Chapter 24
Nutrition, Metabolism, and Body Temperature Regulation
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24
Homeostatic Imbalance 24.6
When sweating is heavy and prolonged, especially in untrained
individuals, losses of water and NaCl may cause painful muscle
spasms called
heat cramps
. Te solution is simple: Drink fluids.
Role of the Hypothalamus
Although other brain regions contribute, the hypothalamus,
particularly its
preoptic
region, is the main integrating center for
thermoregulation. ±ogether the
heat-loss center
(located more
anteriorly) and the
heat-promoting center
make up the brain’s
thermoregulatory centers
.
Te hypothalamus receives afferent input from (1)
periph-
eral
thermoreceptors
located in the shell (the skin), and (2)
central thermoreceptors
sensitive to blood temperature and
located in the body core including the anterior portion of the
hypothalamus. Much like a thermostat, the hypothalamus re-
sponds to this input by reflexively initiating appropriate heat-
promoting or heat-loss activities.
Te central thermoreceptors are more critically located than
the peripheral ones, but varying inputs from the shell proba-
bly alert the hypothalamus of the need to prevent temperature
changes in the core. In other words, they allow the hypothala-
mus to anticipate changes to be made.
Heat-Promoting Mechanisms
When the external temperature is low or blood temperature falls
for any reason, the heat-promoting center is activated. It triggers
one or more of the following four mechanisms to maintain or
increase core body temperature (
Figure 24.26
, bottom).
1.
Constriction of cutaneous blood vessels.
Activation of
the sympathetic vasoconstrictor fibers serving the blood
body uses four mechanisms of heat transfer—radiation, con-
duction, convection, and evaporation.
Radiation
Radiation
is the loss of heat in the form of infrared
waves (thermal energy). Any object that is warmer than objects
in its environment—for example, a radiator and (usually) the
body—will transfer heat to those objects. Under normal condi-
tions, close to half of your body heat loss occurs by radiation.
Because radiant energy flows from warmer to cooler, radia-
tion explains why a cold room warms up shortly aFer it fills with
people (the “body heat furnace,” so to speak). Your body also
gains heat by radiation, as demonstrated by the way your skin
warms when you sunbathe.
Conduction
Conduction
transfers heat from a warmer ob-
ject to a cooler one when the two are in direct contact with
each other. ²or example, when you step into a hot tub, some of
the heat of the water transfers to your skin, and warm buttocks
transfer heat to the seat of a chair by conduction.
Convection
Convection
is the process that occurs because
warm air expands and rises and cool air, being denser, falls.
Consequently, the warmed air enveloping the body is continu-
ally replaced by cooler air molecules. Convection substantially
enhances heat transfer from the body surface to the air because
the cooler air absorbs heat by conduction more rapidly than the
already-warmed air.
±ogether, conduction and convection account for 15–20% of
heat loss to the environment. Tese processes are enhanced by
anything that moves air more rapidly across the body surface,
such as wind or a fan, in other words, by
forced convection
.
Evaporation
Te fourth mechanism by which the body
loses heat is
evaporation
. Water evaporates because its mol-
ecules absorb heat from the environment and become energetic
enough—in other words, vibrate fast enough—to escape as a
gas, which we know as water vapor. Te heat absorbed by water
during evaporation is called
heat of vaporization
. Te evapo-
ration of water from body surfaces removes large amounts of
body heat. Every gram of water that evaporates removes about
0.58 kcal of heat from the body.
Tere is a basal level of body heat loss due to the continuous
evaporation of water from the lungs and oral mucosa, and through
the skin. Te unnoticeable water loss occurring via these routes is
called
insensible water loss
, and the accompanying heat loss is
in-
sensible heat loss
. Insensible heat loss dissipates about 10% of the
basal heat production of the body and is a constant not subject to
body temperature controls. When necessary, however, the body’s
control mechanisms do initiate heat-promoting activities to coun-
terbalance this insensible heat loss.
Evaporative heat loss becomes an active or
sensible
process
when body temperature rises and sweating produces increased
amounts of water for vaporization. Extreme emotional states
activate the sympathetic nervous system, causing body tem-
perature to rise by one degree or so, and vigorous exercise can
raise body temperature as much as 2–3°C (3.6–5.4°²). Vigorous
muscular activity can produce and evaporate 1–2 L/h of per-
spiration, removing 600–1200 kcal of heat from the body each
hour. Tis is more than 30 times the amount of heat lost via
insensible heat loss!
Figure 24.25
Mechanisms of heat exchange.
A woman
enjoying a hot tub illustrates several mechanisms of heat exchange
between the body and its environment. (1) Conduction: Heat
transfers from the hot water to the skin. (2) Radiation: Heat transfers
from the exposed part of the body to the cooler air. (3) Convection:
Warmed air moves away from the body via breezes. There may also
be (4) evaporation of perspiration: Excess body heat is carried away.
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