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Known for its clear presentation style, single-author voice, and focus on content most relevant to clinical and pre-clinical students, Guyton and Hall Textbook of Medical Physiology, 14th Edition, employs a distinctive format to ensure maximum learning and retention of complex concepts. A larger font size emphasizes core information, while supporting information, including clinical examples, are detailed in smaller font and highlighted in pale blue – making it easy to quickly skim the essential text or pursue more in-depth study. This two-tone approach, along with other outstanding features, makes this bestselling text a favorite of students worldwide.
Jahr:
2020
Auflage:
14th Edition
Verlag:
Elsevier
Sprache:
english
Seiten:
1028
ISBN 13:
9780323758383
Serien:
Guyton Physiology
Datei:
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NOTE TO INSTRUCTORS
Contact your Elsevier Sales Representative for teaching
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Hall Textbook of ­Medical Physiology, 14e, or request these
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http://evolve.elsevier.com/Hall/physiology/

14TH EDITION

Guyton and Hall
Textbook of Medical Physiology
John E. Hall, PhD
Arthur C. Guyton Professor and Chair
Department of Physiology and Biophysics
Director, Mississippi Center for Obesity Research
University of Mississippi Medical Center
Jackson, Mississippi

Michael E. Hall, MD, MS
Associate Professor
Department of Medicine, Division of
Cardiovascular Diseases
Associate Vice Chair for Research
Department of Physiology and Biophysics
University of Mississippi Medical Center
Jackson, Mississippi

Elsevier
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Ste 1800
Philadelphia, PA 19103-­2899
GUYTON AND HALL TEXTBOOK OF MEDICAL PHYSIOLOGY,
FOURTEENTH EDITION  ISBN: 978-­0-­323-­59712-­8
INTERNATIONAL EDITION  ISBN: 978-­0-­323-­67280-­1
Copyright © 2021 by 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. Details on how to seek permission, further information about the
Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance
Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions.
This book and the individual contributions contained in it are protected under copyright by the Publisher
(other than as may be noted herein).

Notice
Practitioners and researchers must always rely on their own experience and knowledge in evaluating and
using any information, methods, compounds or experiments described herein. Because of rapid advances
in the medical sciences, in particular, independent verification of diagnoses and drug dosages should be
ma; de. To the fullest extent of the law, no responsibility is assumed by Elsevier, authors, editors or
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negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas
contained in the material herein.

Previous editions copyrighted 2016, 2011, 2006, 2000, 1996, 1991, 1986, 1981, 1976, 1971, 1966, 1961, and 1956.
Library of Congress Control Number: 2020936245

Publisher: Elyse O’Grady
Senior Content Development Specialist: Jennifer Shreiner
Publishing Services Manager: Julie Eddy
Project Manager: Grace Onderlinde
Design Direction: Margaret Reid
Printed in Canada
Last digit is the print number: 9

8

7 6

5

4

3

2 1

To
Our Families
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

Preface
The first edition of the Textbook of Medical Physiology was
written by Arthur C. Guyton almost 65 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.
He 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.
Dr. John Hall worked closely with Dr. Guyton for
almost 30 years and had the privilege of writing parts of
the 9th and 10th editions and of assuming sole responsibility for completing the subsequent editions.
Dr. Michael Hall has joined in the preparation of the
14th edition of the Textbook of Medical Physiology. He is
a physician trained in internal medicine, cardiology, and
physiology and has brought new insights that have helped
greatly to achieve 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
researchers continue to unravel new mysteries of body
functions. Advances in molecular and cellular physiology
have made it possible to explain some physiology principles in the terminology of molecular and physical sciences
rather than in merely a series of separate and unexplained
biological phenomena. However, the molecular events
that underpin the functions of the body’s cells provide
only a partial explanation of human physiology. The total
function of the human body requires complex control
systems that communicate with each other and coordinate the molecular functions of the body’s cells, tissues,
and organs in health and disease.
The Textbook of Medical Physiology is not a reference
book that attempts to provide a compendium of the most
recent advances in physiology. It is a book that continues the tradition of being written for students. It focuses
on the basic principles of physiology needed to begin a
career in the health care professions, such as medicine,
dentistry, and nursing, as well as graduate studies in the
biological and health sciences. It should also be useful
to physicians and health care professionals who wish to

review the basic principles needed for understanding the
pathophysiology of human disease. We have attempted to
maintain the same unified organization of the text that
has been useful to students in the past and to ensure that
the book is comprehensive enough that students will continue to use it during their professional careers.
Our hope is that the Textbook of Medical Physiology
conveys the majesty of the human body and its many
functions and that it stimulates students to study physiology throughout their careers. Physiology links the basic
sciences and medicine. The great beauty of physiology is
that it integrates the individual functions of all the body’s
different cells, tissues, and organs into a functional whole,
the human body. Indeed, the human body is much more
than the sum of its parts, and life relies upon this total
function, not just on the function of individual body parts
in isolation from the others.
This brings us to an important question: How are the
separate organs and systems coordinated to maintain
proper function of the entire body? Fortunately, our bodies are endowed with a vast network of feedback controls
that achieve the necessary balances without which we
would be unable to live. Physiologists call this high level
of internal bodily control homeostasis. In disease states,
functional balances are often seriously disturbed, and
homeostasis is impaired. When even a single disturbance
reaches a limit, the whole body can no longer live. One of
the goals of this text is to emphasize the effectiveness and
beauty of the body’s homeostasis mechanisms as well as
to present their abnormal functions in disease.
Another objective is to be as accurate as possible. Suggestions and critiques from many students, physiologists,
and clinicians throughout the world have checked factual
accuracy as well as balance in the text. Even so, because
of the likelihood of error in sorting through many thousands of bits of information, we issue a further request
for all readers to send notations of error or inaccuracy to
us. Physiologists understand the importance of feedback
for proper function of the human body; feedback is also
important for progressive improvement of a textbook of
physiology. To the many persons who have already helped,
we express sincere thanks. Your feedback has helped to
improve the text.
vii

Preface

A brief explanation is needed about several features
of the 14th edition. Although many of the chapters have
been revised to include new principles of physiology and
new figures to illustrate these principles, the text length
has been closely monitored to limit the book’s size so
that it can be used effectively in physiology courses for
medical students and health care professionals. New
references have been chosen primarily for their presentation of physiological principles, for the quality of
their own references, and for their easy accessibility.
The selected bibliography at the end of the chapters lists
mainly review papers from recently published scientific
journals that can be freely accessed from the PubMed site
at https://www.ncbi.nlm.nih.gov/pubmed/. Use of these
references, as well as cross-­references from them, provides much more extensive coverage of the entire field of
physiology.
Our effort to be as concise as possible has, unfortunately, necessitated a more simplified and dogmatic
presentation of many physiological principles than we
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 of the book is that the print is set
in two sizes. The material in large print constitutes the
fundamental physiological information that students
will require in virtually all of their medical studies. The
material in small print and highlighted with a pale lavender background (or identified by beginning and ending
double gray arrowheads in the ebook version) is of several
different kinds: (1) anatomic, chemical, and other information that is needed for immediate discussion but that

viii

most students will learn in more detail in other courses;
(2) physiological information of special importance to
certain fields of clinical medicine; and (3) information
that will be of value to those students who wish to study
specific physiological mechanisms more deeply.
The ebook version provides links to additional content
including video animations and self-­assessment questions
that can be accessed with computers, smart phones, and
electronic tablets. For additional self-­assessment beyond
these textbook supplements, the reader may consider
using a copy of Guyton and Hall Physiology Review, which
includes more than 1000 practice questions referenced to
the textbook. We hope that these ancillary materials will
assist readers in testing their understanding of basic principles of physiology.
We express sincere thanks to many persons who have
helped to prepare this book, including our colleagues in
the Department of Physiology and Biophysics at the University of Mississippi Medical Center who provided valuable suggestions. The members of our faculty and a brief
description of the research and educational activities of the
department can be found at http://physiology.umc.edu/.
We are especially grateful to Stephanie Lucas for excellent
assistance and to James Perkins for excellent illustrations.
We also thank Elyse O’Grady, Jennifer Shreiner, Grace
Onderlinde, Rebecca Gruliow, and the entire Elsevier
team for continued editorial and production excellence.
Finally, we thank the many readers who continue to
help us improve the Textbook of Medical Physiology. We
hope that you enjoy the current edition and find it even
more useful than previous editions.
John E. Hall
Michael E. Hall

CHAPTER

1

Physiology is the science that seeks to explain the physical and chemical mechanisms that are responsible for the
origin, development, and progression of life. Each type
of life, from the simplest 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, invertebrate physiology, vertebrate physiology, mammalian physiology, human physiology, and many more subdivisions.
Human Physiology. The science of human physiology

attempts to explain the specific characteristics and mechanisms of the human body that make it a living being. The
fact that we remain alive is the result of complex control
systems. 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.
The fact that we are sensing, feeling, and knowledgeable
beings is part of this automatic sequence of life; these special attributes allow us to exist under widely varying conditions that otherwise would make life impossible.
Human physiology links the basic sciences with medicine
and integrates multiple functions of the cells, tissues, and
organs into the functions of the living human being. This integration requires communication and coordination by a vast
array of control systems that operate at every level—from the
genes that program synthesis of molecules to the complex
nervous and hormonal systems that coordinate functions of
cells, tissues, and organs throughout the body. Thus, the coordinated functions of the human body are much more than the
sum of its parts, and life in health, as well as in disease states,
relies on this total function. Although the main focus of this
book is on normal human physiology, we will also discuss,
to some extent, pathophysiology, which is the study of disordered body function and the basis for clinical medicine.

CELLS ARE THE LIVING UNITS OF THE
BODY
The basic living unit of the body is the cell. Each tissue or
organ is an aggregate of many different cells held together
by intercellular supporting structures.

Each type of cell is specially adapted to perform one
or a few particular functions. For example, the red blood
cells, numbering about 25 trillion in each person, transport oxygen from the lungs to the tissues. Although the
red blood cells are the most abundant of any single type of
cell in the body, there are also trillions of additional cells
of other types that perform functions different from those
of the red blood cell. The entire body, then, contains about
35 to 40 trillion human cells.
The many cells of the body often differ markedly from
one another but all have certain basic characteristics that
are alike. For example, oxygen reacts with carbohydrate,
fat, and protein to release the energy required for all cells
to function. Furthermore, the general chemical mechanisms for changing nutrients into energy are basically
the same in all cells, and all cells deliver products of their
chemical reactions into the surrounding fluids.
Almost all cells also have the ability to reproduce additional cells of their own type. Fortunately, when cells of a
particular type are destroyed, the remaining cells of this type
usually generate new cells until the supply is replenished.
Microorganisms Living in the Body Outnumber Human Cells. In addition to human cells, trillions of microbes

inhabit the body, living on the skin and in the mouth, gut,
and nose. The gastrointestinal tract, for example, normally
contains a complex and dynamic population of 400 to 1000
species of microorganisms that outnumber our human
cells. Communities of microorganisms that inhabit the
body, often called microbiota, can cause diseases, but most
of the time they live in harmony with their human hosts
and provide vital functions that are essential for survival of
their hosts. Although the importance of gut microbiota in
the digestion of foodstuffs is widely recognized, additional
roles for the body’s microbes in nutrition, immunity, and
other functions are just beginning to be appreciated and
represent an intensive area of biomedical research.

EXTRACELLULAR FLUID—THE
“INTERNAL ENVIRONMENT”
About 50% to 70% of the adult human body is fluid, mainly
a water solution of ions and other substances. Although
3

UNIT I

Functional Organization of the Human Body
and Control of the “Internal Environment”

UNIT I Introduction to Physiology: The Cell and General Physiology

most of this fluid is inside the cells and is called intracellular fluid, about one-­third is in the spaces outside the cells
and is called extracellular fluid. This extracellular fluid is
in constant motion throughout the body. It is transported
rapidly in the circulating blood and then mixed between
the blood and tissue fluids by diffusion through the capillary walls.
In the extracellular fluid are the ions and nutrients
needed by the cells to maintain life. Thus, all cells live in
essentially the same environment—the extracellular fluid.
For this reason, the extracellular fluid is also called the
internal environment of the body, or the milieu intérieur, a
term introduced by the great 19th-­century French physiologist Claude Bernard (1813–1878).
Cells are capable of living and performing their special functions as long as the proper concentrations of
oxygen, glucose, different ions, amino acids, fatty substances, and other constituents are available in this internal environment.
Differences in Extracellular and Intracellular Fluids.

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

HOMEOSTASIS—MAINTENANCE OF
A NEARLY CONSTANT INTERNAL
ENVIRONMENT
In 1929, the American physiologist Walter Cannon
(1871–1945) coined the term homeostasis to describe the
maintenance of nearly constant conditions in the internal
environment. Essentially, all organs and tissues of the body
perform functions that help maintain these relatively constant conditions. For example, the lungs provide oxygen
to the extracellular fluid to replenish the oxygen used by
the cells, the kidneys maintain constant ion concentrations, and the gastrointestinal system provides nutrients
while eliminating waste from the body.
The various ions, nutrients, waste products, and other
constituents of the body are normally regulated within a
range of values, rather than at fixed values. For some of the
body’s constituents, this range is extremely small. Variations in the blood hydrogen ion concentration, for example, are normally less than 5 nanomoles/L (0.000000005
moles/L). The blood sodium concentration is also tightly
4

regulated, normally varying only a few millimoles per liter,
even with large changes in sodium intake, but these variations of sodium concentration are at least 1 million times
greater than for hydrogen ions.
Powerful control systems exist for maintaining concentrations of sodium and hydrogen ions, as well as for most
of the other ions, nutrients, and substances in the body at
levels that permit the cells, tissues, and organs to perform
their normal functions, despite wide environmental variations and challenges from injury and diseases.
Much of this text is concerned with how each organ or
tissue contributes to homeostasis. Normal body functions
require integrated actions of cells, tissues, organs, and
multiple nervous, hormonal, and local control systems
that together contribute to homeostasis and good health.
Homeostatic Compensations in Diseases. Disease is

often considered to be a state of disrupted homeostasis.
However, even in the presence of disease, homeostatic
mechanisms continue to operate and maintain vital functions through multiple compensations. In some cases,
these compensations may lead to major deviations of the
body’s functions from the normal range, making it difficult to distinguish the primary cause of the disease from
the compensatory responses. For example, diseases that
impair the kidneys’ ability to excrete salt and water may
lead to high blood pressure, which initially helps return
excretion to normal so that a balance between intake and
renal excretion can be maintained. This balance is needed
to maintain life, but, over long periods of time, the high
blood pressure can damage various organs, including the
kidneys, causing even greater increases in blood pressure
and more renal damage. Thus, homeostatic compensations that ensue after injury, disease, or major environmental challenges to the body may represent trade-­offs
that are necessary to maintain vital body functions but,
in the long term, contribute to additional abnormalities
of body function. The discipline of pathophysiology seeks
to explain how the various physiological processes are altered in diseases or injury.
This chapter outlines the different functional systems
of the body and their contributions to homeostasis. We
then briefly discuss 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 the body in two
stages. The first stage is movement of blood through the
body in the blood vessels. 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 circuit an average

Chapter 1

Functional Organization of the Human Body and Control of the “Internal Environment”

Arteriole

Lungs

UNIT I

O2

CO2
Right
heart
pump

Left
heart
pump
Venule
Gut
Figure 1-2. Diffusion of fluid and dissolved constituents through the
capillary walls and interstitial spaces.

Nutrition
and
excretion

Kidneys

Regulation
of
electrolytes

Excretion

That is, the fluid and dissolved molecules are continually
moving and bouncing in all directions in the plasma and
fluid in the intercellular spaces, as well as through capillary pores. Few cells are located more than 50 micrometers from a capillary, which ensures diffusion of almost
any substance from the capillary to the cell within a few
seconds. Thus, the extracellular fluid everywhere in the
body—both that of the plasma and that of the interstitial
fluid—is continually being mixed, thereby maintaining
homogeneity of extracellular fluid throughout the body.

ORIGIN OF NUTRIENTS IN THE
EXTRACELLULAR FLUID
Respiratory System. Figure 1-1 shows that each time

Venous end

Arterial end

Capillaries
Figure 1-1. General organization of the circulatory system.

blood passes through the body, it also flows through the
lungs. The blood picks up oxygen in alveoli, thus acquiring
the oxygen needed by cells. The membrane between the
alveoli and the lumen of the pulmonary capillaries, the
alveolar membrane, is only 0.4 to 2.0 micrometers thick,
and oxygen rapidly diffuses by molecular motion through
this membrane into the blood.
Gastrointestinal Tract. A large portion of the blood pumped

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 blood capillaries, continual
exchange of extracellular fluid 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 capillary walls are permeable to most molecules in
the blood plasma, with the exception of plasma proteins,
which are too large to pass through capillaries readily.
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 in Figure 1-2.
This process of diffusion is caused by kinetic motion
of the molecules in the plasma and the interstitial fluid.

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
ingested food into the extracellular fluid of the blood.

Liver and Other Organs That Perform Primarily Metabolic Functions. Not all substances absorbed from the

gastrointestinal tract can be used in their absorbed form
by the cells. The liver changes the chemical compositions
of many of these substances to more usable forms, and
other tissues of the body—fat cells, gastrointestinal mucosa, kidneys, and endocrine glands—help modify the
absorbed substances or store them until they are needed.
The liver also eliminates certain waste products produced
in the body and toxic substances that are ingested.
5

UNIT I Introduction to Physiology: The Cell and General Physiology

Musculoskeletal System. How does the musculoskeletal

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

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 lung alveoli; the respiratory movement of air into and out of the lungs carries
carbon dioxide to the atmosphere. Carbon dioxide is the
most abundant of all the metabolism products.
Kidneys. Passage of blood through the kidneys removes

most of the other substances from the plasma besides carbon dioxide that are not needed by cells. These substances include different end products of cellular metabolism,
such as urea and uric acid; they also include excesses of
ions and water from the food that accumulate in the extracellular fluid.
The kidneys perform their function first by filtering
large quantities of plasma through the glomerular capillaries into the tubules and then reabsorbing into the blood
substances needed by the body, such as glucose, amino
acids, appropriate amounts of water, and many of the
ions. Most of the other substances that are not needed
by the body, especially metabolic waste products such
as urea and creatinine, 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 many functions of the liver is detoxifi-

cation or removal of ingested drugs and chemicals. The
liver secretes many of these wastes into the bile to be
eventually eliminated in the feces.

REGULATION OF BODY FUNCTIONS
Nervous System. The nervous system is composed of

three major parts—the sensory input portion, the central
nervous system (or integrative portion), and the motor output portion. Sensory receptors detect the state of the body
and its surroundings. For example, receptors in the skin
alert us whenever an object touches the skin. The eyes
are sensory organs that give us 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 stores information, generates thoughts,
creates ambition, and determines reactions that the body
6

performs in response to the sensations. Appropriate signals are then transmitted through the motor output portion of the nervous system to carry out one’s desires.
An important segment of the nervous system is called
the autonomic system. It operates at a subconscious level
and controls many functions of internal organs, including
the level of pumping activity by the heart, movements of
the gastrointestinal tract, and secretion by many of the
body’s glands.
Hormone Systems. Located in the body are endocrine

glands, organs and tissues that secrete chemical substances called hormones. Hormones are transported in
the extracellular fluid to other parts of the body to help
regulate cellular function. For example, thyroid hormone
increases the rates of most chemical reactions in all cells,
thus helping set the tempo of bodily activity. Insulin controls glucose metabolism, adrenocortical hormones control sodium and potassium ions and protein metabolism,
and parathyroid hormone controls bone calcium and
phosphate. Thus, the hormones provide a regulatory system that complements the nervous system. The nervous
system controls many muscular and secretory activities
of the body, whereas the hormonal system regulates many
metabolic functions. The nervous and hormonal systems
normally work together in a coordinated manner to control essentially all the organ systems of the body.

PROTECTION OF THE BODY
Immune System. The immune system includes white

blood cells, tissue cells derived from white blood cells, the
thymus, lymph nodes, and lymph vessels that protect the
body from pathogens such as bacteria, viruses, parasites,
and fungi. The immune system provides a mechanism for
the body to carry out the following: (1) distinguish its own
cells from harmful foreign cells and substances; and (2)
destroy the invader by phagocytosis or by producing sensitized lymphocytes or specialized proteins (e.g., antibodies)
that destroy or neutralize the invader.
Integumentary System. The skin and its various ap-

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

REPRODUCTION
Although reproduction is sometimes not considered a
homeostatic function, it helps maintain homeostasis by
generating new beings to take the place of those that are

Chapter 1

Functional Organization of the Human Body and Control of the “Internal Environment”

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 to help maintain the automaticity and continuity of life.

The human body has thousands of control systems. Some
of the most intricate of these systems are genetic control
systems that operate in all cells to help regulate intracellular and extracellular functions. This subject is discussed
in Chapter 3.
Many other control systems operate within the organs
to regulate functions of the individual parts of the organs;
others operate throughout the entire body to control the
interrelationships between the organs. For example, the
respiratory system, operating in association with the
nervous system, regulates the concentration of carbon
dioxide in the extracellular fluid. The liver and pancreas
control glucose concentration 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 cells, the body has a special control mechanism to
maintain an almost exact and constant oxygen concentration in the extracellular fluid. This mechanism depends
principally on the chemical characteristics of hemoglobin,
which is present in red blood cells. Hemoglobin combines with oxygen as the blood passes through the lungs.
Then, as the blood passes through the tissue capillaries,
hemoglobin, because of its own strong chemical affinity
for oxygen, does not release oxygen into the tissue fluid
if too much oxygen is already there. However, if oxygen
concentration in the tissue fluid is too low, sufficient oxygen is released to re-­establish an adequate concentration.
Thus, regulation of oxygen concentration in the tissues
relies to a great extent on the chemical characteristics of
hemoglobin. 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 oxidative reactions in cells. If all the
carbon dioxide formed in the cells continued to accumulate in the tissue fluids, all energy-­giving reactions of the
cells would cease. Fortunately, a higher than normal carbon dioxide concentration in the blood excites the respiratory center, causing a person to breathe rapidly and deeply.
This deep rapid breathing increases expiration of carbon
dioxide and, therefore, removes excess carbon dioxide
from the blood and tissue fluids. This process continues
until the concentration returns to normal.

Error signal
Brain medulla
Vasomotor
centers

Effectors

Sympathetic
nervous system

Blood vessels
Heart

UNIT I

CONTROL SYSTEMS OF THE BODY

Reference
set point

Feedback signal
Baroreceptors

Arterial
pressure

Sensor

Controlled variable

Figure 1-3. Negative feedback control of arterial pressure by the arterial baroreceptors. Signals from the sensor (baroreceptors) are sent
to the medulla of the brain, where they are compared with a reference set point. When arterial pressure increases above normal, this
abnormal pressure increases nerve impulses from the baroreceptors
to the medulla of the brain, where the input signals are compared
with the set point, generating an error signal that leads to decreased
sympathetic nervous system activity. Decreased sympathetic activity
causes dilation of blood vessels and reduced pumping activity of the
heart, which return arterial pressure toward normal.

Regulation of Arterial Blood Pressure. Several systems

contribute to arterial blood pressure regulation. One of
these, the baroreceptor system, is an excellent example of
a rapidly acting control mechanism (Figure 1-3). In the
walls of the bifurcation region of the carotid arteries in
the neck, and also in the arch of the aorta in the thorax,
are many nerve receptors called baroreceptors that are
stimulated by stretch of the arterial wall. When arterial
pressure rises too high, the baroreceptors send barrages
of nerve impulses to the medulla of the brain. Here, these
impulses inhibit the vasomotor center, which in turn decreases the number of impulses transmitted from the
vasomotor center through the sympathetic nervous system to the heart and blood vessels. Lack of these impulses
causes diminished pumping activity by the heart and dilation of peripheral blood vessels, allowing increased blood
flow through the vessels. Both these effects decrease the
arterial pressure, moving it back toward normal.
Conversely, a decrease in arterial pressure below normal relaxes the stretch receptors, allowing the vasomotor
center to become more active than usual, thereby causing
vasoconstriction and increased heart pumping. The initial
decrease in arterial pressure thus initiates negative feedback mechanisms that raise arterial pressure back toward
normal.

Normal Ranges and Physical
Characteristics of Important Extracellular
Fluid Constituents
Table 1-1 lists some important constituents and physical
characteristics of extracellular fluid, along with their normal values, normal ranges, and maximum limits without
causing death. Note the narrowness of the normal range
for each one. Values outside these ranges are often caused
by illness, injury, or major environmental challenges.
7

UNIT I Introduction to Physiology: The Cell and General Physiology
Table 1-1  Important Constituents and Physical Characteristics of Extracellular Fluid
Constituent

Normal Value

Normal Range

Approximate Short-­Term Nonlethal Limit

Unit

Oxygen (venous)

40

25–40

10–1000

mm Hg

Carbon dioxide (venous)

45

41–51

5–80

mm Hg

Sodium ion

142

135–145

115–175

mmol/L

Potassium ion

4.2

3.5–5.3

1.5–9.0

mmol/L

Calcium ion

1.2

1.0–1.4

0.5–2.0

mmol/L

Chloride ion

106

98–108

70–130

mmol/L

Bicarbonate ion

24

22–29

8–45

mmol/L

Glucose

90

70–115

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 (venous)

7.4

7.3–7.5

6.9–8.0

pH

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

CHARACTERISTICS OF CONTROL SYSTEMS
The aforementioned examples of homeostatic control
mechanisms are only a few of the many thousands in the
body, all of which have some common characteristics, as
explained in this section.

Negative Feedback Nature of Most
Control Systems
Most control systems of the body act by negative feedback, which can 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
8

the extracellular fluid carbon dioxide concentration
because the lungs expire greater amounts of carbon dioxide from the body. Thus, the high concentration of carbon
dioxide initiates events that decrease the concentration
toward normal, which is negative to the initiating stimulus. Conversely, a carbon dioxide concentration that falls
too low results in 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 reduced
pressure, or a low pressure causes a series of reactions that
promote increased pressure. In both cases, these effects
are negative with respect to the initiating stimulus.
Therefore, in general, if some factor becomes excessive or deficient, a control system initiates negative feedback, which consists of a series of changes that return
the factor toward a certain mean value, thus maintaining
homeostasis.
Gain of a Control System. The degree of effectiveness

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

Correction
Error

Thus, in the baroreceptor system example, the correction is −50 mm Hg, and the error persisting is +25 mm Hg.
Therefore, the gain of the person’s baroreceptor system

Chapter 1

Functional Organization of the Human Body and Control of the “Internal Environment”

Return to
normal

4
Bled 1 liter
3

2

Bled 2 liters

overcome by the negative feedback control mechanisms
of the body, and the vicious cycle then fails to develop.
For example, if the person in the aforementioned example
bleeds only 1 liter instead of 2 liters, the normal negative
feedback mechanisms for controlling cardiac output and
arterial pressure can counterbalance the positive feedback
and the person can recover, as shown by the dashed curve
of Figure 1-4.
Positive Feedback Can Sometimes Be Useful. The body

1
Death

0
1

2

3

Hours
Figure 1-4. Recovery of heart pumping caused by negative feedback
after 1 liter of blood is removed from the circulation. Death is caused
by positive feedback when 2 liters or more blood is removed.

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 physiological control systems
are much greater than that of the baroreceptor system.
For example, the gain of the system controlling internal
body temperature when a person is exposed to moderately cold weather is about −33. Therefore, one can see
that the temperature control system is much more effective than the baroreceptor pressure control system.

Positive Feedback May Cause Vicious
Cycles and Death
Why do most control systems of the body operate by
negative feedback rather than by positive feedback? If
one considers the nature of positive feedback, it is obvious that positive feedback leads to instability rather than
stability and, in some cases, can cause death.
Figure 1-4 shows an example in which death can ensue
from positive feedback. This figure depicts the pumping
effectiveness of the heart, showing the heart of a healthy
human pumping about 5 liters of blood per minute. If the
person suddenly bleeds a total of 2 liters, the amount of
blood in the body is decreased to such a low level that
not enough blood is available for the heart to pump effectively. As a result, the arterial pressure falls, and the flow
of blood to the heart muscle through the coronary vessels diminishes. This scenario results in weakening of the
heart, further diminished pumping, a further decrease
in coronary blood flow, and still more weakness of the
heart; the cycle repeats itself again and again until death
occurs. Note that each cycle in the feedback results in
further weakening of the heart. In other words, the initiating stimulus causes more of the same, which is positive
feedback.
Positive feedback is sometimes known as a “vicious
cycle,” but a mild degree of positive feedback can be

sometimes 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. Some of these enzymes act on other inactivated enzymes of the immediately adjacent blood, thus
causing more blood clotting. This process continues until
the hole in the vessel is plugged and bleeding no longer
occurs. On occasion, this mechanism can get out of hand
and cause formation of unwanted clots. In fact, this is what
initiates most acute heart attacks, which can be 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 situation in which positive feedback is valuable. When uterine contractions become
strong enough for the baby’s head to begin pushing
through the cervix, stretching of the cervix sends signals
through the uterine muscle back to the body of the uterus,
causing even more powerful contractions. Thus, the uterine contractions stretch the cervix, and cervical stretch
causes stronger contractions. When this process becomes
powerful enough, the baby is born. If they are 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. Stimulation of the membrane of a nerve fiber causes slight leakage of sodium ions
through sodium channels in the nerve membrane to the
fiber’s interior. The sodium ions entering the fiber then
change the membrane potential, which, in turn, causes
more opening of channels, more change of potential, still
more opening of channels, and so forth. Thus, a slight leak
becomes an explosion of sodium entering the interior of
the nerve fiber, which creates the nerve action potential.
This action potential, in turn, causes electrical current to
flow along the outside and inside of the fiber and initiates
additional action potentials. This process continues 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 is part of an overall negative feedback
process. For example, in the case of blood clotting, the
positive feedback clotting process is a negative feedback
process for the 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.
9

UNIT I

Pumping effectiveness of heart
(Liters pumped per minute)

5

UNIT I Introduction to Physiology: The Cell and General Physiology

More Complex Types of Control
Systems—Feed-­Forward and Adaptive
Control
Later in this text, when we study the nervous system, we
shall see that this system contains great numbers of interconnected control mechanisms. Some are simple feedback
systems similar to those already discussed. Many are not.
For example, some movements of the body occur so rapidly that there is not enough time for nerve signals to travel
from the peripheral parts of the body all the way to the
brain and then back to the periphery again to control the
movement. Therefore, the brain uses a mechanism called
feed-­forward control to cause required muscle contractions. Sensory nerve signals from the moving parts apprise
the brain about whether the movement is performed correctly. If not, the brain corrects the feed-­forward signals
that it sends to the muscles the next time the movement
is required. Then, if still further correction is necessary,
this process will be performed again for subsequent movements. This process 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, much of this text is devoted to discussing these life-­giving mechanisms.

PHYSIOLOGICAL VARIABILITY
Although some physiological variables, such as plasma
concentrations of potassium, calcium, and hydrogen
ions, are tightly regulated, others, such as body weight
and adiposity, show wide variation among different individuals and even in the same individual at different stages
of life. Blood pressure, cardiac pumping, metabolic rate,
nervous system activity, hormones, and other physiological variables change throughout the day as we move
about and engage in normal daily activities. Therefore,
when we discuss “normal” values, it is with the understanding that many of the body’s control systems are constantly reacting to perturbations, and that variability may
exist among different individuals, depending on body
weight and height, diet, age, sex, environment, genetics,
and other factors.
For simplicity, discussion of physiological functions
often focuses on the “average” 70-­kg young, lean male.
However, the American male no longer weighs an average of 70 kg; he now weighs over 88 kg, and the average
American female weighs over 76 kg, more than the average man in the 1960s. Body weight has also increased substantially in most other industrialized countries during
the past 40 to 50 years.
Except for reproductive and hormonal functions,
many other physiological functions and normal values
are often discussed in terms of male physiology. However,
there are clearly differences in male and female physiology
beyond the obvious differences that relate to reproduction. These differences can have important consequences
10

for understanding normal physiology as well as for treatment of diseases.
Age-­related and ethnic or racial differences in physiology also have important influences on body composition,
physiological control systems, and pathophysiology of
diseases. For example, in a lean young male the total body
water is about 60% of body weight. As a person grows and
ages, this percentage gradually decreases, partly because
aging is usually associated with declining skeletal muscle
mass and increasing fat mass. Aging may also cause a
decline in the function and effectiveness of some organs
and physiological control systems.
These sources of physiological variability—sex differences, aging, ethnic, and racial—are complex but important considerations when discussing normal physiology
and the pathophysiology of diseases.

SUMMARY—AUTOMATICITY OF THE
BODY
The main purpose of this chapter has been to discuss
briefly the overall organization of the body and the means
whereby the different parts of the body operate in harmony. To summarize, the body is actually a social order of
about 35 to 40 trillion cells organized into different functional structures, some of which are called organs. Each
functional structure contributes its share to the maintenance of homeostasis in the extracellular fluid, which is
called the internal environment. As long as normal conditions are maintained in this internal environment, the
cells of the body continue to live and function properly.
Each cell benefits from homeostasis and, in turn, each
cell contributes its share toward the maintenance of
homeostasis. This reciprocal interplay provides continuous automaticity of the body until one or more functional
systems lose their ability to contribute their share of function. When this happens, all the cells of the body suffer.
Extreme dysfunction leads to death; moderate dysfunction leads to sickness.

Bibliography
Adolph EF: Physiological adaptations: hypertrophies and superfunctions. Am Sci 60:608, 1972.
Bentsen MA, Mirzadeh Z, Schwartz MW: Revisiting how the brain
senses glucose-and why. Cell Metab 29:11, 2019.
Bernard C: Lectures on the Phenomena of Life Common to Animals
and Plants. Springfield, IL: Charles C Thomas, 1974.
Cannon WB: Organization for physiological homeostasis. Physiol Rev
9:399, 1929.
Chien S: Mechanotransduction and endothelial cell homeostasis: the
wisdom of the cell. Am J Physiol Heart Circ Physiol 292:H1209, 2007.
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.
Eckel-Mahan K, Sassone-Corsi P: Metabolism and the circadian clock
converge. Physiol Rev 93:107, 2013.

Chapter 1

Functional Organization of the Human Body and Control of the “Internal Environment”
Nishida AH, Ochman H: A great-ape view of the gut microbiome. Nat
Rev Genet 20:185, 2019.
Orgel LE: The origin of life on the earth. Sci Am 271:76,1994.
Reardon C, Murray K, Lomax AE: Neuroimmune communication in
health and disease. Physiol Rev 98:2287-2316, 2018.
Sender R, Fuchs S, Milo R: Revised estimates for the number of human
and bacteria cells in the body. PLoS Biol 14(8):e1002533, 2016.
Smith HW: From Fish to Philosopher. New York: Doubleday, 1961.

11

UNIT I

Guyton AC: Arterial Pressure and Hypertension. Philadelphia: WB
Saunders, 1980.
Herman MA, Kahn BB: Glucose transport and sensing in the maintenance of glucose homeostasis and metabolic harmony. J Clin Invest
116:1767, 2006.
Kabashima K, Honda T, Ginhoux F, Egawa G: The immunological
anatomy of the skin. Nat Rev Immunol 19:19, 2019.
Khramtsova EA, Davis LK, Stranger BE: The role of sex in the genomics of human complex traits. Nat Rev Genet 20: 173, 2019.
Kim KS, Seeley RJ, Sandoval DA: Signalling from the periphery to the
brain that regulates energy homeostasis. Nat Rev Neurosci 19:185,
2018.

CHAPTER

2

Each of the trillions of cells in a human being is a living
structure that can survive for months or years, provided
its surrounding fluids contain appropriate nutrients. Cells
are the building blocks of the body, providing structure
for the body’s tissues and organs, ingesting nutrients and
converting them to energy, and performing specialized
functions. Cells also contain the body’s hereditary code,
which controls the substances synthesized by the cells
and permits them to make copies of themselves.

ORGANIZATION OF THE CELL
A schematic drawing of 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, proteins, lipids, and carbohydrates.
Water. Most cells, except for fat cells, are comprised

mainly of water in a concentration of 70% to 85%. 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, magne-

sium, phosphate, sulfate, bicarbonate, and smaller quantities of sodium, chloride, and calcium. These ions are all
discussed in Chapter 4, which considers the interrelations
between the intracellular and extracellular fluids.
The ions provide inorganic chemicals for cellular reactions and are necessary for the operation of some cellular
control mechanisms. For example, ions acting at the cell
membrane are required for the 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% of the cell mass. These proteins can be divided into
two types, structural proteins and functional proteins.
Structural proteins are present in the cell mainly in the
form of long filaments that are polymers of many individual protein molecules. A prominent use of such intracellular filaments is to form microtubules, which provide
the cytoskeletons of cellular organelles such as cilia, nerve
axons, the mitotic spindles of cells undergoing mitosis,
and a tangled mass of thin filamentous tubules that hold
the parts of the cytoplasm and nucleoplasm together in
their respective compartments. Fibrillar proteins are
found outside the cell, especially in the collagen and elastin fibers of connective tissue, and elsewhere, such as in
blood vessel walls, tendons, and ligaments.
The functional proteins are usually composed of combinations of a few molecules in tubular-­globular form.
These proteins are mainly the enzymes of the cell and, in
contrast to the fibrillar proteins, are often mobile in the
cell fluid. Also, many of them are adherent to membranous structures inside the cell and catalyze specific intracellular chemical reactions. For example, the chemical
reactions that split glucose into its component parts and
then combine these with oxygen to form carbon dioxide and water while simultaneously providing energy for
cellular function are all catalyzed by a series of protein
enzymes.
Lipids. Lipids are several types of substances that are

grouped together because of their common property of
being soluble in fat solvents. Especially important lipids

Cell
membrane
Cytoplasm
Nucleolus
Nuclear
membrane

Nucleoplasm
Nucleus

Figure 2-1. Illustration of cell structures visible with a light microscope.

13

UNIT I

The Cell and Its Functions

UNIT I Introduction to Physiology: The Cell and General Physiology
Chromosomes and DNA

Centrioles
Secretory
granule
Golgi
apparatus
Microtubules
Nuclear
membrane

Cell
membrane
Nucleolus
Glycogen
Ribosomes
Lysosome

Mitochondrion

Rough (granular)
endoplasmic
reticulum

Smooth (agranular)
endoplasmic
reticulum

Microfilaments

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

are phospholipids and cholesterol, which together constitute only about 2% of the total cell mass. Phospholipids
and cholesterol 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 fats. In fat cells (adipocytes), triglycerides often
account for as much as 95% of the cell mass. The fat stored
in these cells represents the body’s main storehouse of
energy-­giving nutrients that can later be used to provide
energy wherever it is needed in the body.
Carbohydrates. Carbohydrates play a major role in cell

nutrition and, as parts of glycoprotein molecules, have
structural functions. Most human cells do not maintain
large stores of carbohydrates; the amount usually averages
only about 1% of their total mass but increases to as much
as 3% in muscle cells and, occasionally, to 6% in liver cells.
However, carbohydrate in the form of dissolved glucose
is always present in the surrounding extracellular fluid so
14

that it is readily available to the cell. Also, a small amount
of carbohydrate is stored in cells as glycogen, an insoluble
polymer of glucose that can be depolymerized and used
rapidly to supply the cell’s energy needs.

CELL STRUCTURE
The cell contains highly organized physical structures
called intracellular organelles, which are critical for cell
function. For example, without one of the organelles, the
mitochondria, more than 95% of the cell’s energy release
from nutrients would cease immediately. The most
important organelles and other structures of the cell are
shown in Figure 2-2.

MEMBRANOUS STRUCTURES OF THE CELL
Most organelles of the cell are covered by membranes
composed primarily of lipids and proteins. These membranes include the cell membrane, nuclear membrane,
membrane of the endoplasmic reticulum, and membranes
of the mitochondria, lysosomes, and Golgi apparatus.

Chapter 2

The Cell and Its Functions

Carbohydrate
Extracellular
fluid

UNIT I

Integral protein

Lipid
bilayer
Peripheral
protein
Intracellular
fluid
Cytoplasm

Integral 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 outside
of the membrane and to additional protein molecules on the inside.

The lipids in membranes provide a barrier that
impedes movement of water and water-­
soluble substances from one cell compartment to another because
water is not soluble in lipids. However, protein molecules often penetrate all the way through membranes,
thus providing specialized pathways, often organized
into actual pores, for passage of specific substances
through membranes. Also, many other membrane
proteins are enzymes, which catalyze a multitude of
different chemical reactions, discussed here and in subsequent chapters.

Cell Membrane
The cell membrane (also called the plasma membrane)
envelops the cell and is a thin, pliable, elastic structure
only 7.5 to 10 nanometers thick. It is composed almost
entirely of proteins and lipids. The approximate composition is 55% proteins, 25% phospholipids, 13% cholesterol,
4% other lipids, and 3% carbohydrates.
The Cell Membrane Lipid Barrier Impedes Penetration by Water-­Soluble Substances. 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 proteins.
The basic lipid bilayer is composed of three main types
of lipids—phospholipids, sphingolipids, and cholesterol.
Phospholipids are the most abundant cell membrane
lipids. One end of each phospholipid molecule is hydrophilic and soluble in water. The other end is hydrophobic and soluble only in fats. The phosphate end of the
phospholipid is hydrophilic, and the fatty acid portion is
hydrophobic.
Because the hydrophobic portions of the phospholipid
molecules are repelled by water but are mutually attracted
to one another, they have a natural tendency to attach to
one another in the middle of the membrane, as shown in
Figure 2-3. The hydrophilic phosphate portions then constitute the two surfaces of the complete cell membrane, in
contact with intracellular water on the inside of the membrane and extracellular water on the outside surface.
The lipid layer in the middle of the membrane is
impermeable to the usual water-­soluble substances, such
as ions, glucose, and urea. Conversely, fat-­soluble substances, such as oxygen, carbon dioxide, and alcohol, can
penetrate this portion of the membrane with ease.
Sphingolipids, derived from the amino alcohol sphingosine, also have hydrophobic and hydrophilic groups and
15

UNIT I Introduction to Physiology: The Cell and General Physiology

are present in small amounts in the cell membranes, especially nerve cells. Complex sphingolipids in cell membranes are thought to serve several functions, including
protection from harmful environmental factors, signal
transmission, and adhesion sites for extracellular proteins.
Cholesterol molecules in membranes are also lipids
because their steroid nuclei are 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 membrane proteins are mainly glycoproteins. There are two types of cell membrane proteins,
integral proteins, which protrude all the way through
the membrane, and peripheral proteins, which are
attached only to one surface of the membrane and do
not penetrate all the way through.
Many of the integral proteins provide structural channels (or pores) through which water molecules and water-­
soluble substances, especially ions, can diffuse between
extracellular and intracellular fluids. These protein channels also have selective properties that allow preferential
diffusion of some substances over others.
Other integral proteins act as carrier proteins for transporting substances that otherwise could not penetrate
the lipid bilayer. Sometimes, these carrier proteins even
transport substances in the direction opposite to their
electrochemical gradients for diffusion, which is called
active transport. Still others act as enzymes.
Integral membrane proteins can also serve as receptors
for water-­soluble chemicals, such as peptide hormones,
that do not easily penetrate the cell membrane. Interaction of cell membrane receptors with specific ligands that
bind to the receptor causes conformational changes in
the receptor protein. This process, 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, relaying the signal from the extracellular part of the receptor to the interior of the cell. In this way, integral proteins spanning the
cell membrane provide a means of conveying information
about the environment to the cell interior.
Peripheral protein molecules are often attached to
integral proteins. These peripheral proteins function
almost entirely as enzymes or as controllers of transport
of substances through cell membrane pores.
Membrane Carbohydrates—The Cell “Glycocalyx.”

Membrane carbohydrates occur almost invariably in combination with proteins or lipids in the form of glycoproteins or glycolipids. In fact, most of the integral proteins
are glycoproteins, and about one-tenth of the membrane
lipid molecules are glycolipids. The glyco-­ portions of
16

these molecules almost invariably protrude to the outside
of the cell, dangling outward from the cell surface. Many
other carbohydrate compounds, called proteoglycans—
which are mainly carbohydrates bound to small protein
cores—are loosely attached to the outer surface of the cell
as well. Thus, the entire outside surface of the cell often
has a loose carbohydrate coat called the glycocalyx.
The carbohydrate moieties attached to the outer surface of the cell have several important functions:
1.	Many of them have a negative electrical charge,
which gives most cells an overall negative surface
charge that repels other negatively charged 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 receptors for binding hormones, such as insulin. When bound, this
combination activates attached internal proteins that
in turn activate a cascade of intracellular enzymes.
4.	Some carbohydrate moieties enter into immune reactions, as discussed in Chapter 35.

CYTOPLASM AND ITS ORGANELLES
The cytoplasm is filled with minute and large dispersed
particles and organelles. The jelly-­like fluid portion of the
cytoplasm in which the particles are dispersed is called
cytosol and contains mainly dissolved proteins, electrolytes, and glucose.
Dispersed in the cytoplasm are neutral fat globules,
glycogen granules, ribosomes, secretory vesicles, and five
especially important organelles—the endoplasmic reticulum, the Golgi apparatus, mitochondria, lysosomes, and
peroxisomes.

Endoplasmic Reticulum
Figure 2-2 shows the endoplasmic reticulum, a network
of tubular structures called cisternae and flat vesicular
structures in the cytoplasm. This organelle helps process molecules made by the cell and transports them to
their specific destinations inside or outside the cell. The
tubules and vesicles interconnect. Also, their walls are
constructed of lipid bilayer membranes that contain large
amounts of proteins, similar to the cell membrane. The
total surface area of this structure in some cells—the liver
cells, for example—can be as much as 30 to 40 times the
cell membrane area.
The detailed structure of a small portion of endoplasmic reticulum is shown in Figure 2-4. The space inside
the tubules and vesicles is filled with endoplasmic matrix,
a watery medium that is different from 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 directed
to other parts of the cell. Also, the vast surface area of this

Chapter 2

The Cell and Its Functions

Golgi vesicles

Ribosome

Matrix

ER vesicles

Endoplasmic
reticulum
Rough (granular)
endoplasmic
reticulum

Smooth (agranular)
endoplasmic
reticulum

Figure 2-5. A typical Golgi apparatus and its relationship to the
endoplasmic reticulum (ER) and the nucleus.

Figure 2-4. Structure of the endoplasmic reticulum.

reticulum and the multiple enzyme systems attached to
its membranes provide the mechanisms for a major share
of the cell’s metabolic functions.
Ribosomes and the Rough (Granular) Endoplasmic
Reticulum. Attached to the outer surfaces of many parts

of the endoplasmic reticulum are large numbers of minute
granular particles called ribosomes. Where these particles
are present, the reticulum is called the rough (granular)
endoplasmic reticulum. The ribosomes are composed of a
mixture of RNA and proteins; they function to synthesize
new protein molecules in the cell, as discussed later in this
chapter and in Chapter 3.
Smooth (Agranular) Endoplasmic Reticulum. Part of

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

Golgi Apparatus
The Golgi apparatus, shown in Figure 2-5, is closely
related to the endoplasmic reticulum. It has membranes
similar to those of the smooth endoplasmic reticulum.
The Golgi apparatus 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 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 [ER vesicles]) continually pinch off from the endoplasmic reticulum and shortly thereafter fuse with the
Golgi apparatus. In this way, substances entrapped in ER

vesicles are transported from the endoplasmic reticulum
to the Golgi apparatus. The transported substances are
then processed in the Golgi apparatus to form lysosomes,
secretory vesicles, and other cytoplasmic components
(discussed later in this chapter).

Lysosomes
Lysosomes, shown in Figure 2-2, are vesicular organelles that form by breaking off from the Golgi apparatus; they then disperse throughout the cytoplasm. The
lysosomes provide an intracellular digestive system that
allows the cell to digest the following: (1) damaged cellular structures; (2) food particles that have been ingested
by the cell; and (3) unwanted matter such as bacteria.
Lysosome are different in various cell types but are usually 250 to 750 nanometers in diameter. They are surrounded by typical lipid bilayer membranes and are filled
with large numbers of small granules, 5 to 8 nanometers
in diameter, which are protein aggregates of as many as
40 different hydrolase (digestive) enzymes. A hydrolytic
enzyme is capable of splitting an organic compound into
two or more parts by combining hydrogen from a water
molecule with one part of the compound and combining the hydroxyl portion of the water molecule with the
other part of the compound. For example, protein is
hydrolyzed to form amino acids, glycogen is hydrolyzed
to form glucose, and lipids are hydrolyzed to form fatty
acids and glycerol.
Hydrolytic enzymes are highly concentrated in lysosomes. Ordinarily, the membrane surrounding the lysosome prevents the enclosed hydrolytic enzymes from
coming into contact with other substances in the cell and
therefore prevents their digestive actions. However, some
conditions of the cell break the membranes of 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
17

UNIT I

Golgi
apparatus

UNIT I Introduction to Physiology: The Cell and General Physiology
Secretory
granules

Outer membrane
Inner membrane
Cristae

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

amino acids and glucose. Some of the specific functions of
lysosomes are discussed later in this chapter.

Peroxisomes
Peroxisomes are physically similar to lysosomes, but
they are different in two important ways. First, they are
believed to be formed by self-­replication (or perhaps by
budding off from the smooth endoplasmic reticulum)
rather than from the Golgi apparatus. Second, they contain oxidases rather than hydrolases. Several of the oxidases are capable of combining oxygen with hydrogen
ions derived from different intracellular chemicals to
form hydrogen peroxide (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 oxidize many substances that
might otherwise be poisonous to the cell. For example,
about half the alcohol that a person drinks is detoxified
into acetaldehyde by the peroxisomes of the liver cells in
this manner. A major function of peroxisomes is to catabolize long-­chain fatty acids.

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

Mitochondria
The mitochondria, shown in Figure 2-2 and Figure 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.
18

Matrix

Outer chamber

Oxidative
phosphorylation
enzymes

Figure 2-7. Structure of a mitochondrion.

Mitochondria are present in all areas of each cell’s
cytoplasm, but the total number per cell varies from less
than 100 up to several thousand, depending on the energy
requirements of the cell. Cardiac muscle cells (cardiomyocytes), for example, use large amounts of energy and have
far more mitochondria than fat cells (adipocytes), which
are much less active and use less energy. Furthermore,
the mitochondria are concentrated in those portions
of the cell responsible for the major share of its energy
metabolism. They are also variable in size and shape.
Some mitochondria are only a few hundred nanometers
in diameter and are globular in shape, whereas others are
elongated and are as large as 1 micrometer in diameter
and 7 micrometers long. Still others are branching and
filamentous.
The basic structure of the mitochondrion, shown
in Figure 2-7, is composed mainly of two lipid bilayer-­
protein membranes, an outer membrane and an inner
membrane. Many infoldings of the inner membrane form
shelves or tubules called cristae onto which oxidative
enzymes are attached. The cristae provide a large surface
area for chemical reactions to occur. In addition, the inner
cavity of the mitochondrion is filled with a matrix that
contains large quantities of dissolved enzymes necessary
for extracting energy from nutrients. These enzymes operate in association with oxidative enzymes on the cristae
to cause oxidation of nutrients, thereby forming carbon
dioxide and water and, at the same time, releasing energy.
The liberated energy is used to synthesize a high-­energy
substance called adenosine triphosphate (ATP). ATP is
then transported out of the mitochondrion and diffuses
throughout the cell to release its own energy wherever it
is needed for performing cellular functions. The chemical
details of ATP formation by the mitochondrion are provided in Chapter 68, but some basic functions of ATP in
the cell are introduced later in this chapter.
Mitochondria are self-­replicative, which means that
one mitochondrion can form a second one, a third one,
and so on whenever the cell needs 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 constituent of the nucleus that

Chapter 2

The Cell and Its Functions

α-Tubulin
monomer
Endoplasmic
reticulum

Ribosome

Cell membrane

β-Tubulin
monomer

Microfilaments

Fibrous protein
dimer

Intermediate
filament
(8-12 nm)

Microfilament
(7 nm)

Microtubule
Two intertwined
F-actin chains

Mitochondrion
Intermediate filament

G-actin
monomer

Figure 2-8. Cell cytoskeleton composed of protein fibers called microfilaments, intermediate filaments, and microtubules.

controls replication of the cell. The DNA of the mitochondrion plays a similar role, controlling replication of the
mitochondrion. Cells that are faced with increased energy
demands—for example, in skeletal muscles subjected to
chronic exercise training—may increase the density of
mitochondria to supply the additional energy required.

Cell Cytoskeleton—Filament and Tubular
Structures
The cell cytoskeleton is a network of fibrillar proteins
organized into filaments or tubules. These originate as
precursor proteins synthesized by ribosomes in the cytoplasm. The precursor molecules then polymerize to form
filaments (Figure 2-8). As an example, large numbers of
actin microfilaments frequently occur in the outer zone
of the cytoplasm, called the ectoplasm, to form an elastic support for the cell membrane. Also, in muscle cells,
actin and myosin filaments are organized into a special
contractile machine that is the basis for muscle contraction, as discussed in Chapter 6.
Intermediate filaments are generally strong ropelike
filaments that often work together with microtubules,
providing strength and support for the fragile tubulin
structures. They are called intermediate because their
average diameter is between that of narrower actin microfilaments and wider myosin filaments found in muscle
cells. Their functions are mainly mechanical, and they are
less dynamic than actin microfilaments or microtubules.

All cells have intermediate filaments, although the protein subunits of these structures vary, depending on the
cell type. Specific intermediate filaments found in various
cells include desmin filaments in muscle cells, neurofilaments in neurons, and keratins in epithelial cells.
A special type of stiff filament composed of polymerized tubulin molecules is used in all cells to construct
strong tubular structures, the microtubules. Figure 2-8
shows typical microtubules of a cell.
Another example of microtubules is the tubular skeletal
structure in the center of each cilium that radiates upward
from the cell cytoplasm to the tip of the cilium. This structure is discussed later in the chapter (see Figure 2-18). Also,
both the centrioles and mitotic spindles of cells undergoing
mitosis are composed of stiff microtubules.
A major function of microtubules is to act as a cytoskeleton, providing rigid physical structures for certain
parts of cells. The cell cytoskeleton not only determines
cell shape but also participates in cell division, allows cells
to move, and provides a tracklike system that directs the
movement of organelles in the cells. Microtubules serve
as the conveyor belts for the intracellular transport of
vesicles, granules, and organelles such as mitochondria.

Nucleus
The nucleus is the control center of the cell and sends
messages to the cell to grow and mature, replicate, or
die. Briefly, the nucleus contains large quantities of DNA,
19

UNIT I

Microtubule
(25 nm)

UNIT I Introduction to Physiology: The Cell and General Physiology
15 nm: Small virus

Pores
Endoplasmic
reticulum

150 nm: Large virus

Nucleoplasm

350 nm: Rickettsia

Nucleolus
Nuclear envelope:
outer and inner
membranes

1 µm Bacterium
Cell

Chromatin material (DNA)
Cytoplasm

Figure 2-9. Structure of the nucleus.

which comprise 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 cell reproduction.
The genes first reproduce to create 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 discussed in Chapter 3.
Unfortunately, the appearance of the nucleus under the
microscope does not provide many clues to the mechanisms whereby the nucleus performs its control activities.
Figure 2-9 shows the light microscopic appearance of the
interphase nucleus (during the period between mitoses),
revealing darkly staining chromatin material throughout
the nucleoplasm. During mitosis, the chromatin material
organizes in the form of highly structured chromosomes,
which can then be easily identified using the light microscope, as illustrated in Chapter 3.
Nuclear Membrane. The nuclear membrane, also called

the nuclear envelope, is actually two separate bilayer
membranes, one inside the other. The outer membrane
is continuous with the endoplasmic reticulum of the cell
cytoplasm, and the space between the two nuclear membranes is also continuous with the space inside the endoplasmic reticulum, as shown in Figure 2-9.
The nuclear membrane is penetrated by several thousand nuclear pores. Large complexes of proteins 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 a
molecular weight of 44,000 to pass through with reasonable ease.
Nucleoli and Formation of Ribosomes. The nuclei of

most cells contain one or more highly staining structures
called nucleoli. The nucleolus, unlike most other organelles discussed here, does not have a limiting membrane.
Instead, it is simply an accumulation of large amounts of
20

5-10 µm+
Figure 2-10. Comparison of sizes of precellular organisms with that
of the average cell in the human body.

RNA and proteins of the types found in ribosomes. The
nucleolus enlarges considerably 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 synthesized RNA is
stored in the nucleoli, but most of it is transported outward through the nuclear pores into the 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 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 forms of life, microorganisms that may have been
similar to present-­day viruses, first appeared on earth.
Figure 2-10 shows the relative sizes of the following: (1)
the smallest known virus; (2) a large virus; (3) a Rickettsia; (4) a bacterium; and (5) a nucleated cell, This demonstrates that the cell has a diameter about 1000 times
that of the smallest virus and therefore a volume about 1
billion times that of the smallest virus. Correspondingly,
the functions and anatomical organization of the cell are
also far more complex than those of the virus.
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 is
capable of reproducing itself under appropriate conditions. Thus, the virus propagates its lineage from generation to generation and is therefore a living structure in the
same way that cells and humans are living structures.
As life evolved, other chemicals in addition to 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

Chapter 2

FUNCTIONAL SYSTEMS OF THE CELL
In the remainder of this chapter, we discuss some functional systems of the cell that make it a living organism.

ENDOCYTOSIS—INGESTION BY THE CELL
If a cell is to live and grow and reproduce, it must obtain
nutrients and other substances from the surrounding fluids. Most substances pass through the cell membrane by
the processes of diffusion and active transport.
Diffusion involves simple movement through the membrane caused by the random motion of the molecules of
the substance. Substances move through cell membrane
pores or, in the case of lipid-­soluble substances, through
the lipid matrix of the membrane.
Active transport involves actually carrying a substance
through the membrane by a physical protein structure
that penetrates all the way through the membrane. These
active transport mechanisms are so important to cell
function that they are presented in detail in Chapter 4.
Large particles enter the cell by a specialized function of the cell membrane called endocytosis (Video 2-­1).
The principal forms of endocytosis are pinocytosis and
phagocytosis. Pinocytosis means the ingestion of minute
particles that form vesicles of extracellular fluid and particulate constituents inside the cell cytoplasm. Phagocytosis means the 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 is especially rapid in some
cells. For example, it occurs so rapidly in macrophages
that about 3% 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 an electron microscope.

Proteins
Clathrin

B

A
Actin and myosin

C

Receptors

Coated pit

UNIT I

around the virus and, inside the membrane, a fluid matrix
appeared. Specialized chemicals then developed inside
the fluid to perform special functions; many protein
enzymes appeared that were capable of catalyzing chemical reactions, thus determining the organism’s activities.
In still later stages of life, particularly in the rickettsial and bacterial stages, organelles developed inside the
organism. These represent physical structures of chemical aggregates that perform functions in a more efficient
manner than what can be achieved by dispersed chemicals throughout the fluid matrix.
Finally, in the nucleated cell, still more complex organelles developed, the most important of which is the
nucleus. The nucleus distinguishes this type of cell from
all lower forms of life; it provides a control center for all
cellular activities and for reproduction of new cells generation after generation, with each new cell having almost
exactly the same structure as its progenitor.

The Cell and Its Functions

Dissolving clathrin

D
Figure 2-11. Mechanism of pinocytosis.

Pinocytosis is the only means whereby most large
macromolecules, such as most proteins, 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 (A–D), showing three molecules of protein
attaching to the membrane. These molecules usually
attach to specialized protein receptors on the surface of
the membrane that are specific for the type of protein
that is to be absorbed. The receptors generally are concentrated in small pits on the outer surface of the cell
membrane, called coated pits. On the inside of the cell
membrane beneath these pits is a latticework of fibrillar
protein called clathrin, as well as other proteins, perhaps
including contractile filaments of actin and myosin. Once
the protein molecules have bound with the receptors, the
surface properties of the local membrane change in such
a way that the entire pit invaginates inward, and fibrillar
proteins surrounding the invaginating pit cause its borders to close over the attached proteins, as well as over a
small amount of extracellular fluid. Immediately thereafter, the invaginated portion of the membrane breaks away
from the surface of the cell, forming a pinocytotic vesicle
inside the cytoplasm of the cell.
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,
which is supplied by ATP, a high-­energy substance discussed later in this chapter. This process also requires the
presence of calcium ions in the extracellular fluid, which
probably react with contractile protein filaments beneath
the coated pits to provide the force for pinching the vesicles away from the cell membrane.
Phagocytosis. Phagocytosis occurs in much the same

way as pinocytosis, except that it involves large particles
rather than molecules. Only certain cells have the capability of phagocytosis—notably, tissue macrophages and
some white blood cells.
21

UNIT I Introduction to Physiology: The Cell and General Physiology
Lysosomes

Pinocytotic or
phagocytic
vesicle
Digestive vesicle

proteins, carbohydrates, lipids, and other substances in the
vesicle. The products of digestion are small molecules of
substances such as amino acids, glucose, and phosphates
that can diffuse through the membrane of the vesicle into
the cytoplasm. What is left of the digestive vesicle, called
the residual body, represents indigestible substances. In
most cases, the residual body is finally excreted through
the cell membrane by a process called exocytosis, which is
essentially the opposite of endocytosis. Thus, the pinocytotic and phagocytic vesicles containing lysosomes can be
called the digestive organs of the cells.

Residual body

Lysosomes and Regression of Tissues and Autolysis
of Damaged Cells. Tissues of the body often regress to
Excretion
Figure 2-12. Digestion of substances in pinocytotic or phagocytic
vesicles by enzymes derived from lysosomes.

Phagocytosis is initiated when a particle such as a bacterium, 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; it is the antibody that attaches to the phagocyte receptors, dragging the bacterium along with it. This
intermediation of antibodies is called opsonization, which
is discussed in Chapters 34 and 35.
Phagocytosis occurs in the following steps:
1.	The cell membrane receptors attach to the surface
ligands of the particle.
2.	The edges of the membrane around the points of
attachment evaginate outward within a fraction of
a second to surround the entire particle; then, progressively more and more membrane receptors attach to the particle ligands. All this occurs suddenly
in a zipper-­like manner to form a closed phagocytic
vesicle.
3.	Actin and other contractile fibrils in the cytoplasm
surround the phagocytic vesicle and contract
around its outer edge, pushing the vesicle to the interior.
4.	The contractile proteins then pinch the stem of the
vesicle so completely that the vesicle separates from
the cell membrane, leaving the vesicle in the cell interior in the same way that pinocytotic vesicles are
formed.

LYSOSOMES DIGEST PINOCYTOTIC AND
PHAGOCYTIC FOREIGN SUBSTANCES
INSIDE THE CELL
Almost immediately after a pinocytotic or phagocytic vesicle appears inside a cell, one or more lysosomes become
attached to the vesicle and empty their acid hydrolases to
the inside of the vesicle, as shown in Figure 2-12. Thus,
a digestive vesicle is formed inside the cell cytoplasm in
which the vesicular hydrolases begin hydrolyzing the
22

a smaller size. For example, this regression occurs in the
uterus after pregnancy, in muscles during long periods of
inactivity, and in mammary glands at the end of lactation.
Lysosomes are responsible for much of this regression.
Another special role of the lysosomes is the removal
of damaged cells or damaged portions of cells from tissues. Damage to the cell—caused by heat, cold, trauma,
chemicals, or any other factor—induces lysosomes to
rupture. The released hydrolases immediately begin to
digest the surrounding organic substances. If the damage
is slight, only a portion of the cell is removed, and the cell
is then repaired. If the damage is severe, the entire cell is
digested, a process called autolysis. In this way, the cell is
completely removed, and a new cell of the same type is
formed, ordinarily 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 cause cellular damage. These agents include the following: (1) lysozyme, which
dissolves the bacterial cell wall; (2) lysoferrin, which binds
iron and other substances before they can promote bacterial
growth; and (3) acid at a pH of about 5.0, which activates the
hydrolases and inactivates bacterial metabolic systems.
Autophagy and Recycling of Cell Organelles.

Lysosomes play a key role in the process of autophagy,
which literally means “to eat oneself.” Autophagy is
a housekeeping process whereby obsolete organelles
and large protein aggregates are degraded and recycled (Figure 2-13). Worn-­o ut cell organelles are
transferred to lysosomes by double-­m embrane structures called autophagosomes, which are formed in the
cytosol. Invagination of the lysosomal membrane and
the formation of vesicles provides another pathway for
cytosolic structures to be transported into the lumen
of lysosomes. Once inside the lysosomes, the organelles are digested, and the nutrients are reused by the
cell. Autophagy contributes to the routine turnover of
cytoplasmic components; it is a key mechanism for
tissue development, cell survival when nutrients are
scarce, and maintenance of homeostasis. In liver cells,
for example, the average mitochondrion normally has
a life span of only about 10 days before it is destroyed.

Chapter 2

The Cell and Its Functions

Proteins Synthesis by the Rough Endoplasmic Reticulum. The rough endoplasmic reticulum is characterized by

Isolation membrane

Lipid Synthesis by the Smooth Endoplasmic Reticulum. The endoplasmic reticulum also synthesizes lipids,

AUTOSOME
FORMATION

Autophagosome

Lysosome

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

DOCKING AND
FUSION WITH
LYSOSOME

Autolysosome

Lysosomal
hydrolase

VESICLE BREAKDOWN AND DEGRADATION
Figure 2-13. Schematic diagram of autophagy steps.

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

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

Golgi Apparatus Functions
Synthetic Functions of the Golgi Apparatus. Although

a major function of the Golgi apparatus is to provide additional processing of substances already formed in the
endoplasmic reticulum, it can also synthesize 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, or nonfibrous
components of the extracellular matrix, outside the cells
in the interstitial spaces, which act as fillers between collagen fibers and cells; (3) they are principal components of
the organic matrix in both cartilage and bone; and (4) they
are important in many cell activities, including migration
and proliferation.
23

UNIT I

VESICLE
NUCLEATION

large numbers of ribosomes attached to the outer surfaces
of the endoplasmic reticulum membrane. As discussed in
Chapter 3, protein molecules are synthesized within the
structures of the ribosomes. The ribosomes extrude some
of the synthesized protein molecules directly into the cytosol, but they also extrude many more through the wall
of the endoplasmic reticulum to the interior of the endoplasmic vesicles and tubules into the endoplasmic matrix.

UNIT I Introduction to Physiology: The Cell and General Physiology
Protein
Ribosomes formation

Glycosylation

Lipid
formation

Lysosomes

Secretory
vesicles

Transport
vesicles

Smooth
Rough
Golgi
endoplasmic endoplasmic apparatus
reticulum
reticulum

Figure 2-14. Formation of proteins, lipids, and cellular vesicles by the
endoplasmic reticulum and Golgi apparatus.

Processing of Endoplasmic Secretions by the Golgi
Apparatus—Formation of Vesicles. Figure 2-14 sum-

marizes the major functions of the endoplasmic reticulum and Golgi apparatus. As substances are formed in
the endoplasmic reticulum, especially proteins, they are
transported through the tubules toward portions of the
smooth endoplasmic reticulum that lie nearest to the
Golgi apparatus. At this point, transport vesicles composed of small envelopes of smooth endoplasmic reticulum continually break away and diffuse to the deepest
layer of the Golgi apparatus. Inside these vesicles are
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 compact the endoplasmic reticular
secretions into highly concentrated packets. As the
secretions pass toward the outermost layers of the
Golgi apparatus, the compaction and processing proceed. Finally, both small and large vesicles continually
break away from the Golgi apparatus, carrying with
them the compacted secretory substances and diffusing throughout the cell.
The following example provides an idea of the timing of these processes. When a glandular cell is bathed
in amino acids, newly formed protein molecules can be
detected in the granular endoplasmic reticulum within 3
to 5 minutes. Within 20 minutes, newly formed proteins
are already present in the Golgi apparatus and, within 1
to 2 hours, the proteins are secreted from the surface of
the cell.
24

Types of Vesicles Formed by the Golgi Apparatus—
Secretory Vesicles and Lysosomes. In a highly secre-

tory cell, the vesicles formed by the Golgi apparatus are
mainly secretory vesicles containing proteins that are secreted through the surface of the cell membrane. These
secretory vesicles first diffuse to the cell membrane and
then fuse with it and empty their substances to the exterior by the mechanism called exocytosis. Exocytosis, in
most cases, is stimulated by entry of calcium ions into
the cell. Calcium ions interact with the vesicular membrane and cause its fusion with the cell membrane, followed by exocytosis—opening of the membrane’s outer
surface and extrusion of its contents outside the cell.
Some vesicles, however, are destined for intracellular
use.
Use of Intracellular Vesicles to Replenish Cellular
Membranes. Some 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 fusion
increases the expanse of these membranes and replenishes the membranes as they are used up. For example, the
cell membrane loses much of its substance every time it
forms a phagocytic or pinocytotic vesicle, and the vesicular membranes of the Golgi apparatus continually replenish the cell membrane.
In summary, the membranous system of the endoplasmic reticulum and Golgi apparatus are highly
metabolic and capable of forming new intracellular
structures and secretory substances to be extruded
from the cell.

THE MITOCHONDRIA EXTRACT ENERGY
FROM NUTRIENTS
The principal substances from which cells extract energy
are foods that react chemically with oxygen—carbohydrates, fats, and proteins. In the human body, essentially all carbohydrates are converted into glucose by the
digestive tract and liver before they reach the other cells
of the body. Similarly, proteins are converted into amino
acids, and