Page 1005 - Fundamentals of anatomy physiology

992 Unit 5 Environmental Exchange gland increases the rate of thyroxine release into the blood.
Thyroxine increases not only the rate of carbohydrate catabo-
temperature is called the Lewis Wave or the hunter’s response. This lism, but also the rate of catabolism of all other nutrients.
is an autonomic response to reduce heat loss at the extremities. These effects develop gradually, over a period of days to weeks.

Nonshivering thermogenesis involves the release of Sources of Individual Variation in Thermoregulation
hormones that increase the metabolic activity of all tissues:
The timing of thermoregulatory responses differs from person
The heat-gain center stimulates the adrenal medullae to person. A person may undergo acclimatization (a-klı.-ma-ti-
ZA. -shun)—a physiological adjustment to a particular environ-
through the sympathetic division of the autonomic nervous ment over time. For example, natives of Tierra del Fuego (off
system. As a result, epinephrine is released. It increases the the southernmost tip of South America) once lived naked in the
rates of glycogenolysis in liver and skeletal muscle, and the snow, but Hawaii residents often unpack their sweaters when
metabolic rate of most tissues. The effects are immediate. the temperature drops below 22°C (72°F).

The preoptic nucleus regulates the production of thyrotropin- Another source of variation is body size. Although heat
production takes place within the mass of the body, heat loss
releasing hormone (TRH) by the hypothalamus. In children,
when body temperatures are below normal, additional TRH
is released. This hormone stimulates the release of thyroid-
stimulating hormone (TSH) by the anterior lobe of the
pituitary gland. In response to this release of TSH, the thyroid

Figure 25–15 Vascular Adaptations for Heat Loss and Conservation.

WARM ENVIRONMENT COLD ENVIRONMENT

Warm Warm blood
blood from returns
to trunk
trunk

37°C 36.5°–37°C

Brachial vein (deep) Heat transfer
Basilic vein (superficial)

Brachial
artery (deep)

Cephalic vein 24°C 23°C
(superficial)

25 Median cubital Cooled blood Cool blood
vein (superficial) to distal returns
to trunk
capillaries

Radial artery (deep) c Countercurrent heat exchange
occurs as heat radiates from the
Basilic vein (superficial) warm arterial blood into the
Radial vein (deep) cooler venous blood flowing in
Ulnar artery (deep) the opposite direction. By the
Ulnar vein (deep) time the arterial blood reaches
distal capillaries, where most
a Circulation through the blood vessels of b Circulation through the blood vessels of heat loss to the environment
the forearm in a warm environment. Blood the forearm in a cold environment. Blood occurs, it is already 13°C cooler
enters the limb in a deep artery and now returns to the trunk by a network of than it was when it left the trunk.
returns to the trunk in a network of deep veins that flow around the artery. This mechanism decreases the
superficial veins that radiate heat to the The amount of heat loss is decreased, as rate of heat loss by conserving
environment through the overlying skin. shown in part (c). body heat within the trunk.