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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.
Differences between the actual body temperature and the set point constitute error signals. The hypothalamus operates through a negative feedback loop, such that its response counteracts, rather than enhances, the deviation from the set point, returning 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.
