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Physiology of Normal Sweating
Humans have up to four million sweat glands distributed over
the body, of which about three million 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 by evaporation. Eccrine glands are in higher density
on the soles of the feet and the forehead, followed by 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.[48]
The secretory portion of eccrine glands produce an ultrafiltrate
that is then processed by cells lining the duct portion, where
reabsorption of sodium occurred, 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.[48]
There are also mixed sweat glands called apoeccrine glands,
found in human axillary and perianal areas. These glands develop
during puberty from eccrine precursors.[48]
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 were of this type.[118]
In addition, in vitro studies have shown the apoeccrine gland
to secrete sweat at a sevenfold higher rate than the eccrine gland does.[120]
In vitro sweat production by the apoeccrine gland was stimulated
by epinephrine and methacholine.[118]
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, from T2 to T4 to the skin of the face, from T4 to T12 to the skin of the trunk, and from T10 to L2 to the skin of the lower limbs.[48]
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.[57]
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.[4] 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.[119]
Emotional sweating is thought to be an atavistic function
that was important when hunting animals or fighting enemies.[57]
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.
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