your sweat glands -- 7/27/16

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Today's selection -- from Grunt by Mary Roach. You have 2.4 million sweat glands:

"Like any complex bioelectrochemical system, the human body works best when its vital components are humming along in a set temperature range. For humans, that's roughly ninety-seven and a half to ninety-nine and a half degrees Fahrenheit. When your core temperature begins to rise, either because it's hot where you are or you're toiling hard, or both, the body takes measures to bring it back to the happy range. First and foremost, it sweats. ...

"Sweat isn't cool. It's warm as blood. It essentially is blood. Sweat comes from plasma, the watery, colorless portion of blood. (A dip in the lake cools by conduction: contact with some­thing colder. Highly effective but not always practical.) Sweat cools by evaporation: offloading your heat into the air. Like this: When you start to overheat, vessels in your skin dilate, encouraging blood to migrate there. From the capillaries of the skin, the hot plasma is offloaded through sweat glands -- 2.4 million or so -- onto the sur­face of the body to evaporate. Evaporation carries heat away from the body, in the form of water vapor.

Replica of human skin with perspiration, vapour entrapments

"It is an efficient system. A human in extreme heat can sweat as much as two kilograms an hour, over a span of a few hours. 'Roughly speaking, 10 kilograms loss of sweat [over the course of a day] is not rare for workers in overheated factories and active soldiers sta­tioned in the tropics,' states the late Yas Kuno, longtime professor of physiology at Nagoya University School of Medicine, in the 1956 edition of Human Perspiration. 'One will be struck with wonder ... when he thinks that such a large amount of sweat is produced from glands which are extremely small in size.' Though humans have, by weight, more than twice as much salivary gland tissue as sweat gland tissue, they are capable of producing six times as much sweat as spit. ...

"If sweating is so effective, why were there 14,577 cases of heat illness among active US Armed Forces personnel between 2007 and 2011? Because they work too damn hard. When sweaters exert them­selves, the muscles they're using begin to demand the blood that the body needs to use for sweating. The mildest consequences of this competition for blood are heat exhaustion and heat syncope-fainting. With blood flowing out to the skin for cooling purposes and, at the same time, into the muscles to deliver oxygen to fuel the body's toil, it becomes harder to maintain the blood pressure needed to pump blood up to the brain. Without enough oxygen-carrying blood reach­ing your brain, you pass out. (Counterintuitively, overheated people sometimes pass out not in the midst of their exertions but when they stop and stand still; this is because contracting the leg muscles helps keep blood from pooling down there.)

"Heat exhaustion is embarrassing but not particularly dangerous. Fainting is both symptom and cure. Once you're horizontal on the ground, the blood flows back into your head and you come to. Someone brings you water and escorts you to the shade and you're fine.

"Heatstroke, however, can kill. Here too, it begins with a competition for blood. On a hot day, when your body is trying to sweat your core temperature down to the safe range and you haven't been drinking enough water to replenish your blood volume, and on top of that you're exercising hard and your muscles are clamoring for oxygen -- and the exercise itself is generating heat -- something has to give. 'The body sacrifices flow to the gut in order to put it where it's needed,' explains Sam Cheuvront, a research physiologist at the US Army Research Institute of Environmental Medicine (USARIEM), part of the Natick Labs complex. The splanchnic organs -- a stu­pendously ugly way to say viscera -- are cut off from the things they need: oxygen, glucose, toxic waste pickup. The technical term is ischemia. It is a killer. The digestive organs start to fail. The gasping gut may begin to leak bacteria into the blood. A systemic inflam­matory response sets in, and multi-organ damage ensues. Delirium, sometimes coma, even death, may follow."