Commonly called sweat, it is an excretion from the SKIN, produced by microscopic sweat-glands, of which there are around 2·5 million, scattered over the surface. There are two di?erent types of sweat-glands, known as eccrine and apocrine. Insensible (that is unnoticed) perspiration takes place constantly by evaporation from the openings of the sweat-glands, well over a litre a day being produced. Sensible perspiration (that is, obvious) – to which the term ‘sweat’ is usually con?ned – occurs with physical exertion and raised body temperature: up to 3 litres an hour may be produced for short periods. Normal sweating maintains the body within its customary temperature range and ensures that the skin is kept adequately hydrated – for example, properly hydrated skin of the palm helps the e?ectiveness of a person’s normal grip.
The chief object of perspiration is to maintain an even body temperature by regulating the heat lost from the body surface. Sweating is therefore increased by internally produced heat, such as muscular activity, or external heat. It is controlled by two types of nerves: vasomotor, which regulate the local blood ?ow, and secretory (part of the sympathetic nervous system) which directly in?uence secretion.
Eccrine sweat is a faintly acid, watery ?uid containing less than 2 per cent of solids. The eccrine sweat-glands in humans are situated in greatest numbers on the soles of the feet and palms of the hands, and with a magnifying glass their minute openings or pores can be seen in rows occupying the summit of each ridge in the skin. Perspiration is most abundant in these regions, although it also occurs all over the body.
Apocrine sweat-glands These start functioning at puberty and are found in the armpits, the eyelids, around the anus in association with the external genitalia, and in the areola and nipple of the breast. (The glands that produce wax in the ear are modi?ed apocrine glands.) The ?ow of apocrine sweat is evoked by emotional stimuli such as fear, anger, or sexual excitement.
Abnormalities of perspiration Decreased sweating may occur in the early stages of fever, in diabetes, and in some forms of glomerulonephritis (see KIDNEYS, DISEASES OF). Some people are unable to sweat copiously, and are prone to HEAT STROKE. EXCESSIVE SWEATING, OR HYPERIDROSIS, may be caused by fever, hyperthyroidism (see THYROID GLAND, DISEASES OF), obesity, diabetes mellitus, or an anxiety state. O?ensive perspiration, or bromidrosis, commonly occurs on the hands and feet or in the armpits, and is due to bacterial decomposition of skin secretions. A few people, however, sweat over their whole body surface. For most of those affected, it is the palmar and/or axillary hyperhidrosis that is the major problem.
Conventional treatment is with an ANTICHOLINERGIC drug. This blocks the action of ACETYLCHOLINE (a neurotransmitter secreted by nerve-cell endings) which relaxes some involuntary muscles and tightens others, controlling the action of sweat-glands. But patients often stop treatment because they get an uncomfortably dry mouth. Aluminium chloride hexahydrate is a topical treatment, but this can cause skin irritation and soreness. Such antiperspirants may help patients with moderate hyperhidrosis, but those severely affected may need either surgery or injections of BOTULINUM TOXIN to destroy the relevant sympathetic nerves to the zones of excessive sweating.... perspiration
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