Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 18Hypothalamic Regulation of Hormonal Functions 285

environmental temperature. At 21 °C, vaporization is a minor
component in humans at rest. As the environmental tempera-
ture approaches body temperature, radiation losses decline
and vaporization losses increase.

TEMPERATURE-REGULATING

MECHANISMS

The reflex and semireflex thermoregulatory responses in hu-
mans are listed in Table 18–4. They include autonomic, so-
matic, endocrine, and behavioral changes. One group of
responses increases heat loss and decreases heat production;
the other decreases heat loss and increases heat production. In
general, exposure to heat stimulates the former group of re-
sponses and inhibits the latter, whereas exposure to cold does
the opposite.
Curling up “in a ball” is a common reaction to cold in ani-
mals and has a counterpart in the position some people
assume on climbing into a cold bed. Curling up decreases the
body surface exposed to the environment. Shivering is an
involuntary response of the skeletal muscles, but cold also
causes a semiconscious general increase in motor activity.
Examples include foot stamping and dancing up and down on
a cold day. Increased catecholamine secretion is an important
endocrine response to cold. Mice unable to make norepineph-
rine and epinephrine because their dopamine β-hydroxylase

gene is knocked out do not tolerate cold; they have deficient

vasoconstriction and are unable to increase thermogenesis in

brown adipose tissue through UCP 1. TSH secretion is

increased by cold and decreased by heat in laboratory ani-

mals, but the change in TSH secretion produced by cold in

adult humans is small and of questionable significance. It is

common knowledge that activity is decreased in hot

weather—the “it’s too hot to move” reaction.

Thermoregulatory adjustments involve local responses as

well as more general reflex responses. When cutaneous blood

vessels are cooled they become more sensitive to catechol-

amines and the arterioles and venules constrict. This local

effect of cold directs blood away from the skin. Another heat-

conserving mechanism that is important in animals living in

cold water is heat transfer from arterial to venous blood in the

limbs. The deep veins (venae comitantes) run alongside the

arteries supplying the limbs and heat is transferred from the

warm arterial blood going to the limbs to the cold venous blood

coming from the extremities (countercurrent exchange; see

Chapter 38). This keeps the tips of the extremities cold but con-

serves body heat.

The reflex responses activated by cold are controlled from

the posterior hypothalamus. Those activated by warmth are

controlled primarily from the anterior hypothalamus,

although some thermoregulation against heat still occurs after

decerebration at the level of the rostral midbrain. Stimulation

of the anterior hypothalamus causes cutaneous vasodilation

and sweating, and lesions in this region cause hyperthermia,

with rectal temperatures sometimes reaching 43 °C (109.4 °F).

Posterior hypothalamic stimulation causes shivering, and the

body temperature of animals with posterior hypothalamic

lesions falls toward that of the environment.

AFFERENTS

The hypothalamus is said to integrate body temperature infor-

mation from sensory receptors (primarily cold receptors) in

the skin, deep tissues, spinal cord, extrahypothalamic portions

of the brain, and the hypothalamus itself. Each of these five in-

puts contributes about 20% of the information that is integrat-

ed. There are threshold core temperatures for each of the main

temperature-regulating responses and when the threshold is

reached the response begins. The threshold is 37 °C for sweat-

ing and vasodilation, 36.8 °C for vasoconstriction, 36 °C for

nonshivering thermogenesis, and 35.5 °C for shivering.

FEVER

Fever is perhaps the oldest and most universally known hall-

mark of disease. It occurs not only in mammals but also in

birds, reptiles, amphibia, and fish. When it occurs in homeo-

thermic animals, the thermoregulatory mechanisms behave as

if they were adjusted to maintain body temperature at a higher

than normal level, that is, “as if the thermostat had been reset”

to a new point above 37 °C. The temperature receptors then

TABLE 18–4 Temperature-regulating mechanisms.

Mechanisms activated by cold

Shivering

Hunger

Increased voluntary activity

Increased secretion of norepinephrine and epinephrine

Decreased heat loss

Cutaneous vasoconstriction

Curling up

Horripilation

Mechanisms activated by heat

Increased heat loss

Cutaneous vasodilation

Sweating

Increased respiration

Decreased heat production

Anorexia

Apathy and inertia