Page 1003 - Fundamentals of anatomy physiology
P. 1003
990 Unit 5 Environmental Exchange
and the control of appetite, the result is a sense of satiation which makes up nearly two-thirds of body weight. Water is
and the suppression of appetite. Ghrelin (GREL-in), a hormone an excellent conductor of heat, so the heat produced in one
secreted from the gastric mucosa, stimulates appetite. Ghrelin region of the body is rapidly distributed by diffusion, as well
levels decline when the stomach is full, and increase during the as through the bloodstream. If body temperature is to remain
fasting state. Blood ghrelin levels tend to be low in obese indi- constant, that heat must be lost to the environment at the same
viduals, suggesting that it plays a role in obesity. Other effects rate it is generated. When environmental conditions rise above
of ghrelin are currently being studied. p. 926 or fall below “ideal,” the body must control the gains or losses
to maintain homeostasis.
Thermoregulation
Mechanisms of Heat Transfer
The BMR estimates the rate of energy use by the body. The energy
that cells do not capture and harness is released as heat. This heat Heat exchange with the environment involves four basic
serves an important homeostatic purpose. Humans are subject processes: (1) radiation, (2) convection, (3) evaporation, and
to vast changes in environmental temperatures, but our complex (4) conduction (Figure 25–14).
biochemical systems have a major limitation. Our enzymes oper-
ate over only a relatively narrow temperature range. Accordingly, Objects warmer than the environment lose heat as radia-
our bodies have anatomical and physiological mechanisms that tion. When you feel the heat from the sun, you are experiencing
keep body temperatures within acceptable limits, regardless of radiant heat. Your body loses heat the same way, but in propor-
environmental conditions. This homeostatic process is called tionately smaller amounts. More than 50 percent of the heat
thermoregulation. Failure to control body temperature can you lose indoors is lost through radiation. The exact amount
result in a series of physiological changes. For example, a body varies with both body temperature and skin temperature.
temperature below 36°C (97°F) or above 40°C (104°F) can
cause disorientation. A temperature above 42°C (108°F) can Convection is heat loss to the cooler air that moves across
cause convulsions and permanent cell damage. the surface of your body. As your body loses heat to the air
next to your skin, that air warms and rises, moving away from
We continuously produce heat as a by-product of me- the surface of the skin. Cooler air replaces it, and this air in
tabolism. When energy use increases due to physical activity, turn becomes warmed. This pattern then repeats. Convection
or when our cells are more active metabolically (as they are accounts for roughly 15 percent of the body’s heat loss indoors
during the absorptive state), additional heat is generated. The but is insignificant as a mechanism of heat gain.
heat produced by biochemical reactions is retained by water,
When water evaporates, it changes from a liquid to a vapor.
Figure 25–14 Mechanisms of Heat Transfer. Evaporation absorbs energy—about 0.58 Cal per gram of
water evaporated—and cools the surface where evaporation
Evaporation Radiation occurs. The rate of evaporation at your skin is highly variable.
Convection
25 Each hour, 20–25 mL of water crosses
epithelia and evaporates from the alve-
Conduction olar surfaces and the surface of the skin.
This insensible water loss remains rela-
tively constant. At rest, it accounts for
roughly 20 percent of your body’s aver-
age indoor heat loss. The sweat glands
responsible for sensible perspiration have
a tremendous scope of activity, ranging
from virtual inactivity to secretory rates
of 2–4 liters (2.1–4.2 quarts) per hour.
p. 181
Conduction is the direct transfer of
energy through physical contact. When
you come into an air-conditioned class-
room and sit on a cold plastic chair, you
are immediately aware of this process.
Conduction is generally not an effective
way of gaining or losing heat. Its im-
pact depends on the temperature of the
object and the amount of skin surface
area involved. When you are lying on
