Fluid given off by the skin as vapour by simple evaporation or as sweat actively secreted from sweat glands to evaporate and cool the body. When the body temperature rises, the sympathetic nervous system stimulates eccrine sweat glands to secrete water to the skin surface, where it cools the body by evaporation. Human eccrine sweat is essentially a dilute solution of sodium chloride with trace amounts of other plasma electrolytes. In extreme conditions, human beings may excrete several litres of such sweat in an hour.
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Perspiration (also called sweating or sometimes transpiration) is the production of a fluid, consisting primarily of water as well as various dissolved solids (chiefly chlorides), that is excreted by the sweat glands in the skin of mammals. Sweat also contains the chemicals or odorants 2-methylphenol (o-cresol) and 4-methylphenol (p-cresol), as well as a small amount of urea.
In humans, sweating is primarily a means of thermoregulation, although it has been proposed that components of male sweat can act as pheromonal cues . Evaporation of sweat from the skin surface has a cooling effect due to the latent heat of evaporation of water. Hence, in hot weather, or when the individual's muscles heat up due to exertion, more sweat is produced. Sweating is increased by nervousness and nausea and decreased by cold. Animals with few sweat glands, such as dogs, accomplish similar temperature regulation results by panting, which evaporates water from the moist lining of the oral cavity and pharynx. Primates and horses have armpits that sweat similarly to those of humans.
There are two situations that our nerves will stimulate sweat glands making us sweat: during physical heat, and emotional heat. Emotionally induced sweating is generally restricted to palms, soles, and sometimes the forehead, while physical heat induced sweating occurs throughout the body.
Sweat is not pure water; it always contains a small amount (0.2 - 1%) of solute. When a person moves from a cold climate to a hot climate, adaptive changes occur in their sweating mechanisms. This process is referred to as acclimatisation: the maximum rate of sweating increases and its solute composition decreases. The volume of water lost in sweat daily is highly variable, ranging from 100 to 8,000 mL/day. The solute loss can be as much as 350 mmol/day (or 90 mmol/day acclimatised) of sodium under the most extreme conditions. In a cool climate & in the absence of exercise, sodium loss can be very low (less than 5 mmols/day). Sodium concentration in sweat is 30-65 mmol/l, depending on the degree of acclimatisation.