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The two kinds of sweat glands Schiefferdecker found in 1922, eccrine and apocrine, are enormously different, in origin, in purpose, in quantity and in quality. Let's begin with the eccrine glands, because they are far and away the more numerous, the more important and the nicer of the two. (As we shall soon see, the apocrine glands may have played a fascinating role in prehistoric human life, and they play an oddly important commercial role today because, interacting with skin bacteria, they're the major producer of the stink that keeps the deodorant industry going. But no one can find that the apocrine glands now perform any remotely useful function in the body's cooling system—or any other system, for that matter.)
There are eccrine glands on every inch of human skin, from two to five million of them, except for the lips and parts of the genitals. A German anatomist named Karl Friedrich Theodor Krause was the first to actually count them. He established in the mid-19th century that there were precisely 2,381,248 sweat glands on the human body, and if they were linked together into a single super-gland, they would occupy eight square inches of space. Though his exactitude was admired, Krause's census was later found to be, at best, approximate; the total varies considerably from individual to individual. Eccrine glands are present from infancy in the same number they will be in maturity. Thus, the density of the glands in the skin of a baby is eight to 10 times that of a full-grown person, and a baby's sweating system is almost as efficient in its first year as it will be in adulthood.
The eccrine glands are so essential to human life that without them most of us would be unable to tolerate temperatures higher than 80°F. Given any amount of physical exertion, a sweatless body could bake itself to death with heat stroke at much cooler temperatures.
Gil Gleim, chief physiologist at the Institute for Sports Medicine and Athletic Trauma at New York's Lenox Hill Hospital, explains the body's cooling system this way: "There's an area in the central brain, the hypothalamus, that senses the increase in the temperature of the blood caused by a change in thermal conditions. This part of the brain then sets up an excitation of the autonomic nervous system which controls involuntary functions such as blood flow, heart rate, the digestive system and all sweat glands. The brain's reaction causes sweat to be secreted from the eccrine glands. The body surface gets rid of heat in three ways—by radiating it off, by conducting it away through touching something cooler, or by convection when air currents move over the body and cool it. Sweating removes heat from the body when it evaporates due to a flow of air over the skin."
The principle of physics involved in sweat's acting as a body coolant is the same one that comes into play when water turns to steam. As James Watt could have told you, heat is needed to cause the transformation of a liquid into a gas in a teakettle, and the same process occurs in a human when body heat makes sweat turn to vapor on the skin. As Kuno wrote: "One of the most important properties of water, insofar as it pertains to biological systems, is its high latent heat of vaporization. This property is fully utilized in perspiration, as it controls the body temperature by withdrawing heat from the body by vaporization of water."
For every gram of water evaporated, .58 of a Calorie of body heat is used up. In this case, we are talking about a Calorie with a capital "C," or a kilocalorie, which is equivalent to 1,000 small "c" calories. A calorie is the amount of heat needed to raise the temperature of one gram of water 1° Centigrade. (When dieters talk of calories, they mean the large "C" kind.) A liter of sweat will dissipate about 600 Calories' worth of heat from the body during exercise, the equivalent of a temperature reduction of 50°F. in a person weighing 165 pounds. Strenuous exercise (such as running or cycling) in hot weather, can result in water losses of as much as two or three liters in an hour. One can even train to sweat better.
According to Dr. David Costill, Director of the Human Performance Laboratory at Ball State University, an athlete can tolerate a maximum water loss of 8½% to 10% of body weight. In a man weighing 145 pounds, that equals 14 pounds or about a gallon and a half. "However, that would be extremely severe," says Costill. "Anytime you dehydrate more than two percent of body weight it begins to affect your pulse rate, rectal temperature and your central nervous system. We measured the actual sweat losses of runners at the U.S. Olympic Trials in Alamosa, Colo. in 1968 and of Amby Burfoot when he won the Boston Marathon that year, and the most that I've seen lost, even in a marathon, is five percent to six percent or about three to five liters." Former Dodger Pitcher Don Newcombe, a noted sweater who had—and has—a far beefier build than any marathoner, claims that on one memorably hot summer afternoon in Montreal, while pitching in a minor league game, he sweated so much he dropped from 220 to 200 pounds. "It took three days of drinking beer to get the weight back," says Newcombe.
Gleim says that athletes are able to produce more sweat—and do it more efficiently—than average people because they ultimately "recruit" more sweat glands to work in the course of repeated training routines. "The same is true of people in hot climates," says Gleim. "They don't have more sweat glands; they simply have more of them working. Sweating can be an adaptive process in this sense."
Similarly, saunas and steam baths cause people to lose enormous amounts of sweat in a short time, but as with strenuous exercise, prolonged stays in such baths require acclimation. And one must beware of overheating; if there is no airflow in the bath, perspiration won't evaporate and the body won't cool efficiently. Despite widespread belief to the contrary, a sauna doesn't offer any real benefits to an athlete, with the possible exception of a jockey, wrestler or boxer trying to make weight. It may, in fact, be harmful. As Costill points out, "It's important to bear in mind that dehydration affects endurance. If you dehydrate in a sauna or steam bath, you decrease your body weight by three to four percent. Studies indicate that this means you also decrease performance in a 5,000- or 10,000-meter race by six to seven percent." Furthermore, says Costill, a sauna can't prepare an athlete for hot-weather competition. "The only way to acclimatize yourself to heat is to do the work under conditions of heat," he says. "You can't train in a sauna or steam bath. To become heat-acclimated, a runner must run in that heat for 90 minutes a day for eight days in a row at a fairly high speed."
Though most people think strenuous exercise is an automatic way to work up a sweat, that may not always be the case. Take swimming. Although tests made during distance events held in very warm water—in the Nile and the Amazon, for instance—indicated that endurance swimmers sweat while swimming, test results in cooler water are ambiguous. No one has been able to establish for sure if a swimmer sweats while performing in cool water. But whatever the water temperature, "the demand for cooling from sweating isn't as great while swimming as in dry-land exercise," Gleim says.