Excess heat from muscle activity is absorbed by the blood. As our core temperature rises, our thermoregulatory system in our brain stem engages cooling mechanisms, such as vasculature of the skin. This dilation of peripheral vasculature increases the blood flow to the skin where the blood can be cooled. At the same time, to further increase the rate of heat loss from the skin, the brain stimulates the sweat glands to produce sweat. The evaporation of sweat from the skin increases the rate of cooling.
Sweat consists primarily of water and some electrolytes, specifically sodium and chloride ions.
As long as we can sweat and the sweat can evaporate, we can continue to cool ourselves efficiently. But if for some reason either the sweating mechanism begins to fail or the sweat cannot evaporate, then the cooling mechanism will fail. On hot, very humid days, our cooling mechanism is extremely inefficient, and it becomes relatively easy to overheat because the sweat cannot evaporate. Additionally, our ability to sweat diminishes as we become dehydrated. For the Backcountry is available from the Adventurous Traveler Bookstore.