sweating

sweating Sweat is secreted by glands in the skin when it is necessary to lose excess heat from the body. They are stimulated to do so by sympathetic nerves of the autonomic nervous system when sensors in the hypothalamus are activated by blood reaching it at a higher than normal temperature. There are also other contexts in which sweating occurs which have a less obvious physiological role — the ‘cold sweat’ of fear, of haemorrhage, and of shock.

There are sweat glands all over the skin, but more densely distributed in some sites than others, and there are also two types, according to the way in which their cells produce secretions.

The secretion of the apocrine sweat glands includes cast-off parts of the sweat-generating cells themselves (the secretion of milk by the glandular tissue of the breasts is similarly apocrine). They are to be found in relatively few places (armpit, groin) and these are the ones responsible for the characteristic odour which nowadays in Western society triggers the application of deodorants rather attracting the attention of the nose — thus perhaps dispensing with a more primitive role in sexual attraction.

The epicrine sweat glands produce secretions by the more general method of extruding liquid from the glandular cells in their depths into the spiral ducts which discharge on the skin surface. They are present over the whole body surface, though more concentrated in some areas, such as the palms of the hands and the soles of the feet. They have an essential function in temperature regulation.

Any moisture on the surface of the skin will evaporate unless the environment is highly humid, and by the laws of physics, evaporation causes cooling. Cooling of the skin in turn causes cooling of the venous blood flowing away from it, and hence cooling of the whole circulating blood. Since body tissues are moist and the skin is not entirely waterproof, some evaporation inevitably takes place all the time, without active stimulation of sweating; this is ‘insensible perspiration’. In contrast to colloquial usage therefore, perspiring is not the same as sweating. Sweat provides additional water for evaporation — and to be effective as a body cooler, sweat must not be wiped away, since that defeats the purpose.

Sweat is not just water. It is well known that sweat, or the residue on the skin after evaporation, tastes salty. It is formed by movement of water and inorganic solutes out of the blood plasma into the cells of the sweat gland and thence out into its duct. Since sodium chloride is the main salt in the plasma, it is a major constituent of sweat. The ducts of the sweat glands are not just passive conduits: they reabsorb some of the salt from the fluid on its way to the surface, thus conserving it for the body fluids, although as the flow of sweat increases, a smaller proportion of the salt is reabsorbed. During work or exercise in a hot environment, loss of water could reach between 1 and 2 litres in an hour. This comes initially from the body's extracellular fluid volume (blood and interstitial fluid).

If too much fluid is lost blood volume becomes depleted, with potentially harmful consequences for cardiac output and blood pressure. Proportionally more water is lost than salt, so the body fluids also tend to become over-concentrated. Unless enough water is drunk to correct this, water moves by osmosis out of cells, causing generalized dehydration of the tissues. Thirst promotes intake of replacement fluid and there is also a physiological mechanism which helps to conserve water. When body fluids become concentrated, more antidiuretic hormone is released from the posterior lobe of the pituitary gland, and acts in the kidneys to promote retention of water.

Acclimatization has important effects on sweating. Going to a hot climate and indulging in heavy work or exercise will lead over a period of weeks to an increased maximal sweating rate, assisting temperature control, and accompanied by an increase in the reabsorption of salt. This latter may seem unhelpful, since it tends to exaggerate the disporportionate loss of water relative to salt, and hence the concentration of the extracellular fluid and generalized dehydration. But it enhances thirst, and all is well if water is available. The conservation of salt can be important. Although in a modern Western diet salt is plentiful — sometimes harmfully superfluous — its normal concentration in the body fluids is essential to the maintenance of a normal blood volume, and hence of an effective circulation. In man the hunter, sweating in a hot climate, in regions where salt is scanty in the diet, it is a precious commodity, and the physiological mechanism evolved to conserve it can cut its loss in urine and in sweat to virtually nil. When sweating starts to deplete blood volume, the adrenal glands receive signals to increase the output of the hormone aldosterone; this acts both on kidney tubule cells and on sweat duct cells, causing them to retrieve more salt from the escaping fluid.

Sheila Jennett

Bibliography

Bursztyn, P. G. (1990) Physiology for sportspeople — a serious user's guide to the body. Manchester University Press. Manchester and New York.

See also adrenal glands; body fluids; skin; temperature regulation.