Essentials of Anatomy and Physiology

Body Temperature

1. Normal range is 96.5°to 99.5°F (36°to 38°C),
   with an average of 98.6°F (37°C).


2. Normal fluctuation in 24 hours is 1°to 2°F.


3. Temperature regulation in infants and the elderly is
   not as precise as it is at other ages.

Heat Production
Heat is one of the energy products of cell respiration.
Many factors affect the total heat actually produced
(see Table 17–1).

1. Thyroxine from the thyroid gland—the most im-
   portant regulator of daily heat production. As meta-
   bolic rate decreases, more thyroxine is secreted to
   increase the rate of cell respiration.


2. Stress—sympathetic impulses and epinephrine and
   norepinephrine increase the metabolic activity of
   many organs, increasing the production of ATP
   and heat.


3. Active organs continuously produce heat. Skeletal
   muscle tone produces 25% of the total body heat at
   rest. The liver provides up to 20% of the resting
   body heat.


4. Food intake increases the activity of the digestive
   organs and increases heat production.


5. Changes in body temperature affect metabolic rate.

A fever increases the metabolic rate, and more heat

is produced; this may become detrimental during

very high fevers.

Heat Loss (see Table 17–2)

1. Most heat is lost through the skin.


2. Blood flow through the dermis determines the
   amount of heat that is lost by radiation, conduc-
   tion, and convection.


3. Vasodilation in the dermis increases blood flow and
   heat loss; radiation and conduction are effective
   only if the environment is cooler than the body.


4. Vasoconstriction in the dermis decreases blood
   flow and conserves heat in the core of the body.


5. Sweating is a very effective heat loss mechanism;
   excess body heat evaporates sweat on the skin sur-
   face; sweating is most effective when the atmos-
   pheric humidity is low.


6. Sweating also has a disadvantage in that water is
   lost and must be replaced to prevent serious dehy-
   dration.


7. Heat is lost from the respiratory tract by the evap-
   oration of water from the warm respiratory
   mucosa; water vapor is part of exhaled air.


8. A very small amount of heat is lost as urine and
   feces are excreted at body temperature.

Body Temperature and Metabolism 413

STUDY OUTLINE

increased secretion of thyroxine and a higher meta-

bolic rate.

AGING AND METABOLISM

As mentioned in the previous section, metabolic rate
decreases with age. Elderly people who remain active,
however, can easily maintain a metabolic rate (energy
production) adequate for their needs as long as their
general health is good. Some elderly people subject to
physical or emotional disability, however, may be at
risk for malnutrition. Caregivers may assess such a risk
by asking how often the person eats every day; if
appetite is good, fair, or poor; and how the food tastes.
These simple questions may help ensure adequate
nutrition.
Sensitivity to external temperature changes may
decrease with age, and the regulation of body temper-
ature is no longer as precise. Sweat glands are not as

active, and prolonged high environmental tempera-

tures are a real danger for elderly people. In August

2003, in Europe, an unusually long and severe heat

wave was the cause of at least 25,000 deaths. Most of

those who died were elderly.

SUMMARY

Food is needed for the synthesis of new cells and tis-

sues, or is utilized to produce the energy required for

such synthesis reactions. As a consequence of metabo-

lism, heat energy is released to provide a constant

body temperature and permit the continuation of

metabolic activity. The metabolic pathways described

in this chapter are only a small portion of the body’s

total metabolism. Even this simple presentation, how-

ever, suggests the great chemical complexity of the

functioning human being.