If you've ever run a marathon on a hot day, you know just how important it is to stay cool. Your body features several mechanisms that help you dissipate that excess heat. Chief among these is sweating, a simple yet effective means to wick heat away from the surface of your skin.
Eccrine and Apocrine
Your body has two types of sweat glands. The first of these are apocrine glands, found primarily under your armpits and in the groin. The thick fluid secreted by these glands is broken down by bacteria, yielding strong and often unpleasant odors. The bulk of your sweat glands are eccrine, and these play the more important role in temperature control. Inside an eccrine gland, cells secrete a dilute solution into a duct where sodium ions are reabsorbed, making the sweat that reaches the surface of the skin even more dilute.
Control
The hypothalamus is the part of your brain that controls your internal thermostat. Running a marathon burns up lots of glucose, releasing energy in the form of heat. As your temperature starts to rise above 98 degrees Fahrenheit, your hypothalamus responds by activating cooling mechanisms. One such mechanism is vasodilation, an increase in the diameter of blood vessels near the surface of the skin. Vasodilation increases the rate at which you lose heat to your environment.
Sweating
The other main mechanism for heat loss in humans is sweating. Sweating is a very efficient way to lose heat quickly, because water has a high latent heat of vaporization. This means that it takes a lot of heat energy to vaporize a given amount of water -- much more energy than it would take to vaporize an equal amount of acetone, for example. Each gram of water that evaporates absorbs 0.58 kilocalories of heat energy from your skin.
Considerations
Both vasodilation and sweating are crucial in controlling your body temperature during a marathon. At 98 F, each liter of blood that flows near the skin surface can release as much as much as a kilocalorie of heat to the environment. Sweating is a very efficient way to lose heat, but the rate varies depending on humidity. The more humid the air, the slower the rate at which your sweat will evaporate, and thus the less heat loss will occur.
