Essentials of Anatomy and Physiology

organs produce ATP for peristalsis and for the syn-

thesis of digestive enzymes.

5. Changes in body temperature also have an effect
   on metabolic rate and heat production. This
   becomes clinically important when a person has
   a fever, an abnormally high body tempera-
   ture. The higher temperature increases the meta-
   bolic rate, which increases heat production and
   elevates body temperature further. Thus, a high
   fever may trigger a vicious cycle of ever-increasing
   heat production. Fever is discussed later in this
   chapter.
   The factors that affect heat production are summa-
   rized in Table 17–1.

HEAT LOSS

The pathways of heat loss from the body are the skin,
the respiratory tract, and, to a lesser extent, the uri-
nary and digestive tracts.

Heat Loss through the Skin

Because the skin covers the body, most body heat is

lost from the skin to the environment. When the envi-

ronment is cooler than body temperature (as it usually

is), heat loss is unavoidable. The amount of heat that

is lost is determined by blood flow through the skin

and by the activity of sweat glands.

Blood flow through the skin influences the amount

of heat lost by the processes of radiation, conduction,

and convection. Radiationmeans that heat from the

body is transferred to cooler objects not touching the

skin, much as a radiator warms the contents of a room

(radiation starts to become less effective when the

environmental temperature rises above 88°F). Con-

ductionis the loss of heat to cooler air or objects, such

as clothing, that touch the skin. Convectionmeans

that air currents move the warmer air away from the

skin surface and facilitate the loss of heat; this is why a

fan makes us feel cooler on hot days. Loss of heat by

convection also gives us the “wind chill factor” we

hear about in winter. A cold day that is windy will feel

colder than a cold day when the air is still, because the

wind blows the slightly warmer air surrounding the

body away, replacing it with colder air.

As you may recall from Chapter 5, the tempera-

ture of the skin and the subsequent loss of heat are

determined by blood flow through the skin. The arte-

rioles in the dermis may constrict or dilate to decrease

or increase blood flow. In a cold environment, vaso-

constrictiondecreases blood flow through the dermis

and thereby decreases heat loss. In a warm environ-

ment, vasodilationin the dermis increases blood flow

to the body surface and loss of heat to the environ-

ment.

The other mechanism by which heat is lost from

the skin is sweating. The eccrine sweat glands

secrete sweat (water) onto the skin surface, and excess

body heat evaporates the sweat. Think of running

water into a hot frying pan; the pan is rapidly cooled

as its heat vaporizes the water. Although sweating is

not quite as dramatic (no visible formation of steam),

the principle is just the same.

Sweating is most efficient when the humidity of the

surrounding air is low. Humidity is the percentage of

the maximum amount of water vapor the atmosphere

can contain. A humidity reading of 90% means that

the air is already 90% saturated with water vapor and

can hold little more. In such a situation, sweat does

not readily evaporate, but instead remains on the skin

even as more sweat is secreted. If the humidity is 40%,

Body Temperature and Metabolism 397

Table 17–1 FACTORS THAT AFFECT

HEAT PRODUCTION

Factor Effect

Thyroxine

Epinephrine and

sympathetic

stimulation

Skeletal muscles

Liver

Food intake

Higher body

temperature

• The most important regulator
  of day-to-day metabolism;
  increases use of foods for ATP
  production, thereby increasing
  heat production


• Important in stress situations;
  increases the metabolic activity
  of many organs; increases ATP
  and heat production


• Normal muscle tone requires
  ATP; the heat produced is
  about 25% of the total body
  heat at rest


• Always metabolically active;
  produces as much as 20% of
  total body heat at rest


• Increases activity of the GI
  tract; increases ATP and heat
  production


• Increases metabolic rate, which
  increases heat production,
  which further increases meta-
  bolic rate and heat production;
  may become detrimental dur-
  ing high fevers