The sweat glands are simple, usually unbranched tubular glands of the skin in man and mammalian animals (except moles, pangolins, sloths, and whales) that produce and excrete sweat. There are two types of sweat glands: eccrine glands, which secret salty watery fluid, and apocrine glands, which produce oily fluid containing organic matter that can be smelly. Because they are sensitive to adrenaline, apocrine sweat glands are involved in emotional sweating in humans as they are induced by anxiety, stress, fear, sexual stimulation, and pain. The total number of sweat glands in man is from 2 to 5 million. Their number and size vary in different parts of the body.
The sweat glands consist of an unbranched excretory duct and an acinus, which, like the beginning of the excretory duct, is rolled into a ball located on the boundary between the reticular layer of the skin and the subcutaneous tissue. The excretory ducts open through apertures, or sweat pores, to the surface of the skin, and on hairy parts of the body usually into the hair follicles. The secretory section consists of a single layer of prismatic cells with tiny vacuoles, droplets of fat, and granules of glycogen and pigment. The appended myoepithelial cells, which excrete sweat by their contractions, are located on the basal membrane.
Sweat glands are classified according to type of secretion as ordinary eccrine glands, which are most highly developed among humans and primates, and specific apocrine glands of the majority of mammals. The special varieties of sweat glands of the eyelids and of the ears produce cerumen. In old age the number of sweat glands in man diminishes, and their secretory section is curtailed. The sweat glands are innervated by sympathetic nerves, whose endings secrete acetylcholine when they are stimulated— that is, they are cholinergic, as are the parasympathetic nerves. By producing a large quantity of sweat, the sweat glands regulate heat emission; excrete the products of nitrogen metabolism and salts of alkaline metals, mainly sodium chloride; and moisten the surface of the skin.
