Homeostasis: A Framework for Human Physiology
13
Processes Related to Homeostasis
Adaptation and Acclimatization
The term
adaptation
denotes a characteristic that favors sur-
vival in specifi c environments. Homeostatic control systems
are inherited biological adaptations. An individual’s ability
to respond to a particular environmental stress is not fi xed,
however, but can be enhanced by prolonged exposure to
that stress. This type of adaptation—the improved function-
ing of an already existing homeostatic system—is known as
acclimatization.
Let us take sweating in response to heat exposure as
an example and perform a simple experiment. On day 1 we
expose a person for 30 min to a high temperature and ask her
to do a standardized exercise test. Body temperature rises, and
sweating begins after a certain period of time. The sweating
provides a mechanism for increasing heat loss from the body
and thus tends to minimize the rise in body temperature in a
hot environment. The volume of sweat produced under these
conditions is measured. Then, for a week, our subject enters
the heat chamber for 1 or 2 h per day and exercises. On day 8,
her body temperature and sweating rate are again measured
during the same exercise test performed on day 1. The striking
fi nding is that the subject begins to sweat sooner and much
more profusely than she did on day 1. As a consequence, her
body temperature does not rise to nearly the same degree. The
subject has become acclimatized to the heat. She has under-
gone an adaptive change induced by repeated exposure to the
heat and is now better able to respond to heat exposure.
The precise anatomical and physiological changes that
bring about increased capacity to withstand change during
acclimatization are highly varied. Typically, they involve an
increase in the number, size, or sensitivity of one or more of
the cell types in the homeostatic control system that mediate
the basic response.
Acclimatizations are usually completely reversible. Thus,
if the daily exposures to heat are discontinued, our subject’s
sweating rate will revert to the preacclimatized value within a
relatively short time. If an acclimatization is induced very early
in life, however, at a critical period for development of a struc-
ture or response, it is termed a
developmental acclimatiza-
tion
and may be irreversible. For example, the barrel-shaped
chests of natives of the Andes Mountains do not represent a
genetic difference between them and their lowland compatri-
ots. Rather, this is an irreversible acclimatization induced dur-
ing the fi rst few years of their lives by their exposure to the
high-altitude, low-oxygen environment. The increase in chest
size refl ects the increase in lung size and function. The altered
chest size remains even if the individual moves to a lowland
environment later in life and stays there. Lowland persons who
have suffered oxygen deprivation from heart or lung disease
during their early years show precisely the same chest shape.
Biological Rhythms
As noted earlier, a striking characteristic of many body func-
tions is the rhythmical changes they manifest. The most com-
mon type is the
circadian rhythm,
which cycles approximately
once every 24 h. Waking and sleeping, body temperature, hor-
mone concentrations in the blood, the excretion of ions into
the urine, and many other functions undergo circadian varia-
tion (
Figure 1–9
).
What have biological rhythms to do with homeostasis?
They add an anticipatory component to homeostatic control
systems, in effect a feedforward system operating without
detectors. The negative-feedback homeostatic responses we
described earlier in this chapter are
corrective
responses. They
are initiated
after
the steady state of the individual has been
perturbed. In contrast, biological rhythms enable homeostatic
mechanisms to be utilized immediately and automatically by
activating them at times when a challenge is
likely
to occur but
before it actually does occur. For example, there is a rhythm in
the urinary excretion of potassium—excretion is high during
the day and low at night. This makes sense because we ingest
potassium in our food during the day, not at night when we
are asleep. Therefore, the total amount of potassium in the
body fl uctuates less than if the rhythm did not exist.
A crucial point concerning most body rhythms is that
they are internally driven. Environmental factors do not drive
the rhythm but rather provide the timing cues important for
entrainment
, or setting of the actual hours of the rhythm. A
classic experiment will clarify this distinction.
38
Lights on
Lights off
37
36
15
10
5
0
15
10
5
0
3
2
1
Urinary potassium
(mM)
Plasma cortisol
(
g/100 ml)
Plasma growth
hormone (ng/ml)
Body
temperature (
°
C)
Figure 1–9
Circadian rhythms of several physiological variables in a human
subject with room lights on (open bars at top) for 16 h and off (blue
bars at top) for 8 h. Growth hormone and cortisol are hormones
that regulate metabolism.
Adapted from Moore-Ede and Sulzman.
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