Eccrine sweat glands
Eccrine sweat glands (sometimes merocrine sweat glands) are coiled tubular glands derived from the outer layer of skin but extending into the inner layer used for body temperature regulation. In humans and many other species, they are distributed over almost the entire surface of the body, but are particularly abundant on the palms of hands, soles of feet, and on the forehead. They are lacking in some marine and fur-bearing species. The sweat glands are controlled by sympathetic cholinergic nerves, which are controlled by a center in the hypothalamus. The hypothalamus senses core temperature directly, and also has input from temperature receptors in the skin and modifies the sweat output, along with other thermoregulatory processes.
Substances are secreted by exocytosis. Human eccrine sweat is composed chiefly of water with various salts and organic compounds in solution. It contains minute amounts of fatty materials, urea, and other wastes. The concentration of sodium varies from 35–65 mmol/l, and is lower in people accustomed to a hot environment and in people adapted to physical exercise. The sweat of other species differs in composition.