Perspiration

"Sweat" redirects here. For other uses, see Sweat (disambiguation).

"Perspire" redirects here. For the song by Gigolo Aunts, see Tales from the Vinegar Side.

Sweating
Sweating after exercise.
Classification and external resources
Specialty	Dermatology
ICD-10	R61
ICD-9-CM	780.8
MedlinePlus	003218
Patient UK	Perspiration
MeSH	D013546

Perspiration, also known as sweating 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 (exceptions include humans and horses) produce large amounts of sweat in order to cool down.

Definitions

• Perspiration in humans and other animals is sometimes called transpiration, although that word is usually used in reference to plants.
• The words diaphoresis and hidrosis both can mean either perspiration (in which sense they are synonymous with sweating) 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.
• Focal hyperhidrosis is increased or excessive sweating in certain regions such as the underarms, 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).
• Hidromeiosis is a reduction in sweating that is due to blockages of sweat glands in humid conditions.

Signs and symptoms

The most commonly experienced symptom of excessive sweating is body odor. Odor develops due to yeast or bacteria that live naturally on the skin. As the skin becomes moist and mixes with these, odor is released. Another explanation is when the apocrine gland release sweat directly into the tubule of the glands. When placed under stress, these tubules contract and sweat is pushed to the surface of the skin. Bacteria will begin to break down and odor is released. Medications that are used for other treatments and diet will also affect odor. Medical conditions like kidney failure and diabetic ketoacidosis will have the same effect. Areas that produce excessive sweat usually appear pink or white, but, in severe cases, may appear cracked, scaly, and soft.

Causes

Woman perspiring

Diaphoresis is a non-specific symptom or sign, which means that it has many possible causes.

Physiological causes

Normal physical causes of diaphoresis include physical exertion, menopause, fever, spicy foods, and high environmental temperature. Strong emotions (anger, fear, anxiety etc.) and remembrance of past trauma can also trigger profuse sweating.

The vast majority of sweat glands in the body are innervated by sympathetic "cholinergic" neurons. Sympathetic postganglionic neurons usually secrete norepinephrine and are named sympathetic adrenergic neurons. However, when sympathetic postganglionic neurons innervate sweat glands they secrete acetylcholine and hence are termed sympathetic "cholinergic" neurons. Sweat glands, piloerector muscles and some blood vessels are innervated by sympathetic cholinergic neurons.

Pathological causes

Diaphoresis may be associated with some abnormal conditions, such as hyperthyroidism and shock. If it is accompanied by unexplained weight loss or fever or by palpitations, shortness of breath, or chest discomfort, it suggests serious illness.

Diaphoresis is also seen in an acute myocardial infarction (heart attack), from the increased firing of the sympathetic nervous system, and is frequent in serotonin syndrome. Diaphoresis can also be caused by many types of infections, often accompanied by fever and/or chills. Most infections can cause some degree of diaphoresis and it is a very common symptom in some serious infections such as malaria and tuberculosis. In addition, pneumothorax can cause diaphoresis with splinting of the chest wall. Neuroleptic malignant syndrome and other malignant diseases (e.g. leukemias) can also cause diaphoresis.

Diabetics relying on insulin shots or oral medications may have low blood sugar (hypoglycemia), which can also cause diaphoresis.

Drugs (including caffeine, morphine, alcohol, antidepressants and certain antipsychotics) may be causes, as well as withdrawal from alcohol, benzodiazepines, nonbenzodiazepines or narcotic painkiller dependencies. Sympathetic nervous system stimulants such as cocaine and amphetamines have also been associated with diaphoresis. Diaphoresis due to ectopic catecholamine is a classic symptom of a pheochromocytoma, a rare tumor of the adrenal gland. Acetylcholinesterase inhibitors (e.g. some insecticides) also cause contraction of sweat gland smooth muscle leading to diaphoresis.

Mercury is well known for its use as a diaphoretic, and was widely used in the 19th and early 20th century by physicians to "purge" the body of an illness. However, due to the high toxicity of mercury, secondary symptoms would manifest, which were erroneously attributed to the former disease that was being treated with mercurials.

Infantile acrodynia (childhood mercury poisoning) is characterized by excessive perspiration. A clinician should immediately consider acrodynia in an afebrile child who is sweating profusely.

Hyperhidrosis

Further information: Hyperhidrosis

In some people, the body's mechanism for cooling itself is overactive, so overactive that they may sweat four or five times more than is necessary, or normal. Millions of people are affected by this condition, but more than half never receive treatment due to embarrassment or lack of awareness. While it most commonly affects the armpits, feet, and hands, it is possible for someone to experience this condition over their whole body. The face is another common area for hyperhidrosis to be an issue. Sweating uncontrollably is not always expected and may be embarrassing to sufferers of the condition. It can cause both physiological and emotional problems in patients. It is generally an inherited problem that is found in each ethnic group. It is not life-threatening, but it is threatening to a person's quality of life. A treatment modality is clipping of the sympathic nerve at the level of T4 by thoracoscopic means.

Night sweats

Further information: Sleep hyperhidrosis

Mechanism

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 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.

Composition

Sweat stain on a white cotton T-shirt

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), and 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), and 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 concentrated). Sweat typically is found at moderately acidic to neutral pH levels, typically between 4.5 and 7.0.

A microfluidic model of the eccrine sweat gland was published, which provides details on what solutes partition into sweat, their mechanisms of partitioning, and their fluidic transport to the skin surface.

Fabrics and food

Some fabrics, as polyester and nylon, conserve the heat and create more perspiration, as contrasted with, for example, cotton, linen and silk.

Caffeine, alcohol, onions, garlic and capsaicin are sweat triggers in humans.

Artificial perspiration

Artificial skin capable of sweating similar to natural sweat rates and with the surface texture and wetting properties of regular skin has been developed for research purposes.

See also

• Apocrine
• Body odor
• Deodorant
• Hidradenitis suppurativa
• Hyperhydrosis
• Hyperthermia
• Hyponatremia
• Pheromones
• Sweat Diagnostics
• Sauna suit
• Sweat therapy