distal Tubular Flow Rate Stimulates Potassium Secretion. A rise in distal tubular flow rate, as occurs with volume expansion, high sodium intake, or treatment with some diuretics, stimulates potassium secretion (Figure 29-9). Conversely, a decrease in distal tubular flow rate, as caused by sodium depletion, reduces potassium secretion.
The effect of tubular flow rate on potassium secretion in the distal and collecting tubules is strongly influenced by potassium intake. When potassium intake is high, increased tubular flow rate has a much greater effect to stimulate potassium secretion than when potassium intake is low (see Figure 29-9).
The mechanism for the effect of high-volume flow rate is as follows: When potassium is secreted into the tubular fluid, the luminal concentration of potassium increases, thereby reducing the driving force for potassium diffusion across the luminal membrane. With increased tubular flow rate, the secreted potassium is continuously flushed down the tubule, so the rise in tubular potassium con- centration becomes minimized. Therefore, net potassium secretion is stimulated by increased tubular flow rate.
The effect of increased tubular flow rate is especially important in helping to preserve normal potassium excretion during changes in sodium intake. For example, with a high sodium intake, there is decreased aldoster- one secretion, which by itself would tend to decrease the rate of potassium secretion and, therefore, reduce urinary excretion of potassium. However, the high distal tubular flow rate that occurs with a high sodium intake tends to increase potassium secretion (Figure 29-10), as discussed in the previous paragraph. Therefore, the two effects of high sodium intake, decreased aldosterone secretion and
the high tubular flow rate, counterbalance each other, so there is little change in potassium excretion. Likewise, with a low sodium intake, there is little change in potas- sium excretion because of the counterbalancing effects of increased aldosterone secretion and decreased tubular flow rate on potassium secretion.
P la
sm a
p o
ta ss
iu m
c o
n ce
n tr
at io
n (m
E q
/d ay
)
3.8
4.8
4.6
4.4
4.2
4.0
0 30 60 90 120 150 180 210
Potassium intake (mEq/day)Potassium intake (mEq/day)
Aldosterone system blocked
Aldosterone system blocked
NormalNormal
Figure 29-8 Effect of large changes in potassium intake on extra- cellular fluid potassium concentration under normal conditions (red line) and after the aldosterone feedback had been blocked (blue line). Note that after blockade of the aldosterone system, regulation of potassium concentration was greatly impaired. (Courtesy Dr. David B. Young.)
P o
ta ss
iu m
s ec
re ti
o n
r at
e (p
m o
l/m in
)
10
20
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40
70
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0 5 10 15 20 25 30
Tubular flow rate (nl/min)Tubular flow rate (nl/min)
Normal potassium dietNormal potassium diet
Low potassium dietLow potassium diet
High potassium dietHigh potassium diet
Figure 29-9 Relationship between flow rate in the cortical collect- ing tubules and potassium secretion and the effect of changes in potassium intake. Note that a high dietary potassium intake greatly enhances the effect of increased tubular flow rate to increase potas- sium secretion. The shaded bar shows the approximate normal tubular flow rate under most physiological conditions. (Data from Malnic G, Berliner RW, Giebisch G. Am J Physiol 256:F932, 1989.)
− +
Na+ intake
Aldosterone GFR
Unchanged K+
excretion
Distal tubular flow rate
K+ secretion
Cortical collecting ducts
Proximal tubular Na+
reabsorption
Figure 29-10 Effect of high sodium intake on renal excretion of potassium. Note that a high-sodium diet decreases plasma aldosterone, which tends to decrease potassium secretion by the cortical collecting tubules. However, the high-sodium diet simul- taneously increases fluid delivery to the cortical collecting duct, which tends to increase potassium secretion. The opposing effects of a high-sodium diet counterbalance each other, so there is little change in potassium excretion.
Chapter 29 Renal Regulation of Potassium, Calcium, Phosphate, and Magnesium
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Acute Acidosis Decreases Potassium Secretion. Acute increases in hydrogen ion concentration of the extracellular fluid (acidosis) reduce potassium secretion, whereas decreased hydrogen ion concentration (alkalo- sis) increases potassium secretion. The primary mecha- nism by which increased hydrogen ion concentration inhibits potassium secretion is by reducing the activ- ity of the sodium-potassium ATPase pump. This in turn decreases intracellular potassium concentration and sub- sequent passive diffusion of potassium across the luminal membrane into the tubule.
With more prolonged acidosis, lasting over a period of several days, there is an increase in urinary potassium excretion. The mechanism for this effect is due in part to an effect of chronic acidosis to inhibit proximal tubular sodium chloride and water reabsorption, which increases distal volume delivery, thereby stimulating the secretion of potassium. This effect overrides the inhibitory effect of hydrogen ions on the sodium-potassium ATPase pump. Thus, chronic acidosis leads to a loss of potassium, whereas acute acidosis leads to decreased potassium excretion.
Beneficial Effects of a Diet High in Potassium and Low in Sodium Content. For most of human history, the typical diet has been one that is low in sodium and high in potassium content, compared with the typical modern diet. In isolated populations that have not experienced industrialization, such as the Yanomamo tribe living in the Amazon of Northern Brazil, sodium intake may be as low as 10 to 20 mmol/day while potassium intake may be as high as 200 mmol/day. This is due to their consumption of a diet containing large amounts of fruits and vegetables and no processed foods. Populations consuming this type of diet typically do not experience age-related increases in blood pressure and cardiovascular diseases.
With industrialization and increased consumption of processed foods, which often have high sodium and low potassium content, there have been dramatic increases in sodium intake and decreases in potassium intake. In most industrialized countries potassium consumption averages only 30 to 70 mmol/day while sodium intake averages 140 to 180 mmol/day.
Experimental and clinical studies have shown that the combination of high sodium and low potassium intake increases the risk for hypertension and associated cardio- vascular and kidney diseases. A diet rich in potassium, however, seems to protect against the adverse effects of a high-sodium diet, reducing blood pressure and the risk for stroke, coronary artery disease, and kidney disease. The ben- eficial effects of increasing potassium intake are especially apparent when combined with a low-sodium diet.
Dietary guidelines published by U.S. National Academy of Sciences, the American Heart Association, and other organizations recommend reducing dietary intake sodium chloride to around 65 mmol/day (corresponding to 1.5 g/day of sodium or 3.8 g/day sodium chloride), while increasing potassium intake to 120 mmol/day (4.7 g/day) for healthy adults.
Control of Renal Calcium Excretion and Extracellular Calcium Ion Concentration
The mechanisms for regulating calcium ion concentra- tion are discussed in detail in Chapter 79, along with the endocrinology of the calcium-regulating hormones, parathyroid hormone (PTH), and calcitonin. Therefore, calcium ion regulation is discussed only briefly in this chapter.
Extracellular fluid calcium ion concentration nor- mally remains tightly controlled within a few percent- age points of its normal level, 2.4 mEq/L. When calcium ion concentration falls to low levels (hypocalcemia), the excitability of nerve and muscle cells increases markedly and can in extreme cases result in hypocalcemic tetany. This is characterized by spastic skeletal muscle contrac- tions. Hypercalcemia (increased calcium concentration) depresses neuromuscular excitability and can lead to cardiac arrhythmias.
About 50 percent of the total calcium in the plasma (5 mEq/L) exists in the ionized form, which is the form that has biological activity at cell membranes. The remainder is either bound to the plasma proteins (about 40 percent) or complexed in the non-ionized form with anions such as phosphate and citrate (about 10 percent).
Changes in plasma hydrogen ion concentration can influence the degree of calcium binding to plasma pro- teins. With acidosis, less calcium is bound to the plasma proteins. Conversely, in alkalosis, a greater amount of cal- cium is bound to the plasma proteins. Therefore, patients with alkalosis are more susceptible to hypocalcemic tetany.
As with other substances in the body, the intake of cal- cium must be balanced with the net loss of calcium over the long term. Unlike ions such as sodium and chloride, how- ever, a large share of calcium excretion occurs in the feces. The usual rate of dietary calcium intake is about 1000 mg/ day, with about 900 mg/day of calcium excreted in the feces. Under certain conditions, fecal calcium excretion can exceed calcium ingestion because calcium can also be secreted into the intestinal lumen. Therefore, the gastrointestinal tract and the regulatory mechanisms that influence intestinal calcium absorption and secretion play a major role in calcium homeo- stasis, as discussed in Chapter 79.
Almost all the calcium in the body (99 percent) is stored in the bone, with only about 0.1 percent in the extracellular fluid and 1.0 percent in the intracellular fluid and cell organelles. The bone, therefore, acts as a large reservoir for storing calcium and as a source of calcium when extracellular fluid calcium concentration tends to decrease.
One of the most important regulators of bone uptake and release of calcium is PTH. When extracellular fluid calcium concentration falls below normal, the parathyroid glands are directly stimulated by the low calcium levels to pro- mote increased secretion of PTH. This hormone then acts directly on the bones to increase the resorption of bone salts (release of salts from the bones) and to release large
Unit V The Body Fluids and Kidneys
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amounts of calcium into the extracellular fluid, thereby returning calcium levels back toward normal. When calcium ion concentration is elevated, PTH secretion decreases, so almost no bone resorption occurs; instead, excess calcium is deposited in the bones. Thus, the day-to-day regulation of calcium ion concentration is mediated in large part by the effect of PTH on bone resorption.
The bones, however, do not have an inexhaustible sup- ply of calcium. Therefore, over the long term, the intake of calcium must be balanced with calcium excretion by the gastrointestinal tract and the kidneys. The most important regulator of calcium reabsorption at both of these sites is PTH. Thus, PTH regulates plasma calcium concentration through three main effects: (1) by stimulating bone resorp- tion; (2) by stimulating activation of vitamin D, which then increases intestinal reabsorption of calcium; and (3) by directly increasing renal tubular calcium reabsorption (Figure 29-11). The control of gastrointestinal calcium reabsorption and calcium exchange in the bones is dis- cussed elsewhere, and the remainder of this section focuses on the mechanisms that control renal calcium excretion.
Control of Calcium Excretion by the Kidneys
Calcium is both filtered and reabsorbed in the kidneys but not secreted. Therefore, the rate of renal calcium excre- tion is calculated as
Renal calcium excretion = Calcium filtered − Calcium reabsorbed
Only about 50 percent of the plasma calcium is ionized, with the remainder being bound to the plasma proteins or complexed with anions such as phosphate. Therefore, only about 50 percent of the plasma calcium can be filtered at the glomerulus. Normally, about 99 percent of the filtered cal- cium is reabsorbed by the tubules, with only about 1 percent of the filtered calcium being excreted. About 65 percent of the filtered calcium is reabsorbed in the proximal tubule, 25 to 30 percent is reabsorbed in the loop of Henle, and 4 to 9 percent is reabsorbed in the distal and collecting tubules. This pattern of reabsorption is similar to that for sodium.
As is true with the other ions, calcium excretion is adjusted to meet the body’s needs. With an increase
in calcium intake, there is also increased renal calcium excretion, although much of the increase of calcium intake is eliminated in the feces. With calcium depletion, calcium excretion by the kidneys decreases as a result of enhanced tubular reabsorption.
Proximal Tubular Calcium Reabsorption. Most of the calcium reabsorption in the proximal tubule occurs through the paracellular pathway, dissolved in water and carried with the reabsorbed fluid as it flows between the cells. Only about 20% of proximal tubular calcium reab- sorption occurs through the transcellular pathway in two steps: (1) calcium diffuses from the tubular lumen into the cell down an electrochemical gradient due to the much higher concentration of calcium in the tubular lumen, compared with the epithelial cell cytoplasm, and because the cell interior has a negative relative to the tubular lumen; (2) calcium exits the cell across the basolateral membrane by a calcium-ATPase pump and by sodium- calcium counter-transporter (Figure 29-12).
Loop of Henle and Distal Tubule Calcium Reabsorp- tion. In the loop of Henle, calcium reabsorption is restricted to the thick ascending limb. Approximately 50% of calcium reabsorption in the thick ascending limb occurs through the paracellular route by passive diffusion due to the slight positive charge of the tubular lumen relative to the interstitial fluid. The remaining 50% of calcium reab- sorption in the thick ascending limb occurs through the transcellular pathway, a process that is stimulated by PTH.
In the distal tubule, calcium reabsorption occurs almost entirely by active transport through the cell mem- brane. The mechanism for this active transport is simi- lar to that in the proximal tubule and thick ascending limb and involves diffusion across the luminal membrane through calcium channels and exit across the basolat- eral membrane by a calcium-ATPase pump, as well as a
Vitamin D3 activation PTH
[Ca++]
Intestinal Ca++
reabsorption Renal Ca++
reabsorption Ca++ release from bones
Figure 29-11 Compensatory responses to decreased plasma ion- ized calcium concentration mediated by parathyroid hormone (PTH) and vitamin D.
ATP
3 Na+ 3 Na+
Ca++Ca++
Ca++Ca++ Ca++Ca++
H2OH2O
H2OH2O
Ca++Ca++
Ca++Ca++
Renal interstitial
fluid
Renal interstitial
fluid
Tubular lumen Tubular lumenProximal tubular cellsProximal tubular cells
Figure 29-12 Mechanisms of calcium reabsorption by paracellu- lar and transcellular pathways in the proximal tubular cells.
Chapter 29 Renal Regulation of Potassium, Calcium, Phosphate, and Magnesium
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sodium-calcium counter transport mechanism. In this segment, as well as in the loops of Henle, PTH stimu- lates calcium reabsorption. Vitamin D (Calcitrol) and cal- citonin also stimulate calcium reabsorption in the thick ascending limb of Henle’s loop and in the distal tubule, although these hormones are not as important quantita- tively as PTH in reducing renal calcium excretion.
Factors that Regulate Tubular Calcium Reabsorp- tion. One of the primary controllers of renal tubular calcium reabsorption is PTH. Increased levels of PTH stimulate calcium reabsorption in the thick ascending loops of Henle and distal tubules, which reduces urinary excretion of calcium. Conversely, reduction of PTH pro- motes calcium excretion by decreasing reabsorption in the loops of Henle and distal tubules.
In the proximal tubule, calcium reabsorption usually parallels sodium and water reabsorption and is inde- pendent of PTH. Therefore, in instances of extracellular volume expansion or increased arterial pressure—both of which decrease proximal sodium and water reab- sorption—there is also reduction in calcium reabsorp- tion and, consequently, increased urinary excretion of calcium. Conversely, with extracellular volume contrac- tion or decreased blood pressure, calcium excretion decreases primarily because of increased proximal tubular reabsorption.
Another factor that influences calcium reabsorption is the plasma concentration of phosphate. Increased plasma phosphate stimulates PTH, which increases calcium reab- sorption by the renal tubules, thereby reducing calcium excretion. The opposite occurs with reduction in plasma phosphate concentration.
Calcium reabsorption is also stimulated by metabolic acidosis and inhibited by metabolic alkalosis. Most of the effect of hydrogen ion concentration on calcium excre- tion results from changes in calcium reabsorption in the distal tubule.
A summary of the factors that are known to influ- ence calcium excretion by the renal tubules is shown in Table 29-2.
Regulation of Renal Phosphate Excretion
Phosphate excretion by the kidneys is controlled pri- marily by an overflow mechanism that can be explained as follows: The renal tubules have a normal transport
maximum for reabsorbing phosphate of about 0.1 mM/ min. When less than this amount of phosphate is present in the glomerular filtrate, essentially all the filtered phos- phate is reabsorbed. When more than this is present, the excess is excreted. Therefore, phosphate normally begins to spill into the urine when its concentration in the extra- cellular fluid rises above a threshold of about 0.8 mM/L, which gives a tubular load of phosphate of about 0.1 mM/ min, assuming a GFR of 125 ml/min. Because most peo- ple ingest large quantities of phosphate in milk products and meat, the concentration of phosphate is usually main- tained above 1 mM/L, a level at which there is continual excretion of phosphate into the urine.
The proximal tubule normally reabsorbs 75 to 80 per- cent of the filtered phosphate. The distal tubule reabsorbs about 10 percent of the filtered load, and only very small amounts are reabsorbed in the loop of Henle, collecting tubules, and collecting ducts. Approximately 10 percent of the filtered phosphate is excreted in the urine.
In the proximal tubule, phosphate reabsorption occurs mainly through the transcellular pathway. Phosphate enters the cell from the lumen by a sodium-phosphate co-transporter and exits the cell across the basolateral membrane by a process that is not well understood but may involve a counter transport mechanism in which phosphate is exchanged for an anion.
Changes in tubular phosphate reabsorptive capacity can also occur in different conditions and influence phosphate excretion. For instance, a diet low in phosphate can, over time, increase the reabsorptive transport maxi- mum for phosphate, thereby reducing the tendency for phosphate to spill over into the urine.
PTH can play a significant role in regulating phosphate concentration through two effects: (1) PTH promotes bone resorption, thereby dumping large amounts of phos- phate ions into the extracellular fluid from the bone salts, and (2) PTH decreases the transport maximum for phos- phate by the renal tubules, so a greater proportion of the tubular phosphate is lost in the urine. Thus, whenever plasma PTH is increased, tubular phosphate reabsorp- tion is decreased and more phosphate is excreted. These interrelations among phosphate, PTH, and calcium are discussed in more detail in Chapter 79.
Control of Renal Magnesium Excretion and Extracellular Magnesium Ion Concentration
More than one half of the body’s magnesium is stored in the bones. Most of the rest resides within the cells, with less than 1 percent located in the extracellular fluid. Although the total plasma magnesium concentration is about 1.8 mEq/L, more than one half of this is bound to plasma proteins. Therefore, the free ionized concentra- tion of magnesium is only about 0.8 mEq/L.
The normal daily intake of magnesium is about 250 to 300 mg/day, but only about one half of this intake is absorbed by the gastrointestinal tract. To maintain
↓ Calcium Excretion ↑ Calcium Excretion
↑Parathyroid hormone (PTH) ↓ PTH ↓ Extracellular fluid volume ↑ Extracellular fluid volume ↓ Blood pressure ↑ Blood pressure ↑ Plasma phosphate ↓ Plasma phosphate Metabolic acidosis Metabolic alkalosis Vitamin D
3
Table 29-2 Factors That Alter Renal Calcium Excretion
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Unit V The Body Fluids and Kidneys
magnesium balance, the kidneys must excrete this absorbed magnesium, about one half the daily intake of magnesium, or 125 to 150 mg/day. The kidneys normally excrete about 10 to 15 percent of the magnesium in the glomerular filtrate.
Renal excretion of magnesium can increase markedly during magnesium excess or decrease to almost nil during magnesium depletion. Because magnesium is involved in many biochemical processes in the body, including activa- tion of many enzymes, its concentration must be closely regulated.
Regulation of magnesium excretion is achieved mainly by changing tubular reabsorption. The proximal tubule usually reabsorbs only about 25 percent of the filtered magnesium. The primary site of reabsorption is the loop of Henle, where about 65 percent of the filtered load of magnesium is reabsorbed. Only a small amount (usually <5 percent) of the filtered magnesium is reabsorbed in the distal and collecting tubules.
The mechanisms that regulate magnesium excretion are not well understood, but the following disturbances lead to increased magnesium excretion: (1) increased extracellular fluid magnesium concentration, (2) extra- cellular volume expansion, and (3) increased extracellular fluid calcium concentration.
Integration of Renal Mechanisms for Control of Extracellular Fluid
Extracellular fluid volume is determined mainly by the balance between intake and output of water and salt. In many instances, salt and fluid intakes are dictated by a person’s habits rather than by physiologic control mecha- nisms. Therefore, the burden of extracellular volume reg- ulation is usually placed on the kidneys, which must adapt their excretion of salt and water to match intake of salt and water under steady-state conditions.
In discussing the regulation of extracellular fluid vol- ume, we consider the factors that regulate the amount of sodium chloride in the extracellular fluid because changes in extracellular fluid sodium chloride content usually cause parallel changes in extracellular fluid vol- ume, provided the antidiuretic hormone (ADH)-thirst mechanisms are operative. When the ADH-thirst mecha- nisms are functioning normally, a change in the amount of sodium chloride in the extracellular fluid is matched by a similar change in the amount of extracellular water, so osmolality and sodium concentration are maintained relatively constant.
Sodium Intake and Excretion Are Precisely Matched Under Steady-State Conditions
An important consideration in overall control of sodium excretion—or excretion of most electrolytes, for that matter—is that under steady-state conditions, excre- tion by the kidneys is determined by intake. To maintain life, a person must, over the long term, excrete almost
precisely the amount of sodium ingested. Therefore, even with disturbances that cause major changes in kidney function, balance between intake and output of sodium usually is restored within a few days.
If disturbances of kidney function are not too severe, sodium balance may be achieved mainly by intrarenal adjustments with minimal changes in extracellular fluid volume or other systemic adjustments. But when per- turbations to the kidneys are severe and intrarenal com- pensations are exhausted, systemic adjustments must be invoked, such as changes in blood pressure, changes in circulating hormones, and alterations of sympathetic ner- vous system activity.
These adjustments can be costly in terms of overall homeostasis because they cause other changes throughout the body that may, in the long run, be damaging. For exam- ple, impaired kidney function may lead to increased blood pressure that, in turn, helps to maintain normal sodium excretion. Over the long term the high blood pressure may cause injury to the blood vessels, heart, and other organs. These compensations, however, are necessary because a sustained imbalance between fluid and electrolyte intake and excretion would quickly lead to accumulation or loss of electrolytes and fluid, causing cardiovascular collapse within a few days. Thus, one can view the systemic adjust- ments that occur in response to abnormalities of kidney function as a necessary trade-off that brings electrolyte and fluid excretion back in balance with intake.
Sodium Excretion Is Controlled by Altering Glomerular Filtration or Tubular Sodium Reabsorption Rates
The two variables that influence sodium and water excre- tion are the rates of glomerular filtration and tubular reabsorption:
Excretion = Glomerular filtration − Tubular reabsorption
GFR normally is about 180 L/day, tubular reabsorp- tion is 178.5 L/day, and urine excretion is 1.5 L/day. Thus, small changes in GFR or tubular reabsorption potentially can cause large changes in renal excretion. For example, a 5 percent increase in GFR (to 189 L/day) would cause a 9 L/day increase in urine volume, if tubular compensa- tions did not occur; this would quickly cause catastrophic changes in body fluid volumes. Similarly, small changes in tubular reabsorption, in the absence of compensatory adjustments of GFR, would also lead to dramatic changes in urine volume and sodium excretion. Tubular reabsorp- tion and GFR usually are regulated precisely, so excretion by the kidneys can be exactly matched to intake of water and electrolytes.
Even with disturbances that alter GFR or tubular reab- sorption, changes in urinary excretion are minimized by various buffering mechanisms. For example, if the kidneys become greatly vasodilated and GFR increases (as can occur with certain drugs or high fever), this raises sodium chloride delivery to the tubules, which in turn leads to at least two intrarenal compensations: (1) increased tubular
Chapter 29 Integration of Renal Mechanisms
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reabsorption of much of the extra sodium chloride filtered, called glomerulotubular balance, and (2) macula densa feedback, in which increased sodium chloride delivery to the distal tubule causes afferent arteriolar constriction and return of GFR toward normal. Likewise, abnormali- ties of tubular reabsorption in the proximal tubule or loop of Henle are partially compensated for by these same intrarenal feedbacks.
Because neither of these two mechanisms operates per- fectly to restore distal sodium chloride delivery all the way back to normal, changes in either GFR or tubular reab- sorption can lead to significant changes in urine sodium and water excretion. When this happens, other feedback mechanisms come into play, such as changes in blood pressure and changes in various hormones, and eventu- ally return sodium excretion to equal sodium intake. In the next few sections, we review how these mechanisms operate together to control sodium and water balance and in so doing act also to control extracellular fluid volume. All these feedback mechanisms control renal excretion of sodium and water by altering either GFR or tubular reabsorption.
Importance of Pressure Natriuresis and Pressure Diuresis in Maintaining Body Sodium and Fluid Balance
One of the most basic and powerful mechanisms for the maintenance of sodium and fluid balance, as well as for controlling blood volume and extracellular fluid volume, is the effect of blood pressure on sodium and water excre- tion—called the pressure natriuresis and pressure diure- sis mechanisms, respectively. As discussed in Chapter 19, this feedback between the kidneys and the circulatory sys- tem also plays a dominant role in long-term blood pres- sure regulation.
Pressure diuresis refers to the effect of increased blood pressure to raise urinary volume excretion, whereas pres- sure natriuresis refers to the rise in sodium excretion that occurs with elevated blood pressure. Because pressure diuresis and natriuresis usually occur in parallel, we refer to these mechanisms simply as “pressure natriuresis” in the following discussion.
Figure 29-13 shows the effect of arterial pressure on urinary sodium output. Note that acute increases in blood pressure of 30 to 50 mm Hg cause a twofold to threefold increase in urinary sodium output. This effect is indepen- dent of changes in activity of the sympathetic nervous sys- tem or of various hormones, such as angiotensin II, ADH, or aldosterone, because pressure natriuresis can be dem- onstrated in an isolated kidney that has been removed from the influence of these factors. With chronic increases in blood pressure, the effectiveness of pressure natriure- sis is greatly enhanced because the increased blood pres- sure also, after a short time delay, suppresses renin release and, therefore, decreases formation of angiotensin II and aldosterone. As discussed previously, decreased levels of
angiotensin II and aldosterone inhibit renal tubular reab- sorption of sodium, thereby amplifying the direct effects of increased blood pressure to raise sodium and water excretion.
Pressure Natriuresis and Diuresis Are Key Components of a Renal-Body Fluid Feedback for Regulating Body Fluid Volumes and Arterial Pressure
The effect of increased blood pressure to raise urine out- put is part of a powerful feedback system that operates to maintain balance between fluid intake and output, as shown in Figure 29-14. This is the same mechanism that is discussed in Chapter 19 for arterial pressure control. The extracellular fluid volume, blood volume, cardiac out- put, arterial pressure, and urine output are all controlled at the same time as separate parts of this basic feedback mechanism.
During changes in sodium and fluid intake, this feed- back mechanism helps to maintain fluid balance and to minimize changes in blood volume, extracellular fluid volume, and arterial pressure as follows:
1. An increase in fluid intake (assuming that sodium accom- panies the fluid intake) above the level of urine output causes a temporary accumulation of fluid in the body.
2. As long as fluid intake exceeds urine output, fluid accumulates in the blood and interstitial spaces, caus- ing parallel increases in blood volume and extracellular fluid volume. As discussed later, the actual increases in these variables are usually small because of the effec- tiveness of this feedback.
3. An increase in blood volume raises mean circulatory filling pressure.
4. An increase in mean circulatory filling pressure raises the pressure gradient for venous return.
5. An increased pressure gradient for venous return ele- vates cardiac output.
6. An increased cardiac output raises arterial pressure.
U ri
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Figure 29-13 Acute and chronic effects of arterial pressure on sodium output by the kidneys (pressure natriuresis). Note that chronic increases in arterial pressure cause much greater increases in sodium output than those measured during acute increases in arterial pressure.
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Unit V The Body Fluids and Kidneys
7. An increased arterial pressure increases urine output by way of pressure diuresis. The steepness of the nor- mal pressure natriuresis relation indicates that only a slight increase in blood pressure is required to raise urinary excretion severalfold.
8. The increased fluid excretion balances the increased intake, and further accumulation of fluid is prevented.
Thus, the renal-body fluid feedback mechanism oper- ates to prevent continuous accumulation of salt and water in the body during increased salt and water intake. As long as kidney function is normal and the pressure diuresis mechanism is operating effectively, large changes in salt and water intake can be accommodated with only slight changes in blood volume, extracellular fluid volume, cardiac output, and arterial pressure.
The opposite sequence of events occurs when fluid intake falls below normal. In this case, there is a tendency toward decreased blood volume and extracellular fluid volume, as well as reduced arterial pressure. Even a small decrease in blood pressure causes a large decrease in urine output, thereby allowing fluid balance to be main- tained with minimal changes in blood pressure, blood volume, or extracellular fluid volume. The effectiveness of this mechanism in preventing large changes in blood vol- ume is demonstrated in Figure 29-15, which shows that changes in blood volume are almost imperceptible despite large variations in daily intake of water and electrolytes, except when intake becomes so low that it is not sufficient to make up for fluid losses caused by evaporation or other inescapable losses.
As discussed later, there are nervous and hormonal systems, in addition to intrarenal mechanisms, that can raise sodium excretion to match increased sodium intake even without measureable increases in arterial pressure
in many persons. Other individuals who are more “salt sensitive” have significant increases in arterial pressure with even moderate increases in sodium intake. With prolonged high-sodium intake, lasting over several years, high blood pressure may occur even in those persons who are not initially salt sensitive. When blood pressure does rise, pressure natriuresis provides a critical means of maintaining balance between sodium intake and urinary sodium excretion.
Precision of Blood Volume and Extracellular Fluid Volume Regulation
By studying Figure 29-14, one can see why the blood vol- ume remains almost exactly constant despite extreme changes in daily fluid intake. The reason for this is the following: (1) A slight change in blood volume causes a marked change in cardiac output, (2) a slight change in
Arterial pressure
Extracellular fluid volume
Blood volume
Mean circulatory filling pressure
Venous return
Vascular capacity
Rate of change of extracellular fluid volume
Arterial pressure
R en
al fl
ui d
ex cr
et io
n
Nonrenal fluid loss
Fluid intake
Cardiac output
Heart strength
Total peripheral resistance
Figure 29-14 Basic renal-body fluid feedback mechanism for control of blood volume, extracellular fluid volume, and arterial pressure. Solid lines indicate positive effects, and dashed lines indicate negative effects.
B lo
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it er
s)
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0 1
Blood volume
Normal range
Death
2 3 4 5 6 7 8
Daily fluid intake (water and electrolytes) (L/day)
Daily fluid intake (water and electrolytes) (L/day)
Figure 29-15 Approximate effect of changes in daily fluid intake on blood volume. Note that blood volume remains relatively constant in the normal range of daily fluid intakes.
Chapter 29 Integration of Renal Mechanisms
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cardiac output causes a large change in blood pressure, and (3) a slight change in blood pressure causes a large change in urine output. These factors work together to provide effective feedback control of blood volume.
The same control mechanisms operate whenever there is a blood loss because of hemorrhage. In this case, a fall in blood pressure along with nervous and hormonal factors discussed later cause fluid retention by the kidneys. Other parallel processes occur to reconstitute the red blood cells and plasma proteins in the blood. If abnormalities of red blood cell volume remain, such as occurs when there is deficiency of erythropoietin or other factors needed to stimulate red blood cell production, the plasma volume will simply make up the difference, and the overall blood volume will return essentially to normal despite the low red blood cell mass.
Distribution of Extracellular Fluid Between the Interstitial Spaces and Vascular System
From Figure 29-14 it is apparent that blood volume and extracellular fluid volume are usually controlled in paral- lel with each other. Ingested fluid initially goes into the blood, but it rapidly becomes distributed between the interstitial spaces and the plasma. Therefore, blood vol- ume and extracellular fluid volume usually are controlled simultaneously.
There are circumstances, however, in which the distri- bution of extracellular fluid between the interstitial spaces and blood can vary greatly. As discussed in Chapter 25, the principal factors that can cause accumulation of fluid in the interstitial spaces include (1) increased capillary hydrostatic pressure, (2) decreased plasma colloid osmotic pressure, (3) increased permeability of the capillaries, and (4) obstruction of lymphatic vessels. In all these condi- tions, an unusually high proportion of the extracellular fluid becomes distributed to the interstitial spaces.
Figure 29-16 shows the normal distribution of fluid between the interstitial spaces and the vascular system and the distribution that occurs in edema states. When small amounts of fluid accumulate in the blood as a result of either too much fluid intake or a decrease in renal output of fluid, about 20 to 30 percent of it stays in the blood and increases the blood volume. The remain- der is distributed to the interstitial spaces. When the extracellular fluid volume rises more than 30 to 50 per- cent above normal, almost all the additional fluid goes into the interstitial spaces and little remains in the blood. This occurs because once the interstitial fluid pressure rises from its normally negative value to become posi- tive, the tissue interstitial spaces become compliant and large amounts of fluid then pour into the tissues with- out interstitial fluid pressure rising much more. In other words, the safety factor against edema, owing to a ris- ing interstitial fluid pressure that counteracts fluid accu- mulation in the tissues, is lost once the tissues become highly compliant.
Thus, under normal conditions, the interstitial spaces act as an “overflow” reservoir for excess fluid, sometimes increasing in volume 10 to 30 liters. This causes edema, as explained in Chapter 25, but it also acts as an important overflow release valve for the circulation, protecting the cardiovascular system against dangerous overload that could lead to pulmonary edema and cardiac failure.
To summarize, extracellular fluid volume and blood volume are controlled simultaneously, but the quantita- tive amounts of fluid distribution between the intersti- tium and the blood depend on the physical properties of the circulation and the interstitial spaces, as well as on the dynamics of fluid exchange through the capillary membranes.
Nervous and Hormonal Factors Increase the Effectiveness of Renal-Body Fluid Feedback Control
In Chapter 27, we discuss the nervous and hormonal fac- tors that influence GFR and tubular reabsorption and, therefore, renal excretion of salt and water. These nervous and hormonal mechanisms usually act in concert with the pressure natriuresis and pressure diuresis mechanisms, making them more effective in minimizing the changes in blood volume, extracellular fluid volume, and arterial pressure that occur in response to day-to-day challenges. However, abnormalities of kidney function or of the vari- ous nervous and hormonal factors that influence the kid- neys can lead to serious changes in blood pressure and body fluid volumes, as discussed later.
Sympathetic Nervous System Control of Renal Excretion: Arterial Baroreceptor and Low-Pressure Stretch Receptor Reflexes
Because the kidneys receive extensive sympathetic inner- vation, changes in sympathetic activity can alter renal sodium and water excretion, as well as regulation of
B lo
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Edema
Normal valueNormal value
Death
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Extracellular fluid volume (liters)Extracellular fluid volume (liters)
Figure 29-16 Approximate relation between extracellular fluid volume and blood volume, showing a nearly linear relation in the normal range but also showing the failure of blood volume to con- tinue rising when the extracellular fluid volume becomes exces- sive. When this occurs, the additional extracellular fluid volume resides in the interstitial spaces and edema results.
Unit V The Body Fluids and Kidneys
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extracellular fluid volume under some conditions. For example, when blood volume is reduced by hemorrhage, pressures in the pulmonary blood vessels and other low- pressure regions of the thorax decrease, causing reflex activation of the sympathetic nervous system. This in turn increases renal sympathetic nerve activity, which has several effects to decrease sodium and water excre- tion: (1) constriction of the renal arterioles, with resultant decreased GFR of the sympathetic activation if severe; (2) increased tubular reabsorption of salt and water; and (3) stimulation of renin release and increased angiotensin II and aldosterone formation, both of which further increase tubular reabsorption. And if the reduction in blood vol- ume is great enough to lower systemic arterial pressure, further activation of the sympathetic nervous system occurs because of decreased stretch of the arterial barore- ceptors located in the carotid sinus and aortic arch. All these reflexes together play an important role in the rapid restitution of blood volume that occurs in acute condi- tions such as hemorrhage. Also, reflex inhibition of renal sympathetic activity may contribute to the rapid elimina- tion of excess fluid in the circulation that occurs after eat- ing a meal that contains large amounts of salt and water.
Role of Angiotensin II in Controlling Renal Excretion
One of the body’s most powerful controllers of sodium excretion is angiotensin II. Changes in sodium and fluid intake are associated with reciprocal changes in angio- tensin II formation, and this in turn contributes greatly to the maintenance of body sodium and fluid balances. That is, when sodium intake is elevated above normal, renin secretion is decreased, causing decreased angiotensin II formation. Because angiotensin II has several important effects in increasing tubular reabsorption of sodium, as explained in Chapter 27, a reduced level of angiotensin II decreases tubular reabsorption of sodium and water, thus increasing the kidneys’ excretion of sodium and water. The net result is to minimize the rise in extracellular fluid volume and arterial pressure that would otherwise occur when sodium intake increases.
Conversely, when sodium intake is reduced below nor- mal, increased levels of angiotensin II cause sodium and water retention and oppose reductions in arterial blood pressure that would otherwise occur. Thus, changes in activity of the renin-angiotensin system act as a powerful amplifier of the pressure natriuresis mechanism for main- taining stable blood pressures and body fluid volumes.
Importance of Changes in Angiotensin II in Altering Pressure Natriuresis. The importance of angiotensin II in making the pressure natriuresis mechanism more effective is shown in Figure 29-17. Note that when the angiotensin control of natriuresis is fully functional, the pressure natri- uresis curve is steep (normal curve), indicating that only minor changes in blood pressure are necessary to increase sodium excretion when sodium intake is raised.
In contrast, when angiotensin levels cannot be decreased in response to increased
