Body temperature is regulated by thermal regulatory center of the
hypothalamus, which modifies heat loss from the body, and by heat
production, primarily from muscular activity.
The hypothalamic center
is sensitive to thermal, neural, and humoral stimulation.
There is also
evidence that it responds to changes in perfusing blood temperature of as
little as half a degree Celsius.
A lowering of skin temperature below 32.8 degree C causes a neural
discharge of this center.
A disturbance of thermal regulation characterized by temperature elevation
(fever) is produced by a short polypeptide, interleukin-1, which is
released from macrophages.
There is also a diurnal variation in body temperature of about half a
degree Celsius. Although this natural variation seems to be part of the
biologic clock system, there is no good explanation for this temperature
fluctuation.
Body heat is produced as a result of cellular metabolic activity and
muscular work. It has been estimated that the skeletal muscle and skin
contribute as much as 50% to overall heat production.
Cold stress produces an increase in heat production of 50% to100% by
increasing muscle tone, a modification not associated with significant
physical movement.
Increased heat production beyond this level requires actual muscular
contraction, often in the form of shivering, which can further increase
the heat yield considerably.
Heat loss accounts for 50% of the heat produced, the
remainder of the heat energy provides for the 37 degree
+ 1 degree C body
temperature.
In large part, heat loss is regulated by the volume of
blood that perfuses the superficial vascular arcades in the skin.
Two major factors are involved in the dermal regulatory system:
i)
blood flow to the skin, and ii) the use of that thermal energy to
warm the portion of the skin surface that is wet with perspiration.
Dilatation of these arcades to bring the blood nearer the skin surface
facilitates he transfer of heat, a process that underlies the flushed
appearance during strenuous exercise or hot weather.
The means for heat dissipation from the body are conduction, convection,
and radiation of thermal energy, as well as the evaporation of
perspiration from the surface of the skin.
Under basal conditions, roughly 5% of the cardiac output goes to the skin.
With vasodilatation there is increased heat loss and this value
may reach roughly half of the normal cardiac output. In the reverse
process, environmental cold leads to vasoconstriction and a reduction in
blood flow to the skin to as little as 30 ml/min, an effect seen as
blanching.
Although the skin surface is the major route of
heat loss, smaller quantities of heat energy are lost through the warming
of inspired air and through sweating.
The skin has abundant sweat glands whose orifices deposit perspiration on
the surface.
The evaporation of this fluid contribute to the loss of heat energy by
extracting the heat of vaporization.
At rest a person normally losses about a liter of perspiration a
day.
During strenuous physical activity or in a hot environment, the production
of sweat serves as an important additional source of cooling.
The dermis is also provided with a fatty
layer that serves as an effective insulator. So effective, that aquatic
mammals, which have a thick fatty layer, can flourish in waters that would
literally freeze a human. Even man appears to use body fat as an adaptive
device for cold climates. People living near and above the arctic circle
frequently have thicker dermal fat layers than their southern
counterparts.
Environmental Pathology- Physical Agents: click here
Environmental Pathology - Hypothermia: click here
Environmental Pathology - Hyperthermia: click here
Environmental Pathology - Electric Burns: click here
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