Tuesday, August 14, 2007

Sweat...

I have been sweating A LOT these days at the gym, even in places I never knew I could. Again, ironically, I stumbled across an article in the NY Times about sweating today...

...For sedentary pursuits in temperate weather, people have no need to sweat: excess metabolic heat easily moves from blood vessels at the surface of the skin into the surrounding air. Because the skin is not completely waterproof, some evaporation of water from skin cells adds a little extra cooling.

But when the body’s owner decides to exercise, the muscles generate too much heat for the air to absorb. The same thing happens when the temperature climbs into the 90s: the skin stops losing heat to the air and absorbs it instead. Then temperature-sensing nerves in the skin and the body’s interior tell the brain to unleash a flow of sweat for heavy-duty evaporation and cooling.

Humidity reduces evaporation and makes everyone sweatier. A breeze enhances evaporation and makes skin cooler (unless the air is so hot the body absorbs its heat instead). Dehydration markedly reduces sweat production. So does sunburn.

But individual sweat patterns still vary enormously. Age, sex, genes, weight and shape play a role, said Craig Crandall, a thermoregulation expert at University of Texas Southwestern Medical Center and Presbyterian Hospital, both in Dallas. So does nonexercise activity, and so, according to a pivotal set of sweat studies done during World War II, does clothing, although not in the way one might predict.

Some people have fewer than two million sweat glands; some have as many as four million. Heavy sweaters may have glands five times average size; their big glands are more sensitive to nerve stimuli and make more sweat.

Everyone’s inner temperature cycles around a slightly different genetically determined version of 98.6 set by the hypothalamus, the brain region that serves as thermostat. We run a little cooler in the morning, a little warmer in the late afternoon. Women run about half a degree higher after ovulation...

As for obesity, it is complicated, Dr. Crandall said. Fat may insulate the interior from very hot external temperatures, but it also may compromise heat transfer from interior to skin. Carrying more weight generates more metabolic heat to get rid of. That means more sweat, but research suggests that large people cannot grow more sweat glands to cope with the extra heat load. Radiation of heat from skin to air may become especially important in their heat control. (by: Abigail Zuger, NY Times)

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