Perspiration

Perspiration, also known as sweat, is the fluid secreted by sweat glands in the skin of mammals.^[1]

This article about biology may be excessively human-centric.

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Perspiration
Other names	Sweating, hidrosis, diaphoresis
Droplets of perspiration on the skin
Specialty	Dermatology
Symptoms	Body odor
Complications	Dehydration
Causes	Exercise Fever Heat Hyperthermia Hot flash
Prevention	Drinking water
Treatment	Antiperspirant

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Two types of sweat glands can be found in humans: eccrine glands and apocrine glands.^[2] The eccrine sweat glands are distributed over much of the body and are responsible for secreting the watery, brackish sweat most often triggered by excessive body temperature. Apocrine sweat glands are restricted to the armpits and a few other areas of the body and produce an odorless, oily, opaque secretion which then gains its characteristic odor from bacterial decomposition.

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 litres (0.5–1 US gal) per hour or 10–14 litres (2.5–3.5 US gal) per day, but is less in children prior to puberty.^[3][4][5] 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.

Although sweating is found in a wide variety of mammals,^[6][7] relatively few (apart from humans, horses, some primates and some bovidae) produce sweat in order to cool down.^[8] In horses, such cooling sweat is created by apocrine glands^[9] and contains a wetting agent, the protein latherin which transfers from the skin to the surface of their coats.^[10]

• The words diaphoresis and hidrosis can both mean either perspiration (in which sense they are synonymous with sweating)^[11][12] or excessive perspiration (in which sense they can be either synonymous with hyperhidrosis or differentiable from it only by clinical criteria involved in narrow specialist senses of the words).
• Hypohidrosis is decreased sweating from whatever cause.^[13]
• Focal hyperhidrosis is increased or excessive sweating in certain regions such as the underarm, palms, soles, face, or groin.
• Hyperhidrosis is excessive sweating, usually secondary to an underlying condition (in which case it is called secondary hyperhidrosis) and usually involving the body as a whole (in which case it is called generalized hyperhidrosis).^[13]
• Hidromeiosis is a reduction in sweating that is due to blockages of sweat glands in humid conditions.^[14]
• A substance or medicine that causes perspiration is a sudorific or sudatory.

Sweat contributes to body odor when it is metabolized by bacteria on the skin. Medications that are used for other treatments and diet also affect odor. Some medical conditions, such as kidney failure and diabetic ketoacidosis, can also affect sweat odor.^{[citation needed]}

This section needs additional citations for verification. (March 2016)

Diaphoresis is a non-specific symptom or sign, which means that it has many possible causes. Some causes of diaphoresis include physical exertion, menopause, fever, ingestion of toxins or irritants, and high environmental temperature. Strong emotions (anger, fear, anxiety) and recall of past trauma can also trigger sweating. This is sometimes referred to as flop sweat.^[15]

The vast majority of sweat glands in the body are innervated by sympathetic cholinergic neurons.^[16] Sympathetic postganglionic neurons typically secrete norepinephrine and are named sympathetic adrenergic neurons; however, the sympathetic postganglionic neurons that innervate sweat glands secrete acetylcholine and hence are termed sympathetic cholinergic neurons. Sweat glands, piloerector muscles, and some blood vessels are innervated by sympathetic cholinergic neurons.

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.^{[citation needed]}

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.^{[citation needed]}

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.^[22]

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 sex, genetics, environmental conditions, age and 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 more readily. 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.^[23]

Human sweat is not pure water; though it contains no protein, 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 acclimatization: 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 millilitres per day (0.041 to 3.259 imp fl oz/ks). The solute loss can be as much as 350 mmol/d (or 90 mmol/d acclimatised) of sodium under the most extreme conditions. During average intensity exercise, sweat losses can average up to 2 litres (0.44 imp gal; 0.53 US gal) of water/hour. In a cool climate and in the absence of exercise, sodium loss can be very low (less than 5 mmol/d). Sodium concentration in sweat is 30–65 mmol/L, depending on the degree of acclimatisation.^{[citation needed]}

Horses have a thick, waterproofed, hairy coat that would normally block the rapid translocation of sweat water from the skin to the surface of the hair required for evaporative cooling. To solve this, horses have evolved a detergent-like protein, latherin, that they release at high concentrations in their sweat.^[10] Their perspiration unlike humans is created by apocrine glands.^[9] This protein, by wetting the horses' coat hairs facilitate water flow for cooling evaporation. The presence of this protein can be seen in the lathering that often occurs on the coats of sweating horses, especially when rubbed.^[10] In hot conditions, horses during three hours of moderate-intensity exercise can lose 30 to 35 litres (6.6 to 7.7 imp gal; 7.9 to 9.2 US gal) of water and 100 grams (3.5 oz) of sodium, 198 grams (7.0 oz) of chloride and 45 grams (1.6 oz) of potassium.^[9]

Sweat is mostly water. A microfluidic model of the eccrine sweat gland provides details on what solutes partition into sweat, their mechanisms of partitioning, and their fluidic transport to the skin surface.^[24] Dissolved in the water are trace amounts of minerals, lactic acid, and urea. Although the mineral content varies, some measured concentrations are: sodium (0.9 gram/litre), potassium (0.2 g/L), calcium (0.015 g/L), and magnesium (0.0013 g/L).^[25]

Relative to the plasma and extracellular fluid, the concentration of Na⁺ ions is much lower in sweat (~40 mM in sweat versus ~150 mM in plasma and extracellular fluid). Initially, within eccrine glands sweat has a high concentration of Na⁺ ions. In the sweat ducts, the Na⁺ ions are re-absorbed into tissue by epithelial sodium channels (ENaC) that are located on the apical membrane of epithelial cells that form the duct (see Fig. 9 of the reference).^[2]

Many other trace elements are also excreted in sweat, again an indication of their concentration is (although measurements can vary fifteenfold) zinc (0.4 milligrams/litre), copper (0.3–0.8 mg/L), iron (1 mg/L), chromium (0.1 mg/L), nickel (0.05 mg/L), and lead (0.05 mg/L).^[26][27] 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.^[28] In humans, sweat is hypoosmotic relative to plasma^[29] (i.e. less concentrated). Sweat is found at moderately acidic to neutral pH levels, typically between 4.5 and 7.0.^[30]

Sweat contains many glycoproteins.^[31]