An excretion from the SKIN, produced by the 2.5 million sweat-glands scattered over its surface. 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 confined – occurs with physical exertion and raised body temperature. 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 effectiveness of a person's normal grip.
Sweating is controlled by two types of nerves: vasomotor, which regulate the local blood flow, and secretory (part of the sympathetic nervous system) which directly influence secretion.
is a faintly acid, watery fluid 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.
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 modified apocrine glands.) The flow of apocrine sweat is evoked by emotional stimuli such as fear, anger, or sexual excitement.
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.
This may be caused by fever, hyperthyroidism (see THYROID GLAND, DISEASES OF), obesity, diabetes mellitus, or an anxiety state. Offensive 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 palmar and/or axillary hyperidrosis 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 can be applied to the skin but can cause skin irritation and soreness. Such antiperspirants may help patients with moderate hyperidrosis, 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.