ration. If the body temperature continues to drop, the nervous
system may stimulate muscle cells in the skeletal muscles
throughout the body to contract slightly. This action requires
an increase in the rate of cellular respiration and releases heat
as a by-product. If this response does not raise the body tem-
perature to normal, small groups of muscles may rhythmically
contract with greater force, causing the person to shiver, gen-
erating more heat.
summarizes the body’s tem-
perature-regulating mechanism, and Clinical Application 6.4
examines two causes of elevated body temperature.
Problems in Temperature Regulation
The body’s temperature-regulating mechanism does not
always operate satisfactorily, and the consequences may be
dangerous. For example, air can hold only a limited volume
of water vapor, so on a hot, humid day, the air may become
nearly saturated with water. At such times, the sweat glands
may be activated, but the sweat cannot quickly evaporate. The
skin becomes wet, but the person remains hot and uncomfort-
able. Body temperature may rise, in a condition called hyper-
thermia. In addition, if the air temperature is high, heat loss by
radiation is less effective. If the air temperature exceeds body
temperature, the person may gain heat from the surroundings,
elevating body temperature even higher.
Hypothermia, or lowered body temperature, can result
from prolonged exposure to cold or as part of an illness. It can
be extremely dangerous. Hypothermia begins with shivering
and a feeling of coldness, but if not treated, progresses to men-
tal confusion; lethargy; loss of reﬂ exes and consciousness; and,
eventually, a shutting down of major organs. If the temperature
in the body’s core drops just a few degrees, fatal respiratory
failure or heart arrhythmia may result. However, the extremi-
ties can withstand drops of 20°F to 30°F below normal.
Certain people are at higher risk for developing hypo-
thermia due to less adipose tissue in the subcutaneous layer
beneath the skin (less insulation). These include the very
old, very thin individuals, and the homeless. The very young
with undeveloped nervous systems have dif±
their body temperature. Dressing appropriately and staying
active in the cold can prevent hypothermia. A person suf-
fering from hypothermia must be warmed gradually so that
respiratory and cardiovascular functioning remain stable.
cells include skeletal and cardiac muscle cells and the cells
of certain glands, such as the liver.
When body temperature rises above the set point, nerve
impulses stimulate structures in the skin and other organs to
release heat. For example, during physical exercise, active
muscles release heat, which the blood carries away. The
warmed blood reaches the part of the brain (the hypothala-
mus) that controls the body’s temperature set point, which
signals muscles in the walls of dermal blood vessels to relax.
As these vessels dilate (vasodilation), more blood enters
them, and some of the heat the blood carries escapes to the
outside. At the same time, deeper blood vessels contract
(vasoconstriction), diverting blood to the surface, and the
skin reddens. The heart is stimulated to beat faster, moving
more blood out of the deeper regions.
The primary means of body heat loss is
shun), by which infrared heat rays escape from
warmer surfaces to cooler surroundings. These rays radiate in
all directions, much like those from the bulb of a heat lamp.
Conduction and convection release less heat. In
shun), heat moves from the body directly into
the molecules of cooler objects in contact with its surface.
For example, heat is lost by conduction into the seat of a
chair when a person sits down. The heat loss continues as
long as the chair is cooler than the body surface touching it.
Heat is also lost by conduction to the air molecules that con-
tact the body. As air becomes heated, it moves away from
the body, carrying heat with it, and is replaced by cooler air
moving toward the body. This type of continuous circulation
of air over a warm surface is
Still another means of body heat loss is
shun). When the body temperature rises above
normal, the nervous system stimulates eccrine sweat glands
to release sweat onto the surface of the skin. As this ﬂ
evaporates (changes from a liquid to a gas), it carries heat
away from the surface, cooling the skin.
When body temperature drops below the set point, the
brain triggers different responses in the skin structures.
Muscles in the walls of dermal blood vessels are stimulated
to contract; this decreases the flow of heat-carrying blood
through the skin, which loses color, and helps reduce heat loss
by radiation, conduction, and convection. At the same time,
sweat glands remain inactive, decreasing heat loss by evapo-
Groups of specialized epithelial cells
Keep hair soft, pliable, waterproof
Near or connected to hair follicles, everywhere but on
palms and soles
Eccrine sweat glands
Abundant sweat glands with odorless
Lower body temperature
Originate in deep dermis or subcutaneous layer and
open to surface on forehead, neck, and back
Apocrine sweat glands
Less numerous sweat glands with
secretions that develop odors
Wet skin during pain, fear,
emotional upset, and sexual arousal
Near hair follicles in armpit, groin, around nipples
ed sweat glands
External ear canal
ed sweat glands