Regulation of Organic Metabolism and Energy Balance
591
body is determined by the net difference between heat gain
(from the environment and produced in the body) and heat
loss. Maintaining a stable body temperature means that, in
the steady state, heat production must equal heat loss. Some
of the basic principles of heat gain were described earlier in
this chapter in the section on metabolic rate, and those gov-
erning heat loss are described next. Then we will present
the refl
exes that act upon these processes to regulate body
temperature.
Mechanisms of Heat Loss or Gain
The surface of the body can lose heat to the external environ-
ment by radiation, conduction, convection, and by the evapo-
ration of water (
Figure 16–17
). Before defi ning each of these
processes, however, it must be emphasized that radiation, con-
duction, and convection can under certain circumstances lead
to heat
gain
instead of loss.
Radiation
is the process by which the surfaces of all
objects constantly emit heat in the form of electromagnetic
waves. The rate of emission is determined by the temperature
of the radiating surface. Thus, if the body surface is warmer
than the various surfaces in the environment, net heat is lost
from the body, the rate being directly dependent upon the
temperature difference between the surfaces. Conversely, the
body gains heat by absorbing electromagnetic energy emitted
by the sun.
Conduction
is the loss or gain of heat by transfer of
thermal energy during collisions between adjacent molecules.
In essence, heat is “conducted” from molecule to molecule.
The body surface loses or gains heat by conduction through
direct contact with cooler or warmer substances, including
the air or water. Not all substances, however, conduct heat
equally. Water is a better conductor of heat than is air and,
thus, more heat is lost from the body in water than in air of
similar temperature.
Convection
is the process whereby conductive heat
loss or gain is aided by movement of the air or water next
to the body. For example, air next to the body is heated by
conduction. Because warm air is less dense than cool air, the
heated air around the body surface rises, thus carrying away
the heat just taken from the body. The air that moves away is
replaced by cooler air, which in turn follows the same pattern.
Convection is always occurring because warm air is less dense
and therefore rises, but it can be greatly facilitated by external
forces such as wind or fans. Thus, convection aids conductive
heat exchange by continuously maintaining a supply of cool
air. Therefore, in the rest of this chapter, the term
conduction
will also imply convection.
Evaporation
of water from the skin and membranes lin-
ing the respiratory tract is the other major process causing loss
of body heat. A very large amount of energy—600 kcal/L—is
required to transform water from the liquid to the gaseous
state. Thus, whenever water vaporizes from the body’s surface,
the heat required to drive the process is conducted from the
surface, thereby cooling it.
Temperature-Regulating Refl exes
Temperature regulation offers a classic example of a biologi-
cal control system. The balance between heat production and
heat loss is continuously being disturbed, either by changes in
metabolic rate (exercise being the most powerful infl uence) or
by changes in the external environment (e.g., air temperature)
that alter heat loss or gain. The resulting changes in body
temperature are detected by thermoreceptors. These receptors
initiate refl exes that change the output of various effectors so
that heat production and/or loss are changed and body tem-
perature is restored toward normal.
Figure 16–18
summarizes the components of these
refl exes. There are two categories of thermoreceptors, one in
the skin (
peripheral thermoreceptors
) and the other (
cen-
tral thermoreceptors
) in deep body structures, including
abdominal organs and thermoreceptive neurons in the hypo-
thalamus. Because it is the core body temperature, not the
skin temperature, that is kept constant, the central thermore-
ceptors provide the essential negative feedback component of
the refl exes. The peripheral thermoreceptors provide feedfor-
ward information, as described in Chapter 1, and also account
for the ability to identify a hot or cold area of the skin.
The hypothalamus serves as the primary overall integra-
tor of the refl exes, but other brain centers also exert some con-
trol over specifi c components of the refl exes. Output from the
Radiation
Evaporation
Convection
Warm air
rising
Conduction
Water above body
temperature
Cool air
coming in to
replace warm
air that has
risen
Figure 16–17
Mechanisms of heat transfer.
Figure 16–17
physiological
inquiry
Evaporation is an important mechanism for eliminating heat,
particularly on a hot day or when exercising. What are some of
the negative consequences of this mechanism of heat loss?
Answer can be found at end of chapter.
previous page 619 Vander's Human Physiology The Mechanisms of Body Function read online next page 621 Vander's Human Physiology The Mechanisms of Body Function read online Home Toggle text on/off