Sweat

The body's cooling system

Perspiration is the body-cooling process in which water is released from the intact skin of most mammals. Perspiration leaves the skin surface in one of two ways:

• Simple evaporation from the epidermis (insensible perspiration)
• Active secretion by specialized glands as sweat, which evaporates and exerts a cooling effect by carrying excess heat away from the body

Sweat glands are found in most mammals, but are the chief means of heat dissipation only in primates and certain hoofed animals. Under heavy exertion, an adult human can sweat two to four liters per hour, or 10–14 liters per day.

Types and composition

Two distinct types of sweat glands have been identified in humans:

• Eccrine sweat glands are found over much of the body, where they open directly to pores on the skin surface and secrete high-water-content sweat, typically in response to excessive body heat.
• Apocrine sweat glands are found in areas with concentrations of hair follicles (such as the scalp, armpits and groin) where they secrete into the hair follicle an oily sweat that's scentless until bacterial decomposition imparts a characteristic odor.

Chemical analysis has revealed the primary constituents of human sweat:

• Water (about 99 percent)
• Trace minerals (including sodium, potassium, calcium, magnesium, zinc, copper, iron, nickel, chromium, lead)
• Lactic acid
• Urea