REGULATION OF BODY TEMPERATURE
The hypothalamus is responsible for the regulation of body temperature and is considered the “thermo-stat” of the body. As the thermostat, the hypothalamus maintains the “setting” of body temperature by bal-ancing heat production and heat loss to keep the body at the set temperature.
To do this, the hypothalamus must receive infor-mation about the temperature within the body and about the environmental temperature. Specialized neurons of the hypothalamus detect changes in the temperature of the blood that flows through the brain. The temperature receptors in the skin provide infor-mation about the external temperature changes to which the body is exposed. The hypothalamus then integrates this sensory information and promotes the necessary responses to maintain body temperature within the normal range.
In a warm environment or during exercise, the body temperature tends to rise, and greater heat loss is needed. This is accomplished by vasodilation in the dermis and an increase in sweating. Vasodilation brings more warm blood close to the body surface, and heat is lost to the environment. However, if the environmental temperature is close to or higher than body temperature, this mechanism becomes ineffec-tive. The second mechanism is increased sweating, in which excess body heat evaporates the sweat on the skin surface. As mentioned previously, sweating becomes inefficient when the atmospheric humidity is high.
On hot days, heat production may also be decreased by a decrease in muscle tone. This is why we may feel very sluggish on hot days; our muscles are even slightly less contracted than usual and are slower to respond.
In a cold environment, heat loss from the body is unavoidable but may be reduced to some extent. Vasoconstriction in the dermis shunts blood away from the body surface, so that more heat is kept in the core of the body. Sweating decreases, and will stop completely if the temperature of the hypothalamus falls below about 98.6°F. (Remember that the internal temperature of the brain is higher than an oral tem-perature, and is less subject to any changes in environ-mental temperature.)
If these mechanisms are not sufficient to prevent the body temperature from dropping, more heat may be produced by increasing muscle tone. When this greater muscle tone becomes noticeable and rhythmic, it is called shivering and may increase heat production by as much as five times the normal.
People also have behavioral responses to cold, and these too are important to prevent heat loss. Such things as putting on a sweater or going indoors reflect our awareness of the discomfort of being cold. For people (we do not have thick fur as do some other mammals), these voluntary activities are of critical importance to the prevention of excessive heat loss when it is very cold.