sweat or perspiration, fluid secreted by the sweat glands of mammalian skin and containing water, salts, and waste products of body metabolism such as urea. The dissolved solid content of sweat is only one eighth that of an equal volume of urine, the body's main vehicle of salt excretion; however, excessive sweating may produce severe salt loss (see heat exhaustion). Human sweat glands are of two types, eccrine and apocrine. The eccrine glands, found everywhere on the body surface, are vital to the regulation of body temperature. Evaporation of the sweat secreted by the eccrines cools the body, dissipating the heat generated by metabolic processes. The release of such sweat is usually imperceptible; yet even in cool weather an individual will lose from 1 pt to 3 qt of fluid per day. Only when environmental conditions are especially hot or humid, or during periods of exercise or emotional stress, does the output of sweat exceed the rate of evaporation, so that noticeable beads of moisture appear on the skin. When such conditions are extreme, the body may lose up to 20 qt of fluid per day. Production of sweat is controlled by the temperature-regulating center of the hypothalamus. The apocrine glands, which occur only in the armpits and about the ears, nipples, navel, and anogenital region, are scent glands. They function in response to stress or sexual stimulation, playing no part in temperature regulation. The apocrines exude a sticky fluid quite different from the watery sweat of the eccrines. Apocrine fluid is rich in organic substances that are odorless when fresh but are quickly degraded by bacteria on the skin to produce characteristic odors. Copious sweating in the armpits comes not from the apocrines but from the eccrines interspersed among them.
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