Sweat gland

Sweat glands, or sudoriferous glands, are small tubular structures of the skin that produce sweat. There are two kinds of sweat glands:
 * Eccrine sweat glands are found only in primates and reach their greatest development in humans. They are distributed all over the body (except for the lips, tip of penis and clitoris) although their density varies from region to region.  Humans utilize eccrine sweat glands as primary form of cooling.  The human thermal eccrine system has evolved in concert with bipedalism and development of smooth hairless skin.
 * Apocrine sweat glands are larger, have different mechanism of secretion, and are limited to axilla (armpits) and perianal areas in humans.  Although apocrine glands contribute little to cooling in humans, they are the only effective sweat glands in hoofed animals such as the donkey, cow, horse, and camel.   Most other mammals, such as cats, dogs and pigs, rely on panting or other means for thermal regulation and have sweat glands only in foot pads and snout.  The sweat produced on pads of paws and on palms and soles mostly serves to increase friction and enhance grip.

Both apocrine and eccrine sweat glands contain myoepithelial cells (from Greek myo-, "muscle"), specialized epithelial cells located between the gland cells and the underlying basal lamina. Myoepithelial cell contractions squeeze the gland and discharge the accumulated secretions. The secretory activities of the gland cells and the contractions of myoepithelial cells are controlled by both the autonomic nervous system and by the circulating hormones.

Ceruminous glands, which produce ear wax, and mammary glands, which produce milk, are frequently considered to be modified sweat glands, but they are not. Both ceruminous and mammary glands are true apocrine glands.

Apocrine sweat glands
The name apocrine was originally chosen because it was believed that the gland cells used an apocrine method of secretion. Although it is now known that their secretory products are produced through merocrine secretion, the name has not changed. Apocrine sweat glands are coiled tubular glands that produce a viscous, cloudy and potentially odorous secretion. Apocrine sweat glands discharge in the canals of hair follicles. They begin secreting at puberty; the sweat produced may be acted upon by bacteria, causing a noticeable odor. Apocrine gland secretions may also contain pheromones, chemicals that communicate information to other individuals by altering their hormonal balance. Some research has indicated that the apocrine secretions of mature women can alter the menstrual timing of other women (this is called the McClintock effect), though the research methods used have been criticized. The significance of human pheromones, and the role of apocrine secretions in humans, remains not completely understood.

Eccrine sweat glands
Eccrine sweat glands are smaller than apocrine sweat glands, and they do not extend as deep into the dermis. Eccrine sweat glands are coiled tubular glands that discharge their secretions directly onto the surface of the skin. Their density varies greatly according to body regions, the highest density (>250 glands/cm2) being on soles, palms, and scalp.

The clear secretion produced by eccrine sweat glands is termed sweat or sensible perspiration. Sweat is mostly water, but it does contain some electrolytes, since it is derived from blood plasma, although less concentrated. It therefore contains mainly sodium chloride, but also other electrolytes. The presence of sodium chloride gives sweat a salty taste. The total volume of sweat produced depends on the number of functional glands and the size of the surface opening. The degree of secretory activity is regulated by neural and hormonal mechanisms (men produce greater volumes of sweat than women). When all of the eccrine sweat glands are working at maximum, the rate of perspiration for a human being may exceed three liters per hour, and dangerous fluid and electrolyte losses can occur. For this reason athletes in endurance sports must pause frequently to drink electrolyte-containing fluids.

Eccrine glands have three primary functions:
 * Thermoregulation. Sweat cools the surface of the skin and reduces body temperature.
 * Excretion. Eccrine sweat gland secretion can also provide a significant excretory route for water and electrolytes, as well as for a number of prescription and nonprescription drugs.
 * Protection. Eccrine sweat gland secretion aids in preserving the skin's acid mantle, which helps protect the skin from colonisation from bacteria and other pathogenic organisms.