Thermoregulation

The body temperature of a warm blooded animal must be kept within precise limits so that the organism can still function. In humans, if the body temperature is too high, for example during a fever, dehydration and heatstroke can occur. If the fever is not reduced, eventually the patient will die. The same happens with a low body temperature and is more commonly referred to as hypothermia or exposure. So, how does the body manage to control its temperature?

In mammals, the thermoregulatory center in the brain monitors and controls body temperature. This center has receptors capable of detecting the temperature of the blood flowing through the brain. The skin also has temperature receptors which can monitor the temperature of the skin, and these send messages to the thermoregulatory center. If the core temperature of the body is not at a normal level, the thermoregulatory center sends messages to effectors that carry out actions which will restore it to the correct value.

If the temperature is too high, for example during heavy exercise, the thermoregulatory center sends messages to the sweat glands which cause an increase in the volume of sweat they produce. More sweat on the surface of the skin means an increase in evaporation. This takes away more heat, cooling the skin and therefore cooling the blood flowing through the blood capillaries of the skin. It also sends nerve impulses that cause the surface blood capillaries to dilate (increase in diameter). This process is called vasodilation and it allows more blood to flow through the capillaries, thus losing more heat to the surroundings. It is vasodilation that causes your skin to redden during exercise.

If the temperature falls too low, vasoconstriction occurs, reducing the blood flow through the surface capillaries and reducing heat loss. This can cause your fingers and toes to turn white and go numb. In extreme cases, frostbite occurs. Frostbite is when cells that have no blood supply die because they are getting no oxygen - the dead flesh turns black (gangrene). This can be left to fall off of its own accord or can be removed surgically. The thremoregulatory system also sends out messages to the muscles to vibrate rapidly (shivering) which generates extra heat for the body. A third set of nerve impulses raise the surface hairs of the body in order to trap a layer of air next to the skin for extra insulation.