2-4. HOW IS A PERSON'S BODY TEMPERATURE REGULATED?

a. Hypothalamus. Human beings, like other mammals, have bodies that stay about the same temperature even when the outside (environmental) temperature changes. The body's temperature stays constant because it is constantly monitored by a small area inside the brain called the hypothalamus (hi-po-THAL-ah-mus). When the body begins to cool, the hypothalamus causes the body to produce more heat. When the body becomes too warm, the hypothalamus causes the body to loose heat faster. These heating and cooling actions are very important since tissue damage and even death can result if the body gets too cold or too hot. The hypothalamus receives information concerning the body's temperature from several sources.

(1) Skin. One source of temperature information is the skin. The skin contains many nerves that have special functions. Some nerves protect the body by providing information in the form of pain. Other nerves provide the sense of touch. The hypothalamus uses two other types of nerves. One type senses heat while the other senses cold. These nerves provide information concerning the temperature of the environment.

(2) Hypothalamus. An important source of information concerning the body's actual temperature comes from the hypothalamus itself. Part of the hypothalamus can sense the temperature of the blood flowing through the hypothalamus. During a hyperthermic emergency such as heatstroke, the hypothalamus can be overwhelmed and temporarily shut down, causing the body to lose its ability to cool the body.

b. Cooling Reactions.

(1) Perspiration increases. When perspiration (sweat) on the skin evaporates, the process uses some of the body's heat. The hypothalamus causes the body to perspire more. This increased rate of perspiration then results in more body heat being lost through evaporation.

(2) Blood vessels enlarge. Blood vessels near the surface of the skin loose heat to the environment. The hypothalamus causes these blood vessels to become larger (dilate) when the body is too warm. When the blood vessels enlarge, they loose heat faster. This enlargement causes the skin to have a reddish (flushed) appearance.

c. Warming Reactions.

(1) Muscle activity increases. When the large muscles of the body are active, heat is produced. When the body becomes too cool, the hypothalamus causes the large muscles to contract and relax. These contractions and relaxations cycles, called shivers, produce body heat.

(2) Blood vessels contract. When the body is loosing too much heat, the hypothalamus causes the blood vessels near the surface of the skin to contract (become smaller). The blood vessels' decrease in size causes the vessels to loose heat slower than normal. The contraction of the blood vessels causes the skin to look pale.