Sweat glands

It is estimated that there are around 2.6 million sweat glands in the skin, women have more than men but those in men are more active. There are two different types of sweat glands, eccrine and apocrine.

Eccrine sweat glands
Eccrine sweat glands are widely distributed across the skin surface, although concentrated in palms, soles, forehead and axillae. This type of sweat gland excretes a mildly salty fluid (99% water with small amounts of salts, ammonia, urea and uric acid) directly onto the surface of the skin when the skin surface temperature rises above 35°C (Hinchcliffe et al., 1999). The main purpose of this is to encourage heat loss. The evaporation of 1 L of sweat requires 580 kcal (2,400 KJ) of energy. Heat provides this energy, thus using it up and cooling the skin. Sweat will not evaporate in a very humid climate as the atmosphere is already laden with water.

About 400–500 mL of fluid is lost per day mostly through sweat, although a small amount directly through the surface of the skin, lungs and bucal mucosa, known as insensible water loss. If the ambient temperature is particularly hot or the person is exercising, fluid loss may rise to as much as 12 L. Sweating from eccrine glands is also stimulated by fear particularly on the palms and soles; this phenomenon forms the basis of lie detector tests. Hot, spicy foods may also induce sweating, known as gustatory sweating.

Apocrine sweat glands
Apocrine sweat glands open into the hair follicle and are found mainly around the nipples, scalp in the groin and axillae. They are inactive in childhood, being activated during puberty under the influence of androgens.

The glands are affected by the sympathetic nervous system, during times of stress, pain or sexual arousal. The secretion is basically the same as eccrine sweat with the addition of proteins and fatty substances which make a thicker, stickier substance. Although this is odourless, it is vulnerable to the activities of bacteria found on the skin, and it is this action that makes the musty smell commonly known as body odour. It is thought that apocrine sweat is the human equivalent of a scent gland which, whilst vital for other animals, seems to have become less so for humans.

Mammary glands that produce breast milk and ceruminous glands that produce ear wax in the ear canal are modified apocrine glands.