Exercise, contract a fever, stand in the summer sun, do anything to increase the body's temperature just a fraction, and you'll quickly appreciate how reliable the body's five million sweat glands are. Nerves called thermoreceptors inside the body and on the skin signal the brain to a rise in temperature. In the brain, the hypothalamus is the body's thermostat. When body temperature rises above 98.6 degrees, the nervous system reacts to get rid of the heat. Blood vessels dilate, opening up like big pipes, rushing more blood to the skin's surface, causing more heat to escape.
Sweat glands pump perspiration through pores.
Cooling occurs when body heat is transferred to the sweat, causing it to evaporate and carry heat away, cooling the skin. Dripping sweat means perspiration can't evaporate fast enough. Sweat is 99 percent water and 1 percent dissolved substances such as salt. About a pint of water is lost through the skin in normal conditions, although as much as three gallons can be lost in a day because of exercise or hot weather. That's why staying hydrated is vital.