Humans perspire over nearly every inch of skin, but anyone with sweaty palms or smelly feet can attest to the fact that sweat glands are most numerous in the palms and soles, with the forehead running a close third. There are two types of sweat, or sudoriferous, glands: eccrine and apocrine. Both are coiled tubules embedded in the dermis or subcutaneous layer composed of simple columnar cells.
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Eccrine sweat glands are distributed widely over the body — an average adult has roughly 3 million of them — and produce the watery, salty secretion you know as sweat. Each gland’s duct passes through the epidermis to the skin’s surface, where it opens as a sweat pore.
The sympathetic division of the autonomic nervous system controls when and how much perspiration is secreted depending on how hot the body becomes. Sweat helps cool the skin’s surface by evaporating as fast as it forms. About 99 percent of eccrine-type sweat is water, but the remaining 1 percent is a mixture of sodium chloride and other salts, uric acid, urea, amino acids, ammonia, sugar, lactic acid, and ascorbic acid.
Apocrine sweat glands are located primarily in armpits (known as the axillary region) and the external genital area. Usually associated with hair follicles, they produce a white, cloudy secretion that contains organic matter.
Although apocrine-type sweat contains the same basic components as eccrine sweat and also is odorless when first secreted, bacteria quickly begin to break down its additional fatty acids and proteins — explaining the post-exercise underarm stench. In addition to exercise, sexual and other emotional stimuli can cause contraction of cells around these glands, releasing sweat.
