ions and other substances. Although most of this fluid is inside the cells and is
About 60 per cent of the adult human body is fluid, mainly a water solution of
Extracellular Fluid—The “Internal Environment”
cause or another, the remaining cells of this type usually generate new cells until
own kind. Fortunately, when cells of a particular type are destroyed from one
end products of their chemical reactions into the surrounding fluids.
ing nutrients into energy are basically the same in all cells, and all cells deliver
required for cell function. Further, the general chemical mechanisms for chang-
cells, oxygen reacts with carbohydrate, fat, and protein to release the energy
all of them have certain basic characteristics that are alike. For instance, in all
Although the many cells of the body often differ markedly from one another,
cells.
from those of the red cell. The entire body, then, contains about 100 trillion
cells are the most abundant of any single type of cell in the body, there are
human being, transport oxygen from the lungs to the tissues. Although the red
functions. For instance, the red blood cells, numbering 25 trillion in each
different cells held together by intercellular supporting structures.
The basic living unit of the body is the cell. Each organ is an aggregate of many
these special attributes allow us to exist under widely varying conditions.
feeling, and knowledgeable beings is part of this automatic sequence of life;
the human being is actually an automaton, and the fact that we are sensing,
for warmth. Other forces cause us to seek fellowship and to reproduce. Thus,
us seek food and fear makes us seek refuge. Sensations of cold make us look
very fact that we remain alive is almost beyond our control, for hunger makes
acteristics and mechanisms of the human body that make it a living being. The
human physiology,
Human Physiology.
and many more subdivisions.
human physiology,
physiology, cellular physiology, plant physiology,
viral physiology, bacterial
characteristics. Therefore, the vast field of physiol-
complicated human being, has its own functional
life, from the simple virus to the largest tree or the
development, and progression of life. Each type of
The goal of physiology is to explain the physical and
C
H
A
P
T
E
R
1
3
Functional Organization of the
Human Body and Control of the
“Internal Environment”
chemical factors that are responsible for the origin,
ogy can be divided into
In
we attempt to explain the specific char-
Cells as the Living Units of the Body
Each type of cell is specially adapted to perform one or a few particular
about 75 trillion additional cells of other types that perform functions different
Almost all cells also have the ability to reproduce additional cells of their
the supply is replenished.
called intracellular fluid, about one third is in the spaces outside the cells and
As blood passes through the blood capillaries,
when a person is extremely active.
Figure 1–1 shows the overall circulation of blood.
cells.
blood through the body in the blood vessels, and the
the body in two stages. The first stage is movement of
Extracellular Fluid Transport and
support of one another.
briefly outline the basic theory of the body’s control
to homeostasis are outlined in this chapter; then we
homeostasis. To begin this discussion, the different
system provides nutrients.
stant ion concentrations, and the gastrointestinal
oxygen used by the cells, the kidneys maintain con-
constant conditions. For instance, the lungs provide
The term
and intracellular fluids. These transport processes are
extracellular fluid. Special mechanisms for transport-
potassium, magnesium,
the extracellular fluid; specifically, it contains large
The intracellular fluid differs significantly from
excreted, plus other cellular waste products that are
carbon dioxide
amino acids.
oxygen, glucose, fatty acids,
for the cells, such as
sodium, chloride,
The extracellular fluid contains large amounts of
fatty substances, and other constituents are available
trations of oxygen, glucose, different ions, amino acids,
Cells are capable of living, growing, and performing
ago by the great 19th-century French physiologist
milieu intérieur,
of the body, or the
cellular fluid. For this reason, the extracellular fluid is
needed by the cells to maintain cell life. Thus, all cells
through the capillary walls.
constant motion throughout the body. It is transported
This extracellular fluid is in
Introduction to Physiology: The Cell and General Physiology
4
Unit I
is called extracellular fluid.
rapidly in the circulating blood and then mixed
between the blood and the tissue fluids by diffusion
In the extracellular fluid are the ions and nutrients
live in essentially the same environment—the extra-
also called the internal environment
a term introduced more than 100 years
Claude Bernard.
their special functions as long as the proper concen-
in this internal environment.
Differences Between Extracellular and Intracellular Fluids.
and bicarbonate ions plus nutrients
and
It also contains
that is
being transported from the cells to the lungs to be
being transported to the kidneys for excretion.
amounts of
and phosphate ions
instead of the sodium and chloride ions found in the
ing ions through the cell membranes maintain the ion
concentration differences between the extracellular
discussed in Chapter 4.
“Homeostatic” Mechanisms of
the Major Functional Systems
Homeostasis
homeostasis is used by physiologists to mean
maintenance of nearly constant conditions in the inter-
nal environment. Essentially all organs and tissues of
the body perform functions that help maintain these
oxygen to the extracellular fluid to replenish the
A large segment of this text is concerned with the
manner in which each organ or tissue contributes to
functional systems of the body and their contributions
systems that allow the functional systems to operate in
Mixing System—The Blood
Circulatory System
Extracellular fluid is transported through all parts of
second is movement of fluid between the blood capil-
laries and the intercellular spaces between the tissue
All the blood in the circulation traverses the entire cir-
culatory circuit an average of once each minute when
the body is at rest and as many as six times each minute
continual exchange of extracellular fluid also occurs
between the plasma portion of the blood and the
Venous
Right
heart
pump
Left
heart
pump
Gut
Lungs
Kidneys
Excretion
Regulation
of
electrolytes
end
Arterial
end
Capillaries
Nutrition and excretion
O
2
CO
2
General organization of the circulatory system.
Figure 1–1
the body or the state of the surroundings. For instance,
), and the
three major parts: the
The nervous system is composed of
Nervous System.
Regulation of Body Functions
the renal tubules into the urine.
such as urea, are reabsorbed poorly and pass through
by the body, especially the metabolic end products
ions. Most of the other substances that are not needed
acids, appropriate amounts of water, and many of the
substances needed by the body, such as glucose, amino
The kidneys perform their function by first filtering
of cellular metabolism, such as urea and uric acid; they
cells. These substances include different end products
carries the carbon dioxide to the atmosphere. Carbon
is released from the blood into the lung alveoli; the res-
carbon dioxide
that blood picks up oxygen in the lungs,
static mechanisms, could be destroyed instantaneously.
without which the entire body, along with its homeo-
motility for protection against adverse surroundings,
nutrition. The musculoskeletal system also provides
obvious and simple: Were it not for the muscles, the
the homeostatic functions of the body? The answer is
asked, How does the musculoskeletal system fit into
mucosa, kidneys, and endocrine glands—help modify
other tissues of the body—fat cells, gastrointestinal
many of these substances to more usable forms, and
cells. The liver changes the chemical compositions of
amino acids,
carbohydrates, fatty acids,
nutrients, including
the gastrointestinal tract. Here different dissolved
Gastrointestinal Tract.
diffuse through walls of the tissue capillaries.
0.4 to 2.0 micrometers thick, and oxygen diffuses by
alveolar membrane,
pulmonary capillaries, the
needed by the cells. The
oxygen
the lungs. The blood picks up oxygen in the alveoli,
blood passes through the body, it also flows through
Figure 1–1 shows that each time the
Respiratory System.
extracellular fluid throughout the body.
stitial fluid—is continually being mixed, thereby
seconds. Thus, the extracellular fluid everywhere in the
a capillary, which ensures diffusion of almost any sub-
Few cells are located more than 50 micrometers from
cellular spaces, and also through the capillary pores.
the interstitial fluid. That is, the fluid and dissolved
the arrows. This process of diffusion is caused by
between the blood and the tissue spaces, as shown by
protein molecules. Therefore, large amounts of fluid
of the blood, with the exception of the large plasma
process is shown in Figure 1–2. The walls of the capil-
interstitial fluid that fills the intercellular spaces. This
Functional Organization of the Human Body and Control of the “Internal Environment”
Chapter 1
5
laries are permeable to most molecules in the plasma
and its dissolved constituents diffuse back and forth
kinetic motion of the molecules in both the plasma and
molecules are continually moving and bouncing in all
directions within the plasma and the fluid in the inter-
stance from the capillary to the cell within a few
body—both that of the plasma and that of the inter-
maintaining almost complete homogeneity of the
Origin of Nutrients in the
Extracellular Fluid
thus acquiring the
membrane between the alveoli and the lumen of the
is only
molecular motion through the pores of this membrane
into the blood in the same manner that water and ions
A large portion of the blood
pumped by the heart also passes through the walls of
and
are absorbed from the ingested food into
the extracellular fluid of the blood.
Liver and Other Organs That Perform Primarily Metabolic Func-
tions.
Not all substances absorbed from the gastroin-
testinal tract can be used in their absorbed form by the
the absorbed substances or store them until they are
needed.
Musculoskeletal System.
Sometimes the question is
body could not move to the appropriate place at the
appropriate time to obtain the foods required for
Removal of Metabolic End Products
Removal of Carbon Dioxide by the Lungs.
At the same time
piratory movement of air into and out of the lungs
dioxide is the most abundant of all the end products
of metabolism.
Kidneys.
Passage of the blood through the kidneys
removes from the plasma most of the other substances
besides carbon dioxide that are not needed by the
also include excesses of ions and water from the food
that might have accumulated in the extracellular fluid.
large quantities of plasma through the glomeruli into
the tubules and then reabsorbing into the blood those
sensory input portion, the central
nervous system (or integrative portion
motor
output portion. Sensory receptors detect the state of
Venule
Arteriole
walls and through the interstitial spaces.
Diffusion of fluid and dissolved constituents through the capillary
Figure 1–2
heart and blood vessels. Lack of these impulses causes
vasomotor center,
the medulla of the brain. Here these impulses inhibit
arterial wall. When the arterial pressure rises too high,
baroreceptors,
aorta in the thorax, are many nerve receptors called
carotid arteries in the neck, and also in the arch of the
nism. In the walls of the bifurcation region of the
One of these, the
tribute to the regulation of arterial blood pressure.
tissue fluids. This process continues until the concen-
increases expiration of carbon dioxide and, therefore,
causing a person to breathe rapidly and deeply. This
excites the respiratory center,
nately, a higher than normal carbon dioxide concen-
halt all energy-giving reactions of the cells. Fortu-
cells continued to accumulate in the tissue fluids, the
tions in cells. If all the carbon dioxide formed in the
fluid is regulated in a much different way. Carbon
oxygen-buffering function of hemoglobin.
characteristics of hemoglobin itself. This regulation is
centration. Thus, regulation of oxygen concentration
centration in the tissue fluid is too low, sufficient
much oxygen is already there. But if the oxygen con-
passes through the tissue capillaries, hemoglobin,
blood passes through the lungs. Then, as the blood
blood cells. Hemoglobin combines with oxygen as the
oxygen concentration in the extracellular fluid. This
substances required for chemical reactions in the cells,
potassium, phosphate, and other ions in the extracel-
kidneys regulate concentrations of hydrogen, sodium,
tion of glucose in the extracellular fluid, and the
fluid. The liver and pancreas regulate the concentra-
association with the nervous system, regulates the
For instance, the respiratory system, operating in
to control the interrelations between the organs.
of the organs; others operate throughout the entire
within the
cellular function as well as extracellular function. This
it. The most intricate of these are the genetic control
The human body has thousands of control systems in
Control Systems of the Body
ity and continuity of life.
the final analysis, essentially all body structures are
but it illustrates that, in
homeostasis,
those that are dying. This may sound like a permissive
static function. It does, however, help maintain home-
bolic functions.
mainly muscular and secretory activities of the body,
the nervous system. The nervous system regulates
controls bone calcium and phosphate. Thus, the hor-
and protein metabolism; and parathyroid hormone
cortical hormones control sodium ion, potassium ion,
activity. Insulin controls glucose metabolism; adreno-
tions in all cells, thus helping to set the tempo of bodily
to help regulate cellular function. For instance, thyroid
hormones.
by many of the body’s glands.
movements of the gastrointestinal tract, and secretion
and controls many functions of the internal organs,
out one’s desires.
that the body performs in response to the sensations.
thoughts, create ambition, and determine reactions
cord. The brain can store information, generate
area. The ears also are sensory organs. The central
touches the skin at any point. The eyes are sensory
Introduction to Physiology: The Cell and General Physiology
6
Unit I
receptors in the skin apprise one whenever an object
organs that give one a visual image of the surrounding
nervous system is composed of the brain and spinal
Appropriate signals are then transmitted through the
motor output portion of the nervous system to carry
A large segment of the nervous system is called the
autonomic system. It operates at a subconscious level
including the level of pumping activity by the heart,
Hormonal System of Regulation.
Located in the body are
eight major endocrine glands that secrete chemical
substances called
Hormones are trans-
ported in the extracellular fluid to all parts of the body
hormone increases the rates of most chemical reac-
mones are a system of regulation that complements
whereas the hormonal system regulates many meta-
Reproduction
Sometimes reproduction is not considered a homeo-
ostasis by generating new beings to take the place of
usage of the term
organized such that they help maintain the automatic-
systems that operate in all cells to help control intra-
subject is discussed in Chapter 3.
Many other control systems operate
organs to control functions of the individual parts
body
concentration of carbon dioxide in the extracellular
lular fluid.
Examples of Control Mechanisms
Regulation of Oxygen and Carbon Dioxide Concentrations in the
Extracellular Fluid.
Because oxygen is one of the major
it is fortunate that the body has a special control
mechanism to maintain an almost exact and constant
mechanism depends principally on the chemical char-
acteristics of hemoglobin, which is present in all red
because of its own strong chemical affinity for oxygen,
does not release oxygen into the tissue fluid if too
oxygen is released to re-establish an adequate con-
in the tissues is vested principally in the chemical
called the
Carbon dioxide concentration in the extracellular
dioxide is a major end product of the oxidative reac-
mass action of the carbon dioxide itself would soon
tration in the blood
removes excess carbon dioxide from the blood and
tration returns to normal.
Regulation of Arterial Blood Pressure.
Several systems con-
baroreceptor system, is a simple and
excellent example of a rapidly acting control mecha-
which are stimulated by stretch of the
the baroreceptors send barrages of nerve impulses to
the
which in turn decreases the
number of impulses transmitted from the vasomotor
center through the sympathetic nervous system to the
diminished pumping activity by the heart and also
The degree of effectiveness
maintaining homeostasis.
return the factor toward a certain mean value, thus
feedback,
sive or deficient, a control system initiates
Therefore, in general, if some factor becomes exces-
initiating stimulus.
instances, these effects are negative with respect to the
of reactions that promote an elevated pressure. In both
a lowered pressure, or a low pressure causes a series
In the arterial pressure–regulating mechanisms, a
stimulus.
tration. This response also is negative to the initiating
too low, this causes feedback to increase the concen-
Conversely, if the carbon dioxide concentration falls
to the initiating stimulus.
normal, which is
words, the high concentration of carbon dioxide initi-
amounts of carbon dioxide from the body. In other
This, in turn, decreases the extracellular fluid carbon
tration, a high concentration of carbon dioxide in the
viously. In the regulation of carbon dioxide concen-
back,
Control Systems
Negative Feedback Nature of Most
common. These characteristics are explained in this
the body, all of which have certain characteristics in
The aforementioned examples of homeostatic control
controls, serious body malfunction or death can result.
operating in health; in the absence of any one of these
These examples should give one an appreciation for
even convulsions.
falls below one half of normal, a person frequently
peripheral nerves. When the glucose concentration
centration falls below about one half of normal, a
severely depressed. Also, when the calcium ion con-
more times normal, the heart muscle is likely to be
nerves’ inability to carry signals. Alternatively, if
factor is the potassium ion concentration, because
about 0.5 on either side of normal. Another important
narrow range for acid-base balance in the body, with
lar metabolism that destroys the cells. Note also the
normal can lead to a vicious cycle of increasing cellu-
in the body temperature of only 11°F (7°C) above
malities can cause death. For example, an increase
ranges are usually caused by illness.
the normal range for each one. Values outside these
limits without causing death. Note the narrowness of
with their normal values, normal ranges, and maximum
physical characteristics of extracellular fluid, along
Table 1–1 lists the more important constituents and
of Important Extracellular Fluid Constituents
Normal Ranges and Physical Characteristics
pumping, and raising arterial pressure back toward
normal relaxes the stretch receptors, allowing the
Conversely, a decrease in arterial pressure below
increased blood flow through the vessels. Both of these
dilation of the peripheral blood vessels, allowing
Functional Organization of the Human Body and Control of the “Internal Environment”
Chapter 1
7
effects decrease the arterial pressure back toward
normal.
vasomotor center to become more active than usual,
thereby causing vasoconstriction and increased heart
normal.
Most important are the limits beyond which abnor-
a normal pH value of 7.4 and lethal values only
whenever it decreases to less than one third normal,
a person is likely to be paralyzed as a result of the
the potassium ion concentration increases to two or
person is likely to experience tetanic contraction of
muscles throughout the body because of the sponta-
neous generation of excess nerve impulses in the
develops extreme mental irritability and sometimes
the extreme value and even the necessity of the
vast numbers of control systems that keep the body
Characteristics of Control Systems
mechanisms are only a few of the many thousands in
section.
Most control systems of the body act by negative feed-
which can best be explained by reviewing some
of the homeostatic control systems mentioned pre-
extracellular fluid increases pulmonary ventilation.
dioxide concentration because the lungs expire greater
ates events that decrease the concentration toward
negative
high pressure causes a series of reactions that promote
negative
which consists of a series of changes that
“Gain” of a Control System.
with which a control system maintains constant
Table 1–1
Body temperature
98.4 (37.0)
98–98.8 (37.0)
65–110 (18.3–43.3)
Glucose
85
75–95
20–1500
mg/dl
Bicarbonate ion
28
24–32
8–45
mmol/L
Chloride ion
108
103–112
70–130
mmol/L
Calcium ion
1.2
1.0–1.4
0.5–2.0
mmol/L
Potassium ion
4.2
3.8–5.0
1.5–9.0
mmol/L
Sodium ion
142
138–146
115–175
mmol/L
Carbon dioxide
40
35–45
5–80
mm Hg
Oxygen
40
35–45
10–1000
mm Hg
Normal Value
Normal Range
Approximate Short-Term
Unit
Important Constituents and Physical Characteristics of Extracellular Fluid
Nonlethal Limit
∞F (∞C)
Acid-base
7.4
7.3–7.5
6.9–8.0
pH
positive feedback.
more of the same, which is
the heart. In other words, the initiating stimulus causes
cycle in the feedback results in further weakening of
again and again until death occurs. Note that each
more weakness of the heart; the cycle repeats itself
a further decrease in coronary blood flow, and still
weakening of the heart, further diminished pumping,
through the coronary vessels diminishes. This results in
pressure falls, and the flow of blood to the heart muscle
the heart to pump effectively. As a result, the arterial
liters, the amount of blood in the body is decreased to
of blood per minute. If the person is suddenly bled 2
pumping effectiveness of the heart, showing that the
ensue from positive feedback. This figure depicts the
Figure 1–3 shows an example in which death can
the nature of positive feedback, one immediately sees
One might ask the question, Why do essentially all
Vicious Cycles and Death
to moderately cold weather is about –33. Therefore,
system. For instance, the gain of the system controlling
The gains of some other physiologic control systems
by +25, or –2. That is, a disturbance that increases or
Hg. Therefore, the gain of the person’s baroreceptor
Thus, in the baroreceptor system example, the correc-
tive in preventing change. The gain of the system is
in pressure of +25 mm Hg, called the “error,” which
175 mm Hg to 125 mm Hg. There remains an increase
caused a “correction” of –50 mm Hg—that is, from
25 mm Hg. Thus, the feedback control system has
tioning, and this time the pressure increases only
level of 100 mm Hg up to 175 mm Hg. Then, let us
ing, and the arterial pressure rises from the normal
feedback. For instance, let us assume that a large
Introduction to Physiology: The Cell and General Physiology
8
Unit I
conditions is determined by the gain of the negative
volume of blood is transfused into a person whose
baroreceptor pressure control system is not function-
assume that the same volume of blood is injected into
the same person when the baroreceptor system is func-
means that the control system is not 100 per cent effec-
then calculated by the following formula:
tion is –50 mm Hg and the error persisting is +25 mm
system for control of arterial pressure is –50 divided
decreases the arterial pressure does so only one third
as much as would occur if this control system were not
present.
are much greater than that of the baroreceptor
internal body temperature when a person is exposed
one can see that the temperature control system is
much more effective than the baroreceptor pressure
control system.
Positive Feedback Can Sometimes Cause
control systems of the body operate by negative feed-
back rather than positive feedback? If one considers
that positive feedback does not lead to stability but to
instability and often death.
heart of a healthy human being pumps about 5 liters
such a low level that not enough blood is available for
Gain
Correction
Error
=
begin pushing through the cervix, stretch of the cervix
feedback plays a valuable role. When uterine contrac-
attacks, which are caused by a clot beginning on the
In fact, this is what initiates most acute heart
hand and cause the formation of unwanted clots.
occurs. On occasion, this mechanism can get out of
more blood clotting. This process continues until the
of the immediately adjacent blood, thus causing
are activated within the clot itself. Some
and a clot begins to form, multiple enzymes called
positive feedback. When a blood vessel is ruptured
tage. Blood clotting is an example of a valuable use of
instances, the body uses positive feedback to its advan-
Figure 1
would recover, as shown by the dashed curve of
liters, the normal negative feedback mechanisms for
develop. For instance, if the person in the aforemen-
nisms of the body, and the vicious cycle fails to
cycle,
Positive feedback is better known as a “vicious
” but a mild degree of positive feedback can be
overcome by the negative feedback control mecha-
tioned example were bled only 1 liter instead of 2
controlling cardiac output and arterial pressure would
overbalance the positive feedback and the person
–3.
Positive Feedback Can Sometimes Be Useful.
In some
clot-
ting factors
of these enzymes act on other unactivated enzymes
hole in the vessel is plugged and bleeding no longer
inside surface of an atherosclerotic plaque in a coro-
nary artery and then growing until the artery is
blocked.
Childbirth is another instance in which positive
tions become strong enough for the baby’s head to
sends signals through the uterine muscle back to the
5
4
3
2
1
0
1
Hours
Death
Bled 2 liters
Return to
normal
Bled 1 liter
Pumping effectiveness of heart
(Liters pumped per minute)
2
3
when 2 liters of blood are removed.
liter of blood is removed from the circulation. Death is caused by
Recovery of heart pumping caused by
Figure 1–3
negative feedback after 1
positive feedback
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Physiology. Bethesda: American Physiological Society,
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leads to sickness.
dysfunction leads to death; moderate dysfunction
this happens, all the cells of the body suffer. Extreme
their ability to contribute their share of function.When
share toward the maintenance of homeostasis. This
from homeostasis, and in turn, each cell contributes its
tinue to live and function properly. Each cell bene
in this internal environment, the cells of the body con-
uid, which is called the
organs.
ent functional structures, some of which are called
order of about 100 trillion cells
in harmony. To summarize, the body is actually a
the overall organization of the body and, second, the
The purpose of this chapter has been to point out,
Summary—Automaticity
mechanisms.
depends on all of them. Therefore, a major share of
control systems of the body can be. A person
Thus, one can see how complex the feedback
Adaptive control, in a sense, is delayed negative
subsequent movements. This is called
further correction is needed, this will be done again for
time the movement is required. Then, if still
is performed correctly. If not, the brain corrects the
tractions. That is, sensory nerve signals from the
feed-forward control
movement. Therefore, the brain uses a principle called
discussed. Many are not. For instance, some move-
of interconnected control mechanisms. Some are
Later in this text, when we study the nervous system,
Adaptive Control
More Complex Types of Control Systems—
systems.
blood volume. Also, the positive feedback that causes
clotting, the positive feedback clotting process is a
feedback process. For example, in the case of blood
In each case in which positive feedback is useful, the
ber.
action potentials. This process continues again and
nerve action potential. This action potential in turn
ber, which creates the
still more opening of channels, and so forth. Thus, a
opening of channels, more change of potential,
the membrane potential, which in turn causes more
The sodium ions entering the
s interior.
ber is stimulated, this
for the generation of nerve signals. That is, when
the contractions usually die out, and a few days pass
enough, the baby is born. If it is not powerful enough,
contractions. When this process becomes powerful
the cervix, and the cervical stretch causes stronger
contractions. Thus, the uterine contractions stretch
body of the uterus, causing even more powerful
Functional Organization of the Human Body and Control of the “Internal Environment”
Chapter 1
9
before they begin again.
Another important use of positive feedback is
the membrane of a nerve fi
causes slight leakage of sodium ions through sodium
channels in the nerve membrane to the fiber’
fiber then change
slight leak becomes an explosion of sodium entering
the interior of the nerve fi
causes electrical current to flow along both the outside
and the inside of the fiber and initiates additional
again until the nerve signal goes all the way to the end
of the fi
positive feedback itself is part of an overall negative
negative feedback process for maintenance of normal
nerve signals allows the nerves to participate in
thousands of negative feedback nervous control
we shall see that this system contains great numbers
simple feedback systems similar to those already
ments 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
to cause required muscle con-
moving parts apprise the brain whether the movement
feed-forward signals that it sends to the muscles the
next
adaptive control.
feedback.
’s life
this text is devoted to discussing these life-giving
of the Body
first,
means by which the different parts of the body operate
social
organized into differ-
Each functional structure contributes its share
to the maintenance of homeostatic conditions in the
extracellular fl
internal envi-
ronment. As long as normal conditions are maintained
fits
reciprocal interplay provides continuous automaticity
of the body until one or more functional systems lose
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