Physical Activity and Perspiration

Original Editor - Kapil Narale

Top Contributors - Kapil Narale, Rishika Babburu, Kim Jackson and Uchechukwu Chukwuemeka  

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Introduction

Thermoregulation/Heat Control

It can be said that the main purpose of sweating is to release heat form the body for temperature regulation. During exercise, an excess amount of heat is produced by the working muscles as a byproduct of metabolism. Also during exercise in the outdoors, heat is transferred to from the air to the body, especially when the air temperature is greater than skin temperature.

With sweating, body heat is transferred to water, which is on the surface of the skin.

The energy of sweat vaporization is 580kcal of heat per kg of evaporated sweat.

As mentioned by the Heat Balance Theory, the relationship between the evaporative needs for heat balance and the maximum evaporative capacity of the environment, can determine the amount of sweat produced.

The main forms that the body gains heat is through exercise intensity, which can be described as metabolism, and from heat in the environment. These are the main facilitators of ANS activity. Though, it should be noted that not all sweat is evaporated, and some or much of it may drip off the individual. In humid environments, with condition of low sweat efficiency, a greater sweat rate may be required for a given amount of evaporation.

Skin Health

Eccrine sweat is important in maintaining an epidermal barrier, from release of water, natural moisturizing factors, and antimicrobial peptides onto the skin surface. These natural moisturizing factors include amino acids, lactate, urea, Na⁺, and K⁺, which act as moisturizers for the outer layer of the stratum corneum (outer layer of skin) to stay hydrated. Most of these moisturizers are derived from eccrine sweat, and the amino acids on the skin are derived in the stratum corneum themselves.

Perspiration is shown to increase stratum corneum hydration, which may occur as a result of the moisture transfer fromt he eccrine gland coil directly into the skin before surface sweating begins.

An important therapeutic remedy for reversing dermatitis or dry skin conditions may be to maintain sweating on the surface of the skin.

On a more sensory note, moisturizing/wetting the hands with eccrine sweat on the palmar surfaces may improve tactile sense, and strengthen grip as a fight or flight mechanism.

As mentioned above, the eccrine seat glands produce and excrete antimicrobial peptides such as dermcidin, cathelicidin, and lactoferrin, which all act as a protective mechanism against skin infection.

Physiology of sweat

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. Density of these glands is not uniform throughout the body, as they are the most dens in palms and soles (~250-550 glands/cm²). These are activated by emotional and thermal stimuli.

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 seen that the number of eccrine 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 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 interstitial fluid and the eccrine gland.        

• Apocrine – These are in the axilla, breasts, face, scalp, and perineum. These are larger than eccrine glands, and open into hair follicles instead of on the skin. Similar to the eccrine glands, they are present from birth, 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 in size. 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.  

Mechanism of Perspiration

Secretion

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 binding of acetylcholine to muscarinic receptors on the basolateral membrane o the clear cell

-There is an efflux of KCl through Cl channels in the apical membrane and K channel sin 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.

Control 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. Thermoreceptor sin the abdominal region and surrounding muscles are also involved with controlling sweating. Thermal sweating is controlled by sympathetic cholinergic receptors.

Sweat production is facilitated through the release of acetylcholine from non-myelinated sympathetic class C postganglionic fibers, which bind onto the muscarinic receptors on the sweat gland. (reword)

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.

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 content 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.

Disorders of Sweat Gland Function

There are certain diseases or disorders that can alter the amount that a person sweats…

A chronic condition of Cystic Fibrosis would experience more sweating of Na+ and Cl- content due to a genetic deficiency or absence of functioning CFTR, which would lead to a lower reabsorption rates of Na+ and Cl- in the sweat ducts.

A chronic condition of Addison’s Disease would experience more sweating than normal of Na+ and Cl- due to an impaired adrenal cortex function, which would lead to a lower reabsorption rates of Na+ and Cl- in the sweat ducts.

A chronic condition of Diabetes Mellitus would cause decreased sweating with Type I and Type II diabetes mellitus. Possible mechanisms could be related to autonomic neuropathy and a decreased heat sensitivity, decreased maximal sweat rate, and/or a reduced number of active sweat glands. There is also an inhibited ability to control the heat, especially in higher temperatures and in less fit individuals.

A chronic condition of Multiple Sclerosis would cause decreased sweating due to lesions within the central nervous system, which would lead to a decreased sweat output per gland.

A chronic Spinal Cord Injury would cause decreased or absence of sweating in the on-sensory skin, which can be due to disruption in neural pathways involved in central and peripheral control of sweating. Contrarily, there is an increase in sweating in the sensory skin superior to the spinal lesion.                  

A chronic condition of severe Burns and Skin grafting would cause decreased or absence of sweating in the burned area, due to the removal of dermal and epidermal layers, including the sweat glands. Even as skin grafts heal, there is still a disruption in the ability to sweat.

An acute condition of sunburn would cause decreased sweating in artificially induced mildly sunburned skin

An acute condition of Miliaria rubra (heat rash or prickly heat) would cause decreased sweating due to pore occlusion via keratin plugs, which would cause mechanical blockage of sweat smoothy flowing onto the skin surface. This can be caused by increased humidity, which would cause excess sweat, on the skin surface for longer periods.

An episodic condition of Atopic dermatitis (eczema) would cause decreased sweating on the surface of the skin due to blockage of sweat pores by keratin plugs, sweat leaking into dermal tissue around the glands, and/or histamine-induced sweat prevention. Sweat glucose concentration can be higher than expected with acute atopic dermatitis.

A chronic condition of Anhidrotic ectodermal dysplasia would cause decreased or absence of sweating due to a small genetic disturbance, or eliminated sweat glands throughout the body.

A chronic or episodic condition of Primary hyperhidrosis will produce a greater amount of sweat, primarily affecting one or both axilla, palms, soles, and the head and face (craniofacial areas). This would be induced by neurogenic overactivity of sweat glands, which would otherwise be normal. This occurrence would be from genetic factors.

A chronic or episodic condition of Secondary hyperhidrosis will produce a greater amount of sweat on one or both sides, due to an underlying physiological condition (fever, pregnancy, menopause), pathology (malignancy, infection, cardiovascular disease, endocrine/metabolic, neurologic or psychiatric disorders), or medication.

A long-term tattoo usage can cause decreased sweating rate and higher sweat content of Na+ from  pharmacologically-induced local sweating compared to non-tattooed skin.

With acute or chronic use of medications, chemicals such as Antimuscarinic anticholinergic agents, carbonic anhydrase inhibitors, and tricyclic antidepressants can cause generalized hypohidrosis. As seen above hyperhidrosis is a factor in altered sweating and can be caused by cholinesterase inhibitors, SSRI, opioids, and TCA.  

Sweating induced Deficiencies

Hyponatremia - a condition where the plasma Na+ concentration is less than 135mmol/L. This can be life-threatening based on the plasma Na+ dilution, <125-130mmol/L, and how quickly it decreases. Similar to osmotic activity, the decreased solute concentration in plasma enhances the movement of water from the extracellular to the intracellular space, causing inflammation int he form of swelling in the brain and/or congestion in the lungs.

This can occur in healthy athletes or even in the clinical population.

Plasma Na+ concentration is responsive to changes in body water. Therefore, the main cause of hyponatremia is an increase in body mass due to hydration from water or hypotonic fluid, relative to body water loss.

Plasma Na+ concentration is also somewhat responsive to changes in mass balance of Na+ and K+, mainly from loss of electrolytes through sweat. Overhydration during an event greater than a 4 hour period can cause increased sweat Na+ losses, which can worsen losses in plasma Na+ concentrations.

Hyponatremia is also associated with dehydration, in which a higher sweat Na+ loss was the main cause of a drop in plasma Na+ concentration.

Someone's Na+ sweat concentration has an effect on their risk for resulting with hyponatremia, when long term thermoregulatory sweating is involved.

Cases of 'salty sweat', increased sweat Na+ and Cl- concentrations, also need to be considered. Salty sweat can be seen in heathy individuals and individuals with cystic fibrosis. Despite the causes, it is seen that excessive electrolyte losses from sweating can facilitate Na and Cl imbalances.

Heat and sweating

Exercising in the heat doesn’t necessarily have the same effect on sweating as one may think. Excessive body heat is generally diminished from evaporation of sweat on the skin. With high humidity, water vapour pressure of air is high, thus sweating is not as effective in decreasing the body heat since evaporation cannot efficiently happen.  

In hot conditions it is seen that there is minimal conductive heat exchange between air and skin due to the small temperature difference between the environment and the skin.    

Wind speed is also an important factor. Wind can accelerate heat transfer form the skin, as the flowing air replaces the skin surface air which contains evaporating water, with more dry air, which enhances sweat evaporation.

Quick moving activities such as running or cycling will not have the same effect of high heat and humidity as slower moving activities such as beach volleyball or field events. It is noted that body heat release increases with wind speed, and an increase in ambient temperature reduces air density and air resistance.

The heat and light from the sun also need to be considered. Sunburns can aggravate thermal perceptions from exercise, which would limit thermoregulation from a sweat gland responsiveness.

Micronutrient balance - NaCl

Heat acclimation results in better salt conservation with decreased sweat. After 10 days of heat acclimation, the concentration of sweat Na+ and Cl- can range from about 30% - 60%. In fact, heat acclimation may occur after only two consecutive days of heat exposure, and sweat Na+ concentration will gradually decrease. It is seen that with heat acclimation, sweating rate tends to increase on on the peripheral areas of the body such as the forearms, and not so much on the chest or back.

The mechanism for NaCl conservation can be due an increased responsiveness of the sweat glands to circulating aldosterone. This has an effect on Na+ reabsorption in the eccrine sweat duct by increasing Na-K-ATPase activity. For NaCl conservation to occur with heat acclimation, there needs to be a salt deficit. This would help explain the consumption of electrolytes to maintain balance in the body.

Refuelling? -

It is uncertain whether the consumption of Na+ has an effect on sweat Na+ concentration during exercise. Sweat glands generally respond to salt deficiencies within 1-4 days. It has been shown that it may take many days or weeks for the influence of ingested Na+ to have an effect of sweat Na+ concentration. With much less Na+ ingestion, it was seen that there were no differences or minimal differences on sweat Na+ concentration or the rate of Na removal. It was also foun that tehre were no difference sin Na+ concentration when Na+ was ingested prior to 1.5 hours of exercise.

In terms of minerals, it is seen that minimal intake of various minerals, which are within the body's allowable limits, have a low impact on sweat mineral loss.

Dehydration and Sweating

Hydration is very important before during, and after exercise in the heat, for optimal activity performance. However, there cannot be a set amount of fluid intake or loss, as there is a variable fluid need between individuals. Fluid loss by sweating and hydration status can be assessed by analyzing pre- and post- exercise variations in body weight and urine color and volume.

The consumption of salt before, during, and after activity is important to maintain sodium balance in the body. Specifically in hot environments when sweating and loss of electrolytes is high, this would help to retain and distribute the ingested water throughout the body.          

 

Heat acclimatization would happen with practicing of the activity int eh chosen/desired heat conditions. The acclimatization would be met with a decrease in sweat sodium concentration, an increase in sweat rate, decrease in core body temperature, and decreased heart rate with regular exercise in the heat. Work capacity will also be increased, with the reduction in the risk of exertional heat illnesses.

Ageing and Sweating

Older adults are seen to have a lower sweating response than younger adults. This would be a lower response per activated sweat gland from a particular pharmacological stimulus. It is seen that here is a decline in sweat response throughout adulthood, and there are regional differences within the body with the sweat gland function decline. However, this may be more likely due to a decrease in aerobic fitness and acclimation, as one ages. This may be due to the decrease in sensitivity of sweat gland to cholinergic stimulation.  

It's worth noting that UV and environmental exposures may play a role with sweat gland responsiveness as an individual ages. It’s interesting to note that there isn’t a big difference between sweating rates in older and younger adults with exercise in the heat, except for peak sweating rates in hot and dry climates.  

Thermal tolerance, between older and younger adults, is minimally affected when factors such as fitness level, body composition, and chronic disease, are not considered.

Gender/Sex and Sweating

It is interesting to note that Men have higher sweating rates than Women. They have a higher cholinergic response, and maximal sweating rates, than Women. However, when Men and Women with similar body mass, surface area, and metabolic heat production were compared, differences were only apparent in excess of usual environmental conditions, and metabolic heat production rate, which result in extreme evaporative needs for maintaining thermoregulation.

Sweat gland density is higher in Women than Men, which can be attributed to the lower body surface area n Women. Thus, the lower sweating rates from Women can be a result of lower output per sweat gland. In contrary, higher whole-body sweat rates in Men may be a result of higher body mass, and higher metabolic heat production from higher exercise intensities.

When considering differences between Men and Women, factors such as body size, surface area to mass ratio, heat acclimation status, aerobic capacity, exercise intensity, or environmental conditions all have a greater contribution than the differences of gender and sex in deciding the exercise-heat stress related autonomic responses. These factors can affect the evaporative rate needed for thermoregulation.

Menstrual cycle

Some other possible factors that can affect the stimulation of the autonomic response are maturation/development, altitude/hypoxia, circadian rhythm, and/or menstrual cycle. These alterations, though, in stimulation of regional sweating from an increased body core temperature are not directly related to whole-body sweating when it comes to exercise. In regards to the menstrual cycle, regional sweating during the luteal phase is decreased at a certain body temperature, since there is a greater threshold. Though there aren’t any major differences in whole-body sweating during the different menstrual cycle phases. However, for trained female athletes, exercising in the heat, the different phases of the menstrual cycle don’t have any effect on their physiology or performance.      

Inability to Sweat

The structure of sweat gland 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 time bigger in some individuals, than others. It is seen that the size of individual sweat glands, their response to methacholine, and secretory rate, are all directly related. Aerobic training and heat acclimation are contributors to an increase in sweat gland size and cholinergic responsiveness.      

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