Regulation of Organic Metabolism and Energy Balance
593
uncoupling proteins. These proteins uncouple oxidation from
phosphorylation (Chapter 3) and in effect, make fuel metabo-
lism less effi cient (less ATP is generated). The major product of
this ineffi cient metabolism is heat, which then contributes to
maintaining body temperature in infants. Although very small
amounts of brown adipose tissue are present in adults, its phys-
iological role in adults, if any, is unknown.
Control of Heat Loss by Radiation and Conduction
For purposes of temperature control, the body may be thought
of as a central core surrounded by a shell consisting of skin and
subcutaneous tissue; we will refer to this complex outer shell
simply as skin. The temperature of the central core is regulated
at approximately 37°C, but the temperature of the outer sur-
face of the skin changes markedly.
If the skin and its underlying tissue were a perfect insu-
lator, no heat would ever be lost from the core. The tempera-
ture of the outer skin surface would equal the environmental
temperature, and net conduction would be zero. The skin
is not a perfect insulator, however, so the temperature of its
outer surface generally is somewhere between that of the
external environment and that of the core. Instead of acting as
an insulator, the skin functions as a variable regulator of heat
exchange. Its effectiveness in this capacity is subject to physi-
ological control by a change in blood fl ow. The more blood
reaching the skin from the core, the more closely the skin’s
temperature approaches that of the core. In effect, the blood
vessels can carry heat to the skin surface to be lost to the exter-
nal environment. These vessels are controlled largely by vaso-
constrictor sympathetic nerves, which are refl exly stimulated
in response to cold and inhibited in response to heat. There
is also a population of sympathetic neurons to the skin whose
neurotransmitters cause active vasodilation. Certain areas of
skin participate much more than others in all these vasomotor
responses, and so skin temperatures vary with location.
Finally, there are three
behavioral
mechanisms for alter-
ing heat loss by radiation and conduction: changes in surface
area, changes in clothing, and choice of surroundings. Curling
up into a ball, hunching the shoulders, and similar maneu-
vers in response to cold reduce the surface area exposed to the
environment, thereby decreasing heat loss by radiation and
conduction. In human beings, clothing is also an important
component of temperature regulation, substituting for the
insulating effects of feathers in birds and fur in other mam-
mals. The outer surface of the clothes forms the true “exte-
rior” of the body surface. The skin loses heat directly to the
air space trapped by the clothes, which in turn pick up heat
from the inner air layer and transfer it to the external environ-
ment. The insulating ability of clothing is determined primar-
ily by the thickness of the trapped air layer.
Clothing is important not only at low temperatures,
but also at very high temperatures. When the environmental
temperature is greater than body temperature, conduction
favors heat
gain
rather than heat loss. Heat gain also occurs
by radiation during exposure to the sun. People therefore
insulate themselves in such situations by wearing clothes. The
clothing, however, must be loose to allow adequate movement
of air to permit evaporation. Wearing loose-fi tting clothes is
actually far more cooling than going nude in a hot environ-
ment and during direct exposure to the sun.
The third behavioral mechanism for altering heat loss is
to seek out warmer or colder surroundings, as for example by
moving from a shady spot into the sunlight. Raising or lower-
ing the thermostat of a house or turning on an air conditioner
also fi
ts this category.
Control of Heat Loss by Evaporation
Even in the absence of sweating, there is loss of water by dif-
fusion through the skin, which is not completely waterproof.
A similar amount is lost from the respiratory lining during
expiration. These two losses are known as
insensible water
loss
and amount to approximately 600 ml/day in human
beings. Evaporation of this water can account for a signifi -
cant fraction of total heat loss. In contrast to this passive
water loss, sweating requires the active secretion of fl
uid by
sweat glands
and its extrusion into ducts that carry it to the
skin surface.
Production of sweat is stimulated by sympathetic nerves
to the glands. (These nerves release acetylcholine rather than
the usual sympathetic neurotransmitter norepinephrine.) Sweat
is a dilute solution containing sodium chloride as its major sol-
ute. Sweating rates of over 4 L/h have been reported; the evap-
oration of 4 L of water would eliminate almost 2400 kcal of
heat from the body!
Sweat must evaporate in order to exert its cooling
effect. The most important factor determining evaporation
rate is the water vapor concentration of the air—that is, the
relative humidity. The discomfort suffered on humid days
is due to the failure of evaporation; the sweat glands con-
tinue to secrete, but the sweat simply remains on the skin
or drips off.
Integration of Effector Mechanisms
Table 16–9
summarizes the effector mechanisms regulating
temperature, none of which is an all-or-none response but a
graded, progressive increase or decrease in activity. By alter-
ing heat loss, changes in skin blood fl ow alone can regulate
body temperature over a range of environmental tempera-
tures (approximately 25 to 30°C or 75 to 86°F for a nude
individual) known as the
thermoneutral zone.
At tempera-
tures lower than this, even maximal vasoconstriction cannot
prevent heat loss from exceeding heat production, and the
body must increase its heat production to maintain tempera-
ture. At environmental temperatures above the thermoneu-
tral zone, even maximal vasodilation cannot eliminate heat
as fast as it is produced, and another heat-loss mechanism—
sweating—therefore comes strongly into play. At environ-
mental temperatures above that of the body, heat is actually
added to the body by radiation and conduction. Under such
conditions, evaporation is the sole mechanism for heat loss.
A person’s ability to tolerate such temperatures is determined
by the humidity and by his/her maximal sweating rate. For
example, when the air is completely dry, a person can tolerate
an environmental temperature of 130°C (225°F) for 20 min
or longer, whereas very moist air at 46°C (115°F) is bearable
for only a few minutes.
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