The process of sweating is also called perspiration or diaphoresis . Excessive, pathological secretion of sweat is referred to as hyperhidrosis , while reduced or completely absent sweat production is called hypo- or anhidrosis .
Composition and properties of human sweat
According to the type of their secretion, two types of sweat glands are distinguished: so-called eccrine and apocrine sweat glands.
In humans, the eccrine sweat glands are practically distributed over the entire body and can produce considerable quantities of a clear, odorless secretion that is more than 99 percent water. In addition, sweat contains salts, which in turn consist of electrolytes such as Na + , Cl - , K + ; next to it still Lactate , amino , caproic , caprylic , citric , acetic and propionic acid as well as traces of urea and uric acid . In addition, sugar and ascorbic acid are found in low concentrations in sweat . The pH value is in the acidic range at pH 4.5.
In contrast to this, the apocrine sweat glands only occur in the hairy areas of the body of the armpit and genital region as well as on the (hairless) nipples . They produce small amounts of a milky secretion that contains proteins and lipids and is approximately pH neutral (pH 7.2) (see also vertebrate pheromones ).
Fresh sweat is completely odorless. It is only when long-chain fatty acids are broken down into shorter chains such as butyric acid or formic acid that the typical sweat odor is created. Various bacteria that are part of the natural skin flora are responsible for this. Puberty is an exception : through various hormonal processes in the body, fresh sweat can already smell.
Functions of sweat
Sweating - medically as diaphoresis (. V . Greek διαφέρειν "through wear") or transpiration denotes - is an effective mechanism to excess heat deliver and thus the body temperature to regulate : the heat of evaporation of water during the transition to water vapor is 2400 kJ / liter . The high-volume aqueous secretion of the eccrine glands, which in adults can produce up to 2–4 liters per hour or 10–14 liters per day (10–15 g / min • m²), is used for this. This means that the evaporation of this amount of sweat causes a heat release of 333 W / m² body surface (KOF) - dripping sweat is shed in vain with regard to heat regulation. Without vigorous physical activity or a hot environment, people lose around 100–200 ml of sweat per day.
However, sweat can only evaporate if the water vapor pressure in the air is lower than that on the surface of the skin. The difference in water vapor partial pressures of 1 kPa causes a heat emission of 58 W / m² KOF when there is no wind. The more wind blows, the more heat can be dissipated. The heat given off by sweating is independent of the outside temperature.
Perspiratio insensibilis is a form of water excretion that is imperceptible. It consists of the moisture in the exhaled air and the imperceptible evaporation of water through the skin (diffusion through the skin without involving the sweat glands). This results in a daily loss of 400 to 1000 ml of water and at the same time a heat release that corresponds to approx. 20% of the body heat produced daily at rest. While the loss of fluid through the exhaled air is an unavoidable physical phenomenon, the imperceptible leakage of water serves to hydrate the skin and produce the protective acid mantle .
Profuse sweating with cold skin, so-called cold sweat , is often found in seriously ill patients ( heart attack , pulmonary edema ). Here, sweating does not serve to regulate temperature, but is an accompanying phenomenon.
Sweat also contains sexual fragrances ( pheromones ), so that sweat is also important in reproduction and sexual arousal of the sexual partner. This has been well researched in animals, and pheromones are used in animal breeding to control receptivity (see also Jacobson's organ or vomeronasal organ ).
Its importance in humans is controversial, since the vomeronasal organ , which is used to perceive these odorous substances, regresses to a rudiment during the embryonic period . Nevertheless, there is a great deal of scientific evidence that people's behavior is influenced by components of the odor of sweat, particularly the secretion of the apocrine sweat glands. In humans, this is particularly important in connection with emotional or stress-related sweating.
In contrast to thermoregulatory sweating (= sweating for the purpose of cooling), stress sweating occurs suddenly. This also arises regardless of the ambient temperature, e.g. B. in stressful situations in the office, on "dates" or similar situations. Stress sweat is also often referred to as "cold sweat". The hormones adrenaline and noradrenaline released in the stress reaction constrict the blood vessels in the skin and thus ensure that the blood is redistributed in favor of the muscles. The reduced blood flow to the skin lowers its temperature, and the evaporation of sweat leads to further cooling. In contrast, with thermoregulatory sweating, e.g. B. during physical exertion, the skin blood flow increased in order to dissipate as much heat as possible over the body surface.
Emotional sweating in the armpit involves both eccrine and so-called apocrine sweat glands . Apocrine sweat glands are only found in connection with hair in the armpit and genital region and in the area of the nipple. They play a special role in axillary emotional sweating. Apocrine glands produce a secretion that contains lipids and proteins, as well as substances that skin bacteria can metabolize into volatile molecules . This is then perceived as a typically sweaty smell.
It is believed that such apocrine smells play a role in non-verbal communication . The additional water released by the eccrine sweat glands in the armpit promotes the distribution of apocrine sweat on the skin and on the hair. This enlarges the wetted surface and increases the release of odorous substances.
When stressed, the axillary eccrine and apocrine sweat glands are stimulated to secrete by the autonomic (i.e. the nervous system that cannot be controlled voluntarily) and by stress hormones in the blood ( adrenaline ). The nerve impulse reaches the glands within seconds of perceiving a threat and immediately induces a strong sweat. Significantly larger amounts of sweat are released in the first few minutes (up to 70 mg / min per armpit) than is the case with the rather slow onset of thermal sweating.
In rare cases, the pigment lipofuscin can be deposited in the sweat glands, causing a brightly colored sweat ( chromohidrosis ). This disruption of normal sweat production can be treated with creams containing capsaicin or with botulinum toxin treatment.
Sweating in the sauna
In the sauna , the events described can best be observed: Typically set in the sauna at an ambient temperature of about 90 ° C. The relative humidity is low, but the water vapor pressure on the sweat-covered skin is still far below that of the surroundings, so that the sweat that is formed cannot evaporate, but rather drips off (thermoregulatory ineffective). Since under these conditions all possibilities of heat dissipation are as good as impossible - the heat regulation via convection is excluded due to the high ambient temperature, and the absorbed heat radiation is greater than the radiation-related heat emission - the body core temperature rises relatively quickly.
Because the blood circulation in the skin is intensified, the cardiac output doubles and the pulse rises accordingly. If the recommended duration of a sauna session is not exceeded, the loss of fluid is still limited, provided that it is followed by immediate cooling. An intact cardiovascular system is a prerequisite for this.
However, according to a study, regular sauna visits without infusion and at 60 ° C can improve the symptoms of patients with heart failure .
If an infusion is made, this leads to an additional supply of heat by means of water vapor that condenses on the skin. A scientific study has shown that the water that drips off the body of the sauna visitor after an infusion - depending on the test conditions - consists of 14% to 67% of water that has condensed on the body of the sauna visitor and thus only 33% to 86% of the dripping water actually comes from the sweat of the sauna visitor.
Sweating in animals
Primates , especially humans, as well as horses , hornbears and camels have a particularly large number of sweat glands and sweat a lot. In predators , the distribution of the glands is limited to a few areas of the body, especially the balls of the feet. Pigs and rodents do not have functional sweat glands. These animal species use other cooling options, e.g. B. panting , rolling in or pressing against (damp) ground, licking the fur.
Sweat as an evolutionary advantage
The thermoregulation through sweat brought the early people over the Hominisation a clear advantage when hunting. In contrast to many of its prey animals, Homo erectus probably already had many times more sweat glands and was thus able to hunt endurance . While potential prey (e.g. antelopes ) suffered from exhaustion relatively quickly, as a hunter he could easily cover longer distances.
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