The Apocrine and the Eccrine glands are the 2 important sweat producing glands. The human body contains about 2 million such glands. The eccrine sweat gland is well known and understood than the apocrine gland. The eccrine glands are widely spread around the body and are found within the dermis and epidermis, ending up onto the skin. Whereas the apocrine glands, which are related with the hair follicles, are found mostly in the pubic are, nipples and the axilla. The eccrine gland excretes sweat that constitutes 99% water, while oily and viscous secretions are excreted by the apocrine glands. The eccrine sweat contains vitamins, hormones and other inorganic material as well as NaCl, which is the most concentrated solute present in the secretion. The plasma contains drugs that diffuse through the walls of the sweat gland, entering into the sweat, and after that are consequently detected and quantified. The density of the gland varies greatly with different parts of the body; 60/cm2 on the back, 150-200/cm2 on the wrist and forearm, and as high as 600/cm2 on the feet. The absorption of water into the sweat makes it hypotonic; otherwise an isotonic plasma filtrate is formed by the sweat. At rest, the pH of sweat is 5.8 whereas while exercise, it increases to 6.1 – 6.4.
There are two types of sweat production; insensible sweat production, which is a passive loss of about 300 to 700 ml of body water a day; and the other, is sensible sweat production, to regulate body temperature variation due to exercise, humidity or increased ambient temperature, and in reaction to stress. During vigorous exercise, the sensible sweat production occurs between 2 to 4 liters per hour. This can be recorded even higher but only for short time intervals; when the glands fatigue and results in hypothermia. The trunk excretes about 50% of the total sweat, whereas the legs and the head (including upper extremities) contributes 25% each in sweat excretion. Sweat rates on the hands, face and feet amount to 100ml/meter square per hour, whereas it is 6 – 10 ml/square meter per hour on the trunk, arms and legs. The absorbent pad of the sweat patch, covering an area about 14cm2, can collect 300µL of sweat per day.