Homeostasis: A Framework for Human Physiology
9
Although the resetting of a set point is adaptive in some
cases, in others it simply refl ects the clashing demands of dif-
ferent regulatory systems. This brings us to one more gener-
alization. It is not possible for everything to be held constant
by homeostatic control systems. In our example, body tem-
perature was maintained despite large swings in ambient tem-
perature, but only because the homeostatic control system
brought about large changes in skin blood fl ow and skeletal
muscle contraction. Moreover, because so many properties
of the internal environment are closely interrelated, it is often
possible to keep one property relatively constant only by mov-
ing others away from their usual set point. This is what we
mean by “clashing demands.”
The generalizations we have given about homeostatic
control systems are summarized in
Table 1–2
. One additional
point is that, as is illustrated by the regulation of body tem-
perature, multiple systems often control a single parameter.
The adaptive value of such redundancy is that it provides much
greater fi ne-tuning and also permits regulation to occur even
when one of the systems is not functioning properly because
of disease.
Feedforward Regulation
Another type of regulatory process often used in conjunction
with feedback systems is feedforward. Let us give an example
of feedforward and then defi ne it. The temperature-sensitive
nerve cells that trigger negative feedback regulation of body
temperature when it begins to fall are located inside the
body. In addition, there are temperature-sensitive nerve cells
in the skin, and these cells, in effect, monitor outside tem-
perature. When outside temperature falls, as in our example,
these nerve cells immediately detect the change and relay this
information to the brain. The brain then sends out signals
to the blood vessels and muscles, resulting in heat conserva-
tion and increased heat production. In this manner, com-
pensatory thermoregulatory responses are activated
before
the colder outside temperature can cause the internal body
temperature to fall. In another familiar example, the smell
of food triggers nerve responses from smell receptors in the
nose to the cells of the gastrointestinal system. This prepares
the stomach and intestines for the process of digestion. Thus,
the stomach begins to churn and produce acid even before
we consume any food. Thus,
feedforward
regulation antici-
pates changes in regulated variables such as internal body
temperature or energy availability, improves the speed of the
body’s homeostatic responses, and minimizes fl uctuations in
the level of the variable being regulated—that is, it reduces
the amount of deviation from the set point.
In our examples, feedforward control utilizes a set of
external or internal environmental detectors. It is likely, how-
ever, that many examples of feedforward control are the result
of a different phenomenon—learning. The fi rst times they
occur, early in life, perturbations in the external environment
probably cause relatively large changes in regulated internal
environmental factors, and in responding to these changes
the central nervous system learns to anticipate them and resist
them more effectively. A familiar form of this is the increased
heart rate that occurs in an athlete just before a competition
begins.
Components of Homeostatic
Control Systems
Refl exes
The thermoregulatory system we used as an example in the
previous section, and many of the body’s other homeostatic
control systems, belong to the general category of stimu-
lus-response sequences known as refl exes. Although in some
refl
exes we are aware of the stimulus and/or the response,
many refl exes regulating the internal environment occur with-
out our conscious awareness.
In the most narrow sense of the word, a
refl
ex
is a specifi c
involuntary, unpremeditated, unlearned “built-in” response to
a particular stimulus. Examples of such refl exes include pull-
ing your hand away from a hot object or shutting your eyes as
an object rapidly approaches your face. There are also many
responses, however, that appear automatic and stereotyped but
are actually the result of learning and practice. For example, an
experienced driver performs many complicated acts in operating
a car. To the driver these motions are, in large part, automatic,
stereotyped, and unpremeditated, but they occur only because
a great deal of conscious effort was spent learning them. We
term such refl exes
learned,
or
acquired refl
exes.
In general,
Table 1–2
Some Important Generalizations
About Homeostatic Control Systems
1. Stability of an internal environmental variable is achieved
by balancing inputs and outputs. It is not the absolute
magnitudes of the inputs and outputs that matter, but the
balance between them.
2. In negative feedback systems, a change in the variable being
regulated brings about responses that tend to move the
variable in the direction opposite the original change—that
is, back toward the initial value (set point).
3. Homeostatic control systems cannot maintain complete
constancy of any given feature of the internal environment.
Therefore, any regulated variable will have a more-or-less
narrow range of normal values depending on the external
environmental conditions.
4. The set point of some variables regulated by homeostatic
control systems can be reset—that is, physiologically raised or
lowered.
5. It is not always possible for homeostatic control systems
to maintain constancy in every variable in response to an
environmental challenge. There is a hierarchy of importance,
so that the constancy of certain variables may be altered
markedly to maintain others within their normal range.
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