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Sweat Is Cool

This is an excerpt from Practical Guide to Exercise Physiology by Robert Murray & W. Larry Kenney.

You know what it's like to sweat, but few understand the true importance of sweating during physical activity or heat exposure. Whenever core body temperature rises above the sweat threshold, sweat glands in the skin begin to produce sweat. You were born with roughly two million sweat glands that enable you to survive hot weather and vigorous physical exercise simply because those glands secrete water onto the surface of the skin. As water molecules evaporate from the skin, heat is lost to the environment. In fact, during intense exercise, 80% of the heat produced by muscles is lost to surroundings by the evaporation of sweat. Sweat that drips off the skin, however, provides no cooling.

Even babies can sweat because humans are born with millions of active sweat glands. Sweat glands become more active after puberty.

Sweating does for humans what panting does for dogs, but sweating does it better. Sweating is an effective way to stay safely cool in hot environments. That simple fact explains why humans are better than most animals at being physically active in the heat for prolonged periods. Animals that do not sweat cannot pant fast enough to keep up with the heat production of exercise. In humans, when body temperature increases slightly above normal resting temperature (98.6 ℉, or 37 ℃), the hypothalamus in the brain senses the increase in temperature and signals the sympathetic nervous system to dilate blood vessels in the skin and activate sweat glands. (See figure 10.3.)

You know how uncomfortable you feel on a hot, humid day. The relative humidity of the air influences the rate of heat loss because humidity affects the evaporation of sweat. When the air is completely saturated with water vapor (100% humidity), sweat cannot evaporate from the skin; it literally has no place to go because the surrounding air already contains as much water vapor as it can hold. At the other end of the humidity spectrum, when exercise takes place in a dry, desert environment, sweat can evaporate so quickly from the skin that you hardly notice you're sweating. That rapid evaporation makes for effective heat loss but increases the risk of dehydration because sweaty skin and clothing are a cue that you're losing fluid and should drink to replace it. As humidity climbs above 50%, heat loss is made progressively more difficult, increasing the risk of heat exhaustion and heatstroke.

Figure 10.3 Sweating is a critical part of temperature regulation because sweating during exercise is the primary way in which humans lose heat to the environment and maintain a safe internal body temperature.

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