HUMAN BIOLOGY

tIssUes, orGans, and orGan systeMs 83

hoW is body temperature regulated?

• The hypothalamus (in the brain) regulates physiological
  mechanisms that adjust the body’s core temperature.


• Responses to heat stress include dilation of blood vessels near
  the body surface and evaporative heat loss.


• Responses to cold stress include constriction of blood vessels
  near the body surface, the pilomotor response, shivering, and
  nonshivering heat production.

taKe-hoMe Message

When other responses can’t counteract cold stress,

signals from the hypothalamus step up skeletal muscle

contractions, similar to the low-level contractions that pro-

duce muscle tone. The result is shivering—skeletal muscles

contract ten to twenty times per second, boosting heat pro-

duction throughout the body.

Severe exposure to cold can lead to a hormone-

driven response that speeds up cell metabolism. This

nonshivering heat production occurs in a type of

brownish adipose tissue called “brown fat.” Heat is gen-

erated as the lipid molecules are broken down. In babies

(who can’t shiver) brown fat makes up about 5 percent

of body weight; most adults have some in their neck and

upper chest.

In hypothermia, body core tem perature falls below the

normal range. A drop of only a few degrees leads to men-

tal confusion. Further cooling can cause coma and death.

Some victims of extreme hypothermia, mainly children,

have survived prolonged immersion in ice-cold water. One

reason is that mammals, including humans, have a dive

reflex. When the body is submerged, the heart rate slows

and blood is shunted to the brain and other vital organs.

Freezing often destroys tissues, a condition we call

frostbite. Frozen cells may be saved if thawing is precisely

controlled. This sometimes can be done in a hospital.

When someone has a fever, the hypothalamus has
reset the “thermostat” that dictates what the body’s core
temperature will be. The normal response mechanisms
occur but they maintain a higher temperature. When
a fever “breaks,” peripheral vasodilation and sweating
increase as the body works to restore the normal core
temperature. The controlled increase in core temperature
during a fever seems to boost immune responses, so using
fever-reducing drugs may actually interfere with fever’s
beneficial effects. A severe fever always requires medical
attention because of the dangers it poses.

several responses counteract cold

Table 4.3 also summarizes the major responses to cold
stress, which the hypothalamus also regulates. When
the outside temperature drops, thermoreceptors (thermo-
means “heat”) at the body surface detect the decrease.
When their signals reach the hypothalamus, neurons sig-
nal smooth muscle in the walls of certain skin blood vessels
to contract, and the blood vessels narrow. This narrowing,
called vasoconstriction, reduces blood flow to capillaries
near the body surface, so your body retains heat. When
your hands or feet get cold, as much as 99 percent of the
blood that would otherwise flow to your skin is diverted
(Figure 4.16).
In the pilomotor response to a drop in outside temper-
ature, your body hair can “stand on end.” This happens
because smooth muscle controlling the erection of body
hair is stimulated to contract. This creates a layer of still
air close to the skin that reduces heat loss. (This response
is most effective in mammals with more body hair than
humans!) Heat loss can be restricted even more by behav-
iors that reduce the amount of body surface exposed for
heat exchange, as when you put on a sweater or hold your
arms tightly against your body.

Figure 4.16 A thermogram shows body heat in the vivid colors

of the infrared part of the light spectrum. Red, orange, and

yellow areas are warmest; green, blue, and violet areas are

colder. This thermogram captures temperature differences

between a warm left hand and a cold right hand.

Environmental

Stimulus Main responses Outcome

Table 4.3 Summary of human Responses to Cold

Stress and to heat Stress

Drop in

temperature

Vasoconstriction of blood

vessels in skin; pilomotor

response; behavior changes

(e.g., putting on a sweater)

Heat is

conserved

Increased muscle activity;

shivering; nonshivering

heat production

More heat is

produced

Rise in

temperature

Vasodilation of blood vessels

in skin; sweating; changes

in behavior; heavy breathing

Heat is

dissipated

from body

Reduced muscle activity Less heat is

produced Bruno Boissonnet/Science Source

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