Normal body function requires a relatively constant
body temperature, which is regulated by the body's
thermostat, a region of the brain called the
hypothalamus. The hypothalamus generates a
temperature set point for the body and appears to be
the major site for the integration of temperature
information.
When the body's temperature is warmer than the set
point, the hypothalamus sends instructions to various
organs to cool the body down. When the body gets
too cool, the hypothalamus commands the body to do
the opposite and perform a warming response.
Blood vessels are some of the targets under the
control of the hypothalamus. For example, when the
body is cooler than its set point, the hypothalamus
triggers blood vessels in the skin to con-strict. The
constriction prevents blood from circulating close to
the body's surface and thereby reduces heat loss to
the environment.
When the body is too cool, the hypothalamus also
stimulates shivering. The repeated contractions of
muscle fibers generate heat in the body.
The hypothalamus controls an endocrine organ called
the thyroid gland. When the body is exposed to cold
for a prolonged period of time, the hypothalamus
triggers this gland to release hormones that, in turn,
trigger cells of the body to increase their metabolic
rates. A higher metabolic rate results in a greater
production of body heat.
When the body gets too warm, the hypothalamus
triggers cooling responses. These responses include
dilating blood vessels near the body's surface,
increasing perspiration for evaporative cooling, and
accelerating the rate of breathing, which also cools
the body.
We can describe these actions of the hypothalamus in
a diagram. The hypothalamus receives temperature
information from receptors in the skin, in the viscera,
and in the hypothalamus itself. With this information,
the hypothalamus compares the body's temperature
to its set point.
A key sensor for temperature information appears to
be the hypothalamus itself, although it also integrates
information from other sensors in the body. An
experiment on a ground squirrel, in which the
hypothalamus was either warmed or cooled, provides
evidence that the hypothalamus acts as a temperature
sensor.
When the hypothalamus was cooled, the animal
increased its metabolic rate and thereby elevated its
body temperature. In contrast, when it was warmed,
the squirrel's body temperature dropped. This
experiment demonstrates that the hypothalamus acts
as a temperature sensor in addition to being the
body's thermostat.

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