Physiology of Sweat

Original Editor - Kapil Narale

Top Contributors - Kapil Narale, Uchechukwu Chukwuemeka and Kim Jackson  

Introduction

Human thermoregulation depends significantly on sweat evaporation from the skin surfaces. This becomes clear when sweating is inhibited during periods of hard physical labour or being subjected to hot conditions. Although sweating is widely believed to play a thermoregulatory role, it also serves several other minor homeostatic roles. For instance, it is believed that sweat glands perform an excretory role, comparable to that of the renal system, which is to eliminate excess micronutrients and metabolic waste . Also, body core temperature increases significantly with heat stress in persons with reduced ability to sweat following exerciseor people wearing absorbing protective clothing/equipment , which can cause heat exhaustion or heat stroke if additional sources of cooling are not given.

Types of Sweat Glands

• Eccrine – These are the most distributed throughout the body and are distributed throughout the entire surface area. They produce the most sweat. There are about 2-4 million glands throughout the body. They are found on both glabrous (palms, soles) and non-glabrous parts of the body. The density of these glands is not uniform throughout the body, as they are the densest in palms and soles (~250-550 glands/cm²). Emotional and thermal stimuli activated these.

The density of these glands on non-glabrous skin, like the face, trunk, and limbs, are ~2-5x less than on glabrous skin. These are more variably distributed and mainly function with temperature regulation.

It is known that the number of eccrine sweat glands are fixed throughout life, and start developing in infancy. This explains why the density of these glands decrease as a person develops and grows, and their skin expands. This is known to be inversely proportional to body surface area. Therefore, children have a greater density of sweat glands than adults, and obese people of any age are seen to have a lower density of eccrine sweat glands.

Despite this, having a greater density of sweat glands does not indicate that the person would sweat more. The differences between sweating rate throughout the body, or different areas of the body, would be due to the sweat secretion rate per gland, compared to the total number of active sweat glands. The contents of eccrine sweat are mainly water and NaCl. It also contains chemicals from the interstitial fluid and the eccrine gland.        

Eccrine sweat glands can also be thought of as an excretory organ, since they excrete waste products as well.

• Apocrine – These are larger than eccrine glands, and open into hair follicles, instead of being directly on the skin.

Apocrine sweat glands are present in the: axilla, breasts, face, scalp, and perineum.

Similar to the eccrine glands, they are present from birth/childhood, but unlike the eccrine glands, they do not start secreting until puberty. Apocrine glands produce viscous, lipid-rich sweat, which consists of proteins, and ammonia. This gland can be known as the scent gland, which is involved in producing pheromones, having an important social and sexual function in humans.

• Apoeccrine – These develop from eccrine sweat glands between the ages of ~8-14 years. They share properties of both eccrine and apocrine glands and are medium sized. These are only found in the axillary region.

They are similar to eccrine glands, as the distal duct connects to and empties sweat onto the surface of the skin, and they produce a large amount of salt water secretions.

Here is a video explaining sweat glands, with some additional information:

Composition of Sweat

Although sweat is mainly water and NaCl, there are many other minerals and constituents as a part of its composition. These include:

Micronutrient	Concentration	Parts of the Body	Sweat Gland Mechanism
Sodium	10-90mmol/L	Found in many sites such as forehead, chest, back, upper arm, but not in all individual sites, such as the foot, calf, thigh.	Initial sweat is isotonic with blood plasma – hypotonic sweat is the result.
Chloride	10-90mmol/L	Found in many sites such as forehead, chest, back, upper arm, but not in all individual sites, such as the foot, calf, thigh.	Initially a little hypertonic when compared to blood plasma - resulting sweat is hypotonic.
Potassium	2-8mmol/L		Sweat is closely isotonic with blood plasma.

There are trace amounts of calcium (0.2-2.0 mmol/L), magnesium (0.02-0.4 mmol/L), iron (0.0001-0.03 mmol/L), zinc (0.0001-0.02 mmol/L), and copper (0.0005-0.02 mmol/L). However, regional measures of calcium, magnesium, iron, zinc, and copper, all overestimate the concentration produced throughout the body.

Interstitial fluid is the initial constituent for sweat development, thus many of the components released are derived from interstitial fluid itself. Some, not from interstitial fluid, are byproducts of sweat gland metabolism, and play a role in skin health. Other non-mineral components (some harmful, some not harmful) of sweat include:

• Lactate – there is about 5-40mmol/L secreted. Eccrine sweat gland metabolism produces it.

Interestingly, there is an inverse relationship between sweat rate and the concentration of sweat lactate, however there is a direct relationship between sweat rate and lactate excretion rate. It plays a role as a natural skin moisturizer, in eccrine sweat, and functions to excrete metabolic waste.

• Urea – there is about 4-12mmol/L secreted.

This comes from blood plasma, therefore has a similar concentration. Urea also plays a role as a natural skin moisturizer, in eccrine sweat, and functions to excrete metabolic waste.

• Ethanol – there is about 2-30mmol/L secreted.

This also mainly comes from blood plasma. Ethanol has a role of detoxification in eccrine sweat.

• Ammonia – there is about 1-8mmol/L secreted.

The concentration of ammonia is about 20-50x greater than plasma, and the concentration is inversely related to sweating rate and pH. It mainly comes from plasma NH₃. The function of ammonia in eccrine sweat is the excretion of metabolic waste.

• Bicarbonate – there is about 0.5-5.0mmol/L secreted.

There is less concentration of bicarbonate than blood plasma. With HCO₃ being reabsorbed in the sweat duct, there is acidification of the sweat output. HCO₃ is inversely proportional to sweat rate. Therefore, sweat pH is lower at lower sweat rates. HCO₃ controls the pH of eccrine sweat.

• Glucose – there is about 0.01-0.2mmol/L secreted.

This is the main energy source for eccrine sweat gland secretions.

• Heavy Metals (lead) - there is about 0.0002-0.0006mmol/L secreted.

Concentrations are generally higher than plasma, but its secretion is very minute. Its main function is detoxification.

Other minor constituents include:

• Antibodies (e.g. IgG, IgA) and Antimicrobial peptides (e.g. dermcidin, cathelicidin, lactoferrin)

• Other Proteins (e.g. albumin, α-globulin, γ-globulin)

• Cytokines (e.g. Interleukin-1α, 1β, 6, 8, 31, TNFα)

• Amino acids (e.g. pyrrolidone carboxylic acid, urocanic acid, serine, histadine, ornithine, glycine, alanine, aspartic acid, lysine)

• Proteolytic enzymes

• Persistent Organic Pollutants (e.g. organochlorinated pesticides, polychlorinated biphenyls, perfluorinated compounds)

• Other Toxicants (e.g. BPA, phthalate, polybrominated diphenyl ethers), and also cortisol, neuropeptides, bradykinins, cyclic AMP, angiotensins, and histamines.

For more information on sweat contents, see these resources: Characterization of sweat induced with pilocarpine, physical exercise, and collected passively by metabolomic analysis, Metabolomics analysis of human sweat collected after moderate exercise, and The proteomic and metabolomic characterization of exercise induced sweat for human performance monitoring: A pilot investigation.

Here is a brief video describing the contents of sweat, how to maintain these components, and the benefits of sweating:

Sweat Rate vs. Sweat Content

There is a direct correlation between sweat rate, and total sweat output from the body, and contributors to sweat composition. Whole body sweating rate is derived by the product of the density of active sweat glands and the secretion rate per gland. When someone sweats, the primary response is a rapid increase of sweat gland recruitment, and then a progressive increase of sweat secretion per gland.

Some important aspects of thermoregulatory sweating include:

• Onset (body core temperature threshold), and
• Sensitivity (slope of the relation between sweating rate and the change in body core temperature),

of the sweat response to hyperthermia.

Shifts in sweating temperature onset are central (hypothalamic), and changes in sensitivity are peripheral, at the level of the sweat gland.

The Na⁺ and Cl^- concentrations of the sweat output are determined by the rate of Na⁺ reabsorption in relation to Na⁺ secretion in the clear cells.

Plasma aldosterone concentration, and/or sweat gland sensitivity to aldosterone, affects the Na-K-ATPase activity, which affects Na⁺ reabsorption. Resting plasma aldosterone is controlled by an individual’s long-term physiological condition, which factors in heat acclimation, fitness, and diet. This also changes according to exercise and dehydration.

A study found that as the forearm sweating rate increased, the rate of Na⁺ secretion increased greater than the rate of Na⁺ reabsorption along the sweat duct. With this kind of sweating, stimulated from exercise activity, the concentration of Na⁺ sweat significantly increased. It is seen that the rate of Na⁺ reabsorption increases with an increase in sweating rate. Despite this, the percentage of secreted Na⁺ reabsorbed into the duct decreased with a rise in sweat rate. The quicker the primary sweat levels along the duct, the smaller percentage of Na⁺ that can be reabsorbed.

During exercise or heat stress, a minimum threshold of sweating rate could be needed before the Na⁺ concentration of sweat increases, with an increase in sweating rate. It is seen that factors, such as air temperature or exercise intensity, that respond to an increase in sweating rate, will produce a higher sweat concentration of Na⁺ and Cl^-. This may be applicable to various sites, or to the entire body. It was found that sites with a higher sweating rate of Na⁺ and Cl^- result in a higher concentration of Na⁺ and Cl^-.

Contrarily, there are two parameters, central threshold temperature, and sensitivity, which have a major effect on determining sweat rate. These parameters help outline the effect of nonthermal factors on sweating.

Bicarbonate and Lactate

The reabsorption of bicarbonate, by the sweat gland, for the acid-base balance of blood, is also an important factor. Before the sweat is released onto the skin surface, the fluid in the ductal lumen gains acidity, due to possible secretion of hydrogen ions into the sweat duct.      

The initial pH of sweat is about 7.1-7.4.      

Bicarbonate reabsorption is inversely related to sweat rate. Thus, at low sweat rates, the luminal fluid is exposed to the duct for a long time and is further acidified, which results in a pH of about 4-5. However, at quicker flow rates, pH can remain as high as 6.9.

Since lactate is produced by eccrine sweat gland metabolism, there is a direct relationship between sweat rate and the rate of lactate excretion. The higher the sweat rate, the higher the concentration of lactate excreted. However, because of the increased release of water content, there is an inverse relationship between sweat rate and lactate concentration. Thus, it makes sense that sweat lactate concentration decreases with increased exercise intensity.      

Mechanism of Sweating

The control centre for temperature is in the brain, and thermoreceptors are located all over the body. When body temperature increases above normal resting body temperature, the thermoreceptors send a message to the control centre in the brain. The control centre responds to this by signalling a response towards heat loss, skin blood vessels dilating, and thus sweating occurs, as sweat glands are activated. The control centre is inactivated, and sweating stops once the body returns to a normal temperature. As the sweat evaporates, heat is exchanged with the environment and is lost from the body, thus decreasing skin temperature.

Sweat Gland Size

The structure of sweat glands can play a role in the likelihood and amount of sweating. When the sweat glands are frequently activated, with regular exercise, there is acclimation of the sweat glands in their size and neural/hormonal response. Sweat glands, between individuals, can be of varying sizes, up to five times bigger in some individuals, than others. It is seen that the size of individual sweat glands, their response to methacholine, and their secretory rate, are all directly related. Aerobic training and heat acclimation are contributors to an increase in sweat gland size and cholinergic responsiveness.      

Secretion

The secretion of sweat follows a Na-K-2Cl cotransport model:

• Release of intracellular Ca stores and an influx of extracellular Ca into the cytoplasm is triggered by the binding of acetylcholine to muscarinic receptors on the basolateral membrane of the clear cell.
• There is an efflux of KCl through Cl channels in the apical membrane and K channels in the basolateral membrane.
• This results in cell shrinkage, triggering an influx of Na, K, and Cl, from Na-K-2Cl transporters on the basolateral membrane.
• This then causes efflux of Na and K from the Na-K-ATPase and K channels on the basolateral membrane, and Cl efflux via Cl channels on the apical membrane.    

The rate of Na, Cl, and bicarbonate reabsorption is flow-dependent, where higher sweat rates are directly associated with lower reabsorption rates, thus producing a higher sweat electrolyte concentration.

In a study, it was proposed that the rate of secretion can be dependent on environmental factors such as humidity, wind speed, or air pressure. These environmental factors which can lead to evaporation of sweat from the skin can also facilitate sweat secretion at a given mean body temperature.

Control of Eccrine Sweating & Modifiers of Eccrine Sweating

As mentioned, eccrine sweat glands primarily respond to thermal stimuli, specifically increases in core body temperature. However, skin temperature and increases in skin blood flow are also involved.

Central and skin thermoreceptors sense an increase in body temperature. This information is registered by the preoptic area of the hypothalamus, to trigger the sudomotor response. Thermoreceptors in the abdominal region and surrounding muscles are also involved with controlling sweating. Thermal sweating is controlled by sympathetic cholinergic receptors.

Sweat production is facilitated by the release of acetylcholine from non-myelinated sympathetic class C postganglionic nerve fibers, binding onto the muscarinic receptors on the sweat gland. Secretion of sweat can also occur due to adrenergic receptors, but this is not as dominant as cholinergic receptors.

Eccrine sweat glands can also respond to exercise stimuli of a non-thermal nature which may be controlled by a feed-forward mechanism.

The following chart helps outline modifiers of the control of eccrine sweating:

External and Internal Factors, and Timeframe	Timeframe	Effect on Sweating Rate and/or Composition
Dietary NaCl	Acute/Chronic	No effect on sweat rate with regular intake of NaCl in a short duration, within a time span of 3 days.
Dietary intake of other minerals (Ca, Fe, Zn, Cu) and vitamins (ascorbic acid, thiamine)	Acute/Chronic	No effect on sweat mineral or vitamin concentrations.
Fluid Intake	Acute	Water ingestion results in a reflex transient increase in sweat rate, especially when in an hypohydrated state. There is no effect on sweat Na, K, Cl and lactate concentrations.
Dehydration	Acute	Reduced whole body, and regional, sweating rate, due to the hyperosmolarity-induced increase in threshold for the onset of sweat, and a hypovolemia-induced decrease in sweat sensitivity, with a minor effect. Similar effects are seen with sweat Na and Cl concentrations, with no effects seen on K concentrations.
Alcohol	Acute	There is no effect on the sweating rate. However, sweat ethanol concentration increases with consumption, and increases linearly with an increase in blood alcohol concentrations.
Exercise Intensity	Acute	There is an increase in whole body, and regional, sweating rate with an increase in exercise intensity, since metabolism is directly related to energy expenditure. Na and Cl concentrations increase as exercise intensity increases since the rate of Na and Cl reabsorption is flow dependent, minimal, or has no effect on sweat K concentrations. There is an inverse relationship between sweat rate and sweat lactate, and ammonia concentrations.
Environment	Acute	There is an increase in the whole body, and regional, sweating rate with increased environmental heat stress (increased air temperature, increased solar radiation, and decreased air velocity), at a given workload. Suppression of sweating with a reduction in sweating due to a long-term exposure of humid air. There is an increase in sweat Na and Cl concentrations with an increase in ambient temperatures.
Altitude/Hypoxia	Acute	There is an increase in sweating. There is a reduced sweat sensitivity for regional sweat rate.
Clothing/Protective Equipment	Acute	There is an increase in whole body, and regional, sweat rate due to reduced evaporative, and radiant heat loss from covering the skin. The protective gear can also increase metabolic heat production.
Body Mass	Chronic	There is an increased whole body sweat rate in individuals with a larger body mass, due to an increased metabolic heat production at a given workload during weight-bearing exercises. Also they likely to have lower sweat efficiency.
Heat Acclimation	Chronic	There is an increase in whole body sweat rate. The periphery (limbs/forearm) tend to increase more than the core and center sites. This shows that there is preference towards the periphery, resulting in more distributed sweating throughout the body. There are reduced Na and Cl concentrations, and no changes in other minerals. There are ANS changes due to gland hypertrophy, increased cholinergic and aldosterone sensitivity, and a lower sweat onset threshold.
Aerobic Training	Chronic	There is an increase in whole body, and regional, sweat rate due to increased cholinergic sensitivity and lower sweat onset threshold.
Sex/Gender	Chronic	There is a higher whole body, and regional, sweat rate in Men due to greater cholinergic sensitivity and maximal sweating rate. This can occur only at high evaporative requirements for heat balance. Higher whole body sweat rate in Men can be related to a higher body mass and metabolic heat production, rather than sex/gender. There are minimal differences in sweat Na, Cl, and lactate concentrations between sex/gender.
Menstrual Cycle	Cyclical	There is no effect on whole body sweat rate, but there is a lower regional sweat rate at a given body core temperature during the luteal phase of menstruation. There is no effect on sweat Na, Cl, K concentrations.
Circadian Rhythm	Cyclical	There is an increased sweat threshold in the afternoon, between 12pm and 4pm, as opposed to early morning, between 4 am and 5:30am.
Race/Ethnicity	Chronic	There aren't any race or ethnicity differences in whole body, or regional, sweat rate, or sweat composition. Heat habituation, characterized by lower (activated sweat gland density and sweat gland output) and more efficient sweating (less dripping), are found in people acclimated to hot or tropical environments.
Maturity	Progressive Change	Lower whole body sweat rate and sweat Na concentrations are found in pre-pubertal boys compared to post-pubertal boys.
Ageing	Progressive Change	Reduced whole body, and regional, sweat rate related to decreased sweat gland output is associated with a reduction in aerobic fitness and heat acclimation rather than ageing. During exercise, heat stress differences are more relevant for peak sweating rate (e.g. exercise in hot dry climates). There is no impact of ageing on sweat Na concentration.

Methods of Sweating

In addition to the methods to secrete sweat from the eccrine glands, there may be further constituents/contaminants in the sweat and/or on the skin. This can include remaining contents of the sweat duct, secretion of sebum, epidermal cells, and/or skin surface contaminants. It is seen that there can be NaCl content on the skin with or without sweating, however contents on the skin with sweating may be unnoticeable.    

It is seen that autonomic responses can differ between sweating from:

• Pharmacological methods,
• Passive heating methods, or
• Exercise induced methods.

Evaporation of Sweat

There are three factors that control the evaporation of sweat from the skin:

1. Ambient conditions - air temperature and relative humidity
2. The convective currents around the body
3. Skins surface that is exposed to the environment

Relative humidity contributes the most to the rate of evaporative heat loss, at high environmental temperatures. Higher humidity reduces the rate of evaporation. Therefore, to have sweat evaporate in an efficient manner, it is better to be, or go towards, less humid conditions.

The high relative humidity reduces the relative pressure gradient between someone's skin and the environment. With extremely high humidity, the vapour pressure in the air is similar to the vapour pressure on moist skin, thus the rate of evaporation is decreased. Sweating in a hot and humid environment results in unnecessary water loss, increased body heat, and a higher sweat rate. Sweating itself doesn't cool the skin, but it is the evaporation that actually cools the skin.

Hidromeiosis

When it comes to hidromeiosis, the minimization of sweat secretion rate when the skin is already wet, it was noted that when there is water on the skin, either from sweating or immersion, sweat secretion is halted. In any activity or thermal sweating, it is noted that a threshold rate of sweating must be reached for hidromeiosis to begin. Hidromeiosis does not seem to be an adaptive process, since as sweating begins to cease, the temperature in certain areas of the body may begin to rise. It is seen, too, that dehydration may increase hidromeiosis. In addition, exposure to thermal heat stress may be more of a factor for hidromeiosis, compared to the stress from physical activity. Hidromeiosis usually begins after 1-2 hours of intense physical activity, and as such there is a threshold sweat rate, at which hidromeiosis ceases.

Hidromeiosis is seen to only affect sweat rate after the skin becomes fully wet. Also, the rate at which sweat rate decreases is directly related to the sweat rate when hidromeisos begins. It is important to note that heat adaptation (in the summer) or adaptation to the cold (in the winter) affects the threshold of when hidromeiosis starts. In general, it is possible that hidromeiosis may be the cause of the cessation of sweating in humid environments, as opposed to dry environments.

For an extension on the topic of sweat, check out the following pages:

• Physical Activity and Perspiration, and
• Disorders of Sweating

Here is a quick, very informative TedEd video on sweat, and its various components: