Evaporation is by far the greatest agent in cooling your core temperature, but not the only one. There are 3 others: conduction, convection, and radiation.
There are two main ways your brain can effect heat loss thru your skin: increase the blood flow through the skin, and increased sweat. When blood reaches the surface of your skin, calories are lost to the surrounding environment, and calories are a measure of heat. You are also cooled by evaporation of invisible sweat. But when your temperature rises more, the brain turns on the sweat glands, increasing the sweating.
The exchange of heat from the skin to the environment depnds on several factors. Radiation is the most important, accounting for 60% of heat loss under resting conditions. Your body is a metabolic furnace that constantly radiates heat. You can also gain heat by the sun’s radiation.
You also lose or gain heat by conduction. If you sit on a hot surface, you will gain heat. In contrast, cold water causes heat from the skin to be conducted to the water.
Closely related to conduction is convection, which is moving the warmed water or air away from the skin and replacing it with cooler water or air. That is why a cool breeze helps, but not a hot breeze.
At room temperatures at rest, about 25% of your calories of heat are eliminated by insensible perspiration. As you exercise and increase the need to eliminate heat, you sweat more and the heat loss is increased. That is why humidity causes heat stress. It limits your ability to cool by evaporation. In humid environments, the body must rely mostly on radiation, conduction, and convection to cool.
95% of the calories you consume and eliminate are eliminated through your skin. Only about 25% of the calories expended with exercise are used for the mecanical effort. The other 75% are from heat produced by the muscles. That is why vigorous exercise causes the body temperature to rise.
A healthy body has an amazing ability to adjust to its environment and people can become acclimiated to either heat or cold. Acclimitazation to heat requires 2-6 weeks. Before acclimitazation occurs, the sweat glands will lose a lot of salt when you sweat. Sweat glands adapt to recycle the sweat in a way to permit reabsorption of some salt. But in vigorous exercise, the capacity of the sweat glands to reabsorb salt will be overloaded.
There is a change within the cell structure (mitochondria) that enables the cells to metabolize energy compounds from food more efficiently, using less oxygen. Since delivery of oxygen is dependent upon the capacity of the circulation to increase blood flow, this increased efficiency decreases the overall work the heart must do during heat stress.
(* The Health Letter * August 1991)