HUMAN BIOLOGY

core temperature The
temperature of the head and
torso, normally about 37°C,
or 98.6°F.

endotherm Animal whose
body heat is generated by
the metabolic processes of
its cells.

hyperthermia Condition in
which body core tempera-
ture rises above the normal
range.

hypothermia Condition in
which body core tempera-
ture falls below the normal
range.

nonshivering heat
production When brown
adipose tissue releases
energy as heat rather than
storing it as ATP.

82 Chapter 4

body temperature drops by 10°F. If it drops below 35°C

(95°F), you are courting danger. As enzymes lose their abil-

ity to function, your heart will not beat as often or as effec-

tively, and heat-generating mechanisms such as shivering

stop. At this low core temperature breathing slows, so you

may lose consciousness. Below 80°F the human heart may

stop beating entirely. Given these stark physiological facts,

humans require mechanisms that help maintain the core

body temperature within narrow limits.

excess heat must be eliminated

Table 4.3 summarizes the main responses to heat stress.

They are governed by the hypothalamus, a structure in

the brain in which there are both neurons and endocrine

cells. When core temperature rises above a set point, the

hypothalamus order s blood vessels in the skin to dilate.

This widening, called vasodilation, allows more blood to

flow through the vessels, where the excess heat that blood

carries is dissipated.

The hypothalamus also can activate sweat glands and

increase the amount of body heat lost via evaporation.

With roughly 2.5 million sweat glands in skin, lots of heat

is dissipated when the water in sweat evaporates. With

prolonged heavy sweating the body also loses key salts,

especially sodium chloride. Los-

ing too many of these electrolytes

can make you feel woozy. “Sports

drinks” replenish electrolytes.

Sometimes peripheral blood

flow and evaporative heat loss can’t

adequately counter heat stress.

The result is hyperthermia, in

which the core temperature rises

above normal. A relatively modest

increase causes heat exhaustion, in

which blood pressure drops due to

vasodilation and water losses from

heavy sweating. The skin feels cold

and clammy, and the person may

collapse.

When heat stress is severe

enough to completely break down

the body’s temperature controls,

heat stroke occurs. Sweating stops,

the skin becomes dry, and the core

body temper ature rapidly rises to a

level that can be lethal.

how homeostatic Feedback Maintains the body’s

Core temperature

n    Controls over the body’s core temperature provide good

examples of negative feedback loops.

We humans are endotherms, which

means “heat from inside.” The

b o dy ’s core temperature—the tem-

perature of the head and torso—is

about 37°C, or 98.6°F. It is controlled

mainly by metabolic activity, which

produces heat, and by negative feed-

back loops. These homeostatic con-

trols adjust physiological responses

for conserving or getting rid of heat

(Figure 4.15). We can assist the phys-

iological controls by altering our

behavior—for example, by changing

clothes or switching on a furnace or

an air- conditioner.

Metabolism produces heat. If

that heat were to build up inter-

nally, your core temperature would

steadily rise. Above 41°C (105.8°F),

some enzymes become denatured

and virtually shut down. By the same token, the rate of

enzyme activity generally decreases by at least half when

Figure 4.15 Animated! Homeostatic

controls regulate internal body

temperature. (© Cengage Learning)

Change in skin temperature Change in core temperature

peripheral

thermoreceptors

in skin

central

thermoreceptors

in hypothalamus,

abdominal organs,

and elsewhere

hormonal signals from

“thermostat” centers

in hypothalamus

motor

neurons

skeletal

muscles

voluntary

changes in

behavior

adjustments

in heat gain

or heat loss

adjustments in muscle

activity (in metabolic

heat output)

adjustment in loss

or conservation of

metabolic heat

adjustment

in heat loss

muscle tone,

shivering

vasoconstriction,

vasodilation sweating

smooth muscle in

arterioles in skin

sweat

glands

4.11

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