6
Chapter 1
in the interstitial compartment and restricts proteins mainly
to the plasma. With this understanding of the structural orga-
nization of the body, and the way in which water is distrib-
uted throughout, we turn to a description of how a balance is
achieved in the body’s internal environment.
Homeostasis: A Defi ning Feature
of Physiology
From the earliest days of physiology—at least as early as the
time of Aristotle—physicians recognized that good health
was somehow associated with a balance among the multiple
life-sustaining forces (“humours”) in the body. It would take
millennia, however, for scientists to determine just what it was
that was being balanced, and how this balance was achieved.
The advent of modern tools of science, including the ordinary
microscope, led to the discovery that the human body is com-
posed of trillions of cells, each of which is packaged to permit
movement of certain substances, but not others, across the cell
membrane. Over the course of the nineteenth and twentieth
centuries, it became clear that most cells are in contact with
the interstitial fl
uid. The interstitial fl uid, in turn, was found
to be in a state of fl ux, with water and solutes, such as ions and
gases, moving back and forth through it between the cell inte-
riors and the blood in nearby capillaries (see Figure 1–2).
It was further determined by careful observation that
most of the common physiological variables found in nor-
mal, healthy organisms—blood pressure, body temperature,
and blood-borne factors such as oxygen, glucose, and sodium,
for example—are maintained within a predictable range. This
was true despite external environmental conditions that may
be far from constant. Thus was born the idea, fi rst put forth
by the French physician and physiologist Claude Bernard, of a
constant internal
milieu
that is a prerequisite for good health,
a concept later refi ned by the American physiologist Walter
Cannon, who coined the term
homeostasis
.
Originally,
homeostasis
was defi ned as a state of rea-
sonably stable balance between physiological variables such as
those just described. This simple defi nition cannot give one
a complete appreciation of what homeostasis truly entails,
however. There probably is no such thing as a physiological
variable that is constant over long periods of time. In fact,
some variables undergo fairly dramatic swings around an aver-
age value during the course of a day, yet are still considered
in balance. That is because homeostasis is a
dynamic,
not a
static, process. Consider swings in blood glucose levels over
the course of a day (
Figure 1–3
). After a meal, blood glucose
levels rise considerably. Clearly, such a large change from base-
line cannot be considered stable or static. What is important,
though, is that once blood glucose increases, compensatory
mechanisms restore the glucose level toward the level it was
at before the meal. These homeostatic compensatory mecha-
nisms do not, however, overshoot to any signifi cant degree in
the opposite direction. That is, the blood glucose levels do not
fall below the pre-meal level, or do so only moderately. In the
case of glucose, the endocrine system is primarily responsible
for this adjustment, but a wide variety of control systems may
be initiated to regulate other processes. In later chapters, we
will see how nearly every organ and tissue of the human body
contributes to homeostasis, sometimes in multiple ways, and
usually in concert with each other.
Thus, homeostasis does not imply that a given physi-
ological function or variable is rigidly constant with respect
to time, but that it fl uctuates within a predictable and often
narrow range. When disturbed up or down from the normal
range, it is restored to normal.
What do we mean when we say that something var-
ies within a normal range? This depends on just what we
are monitoring. If the circulating arterial oxygen level of a
healthy person breathing air at sea level is measured, it does
not change much over the course of time, even if the person
exercises. Such a system is said to be tightly controlled and to
demonstrate very little variability or scatter around an average
value. Blood glucose levels, as we have seen, may vary consid-
erably over the course of a day. Yet, if the daily average glucose
level was determined in the same person on many consecu-
tive days, it would be much more predictable over days or even
years than random, individual measurements of glucose over
the course of a single day. In other words, there may be con-
siderable variation in glucose values over short time periods,
but less when they are averaged over long periods of time. This
has led to the concept that homeostasis is a state of
dynamic
constancy.
In such a state, a given variable like blood glucose
may vary in the short term, but is fairly constant when aver-
aged over the long term.
It is also important to realize that a person may be
homeostatic for one variable, but not homeostatic for another.
Homeostasis must be described differently, therefore, for
each variable. For example, as long as the concentration of
sodium in the blood remains within a few percent of its nor-
Blood levels of glucose (mg/dL)
20
40
160
Time of day
12:00
A.M.
Breakfast
6:00
A.M.
12:00
P.M.
6:00
P.M.
12:00
A.M.
60
80
100
120
140
Lunch
Dinner
Figure 1–3
Changes in blood glucose levels during a typical 24-hour period.
Note that glucose increases after each meal, more so after larger
meals, and then returns to the pre-meal level in a short while.
The profi le shown here is that of a person who is homeostatic for
blood glucose, even though levels of this sugar vary considerably
throughout the day.
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