We sweat primarily—and crucially—to cool our body. When sweat evaporates, heat is removed. There are two types of sweat glands in the body. The heat-regulating function appears to be limited to eccrine sweat glands, located all over the body. In contrast, apocrine sweat glands in the armpits, groin, and around the navel respond to emotional and nervous stimuli.
It’s mostly water, with small amounts of sodium, chloride, potassium, and other minerals (collectively called electrolytes) that play an important role in regulating blood pressure and the body’s water balance. Sweat from apocrine glands is different, however, and gives sweating a bad name. It contains proteins and fatty acids, and it mixes with oil and dirt, making it thicker and slightly yellowish. Body odor comes from compounds released by bacteria feeding on organic particles in apocrine sweat.
Sweat rates vary greatly. Sedentary people may sweat anywhere from a negligible amount to two quarts a day. Under extreme conditions, heat and physical exertion can increase the output to as much as two quarts an hour. Factors such as age, sex, genetics, and conditioning affect the amount a person sweats.
Your working muscles burn lots of calories and thus produce heat, which raises body temperature. The body reacts by increasing blood flow to your skin, which helps remove heat by a process called convection, and also causes you to sweat more, which cools you off by its evaporation.
Yes, but the sweat doesn’t cool you off, since it doesn’t evaporate as much and simply drips off. That’s why a hot, humid climate feels more uncomfortable than a hotter but drier climate. Air movement does promote evaporation of sweat, so a fan may help— for instance, while you’re exercising on a stationary bike or lifting weights.
Sweating reduces blood volume by removing water and thus boosts the concentration of sodium in the blood, which stimulates the brain to trigger thirst. But when you exercise or work hard, the thirst mechanism doesn’t keep up with your need for fluid. By the time you’re thirsty, you’ve already lost a lot of fluid. A fluid deficit of even a pint or two can impair physical performance. Always drink before, during, and after prolonged exercise, especially in the heat.
It can be. You can lose excessive amounts of water and sodium during severe and prolonged sweating. When you don’t replace the water lost through perspiration, blood volume drops, you sweat less, and body temperature rises. This can lead to heat cramps, heat exhaustion, or heat stroke (a life-threatening condition). Sports drinks or salty snacks can provide some sodium during endurance exercise. In the long term your normal diet will replace the minerals lost in sweat.
It depends. As you become physically fit, your core temperature will rise more slowly, your heart will be able to beat more slowly at a given workload, and you’ll sweat more efficiently. For instance, you’ll start to sweat at a lower core body temperature and sweat more—assuming you’re fully hydrated when you start. Thus you stay cooler and can exercise more in the heat. Your sweat is more diluted, containing less sodium and other minerals. Just as exercise increases efficiency and size of muscles, it boosts the efficiency of the heat-regulation system and even the size of sweat glands.
On the other hand, sedentary people usually sweat a lot because they deal poorly with the heat—their core temperature rises faster, the heart beats faster, and they sweat more at a given workload than well-conditioned people, and thus they become more fatigued.
Yes, especially when you exercise regularly in hot weather. Changes in your cardiovascular and nervous systems (such as a lower heart rate and increased blood flow to the skin) help you acclimatize to the heat. Your sweat glands become bigger. Most of the adaptation occurs during the first five days, but full acclimatization takes about two weeks. The fitter you are and the more you exercise in the heat, the faster you’ll be able to adapt. After a few days back at normal temperatures, you’ll lose most of these adaptive changes.
Because their cardiovascular system is usually less fit. Thus they tend to pump less blood to the skin (but with a greater strain on the heart), sweat less, and thus are less able to dissipate internal heat. In addition, older people can more easily become dehydrated because they tend not to feel as thirsty as their younger counterparts, so they should make a special effort to drink plenty of fluids. Well-trained, healthy older athletes, however, can usually cope well with the heat.