Physiology of Normal Sweating
Humans have up to four million sweat glands distributed over the body. Approximately three million of these are eccrine sweat glands. Eccrine glands secrete an odorless, clear fluid that serves to aid in the regulation of body temperature by allowing heat loss through evaporation. Eccrine glands are found in higher density on the soles of the feet, the forehead, the palms, and the cheeks.[48,57] Apocrine sweat glands are located in limited areas—the axilla and urogenital regions—and produce a thick, odorless fluid that undergoes bacterial decomposition, leading to substances with strong odors. The ratio of apocrine to eccrine glands is one to one in the axillae and one to ten elsewhere.
The secretory portion of eccrine glands produces an ultrafiltrate that is then processed by cells lining the duct portion, where reabsorption of sodium occurs, leading to hypotonic sweat and conserving electrolytes. An acclimatized person can produce up to several liters of sweat per hour and ten liters per day.[48,57] For nonhuman species, apocrine sweat provides pheromone signaling that is important in mating, parenting, and other interactions; it is unclear what the role is for apocrine sweat in humans.
There are also mixed sweat glands called apoeccrine glands. These are found in human axillary and perianal areas. Apoeccrine glands develop during puberty from eccrine precursors. The function of this type of gland and its role in the pathophysiology of hyperhidrosis is unknown, but in some patients, up to 45% of the axillary sweat glands are of this type. In addition, in vitro studies have shown the apoeccrine gland to secrete sweat at a sevenfold higher rate than the eccrine gland does. (In vitro sweat production by the apoeccrine gland was stimulated by epinephrine and methacholine.)
Both eccrine and apocrine sweat glands are innervated by postganglionic sympathetic fibers. For eccrine glands, the major neurotransmitter is acetylcholine, and for apocrine glands, catecholamines are the major neurotransmitters. Spinal cord segments from T2 to T8 provide innervation to the skin of the upper limbs, T2 to T4 innervate the skin of the face, T4 to T12 innervate the skin of the trunk, and T10 to L2 innervate the skin of the lower limbs.
The thermoregulatory center in the hypothalamus controls body temperature by regulating eccrine sweat output and blood flow to the skin. This center responds not only to changes in core body temperature, but also to hormones, endogenous pyrogens, physical activity, and emotions. Both emotional and physical activities are thought to affect the thermoregulatory center via the limbic system.
The sweat glands on the palms and soles appear to be activated primarily by emotional stimuli, while axillary sweating is stimulated by both thermoregulatory changes as well as emotional stimuli. Since palmar and plantar glands do not differ from sweat glands in other areas with respect to morphology, innervation, and neurotransmitter response, one theory suggests that there is a distinct hypothalamic thermoregulatory center that controls sweating in the palms and soles and sometimes in the axillae. Differing from the regulatory center that controls sweating in other areas, this center is thought to receive input exclusively from the cortex and to be unresponsive to temperature changes. Supporting this theory is the finding that emotional sweating is not seen during sleep or sedation.
Emotional sweating is thought to be an atavistic (ancient or ancestral) human function that was important when hunting animals or fighting enemies. Physiologic amounts of sweat on the palms and soles can improve friction by controlling the humidity of the stratum corneum, leading to an improved grip. Generalized sweating cools the body when intense physical activity is expected. In addition, increased eccrine sweat output in the axillae produced by emotional stimuli will allow natural odors from prior apocrine gland secretion to aerosolize and function as pheromone signals.