|About 90 percent of the body’s heat is produced in the torso area by the major organs and muscle groups. The amount of heat generated is increased as the body works harder. In order to maintain a constant core temperature, the body must either give up or retain this heat as necessary. How this is accomplished depends greatly on the ambient temperature and humidity around you.
Convective body cooling.
Under normal conditions (60º to 80º F ambient temperature),
the circulatory system carries core heat toward the skin’s surface.
Since heat always travels from hot to cold, rather than from cold to
hot, the body heat is carried away as the cooler outside air passes
over the skin. This process is known as convective cooling, since the
heat is removed by the movement of air.
Evaporative body cooling.
As the temperature outside begins to rise, the difference between
normal skin temperature (90º F) and the ambient temperature narrows.
This difference is known as the delta T. As the ambient temperature
rises above 80º F, the delta T is not great enough to allow the body’s
internal heat to flow away from the body by convection.
Instead, the body reacts by cooling itself through
a process known as evaporative cooling. When water fluid is exposed
to warm, dry air, it will evaporate into water vapor. This change of
state is called a phase change and it produces a tremendous cooling
effect. The body creates this phase change by secreting perspiration
from our sweat glands to the surface of the skin.
When the air surrounding the skin is warm and dry,
this is an extremely efficient process. But as the humidity rises,
perspiration can no longer evaporate to water vapor, as the air is
already saturated. This is a dangerous condition, since the body has
no other natural mechanism to give up heat.
Vasoconstriction and the brain.
When the outside temperature drops below 60º F, the body needs
to reverse the process and retain its internally generated heat. This
is accomplished by a process called vasoconstriction. Vasoconstriction
is the restriction of blood flow to the skin surface by contraction
of blood vessels. Since the body’s organs must always have a flow of
blood, vasoconstriction is applied only to those vessels carrying heat
to the skin’s surface.
When the brain is fooled into thinking the temperature
is cool–for example, when ice is applied to the body–vasoconstriction
occurs in an effort to prevent loss of heat, even though the core temperature
is actually rising. This can lead to dizziness and fainting.
More dangerous is the fact that the cool skin
temperature physiologically feels comfortable, so you may actually
work harder, creating an even faster rise in core body temperature
and the risk of cardiac arrest.
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