Page 1004 - Fundamentals of anatomy physiology

cool sand in the shade, conductive losses can be considerable.
When you are standing on the same sand, conductive losses
are negligible.
The Regulation of Heat Gain and Heat Loss
Heat loss and heat gain involve the activities of many systems.
Those activities are coordinated by the heat-loss center and
heat-gain center, respectively, in the preoptic area of the ante-
rior hypothalamus. p. 506 These centers modify the activities
of other hypothalamic nuclei. The overall effect is to control
temperature by influencing two processes: the rate of heat pro-
duction and the rate of heat loss to the environment. Changes
in behavior may also support these processes.
Mechanisms for Increasing Heat Loss. When the temperature
at the preoptic nucleus rises above its set point, the heat-loss
center is stimulated, producing three major effects:
1. The inhibition of the vasomotor center causes peripheral va-
sodilation, and warm blood flows to the surface of the body.
The skin takes on a reddish color, skin temperatures rise,
and radiative and convective losses increase.
2. As blood flow to the skin increases, sweat glands are stimu-
lated to increase their secretory output. The perspiration flows
across the body surface, and evaporative heat losses accel-
erate. Maximal secretion, if completely evaporated, would
remove 2320 Cal per hour.
3. The respiratory centers are stimulated, and the depth of res-
piration increases. Often, the individual begins breathing
through an open mouth rather than through the nasal
passageways. This increases evaporative heat losses through
the lungs.
Mechanisms for Promoting Heat Gain. The function of the
heat-gain center of the brain is to prevent hypothermia (hı.-po. -
THER-me. -uh), or below-normal body temperature. When the
temperature at the preoptic nucleus drops below acceptable
levels, the heat-loss center is inhibited and the heat-gain center
is activated.
Heat Conservation. The sympathetic vasomotor center de-
creases blood flow to the dermis, thereby reducing losses by
radiation, convection, and conduction. The skin cools. With
blood flow restricted, the skin may take on a bluish or pale
color. The epithelial cells are not damaged, because they can
tolerate extended periods at temperatures as low as 25°C
(77°F) or as high as 49°C (120°F).
In addition, blood returning from the limbs is shunted
into a network of deep veins. p. 789 Under warm conditions,
blood flows in a superficial venous network (Figure 25–15a). In
cold conditions, blood is diverted to a network of veins that lie
deep to an insulating layer of subcutaneous fat (Figure 25–15b).
This venous network wraps around the deep arteries, and
Chapter 25 Metabolism and Energetics 991
Clinical Note
Induced Hypothermia Hypothermia may be intention-
ally produced during surgery. The goal is to reduce the
metabolic rate of a particular organ or of the patient’s
entire body. In controlled hypothermia, the patient is first
anesthetized to prevent shivering, which would act to fight
off hypothermia.
During open-heart surgery, the body is typically
cooled to 25°C–32°C (79°F–89°F). This cooling reduces the
metabolic demands of the body, which will be receiving
blood from an external pump or oxygenator. The heart
must be stopped completely during the operation, and it
cannot be well supplied with blood over this period. For
this reason, the heart is perfused with an arresting solution
at 0°–4°C (32°F–39°F) and maintained at a temperature
below 15°C (60°F) during the operation. At these tempera-
tures, the cardiac muscle can tolerate several hours of isch-
emia (inadequate blood supply) without damage.
When cardiac surgery is performed on infants, a deep
hypothermia may be produced by cooling the entire body
to temperatures as low as 11°C (52°F) for an hour or more.
In effect, these conditions are similar to those experienced
by victims of accidental drowning.
heat is conducted from the warm blood flowing outward to
the limbs to the cooler blood returning from the periphery
(Figure 25–15c). This arrangement traps the heat close to the
body core and restricts heat loss. Such exchange between fluids
that are moving in opposite directions is called countercurrent
exchange. (We return to this topic in Chapter 26.)
Heat Generation. We can divide the mechanisms for generat-
ing heat into two broad categories: shivering thermogenesis
and nonshivering thermogenesis. In shivering thermogen-
25esis (ther-mo. -JEN-e-sis), a gradual increase in muscle tone
increases the energy consumption of skeletal muscle tissue
throughout your body. The more energy used, the more heat is
produced. Both agonist and antagonist muscles are involved.
The degree of stimulation varies with the demand.
If the heat-gain center is extremely active, muscle tone in-
creases to the point at which stretch receptor stimulation will
produce brief, oscillatory contractions of antagonistic muscles.
In other words, you begin to shiver. Shivering increases the
workload of the muscles and further elevates oxygen and energy
consumption. The heat that is produced warms the deep vessels,
to which blood has been shunted by the sympathetic vasomotor
center. Shivering can elevate body temperature quite effectively,
increasing the rate of heat generation by as much as 400 percent.
The intermittent cycle of capillary vasoconstriction and
vasodilation in our hands when they are exposed to cold