The mechanism of sweating
Sweat glands distributed all over the body regulate its temperature by their secretion. Heat, however, is not the only effective stimulus: some of the glands, for example, respond only to emotional changes
by Dr. R. K. MACPHERSON
THE Oxford Dictionary of Quotations contains only eleven entries under the headings “sweat” and “sweating.” compared, for example, with nearly eleven hundred under “love” and its derivatives. From this we may conclude either that sweating is a phenomenon which attracts little attention in this country or that it is seldom talked about. Be this as it may, sweating is a matter of considerable physiological importance.
Homoiothermous or “warm-blooded” animals (birds and mammals) maintain a relatively constant body temperature mainly by the regulation of their heat loss. When the environmental temperature is high or the amount of heat produced within the body is large, as in strenuous exercise, the loss of an adequate amount of heat is ultimately dependent upon the evaporation of water from the surface of the body, either from the skin or from the respiratory tract.
Under conditions of thermal stress, birds and most mammals increase the rate of evaporation of water, and hence the loss of heat, by increasing their rate of breathing, that is, by panting. Man, however, does not pant in the heat: he increases his loss of water through the skin by sweating. Other mammals sweat too, but on the whole not efficiently. The cat sweats only on its pads, and the pig from its snout which makes nonsense of “sweating like a pig “—and even the horse is inferior to man in his ability to sweat. This sudorific superiority of man is so marked that it is considered by many to indicate that man was originally an inhabitant of the tropics.
A nude man, completely at rest, will begin to sweat at an air temperature of approximately 30C., but increasing the clothing or the rate of energy expenditure will progressively lower this limit. Sweating occurs even in Arctic environments if the rate of working is sufficiently high.
There is a small uncontrollable loss of water from the body by insensible perspiration, that is, the diffusion of water vapour through the skin—a simple drying out process—but true sweating is an active secretary process in which a definite substance, sweat, is produced by glands situated within the dermis or true skin.
Sweat is essentially a dilute aqueous solution of sodium chloride (common salt), some potassium salts, added to which there are small quantities of other substances—traces of glucose, about the same amount of urea as in plasma, and a variable amount of lactic acid. The concentration of salt in sweat varies with individuals and from time to time in the same individual. It increases after prolonged copious sweating, decreases with acclimatization to heat, and is affected by the amount of salt in the diet. Usually it ranges between 0.1 per cent, and 0.5 per cent. (Sweat is therefore the most dilute of all bodily secretions), although much lower values are found in well-acclimatized subjects on a low salt intake. Because sweat is more dilute than the plasma or extracellular fluid from which it is derived, the sweat glands must do work—the more dilute the sweat the more work per unit volume—so that the rise in salt concentration which follows prolonged copious sweating is probably the effect of fatigue.
The amount of sweat which can be secreted also varies considerably. In general, half a liter an hour can be considered heavy sweating, but some sudorilic athletes can produce up to two liters per hour, though one liter an hour is probably the upper limit for most people. The amount of unevaporated sweat on the skin may be no guide to the amount of sweat being secreted. In a moist tropical climate, when sweat is being produced at half a litre an hour sweat will be dripping from the body, but in a desert climate the same amount can be secreted and the skin feel merely moist. Indeed, high sweat rates occur more frequently in deserts than in the moist tropics—amounts up to 10 liters per day occur in men working in desert conditions. Even higher losses may occur in some hot industries, the classical example being the now extinct race of stokers who tired the boilers of coal-burning ships in the Red Sea.
It is customary to recognize two types of sweat glands, apocrine and eccrine. The apocrine are relatively large glands, few in numbers, found in the armpit and pubic regimes, which secrete a milky, opalescent fluid with a characteristic odour. The manner of secretion and the mechanism of control are entirely different from the ordinary sweat glands and they play no part in temperature regulation. It is probably incorrect to regard them as sweat glands at all: they appear to be related rather to the scent glands found in other mammals.
The true sweat glands, or eccrine sweat glands, are distributed all over the body; their density varies considerably in different parts of the body but is, on the average, about 120 per square centimeter. These are the glands which are concerned in temperature regulation, but not entirely so:
heat is not the only adequate stimulus for secretion. “Cold sweats,” such as the sweating that accompanies sea sickness, nausea and vomiting and the sweating which is such a characteristic prelude to fainting, are familiar enough. Yet another type, gustatory sweating, occurs on the head and neck when pungent substances such as hot curry or capsicums are eaten. The eccrine sweat glands on the palms and soles are said to take no part in temperature regulation but respond only to emotional changes. These forms are; however, relatively unimportant –sweating is essentially a temperature-regulating mechanism.
The sweat glands are supplied by sympathetic nerve fibers contained in the coetaneous nerves, and if these nerves are anaesthetized or cut secretion ceases: but the means by which the secretion of sweat is initiated and controlled is still imperfectly understood. However, it is known that local reflex and central mechanisms involving both the temperature of the skin and the deep body temperature are all concerned.
It was observed as long ago as the end of the last century that if the blood in the carotid artery was warmed the animal responded by sweating. Subsequent research has confirmed that there is a central heat-regulating center in the hypothalamus which behaves after the manner of a simple thermostat; when the temperature of the blood supplying it rises above a fixed value sweating begins. But this is clearly not the whole story, for if a person who is sweating violently is transferred suddenly to a cool environment, sweating ceases rapidly even though the temperature of the body remains high. Furthermore, although there is a time lag between exposure to a hot environment and the onset of sweating, this delay may be so short, especially for well-acclimatized subjects, that the rise in deep body temperature cans only be very, very small.
Clearly then, both skin and deep body temperature are concerned in the control of sweating, but the relation between them is extremely complex. For example, if a man who is sweating while sitting quietly at a temperature of 350 C. is made to do muscular work, his rate of sweating, a might be expected, will increase hut his skin temperature will fall.
In acclimatized subjects the amount of sweat produced is accurately adjusted to the needs of the body, and recent researches have shown that the rate at which sweat is secreted is the best single physiological measure of the stress of a hot environment. It is therefore possible to compare the relative stress of two hot environments by comparing the amount of sweat secreted on exposure to each. A very valuable scale of environmental stress has recently been developed based on the amount of sweat lost in four hours by fit, acclimatized young men.
Quantitative studies of the amount of sweat lost and its salt content are also important because both the salt and the water lost must be replaced. Failure to replace the salt results in a wide range of symptom.s varying from a feeling of lassitude to muscular cramps and even complete collapse.
If one is losing 10 liters of sweat with a salt concentration of 0.25 per cent.. 25 grams of salt must be eaten per day. so that, as the average European intake is 10-15 grams. a considerable salt supplement will be required-—hence the practice of taking extra salt as salt tablets in hot climates. The example cited is, however, exceptional. Such a rate of sweating is extreme and the salt concentration quoted is high for an acclimatized individual, so that the usual European intake of salt is adequate to replace losses except in unusual circumstances. Some indigenous inhabitants of the tropics have a salt intake of one third of the European value and seem to suffer no ill effects. Despite all the work that has been done on the subject, no finality has yet been reached on the vexed question of salt requirements in the heat. Fortunately, in practice one can err on the safe side. Salt is cheap and if an excess is taken the kidneys can deal with it.
Like all bodily functions, the secretion of sweat sometimes goes wrong. Excessive sweating (hyperidrosis), especially of the hands and feet, is not uncommon. Such sweating is emotional and a vicious circle is set up—the more worry the more sweating—so that a man may not be able to follow his occupation or may become a social outcast. Sometimes this sweat, especially on the feet, may develop an offensive odour, a condition known as bromidrosis, with even more unfortunate social consequences.
More important, from the point of view of survival, are those disorders which reduce the amount of sweat secreted. Complete congenital absence of the sweat glands, with a consequent total anidrosis, does occur but is rare. Lesser decrees of anidrosis are common, and are usually secondary to occlusion of the sweat ducts. Any injury to the outer layers of the skin, as for example sunburn, by occluding the mouths of the sweat ducts can produce a degree of secondary anidrosis, and this is the mode of action of the astringent lotions applied to control excessive sweating in restricted areas, for example, the armpits. In prickly heat there is also occlusion of the sweat ducts (how this is brought about is not certain) and when the sufferer is exposed to heat the secreted sweat ruptures the duct and escapes into the tissues. The consequent irritation produces the familiar prickling.
If a substantial area of the skin is affected by anidrosis it may imperil body temperature regulation in the heat. The most dramatic event associated with failure to sweat is the extremely dangerous but fortunately rare disorder known as heat stroke. It occurs typically as the result of strenuous physical exertion, usually under some form of compulsion, by an un-acclimatized subject in very severe heat. It is characterized by hyperpyrexia (a body temperature of 41-420 C. or over) with a hot dry skin, and is rapidly followed, if untreated, by unconsciousness and death. It is not altogether clear whether the high temperature results from a primary failure of the sweating mechanism, or whether the nervous control of sweating is wrecked by the high temperature, but whichever is the initiating cause the result is a vicious circle which, if not interrupted by artificial cooling, is rapidly fatal.
The sweat glands are secretory, not excretory, organs. There is little evidence to support the idea that the skin can act as an accessory kidney. The most careful studies would seem to show that the amount of urea in sweat is not greatly different from that in the blood stream, and passes into the sweat by a process of simple diffusion which also probably accounts for the occurrence of atebrin, morphia, alcohol and even mercury which have been reported in the sweat of subjects taking these drugs. Nevertheless, the belief in sweating as a therapeutic measure dies hard. It is probably the last lingering remnant of the eliminative therapy of former days—the sudorifics, the purges, the emetics, the bleeding, the leeches, the clysters, the cupping and the blistering, that once made sickness such a terrible experience and death a horrifying spectacle.