Your sweat glands run on a different nerve signal from the rest of the system
At full output an adult can sweat 2 to 4 litres an hour. The glands doing it are wired unusually: most sympathetic nerves release norepinephrine, but the ones driving sweat glands release acetylcholine. And the smell people blame on sweat actually comes from skin bacteria breaking down an odourless secretion.
Humans have two kinds of sweat gland. Eccrine glands cover most of the body and produce the watery, slightly salty fluid that cools us. Apocrine glands are confined to the armpits and a few other areas, releasing an oily, opaque liquid with no odour until bacteria get to work on it. Cooling depends on evaporation: as the most energetic water molecules leave the skin they carry heat away, chilling surface blood vessels, and that cooled blood flows back to the core.
Control sits in the hypothalamus, in thermosensitive neurons in its preoptic and anterior regions, with extra input from temperature sensors in the skin. A rise in core temperature triggers far more sweating than the same rise in average skin temperature. Emotional sweating follows a different map from heat sweating: it tends to stay on the palms, soles, armpits and sometimes the forehead, while heat-driven sweat appears all over.
Sweat is mostly water, with 0.2 to 1 percent dissolved material and no protein. Daily losses vary enormously, from about 100 to 8,000 millilitres. Moving from a cold to a hot climate triggers acclimatisation, raising the maximum sweat rate while making the fluid less salty, so sodium concentration ranges from roughly 30 to 65 millimoles per litre depending on how adapted someone is.
Few mammals sweat to cool off; humans, horses, some primates and some bovids are exceptions, while dogs rely on panting. Horses face a problem because their thick waterproof coat would trap sweat, so their apocrine sweat carries latherin, a detergent-like protein that wets the hair and produces the familiar lather. In hot conditions a horse doing three hours of moderate work can shed 30 to 35 litres of water. History offers a darker note: nineteenth-century doctors dosed patients with mercury to make them sweat, then mistook the resulting poisoning for the original illness.
Source: Perspiration
