Homeostasis: A Framework for Human Physiology
15
the gastrointestinal (GI) tract or the lungs. Alternatively,
a substance may be synthesized within the body from other
materials.
The pathways on the right of the fi gure are causes of net
loss from the body. A substance may be lost in the urine, feces,
expired air, or menstrual fl
uid, as well as from the surface of
the body as skin, hair, nails, sweat, or tears. The substance
may also be chemically altered by enzymes and thus removed
by metabolism.
The central portion of the fi
gure illustrates the distribu-
tion of the substance within the body. The substance may be
taken from the pool and accumulated in storage depots, such
as the accumulation of fat in adipose tissue. Conversely, it may
leave the storage depots to reenter the pool. Finally, the sub-
stance may be incorporated reversibly into some other molec-
ular structure, such as fatty acids into plasma membranes.
Incorporation is reversible because the substance is liberated
again whenever the more complex structure is broken down.
This pathway is distinguished from storage in that the incor-
poration of the substance into other molecules produces new
molecules with specifi c functions.
Substances do not necessarily follow all pathways of this
generalized schema. For example, minerals such as sodium
cannot be synthesized, do not normally enter through the
lungs, and cannot be removed by metabolism.
The orientation of Figure 1–10 illustrates two important
generalizations concerning the balance concept: (1) During
any period of time, total-body balance depends upon the rela-
tive rates of net gain and net loss to the body; and (2) the pool
concentration depends not only upon the total amount of the
substance in the body, but also upon exchanges of the sub-
stance
within
the body.
For any substance, three states of total-body balance are
possible: (1) Loss exceeds gain, so that the total amount of the
substance in the body is decreasing, and the person is in
nega-
tive balance;
(2) gain exceeds loss, so that the total amount
of the substance in the body is increasing, and the person is in
positive balance;
and (3) gain equals loss, and the person is
in
stable balance.
Clearly a stable balance can be upset by a change in
the amount being gained or lost in any single pathway in the
schema. For example, increased sweating can cause severe neg-
ative water balance. Conversely, stable balance can be restored
by homeostatic control of water intake and output.
Let us take sodium balance as another example. The
control systems for sodium balance target the kidneys, and the
systems operate by inducing the kidneys to excrete into the
urine an amount of sodium approximately equal to the amount
ingested daily. In this example, we assume for simplicity that
all sodium loss from the body occurs via the urine (although
some is also lost in perspiration). Now imagine a person with
a daily intake and excretion of 7 g of sodium—not an unusual
intake for most Americans—and a stable amount of sodium
in her body (
Figure 1–11
). On day 2 of our experiment, the
subject changes her diet so that her daily sodium consumption
rises to 15 g—a large but still commonly observed intake—
and remains there indefi nitely. On this same day, the kidneys
excrete into the urine somewhat more than 7 g of sodium,
but not all the ingested 15 g. The result is that some excess
sodium is retained in the body that day—that is, the person is
in positive sodium balance. The kidneys do somewhat better
on day 3, but it is probably not until day 4 or 5 that they are
excreting 15 g. From this time on, output from the body once
again equals input, and sodium balance is once again stable.
The delay of several days before stability is reached is quite
typical for the kidneys’ handling of sodium, but should not
be assumed to apply to other homeostatic responses, most of
which are much more rapid.
Although again in stable balance, the woman has per-
haps 2 percent more sodium in her body than was the case
when she was in stable balance ingesting 7 g. It is this 2 per-
cent extra body sodium that constitutes the continuous error
signal to the control systems, driving the kidneys to excrete
15 g/day rather than 7 g/day. Recall the generalization (Table
1–2, no. 3) that homeostatic control systems cannot maintain
complete constancy of the internal environment
in the face of
continued change in the perturbing event
because some change
in the regulated variable (body sodium content, in our exam-
ple) must persist to signal the need to maintain the compen-
sating responses. An increase of 2 percent does not seem large,
but it has been hypothesized that this small gain might facili-
tate the development of high blood pressure in some people.
In summary, homeostasis is a complex, dynamic process
that regulates the adaptive responses of the body to changes
in the external and internal environments. To work prop-
erly, homeostatic systems require a sensor to detect the envi-
ronmental change, and a means to produce a compensatory
response. Because compensatory responses require muscle
activity, behavioral changes, or synthesis of chemical messen-
gers such as hormones, homeostasis is only achieved by the
expenditure of energy. The nutrients that provide this energy,
and the cellular structures and chemical reactions that release
the energy stored in the chemical bonds of the nutrients, are
described in the following two chapters.
Days
g/day
12345
Sodium
ingested
Sodium
excreted
Percent increase
in total body
sodium
0
7
15
0
1
2
Figure 1–11
Effects of a continued change in the amount of sodium ingested on
sodium excretion and total-body sodium balance. Stable sodium
balance is reattained by day 4, but with some gain of total-body
sodium.
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