Homeostasis: A Framework for Human Physiology
7
mal range, sodium homeostasis exists. However, a person in
sodium homeostasis may suffer from other disturbances, such
as abnormally high carbon dioxide levels in the blood result-
ing from lung disease, a condition that could be fatal. Just
one nonhomeostatic variable, among the many that can be
described, can have life-threatening consequences. Typically,
though, if one system becomes dramatically out of balance,
other systems in the body become nonhomeostatic as a conse-
quence. In general, if all the major organ systems are operating
in a homeostatic manner, a person is in good health. Certain
kinds of disease, in fact, can be defi ned as the loss of homeo-
stasis in one or more systems in the body. To elaborate on our
earlier defi nition of physiology, therefore, when homeostasis is
maintained, we refer to physiology; when it is not, we refer to
pathophysiology.
General Characteristics of
Homeostatic Control Systems
The activities of cells, tissues, and organs must be regulated
and integrated with each other so that any change in the extra-
cellular fl uid initiates a reaction to correct the change. The
compensating mechanisms that mediate such responses are
performed by
homeostatic control systems.
Consider an example of the regulation of body tem-
perature. Our subject is a resting, lightly clad man in a room
having a temperature of 20°C and moderate humidity. His
internal body temperature is 37°C, and he is losing heat to
the external environment because it is at a lower tempera-
ture. However, the chemical reactions occurring within the
cells of his body are producing heat at a rate equal to the rate
of heat loss. Under these conditions, the body undergoes no
net gain or loss of heat, and the body temperature remains
constant. The system is in a
steady state,
defi ned as a system
in which a particular variable—temperature, in this case—is
not changing, but energy—in this case, heat—must be added
continuously to maintain a constant condition. Steady state
differs from
equilibrium,
in which a particular variable is
not changing but no input of energy is required to maintain
the constancy. The steady-state temperature in our example
is known as the
set point,
sometimes termed the operating
point, of the thermoregulatory system.
This example illustrates a crucial generalization about
homeostasis. Stability of an internal environmental variable is
achieved by the balancing of inputs and outputs. In the pre-
vious example, the variable (body temperature) remains con-
stant because metabolic heat production (input) equals heat
loss from the body (output).
Now imagine that we lower the temperature of the
room rapidly, say to 5°C, and keep it there. This immedi-
ately increases the loss of heat from our subject’s warm skin,
upsetting the balance between heat gain and loss. The body
temperature therefore starts to fall. Very rapidly, however,
a variety of homeostatic responses occur to limit the fall.
Figure 1–4
summarizes these responses.
The reader is urged
to study Figure 1–4
and its legend carefully because the fi gure
is typical of those used throughout the remainder of the book to
illustrate homeostatic systems, and the legend emphasizes sev-
eral conventions common to such fi
gures.
The fi rst homeostatic response is that blood vessels to the
skin become constricted (narrowed), reducing the amount of
warm blood fl owing through the skin. This reduces heat loss
to the environment and helps maintain body temperature. At
a room temperature of 5°C, however, blood vessel constriction
cannot completely eliminate the extra heat loss from the skin.
Like the person shown in the chapter opening photo,
O
ur sub-
ject curls up in order to reduce the surface area of the skin avail-
able for heat loss. This helps somewhat, but excessive heat loss
still continues, and body temperature keeps falling, although at
a slower rate. Clearly, then, if excessive heat loss (output) cannot
be prevented, the only way of restoring the balance between
heat input and output is to increase input, and this is precisely
what occurs. Our subject begins to shiver, and the chemical
reactions responsible for the skeletal muscular contractions
that constitute shivering produce large quantities of heat.
Return of body temperature toward original value
Heat loss from body
Heat production
Constriction of skin
blood vessels
Shivering
Curling up
(Body’s responses)
Body temperature
Heat loss from body
Room temperature
Begin
Figure 1–4
A homeostatic control system maintains body temperature when
room temperature decreases. This fl ow diagram is typical of those
used throughout this book to illustrate homeostatic systems, and
several conventions should be noted. The “Begin” sign indicates
where to start. The arrows next to each term within the boxes
denote increases or decreases. The arrows connecting any two boxes
in the fi gure denote cause and effect; that is, an arrow can be read as
“causes” or “leads to.” (For example, decreased room temperature
“leads to” increased heat loss from the body.) In general, you
should add the words “tends to” in thinking about these cause-
and-effect relationships. For example, decreased room temperature
tends to cause an increase in heat loss from the body, and curling
up tends to cause a decrease in heat loss from the body. Qualifying
the relationship in this way is necessary because variables like heat
production and heat loss are under the infl uence of many factors,
some of which oppose each other.
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