The facial sweat of a boy
Perspiration (sweating, transpiration, or diaphoresis) is the production of fluids secreted by the sweat glands in the skin of mammals. Two types of sweat glands can be found in humans: Eccrine glands and apocrine glands. The eccrine sweat glands are distributed over much of the body.
In humans, sweating is primarily a means of thermoregulation which is achieved by the water-rich secretion of the eccrine glands. Maximum sweat rates of an adult can be up to 2-4 liters per hour or 10-14 liters per day (10-15 g/min•m²), but is less in children prior to puberty. Evaporation of sweat from the skin surface has a cooling effect due to evaporative cooling. Hence, in hot weather, or when the individual's muscles heat up due to exertion, more sweat is produced. 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 like those of humans. Although sweating is found in a wide variety of mammals, relatively few, such as humans and horses, produce large amounts of sweat in order to cool down.
- Hypohidrosis is decreased sweating from whatever cause.
- Focal hyperhidrosis is increased or excessive sweating in certain regions such as the underarms, palms, soles, face or groin.
- Hyperhidrosis, also referred to as generalized or secondary hyperhidrosis is excessive sweating due to an underlying condition and usually involves the body as a whole.
- Hidromeiosis is a reduction in sweating that is due to blockages of sweat glands in humid conditions.
Sweating allows the body to regulate its temperature. Sweating is controlled from a center in the preoptic and anterior regions of the brain's hypothalamus, where thermosensitive neurons are located. The heat-regulatory function of the hypothalamus is also affected by inputs from temperature receptors in the skin. High skin temperature reduces the hypothalamic set point for sweating and increases the gain of the hypothalamic feedback system in response to variations in core temperature. Overall, however, the sweating response to a rise in hypothalamic ('core') temperature is much larger than the response to the same increase in average skin temperature.
Sweating causes a decrease in core temperature through evaporative cooling at the skin surface. As high energy molecules evaporate from the skin, releasing energy absorbed from the body, the skin and superficial vessels decrease in temperature. Cooled venous blood then returns to the body's core and counteracts rising core temperatures.
There are two situations in which the nerves will stimulate the sweat glands, causing perspiration: during physical heat and during emotional stress. In general, emotionally induced sweating is restricted to palms, soles, armpits, and sometimes the forehead, while physical heat-induced sweating occurs throughout the body.
People have an average of two to four million sweat glands. But how much sweat is released by each gland is determined by many factors, including gender, genetics, environmental conditions, age or your fitness level. Two of the major contributors to sweat rate are an individual's fitness level and weight. If an individual weighs more, sweat rate is likely to increase because the body must exert more energy to function and there is more body mass to cool down. On the other hand, a fit person will start sweating earlier and easier. As someone becomes fit, the body becomes more efficient at regulating the body's temperature and sweat glands adapt along with the body's other systems.
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 the sweating mechanisms of the person. 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. During average intensity exercise, sweat losses can average up to 2 litres of water/hour. In a cool climate and 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.
Sweat contains mainly water. It also contains minerals, lactate, and urea. Mineral composition varies with the individual, their acclimatisation to heat, exercise and sweating, the particular stress source (sauna, etc.), the duration of sweating, and the composition of minerals in the body. An indication of the minerals content is sodium (0.9 gram/liter), potassium (0.2 g/l), calcium (0.015 g/l), magnesium (0.0013 g/l). Also many other trace elements are excreted in sweat, again an indication of their concentration is (although measurements can vary fifteenfold) zinc (0.4 milligrams/liter), copper (0.3–0.8 mg/l), iron (1 mg/l), chromium (0.1 mg/l), nickel (0.05 mg/l), lead (0.05 mg/l). Probably many other less-abundant trace minerals leave the body through sweating with correspondingly lower concentrations. Some exogenous organic compounds make their way into sweat as exemplified by an unidentified odiferous "maple syrup" scented compound in several of the species in the mushroom genus Lactarius. In humans, sweat is hypoosmotic relative to plasma  (i.e. less salty). Sweat typically is found at moderately acidic to neutral pH levels, typically between 4.5 and 7.0.
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