Understanding these problems and how to prevent them requires a knowledge of the body's way of regulating heat, water, and sodium balance. During heavy exercise, the human body can generate ten to twenty times the amount of heat that it does at rest. Since only 20 percent of that heat is used to do work, the rest must be dissipated. Because heat transfer is accomplished by increasing blood flow to the skin, it is vital to keep up the blood volume, which means keeping up hydration. In addition, evaporation of sweat is the body's major mechanism for heat dissipation while exercising.
Sweat is composed mainly of water, with a small amount of sodium.
Dehydration therefore clearly robs the body of its ability to cope with heat stress, and increases the risk of heat illness. In fact, a fluid loss of 1 percent of total body weight can increase the body's core temperature. Common exertional heat illnesses include heat syncope, heat cramps, heat exhaustion, and heatstroke. It may also occur if individuals stand for prolonged periods in a hot environment without moving the legs. It is not dangerous, and is easily treated and easily prevented.