Cells as the Living Units of the Body
The basic
living unit of the body is the cell. Each organ is an aggregate of many
different cells held together by intercellular supporting structures. Each type
of cell is specially adapted to perform one or a few particular functions. For
instance, the red blood cells, numbering 25 trillion in each human being,
transport oxygen from the lungs to the tissues. Although the red cells are the
most abundant of any single type of cell in the body, there are about 75
trillion additional cells of other types that perform functions different from
those of the red cell. The entire body, then, contains about 100 trillion
cells. Although the many cells of the body often differ markedly from one
another, all of them have certain basic characteristics that are alike. For
instance, in all cells, oxygen reacts with carbohydrate, fat, and protein to
release the energy required for cell function. Further, the general chemical
mechanisms for changing nutrients into energy are basically the same in all
cells, and all cells deliver end products of their chemical reactions into the
surrounding fluids. Almost all cells also have the ability to reproduce
additional cells of their own kind. Fortunately, when cells of a particular
type are destroyed from one cause or another, the remaining cells of this type
usually generate new cells until the supply is replenished.
Extracellular
Fluid—the “Internal Environment”
About 60 per
cent of the adult human body is fluid, mainly a water solution of ions and other
substances. Although most of this fluid is inside the cells and is called intracellular
fluid, about one third is in the spaces outside the cells and is called
extracellular fluid. This extracellular fluid is in constant motion throughout
the body. It is transported rapidly in the circulating blood and then mixed
between the blood and the tissue fluids by diffusion through the capillary
walls. In the extracellular fluid are the ions and nutrients needed by the cells
to maintain cell life. Thus, all cells live in essentially the same
environment—the extracellular fluid. For this reason, the extracellular fluid is
also called the internal environment of the body or the milieu intérieur, a
term introduced more than 100 years ago by the great 19th-century French
physiologist Claude Bernard. Cells are capable of living, growing and
performing their special functions as long as the proper concentrations of
oxygen, glucose ,different ions, amino acids, fatty substances, and other
constituents are available in this internal environment.
Differences between
Extracellular and Intracellular Fluids:
The extracellular
fluid contains large amounts of sodium, chloride, and bicarbonate ions plus
nutrients for the cells, such as oxygen, glucose, fatty acids, and amino acids.
It also contains carbon dioxide that is being transported from the cells to the
lungs to be excreted, plus other cellular waste products that are being
transported to the kidneys for excretion. The intracellular fluid differs
significantly from the extracellular fluid; specifically, it contains large
amounts of potassium, magnesium and phosphate ions instead of the sodium and
chloride ions found in the extracellular fluid. Special mechanisms for
transporting ions through the cell membranes maintain the ion concentration
differences between the extracellular and intracellular fluids.
“Homeostatic” Mechanisms
of the Major Functional Systems
Homeostasis
The term
homeostasis is used by physiologists to mean maintenance of nearly constant
conditions in the internal environment. Essentially all organs and tissues of
the body perform functions that help maintain these constant conditions. For
instance, the lungs provide oxygen to the extracellular fluid to replenish the
oxygen used by the cells, the kidneys maintain constant ion concentrations, and
the gastrointestinal system provides nutrients. A large segment of this text is
concerned with the manner in which each organ or tissue contributes to
homeostasis. To begin this discussion, the different functional systems of the
body and their contributions to homeostasis are outlined in this chapter; then
we briefly outline the basic theory of the body’s control systems that allow the
functional systems to operate in support of one another.
Extracellular Fluid
Transport and Mixing System—the Blood Circulatory System
Extracellular
fluid is transported through all parts of the body in two stages. The first stage
is movement of blood through the body in the blood vessels, and the second is
movement of fluid between the blood capillaries and the intercellular spaces
between the tissue cells. The diagram shows the overall circulation of blood.
All the blood in the circulation traverses the entire circulatory circuit an
average of once each minute when the body is at rest and as many as six times
each minute when a person is extremely active. As blood passes through the blood
capillaries, continual exchange of extracellular fluid also occur between the
plasma portion of the blood and the interstitial fluid that fills the
intercellular spaces. The walls of the capillaries are permeable to most
molecules in the plasma of the blood, with the exception of the large plasma
protein molecules. Therefore, large amounts of fluid and its dissolved
constituents diffuse back and forth between the blood and the tissue spaces, as
shown by the arrows. This process of diffusion is caused by kinetic motion of
the molecules in both the plasma and the interstitial fluid. That is, the fluid
and dissolved molecules are continually moving and bouncing in all directions
within the plasma and the fluid in the intercellular spaces, and also through
the capillary pores. Few cells are located more than 50 micrometers from a
capillary, which ensures diffusion of almost any substance from the capillary
to the cell within a few seconds. Thus, the extracellular fluid everywhere in
the body—both that of the plasma and that of the interstitial fluid—is
continually being mixed, thereby maintaining almost complete homogeneity of the
extracellular fluid throughout the body.
Origin of Nutrients
in the Extracellular Fluid
Respiratory System.
The above
diagram shows that each time the blood passes through the body, it also flows
through the lungs. The blood picks up oxygen in the alveoli, thus acquiring the
oxygen needed by the cells. The membrane between the alveoli and the lumen of
the pulmonary capillaries, the alveolar membrane, is only 0.4 to 2.0
micrometers thick, and oxygen diffuses by molecular motion through the pores of
this membrane into the blood in the same manner that water and ions diffuse
through walls of the tissue capillaries.
Gastrointestinal Tract.
A large portion
of the blood pumped by the heart also passes through the walls of the
gastrointestinal tract. Here different dissolved nutrients, including
carbohydrates, fatty acids, and amino acids, are absorbed from the ingested
food into the extracellular fluid of the blood.
Liver and
Other Organs That Perform Primarily Metabolic Functions. Not all substances
absorbed from the gastrointestinal tract can be used in their absorbed form by
the cells. The liver changes the chemical compositions of many of these substances
to more usable forms, and other tissues of the body—fat cells, gastrointestinal
mucosa, kidneys, and endocrine glands—help modify the absorbed substances or
store them until they are needed.
Musculoskeletal System.
Sometimes
the question is asked, How does the musculoskeletal system fit into the
homeostatic functions of the body? The answer is obvious and simple: Were it
not for the muscles, the body could not move to the appropriate place at the
appropriate time to obtain the foods required for nutrition. The
musculoskeletal system also provides motility for protection against adverse
surroundings, without which the entire body, along with its homeostatic
mechanisms, could be destroyed instantaneously.
Removal of
Metabolic End Products
Removal of
Carbon Dioxide by the Lungs. At the same time that blood picks up oxygen in the
lungs, carbon dioxide is released from the blood into the lung alveoli; the
respiratory movement of air into and out of the lungs carries the carbon
dioxide to the atmosphere. Carbon dioxide is the most abundant of all the end
products of metabolism.
Kidneys
Passage of the blood through the kidneys removes from the plasma most of the
other substances besides carbon dioxide that are not needed by the cells. These
substances include different end products of cellular metabolism, such as urea
and uric acid; they also include excesses of ions and water from the food that
might have accumulated in the extracellular fluid. The kidneys perform their
function by first filtering large quantities of plasma through the glomeruli into
the tubules and then reabsorbing into the blood those substances needed by the
body, such as glucose, amino acids, appropriate amounts of water, and many of
the ions. Most of the other substances that are not needed by the body,
especially the metabolic end products such as urea, are reabsorbed poorly and
pass through the renal tubules into the urine.
Regulation
of Body Functions
Nervous System.
The nervous
system is composed of three major parts: the sensory input portion, the central
nervous system (or integrative portion), and the motor output portion. Sensory
receptors detect the state of the body or the state of the surroundings. For
instance, receptors in the skin apprise one whenever an object touches the skin
at any point. The eyes are sensory organs that give one a visual image of the
surrounding area. The ears also are sensory organs. The central nervous system
is composed of the brain and spinal cord. The brain can store information,
generate thoughts, create ambition, and determine reactions that the body
performs in response to the sensations. Appropriate signals are then
transmitted through the motor output portion of the nervous system to carry out
one’s desires. A large segment of the nervous system is called the autonomic
system. It operates at a subconscious level and controls many functions of the
internal organs, including the level of pumping activity by the heart,
movements of the gastrointestinal tract, and secretion by many of the body’s
glands.
Hormonal System of Regulation
Located in
the body are eight major endocrine glands that secrete chemical substances
called hormones. Hormones are transported in the extracellular fluid to all
parts of the body to help regulate cellular function. For instance, thyroid
hormone increases the rates of most chemical reactions in all cells, thus
helping to set the tempo of bodily activity. Insulin controls glucose
metabolism; adrenocortical hormones control sodium ion, potassium ion, and
protein metabolism; and parathyroid hormone controls bone calcium and
phosphate. Thus, the hormones are a system of regulation that complements the
nervous system. The nervous system regulates mainly muscular and secretory
activities of the body, whereas the hormonal system regulates many metabolic
functions.
Control Systems of
the Body
The human
body has thousands of control systems in it. The most intricate of these are
the genetic control systems that operate in all cells to help control
intracellular function as well as extracellular function. Many other control
systems operate within the organs to control functions of the individual parts
of the organs; others operate throughout the entire body to control the
interrelations between the organs. For instance, the respiratory system,
operating in association with the nervous system, regulates the
Concentration
of carbon dioxide in the extracellular fluid. The liver and pancreas regulate
the concentration of glucose in the extracellular fluid, and the kidneys
regulate concentrations of hydrogen, sodium, potassium, phosphate, and other
ions in the extracellular fluid.
Examples of Control
Mechanisms
Regulation
of Oxygen and Carbon Dioxide Concentrations in the Extracellular Fluid. Because
oxygen is one of the major substances required for chemical reactions in the
cells, it is fortunate that the body has a special control mechanism to
maintain an almost exact and constant oxygen concentration in the extracellular
fluid. This mechanism depends principally on the chemical characteristics of
hemoglobin, which is present in all red blood cells. Hemoglobin combines with
oxygen as the blood passes through the lungs. Then, as the blood passes through
the tissue capillaries, hemoglobin, because of its own strong chemical affinity
for oxygen, does not release oxygen into the tissue fluid if too much oxygen is
already there. But if the oxygen concentration in the tissue fluid is too low,
sufficient oxygen is released to re-establish an adequate concentration. Thus,
regulation of oxygen concentration in the tissues is vested principally in the
chemical characteristics of hemoglobin itself. This regulation is called the
oxygen-buffering function of hemoglobin. Carbon dioxide concentration in the
extracellular fluid is regulated in a much different way. Carbon dioxide is a
major end product of the oxidative reactions in cells. If all the carbon
dioxide formed in the cells continued to accumulate in the tissue fluids, the
mass action of the carbon dioxide itself would soon halt all energy-giving
reactions of the cells. Fortunately, a higher than normal carbon dioxide
concentration in the blood excites the respiratory center, causing a person to
breathe rapidly and deeply. This increases expiration of carbon dioxide and,
therefore, removes excess carbon dioxide from the blood and tissue fluids. This
process continues until the concentration returns to normal.
Regulation
of Arterial Blood Pressure.
Several
systems contribute to the regulation of arterial blood pressure. One of these, the
baroreceptor system, is a simple and excellent example of a rapidly acting
control mechanism. In the walls of the bifurcation region of the carotid
arteries in the neck and also in the arch of the aorta in the thorax, are many
nerve receptors called baroreceptors, which are stimulated by stretch of the
arterial wall. When the arterial pressure rises too high, the baroreceptors
send barrages of nerve impulses to the medulla of the brain. Here these
impulses inhibit the vasomotor center, which in turn decreases the number of
impulses transmitted from the vasomotor center through the sympathetic nervous
system to the heart and blood vessels. Lack of these impulses causes diminished
pumping activity by the heart and also dilation of the peripheral blood
vessels, allowing increased blood flow through the vessels. Both of these
effects decrease the arterial pressure back toward normal. Conversely, a
decrease in arterial pressure below normal relaxes the stretch receptors,
allowing the vasomotor center to become more active than usual, thereby causing
vasoconstriction and increased heart pumping, and raising arterial pressure
back toward normal.
Characteristics of Control Systems
The
aforementioned examples of homeostatic control mechanisms are only a few of the
many thousands in the body, all of which have certain characteristics in
common. These characteristics are explained in this section. Negative Feedback
Nature of Most Control Systems Most control systems of the body act by negative
feedback, which can best be explained by reviewing some of the homeostatic
control systems mentioned previously. In the regulation of carbon dioxide
concentration, a high concentration of carbon dioxide in the extracellular fluid
increases pulmonary ventilation. This, in turn, decreases the extracellular fluid
carbon dioxide concentration because the lungs expire greater amounts of carbon
dioxide from the body. In other words, the high concentration of carbon dioxide
initiates events that decrease the concentration toward normal, which is
negative to the initiating stimulus. Conversely, if the carbon dioxide
concentration falls too low, this causes feedback to increase the
concentration. This response also is negative to the initiating stimulus. In
the arterial pressure–regulating mechanisms, a high pressure causes a series of
reactions that promote a lowered pressure, or a low pressure causes a series of
reactions that promote an elevated pressure. In both instances, these effects
are negative with respect to the initiating stimulus. Therefore, in general, if
some factor becomes excessive or deficient, a control system initiates negative
feedback, which consists of a series of changes that return the factor toward a
certain mean value, thus maintaining homeostasis.
“Gain” of a Control System
The degree
of effectiveness with which a control system maintains constant conditions is
determined by the gain of the negative feedback. For instance, let us assume
that a large volume of blood is transfused into a person whose baroreceptor
pressure control system is not functioning, and the arterial pressure rises
from the normal level of 100 mm Hg up to 175 mm Hg. Then, let us assume that
the same volume of blood is injected into the same person when the baroreceptor
system is functioning, and this time the pressure increases only 25 mm Hg.
Thus, the feedback control system has caused a “correction” of –50 mm Hg—that
is, from 175 mm Hg to 125 mm Hg. There remains an increase in pressure of +25
mm Hg, called the “error,” which means that the control system is not 100 per
cent effective in preventing change.
Thus, in the
baroreceptor system example, the correction is –50 mm Hg and the error
persisting is +25 mm Hg. Therefore, the gain of the person’s baroreceptor
system for control of arterial pressure is –50 divided by +25, or –2. That is,
a disturbance that increases or decreases the arterial pressure does so only
one third as much as would occur if this control system were not present. The
gains of some other physiologic control systems are much greater than that of
the baroreceptor system. For instance, the gain of the system controlling
internal body temperature when a person is exposed to moderately cold weather
is about –33. Therefore, one can see that the temperature control system is
much more effective than the baroreceptor pressure control system.
Positive
Feedback Can Sometimes Cause Vicious Cycles and Death One might ask the
question, why do essentially all control systems of the body operate by
negative feedback rather than positive feedback? If one considers the nature of
positive feedback, one immediately sees that positive feedback does not lead to
stability but to instability and often death. Figure 1–3 shows an example in
which death can ensue from positive feedback. This figure depicts the pumping
effectiveness of the heart, showing that the heart of a healthy human being
pumps about 5 liters of blood per minute. If the person is suddenly bled 2
liters, the amount of blood in the body is decreased to such a low level that
not enough blood is available for the heart to pump effectively. As a result,
the arterial pressure falls, and the flow of blood to the heart muscle through
the coronary vessels diminishes. These 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 better known as a “vicious cycle,” but a mild degree of positive
feedback can be overcome by the negative feedback control mechanisms of the body,
and the vicious cycle fails to develop. For instance, if the person in the
aforementioned example were bled only 1 liter instead of 2 liters, the normal
negative feedback mechanisms for controlling cardiac output and arterial
pressure would overbalance the positive feedback and the person would recover.
Positive Feedback Can Sometimes Be Useful
In some
instances, the body uses positive feedback to its advantage. Blood clotting is
an example of a valuable use of positive feedback. When a blood vessel is
ruptured and a clot begins to form, multiple enzymes called clotting factors
are activated within the clot itself. Some of these enzymes act on other
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 the formation of unwanted clots. In fact, this is what initiates most
acute heart attacks, which are caused by a clot beginning on the inside surface
of an atherosclerotic plaque in a coronary artery and then growing until the
artery is blocked. Childbirth is another instance in which positive feedback
plays a valuable role. When uterine contractions become strong enough for the
baby’s head to begin pushing through the cervix, stretch of the cervix sends
signals through the uterine muscle back to the body of the uterus, causing even
more powerful contractions. Thus, the uterine contractions stretch the cervix,
and the cervical stretch causes stronger contractions. When this process
becomes powerful enough, the baby is born. If it is not powerful enough, the
contractions usually die out, and a few days pass before they begin again.
Another important use of positive feedback is for the generation of nerve
signals. That is, when the membrane of a nerve fiber is stimulated, this causes
slight leakage of sodium ions through sodium channels in the nerve membrane to
the fiber’s interior. The sodium ions entering the fiber then change the membrane
potential, which in turn causes more opening of channels, more change of
potential, still more opening of channels, and so forth. Thus, a slight leak
becomes an explosion of sodium entering the interior of the nerve fiber, which
creates the nerve action potential. This action potential in turn causes
electrical current to flow along both the outside and the inside of the fiber and
initiates additional action potentials. This process continues again and again
until the nerve signal goes all the way to the end of the fiber. In each case in
which positive feedback is useful, the positive feedback itself is part of an
overall negative feedback process. For example, in the case of blood clotting,
the positive feedback clotting process is a negative feedback process for
maintenance of normal blood volume. Also, the positive feedback that causes
nerve signals allows the nerves to participate in thousands of negative
feedback nervous control systems.
More Complex
Types of Control Systems— Adaptive Control Later in this text, when we study
the nervous system, we shall see that this system contains great numbers of
interconnected control mechanisms. Some are simple feedback systems similar to
those already discussed. Many are not. For instance, some movements of the body
occur so rapidly that there is not enough time for nerve signals to travel from
the peripheral parts of the body all the way to the brain and then back to the
periphery again to control the movement. Therefore, the brain uses a principle
called feed-forward control to cause required muscle contractions. That is,
sensory nerve signals from the moving parts appraise the brain whether the
movement is performed correctly. If not, the brain corrects the feed-forward
signals that it sends to the muscles the next time the movement is required.
Then, if still further correction is needed, this will be done again for
subsequent movements. This is called adaptive control. Adaptive control, in a
sense, is delayed negative feedback. Thus, one can see how complex the feedback
control systems of the body can be. A person’s life depends on all of them.
Therefore, a major share of this text is devoted to discussing these
life-giving mechanisms.
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| CIRCULATION PATTERN IN LIVING SYSTEM |
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| PARTITION OF VEIN & ARTERIES |
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| COMPONENTS OF EXTRACELLULAR FLUID |
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| EFFICIENCY OF PUMPING ACTION OF HEART |
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