Perspiration, also referred to as sweat, is the fluid secreted by the sweat glands found in mammalian skin.
Humans possess two distinct categories of sweat glands: eccrine and apocrine. Eccrine sweat glands are widely distributed across the body, primarily secreting a watery, saline perspiration typically induced by elevated body temperature. In contrast, apocrine sweat glands are confined to regions such as the axillae and certain other bodily areas, generating an initially odorless, oily, and opaque secretion that acquires its distinctive scent through bacterial decomposition.
For humans, perspiration primarily serves as a mechanism for thermoregulation, facilitated by the aqueous secretions of the eccrine glands. An adult's maximum perspiration rate can reach 2–4 liters (0.5–1 US gal) per hour or 10–14 liters (2.5–3.5 US gal) daily, though this rate is lower in prepubescent children. The evaporation of sweat from the dermal surface induces a cooling effect through evaporative heat loss. Consequently, increased sweat production occurs during hot environmental conditions or when an individual's muscles generate heat from physical exertion. Animals possessing limited sweat glands, such as canines, achieve comparable thermal regulation through panting, which involves the evaporation of water from the moist linings of their oral cavity and pharynx.
While perspiration is observed across numerous mammalian species, only a limited number, including humans, horses, certain primates, and some bovids, generate sweat specifically for thermoregulatory cooling. In equines, this cooling perspiration originates from apocrine glands and incorporates latherin, a protein acting as a wetting agent that migrates from the skin to the surface of their coats.
Definitions
- The terms diaphoresis and hidrosis can denote either general perspiration (rendering them synonymous with sweating) or excessive perspiration. In the latter context, they may be considered synonymous with hyperhidrosis or distinguished from it solely by specific clinical criteria within specialized medical terminology.
- Hypohidrosis refers to a reduction in perspiration, irrespective of its underlying etiology.
- Focal hyperhidrosis describes heightened or excessive perspiration localized to specific anatomical areas, including the axillae, palms, soles, face, or groin.
- Hyperhidrosis denotes excessive perspiration, frequently occurring as a secondary manifestation of an underlying medical condition (termed secondary hyperhidrosis) and typically affecting the entire body (referred to as generalized hyperhidrosis).
- Hidromeiosis signifies a diminished rate of perspiration resulting from the obstruction of sweat glands under humid environmental conditions.
- A substance or pharmaceutical agent that induces perspiration is classified as a sudorific or sudatory.
Signs and symptoms
Perspiration contributes to body odor when metabolized by cutaneous bacteria. Furthermore, pharmaceutical interventions for other conditions and dietary factors can influence odor. Certain medical pathologies, including renal failure and diabetic ketoacidosis, may also alter the characteristic scent of sweat.
Causes
Diaphoresis represents a non-specific symptom or sign, indicating a diverse range of potential etiologies. These include, but are not limited to, physical exertion, menopause, febrile states, exposure to toxins or irritants, and elevated ambient temperatures. Intense emotional states, such as anger, fear, or anxiety, along with the recollection of previous traumatic events, can also provoke perspiration, sometimes colloquially termed 'flop sweat'.
The predominant proportion of sweat glands throughout the body receive innervation from sympathetic cholinergic neurons. While sympathetic postganglionic neurons generally release norepinephrine and are consequently designated as sympathetic adrenergic neurons, those specifically innervating sweat glands secrete acetylcholine, thus earning the classification of sympathetic cholinergic neurons. Sweat glands, piloerector muscles, and certain blood vessels are all innervated by these sympathetic cholinergic neurons.
Pathological Perspiration and Symptoms
Diaphoresis can be linked to various pathological states, including hyperthyroidism and shock. Its co-occurrence with unexplained weight loss, fever or chills, palpitations, dyspnea, unconsciousness, fatigue, dizziness, myalgia, nausea, emesis, diarrhea, or thoracic discomfort indicates the potential presence of a severe underlying medical condition.
Diaphoresis is observed during an acute myocardial infarction (heart attack), stemming from heightened sympathetic nervous system activity, and frequently occurs in serotonin syndrome, a condition that can lead to severe illness or mortality. Various infections can also induce diaphoresis, often accompanied by high fever or chills, potentially resulting in hyperthermia. While most infections may cause some level of diaphoresis, it is a prominent symptom in serious infections like malaria and tuberculosis. Furthermore, pneumothorax can precipitate diaphoresis alongside chest wall splinting. Neuroleptic malignant syndrome and other malignant conditions, such as leukemias, are also known causes of diaphoresis.
Individuals with diabetes who use insulin injections or oral medications may experience hypoglycemia (low blood sugar), which can also induce diaphoresis.
Various pharmacological agents, including caffeine, morphine, alcohol, antidepressants, and certain antipsychotics, can induce diaphoresis. Withdrawal from substances such as alcohol, benzodiazepines, nonbenzodiazepines, or narcotic painkillers can also trigger this condition. Sympathetic nervous system stimulants, including cocaine and amphetamines, are similarly linked to diaphoresis. Ectopic catecholamine production, leading to diaphoresis, is a characteristic symptom of pheochromocytoma, a rare adrenal gland tumor. Additionally, acetylcholinesterase inhibitors, such as some insecticides, cause the contraction of sweat gland smooth muscle, resulting in diaphoresis. Historically, mercury was recognized for its diaphoretic properties and was extensively employed by physicians in the 19th and early 20th centuries to "purge" the body of illness. Nevertheless, mercury's significant toxicity often led to secondary symptoms, which were mistakenly attributed to the original disease being treated with mercurial compounds.
Infantile acrodynia, a condition resulting from childhood mercury poisoning, is characterized by profuse perspiration. Clinicians should promptly consider acrodynia in afebrile children exhibiting excessive sweating.
Certain individuals may develop an allergy to sweat. This allergic reaction is not caused by the sweat itself but rather by an allergenic protein secreted by bacteria residing on the skin. Tannic acid, in conjunction with showering, has been shown to mitigate this allergic response.
Hyperhidrosis
Hyperhidrosis affects millions globally, yet over half of those afflicted do not seek treatment, often due to embarrassment, insufficient awareness, or perceived lack of severity. Although it predominantly impacts the axillae, feet, and hands, generalized hyperhidrosis affecting the entire body is also possible. The face represents another frequent site for this condition. Uncontrolled sweating can be unexpected and distressing for individuals, leading to both physiological and emotional challenges. The condition is typically inherited. While not life-threatening, hyperhidrosis significantly impairs an individual's quality of life. Therapeutic options include antiperspirants, iontophoresis, and surgical excision of sweat glands. For severe cases, botulinum toxin injections or endoscopic thoracic sympathectomy, a surgical procedure involving the cutting of nerves that stimulate excessive sweating, may be considered.
Night Sweats
Night sweats, medically termed nocturnal hyperhidrosis, refer to episodes of excessive perspiration occurring during sleep. Affected individuals may or may not also experience excessive sweating while awake.
Among women over 40, a prevalent cause of night sweats involves hormonal fluctuations associated with menopause and perimenopause. This phenomenon is particularly common throughout the menopausal transition.
Although night sweats can be benign, they may also indicate a serious underlying medical condition. It is crucial to differentiate between night sweats stemming from pathological causes and those resulting from an overly warm sleep environment, such as an excessively hot bedroom or an abundance of bedding. Night sweats attributable to a medical condition or infection are characterized as "severe hot flashes occurring at night that can drench sleepwear and sheets, which are not related to the environment." Certain underlying medical conditions and infections responsible for these severe night sweats can be life-threatening and necessitate immediate investigation by a healthcare professional.
Mechanism
Thermoregulation in the human body is facilitated by the process of sweating. This physiological response is governed by a control center situated within the preoptic and anterior regions of the brain's hypothalamus, which houses specialized thermosensitive neurons. Furthermore, the hypothalamus's thermoregulatory capacity is influenced by afferent signals originating from cutaneous temperature receptors. Elevated skin temperatures lead to a reduction in the hypothalamic set point for sudation and enhance the gain of the hypothalamic feedback mechanism in response to fluctuations in core body temperature. Nevertheless, the sudorific response triggered by an elevation in hypothalamic (core) temperature significantly surpasses that induced by an equivalent increase in average skin temperature.
Sweating contributes to a reduction in core body temperature primarily through evaporative cooling occurring at the epidermal surface. During this process, high-energy molecules dissipate from the skin, releasing absorbed thermal energy from the body, which consequently lowers the temperature of the skin and superficial vasculature. Subsequently, cooled venous blood circulates back to the body's core, effectively mitigating increases in internal temperature.
Nervous system stimulation of sweat glands, leading to perspiration, typically arises in two distinct contexts: exposure to physical heat and periods of emotional stress. Generally, emotionally triggered sudation is localized to the palms, soles, axillae, and occasionally the forehead, whereas perspiration induced by physical heat manifests across the entire body surface.
Human perspiration is not solely composed of water; despite lacking protein, it consistently contains a minor proportion (0.2–1%) of dissolved solutes. Upon an individual's transition from a frigid to a warm climate, the body's sudorific mechanisms undergo adaptive modifications. This phenomenon, termed acclimatization, involves an elevation in the maximum sweating rate and a reduction in the solute concentration of the sweat. The daily volume of water expelled via sweat exhibits considerable variability, spanning from 100 to 8,000 milliliters (0.041 to 3.259 imp fl oz/ks). Under highly extreme conditions, sodium solute loss can reach up to 350mmol/d (or 90mmol/d in acclimatized individuals). During moderate-intensity physical activity, hourly sweat losses can average as much as 2 liters (0.44 imp gal; 0.53 US gal) of water. Conversely, in cool environments and without physical exertion, sodium excretion can be minimal (below 5 mmol/d). The concentration of sodium in sweat typically ranges from 30 to 65 mmol/L, contingent upon the extent of acclimatization.
Equines possess a dense, water-resistant, and hirsute coat, which would ordinarily impede the swift transfer of sweat from the skin to the hair surface, a process essential for effective evaporative cooling. To overcome this physiological challenge, horses have developed latherin, a detergent-like protein secreted in high concentrations within their sweat. Unlike human perspiration, equine sweat is produced by apocrine glands. This protein enhances the wetting of the horses' coat hairs, thereby facilitating water flow for evaporative cooling. The existence of this protein is evidenced by the characteristic lathering frequently observed on the coats of perspiring horses, particularly when subjected to friction. Under hot environmental conditions, horses engaging in three hours of moderate-intensity exercise can experience fluid losses of 30 to 35 liters (6.6 to 7.7 imp gal; 7.9 to 9.2 US gal) of water, alongside electrolyte losses of 100 grams (3.5 oz) of sodium, 198 grams (7.0 oz) of chloride, and 45 grams (1.6 oz) of potassium.
Constituents
Perspiration is predominantly aqueous. A microfluidic model of the eccrine sweat gland elucidates the specific solutes that partition into sweat, their underlying partitioning mechanisms, and their subsequent fluidic transport to the skin's surface. Within this aqueous matrix, trace quantities of minerals, lactic acid, and urea are dissolved. While mineral content fluctuates, representative measured concentrations include: sodium (0.9 gram/litre), potassium (0.2 g/L), calcium (0.015 g/L), and magnesium (0.0013 g/L).
Compared to plasma and extracellular fluid, the concentration of Na+ ions in sweat is considerably lower (approximately 40 mM in sweat versus 150 mM in plasma and extracellular fluid). Initially, sweat produced within the eccrine glands exhibits a high concentration of Na+ ions. However, within the sweat ducts, these Na+ ions are reabsorbed into the surrounding tissue via epithelial sodium channels (ENaC), which are situated on the apical membrane of the epithelial cells forming the duct.
Numerous other trace elements are also excreted in sweat, with their concentrations, despite potential fifteenfold measurement variations, including zinc (0.4 milligrams/litre), copper (0.3–0.8 mg/L), iron (1 mg/L), chromium (0.1 mg/L), nickel (0.05 mg/L), and lead (0.05 mg/L). It is probable that many other less abundant trace minerals are eliminated from the body via perspiration, albeit at proportionally lower concentrations. Certain exogenous organic compounds are also present in sweat, such as an unidentified odiferous 'maple syrup'-scented compound found in several species within the mushroom genus Lactarius. In humans, sweat is hypoosmotic compared to plasma, meaning it is less concentrated. The pH of sweat typically ranges from moderately acidic to neutral, specifically between 4.5 and 7.0.
Sweat comprises a significant number of glycoproteins.
Additional Functions
Antimicrobial Properties
Sweat potentially fulfills an antimicrobial role, akin to other secretory fluids such as earwax, tears, saliva, and milk. This function is facilitated by a combination of glycoproteins that either directly bind to microbes or inhibit their attachment to the skin, thereby appearing to contribute to the innate immune system.
In 2001, investigators at Eberhard-Karls University in Tübingen, Germany, successfully isolated dermcidin, a substantial protein, from human skin. This protein, capable of being cleaved into various antimicrobial peptides, demonstrated efficacy in eradicating several bacterial and fungal species pathogenic to humans, including Escherichia coli, Enterococcus faecalis, Staphylococcus aureus, and Candida albicans. Dermcidin exhibited activity under conditions of high salt concentration and within the acidic pH range characteristic of human sweat, where its concentration ranged from 1 to 10 mg/ml.
Societal and Cultural Aspects
Synthetic Perspiration
For research applications, artificial skin has been engineered to mimic natural sweat rates, surface texture, and wetting characteristics. Furthermore, synthetic perspiration is commercially available for in-vitro experimentation, formulated to include 19 amino acids along with the predominant minerals and metabolites found in natural sweat.
Diagnostic Applications
Significant interest exists in utilizing sweat within wearable technology. Sweat offers the advantage of non-invasive and continuous sampling and sensing via electronic tattoos, bands, or patches. Nevertheless, employing sweat as a diagnostic fluid introduces several challenges, including extremely small sample volumes and the filtration or dilution of larger hydrophilic analytes. Presently, the primary commercial diagnostic application for sweat involves testing infants for cystic fibrosis, which relies on measuring sweat chloride concentrations.
References
References
Ferner S, Koszmagk R, Lehmann A, Heilmann W (1990). '[Reference values of Na(+) and Cl(-) concentrations in adult sweat]'. Journal of Respiratory Diseases (German). 175 (2): 70–5. PMID 2264363.
- Ferner S, Koszmagk R, Lehmann A, Heilmann W (1990). "[Reference values of Na(+) and Cl(-) concentrations in adult sweat]". Zeitschrift für Erkrankungen der Atmungsorgane (in German). 175 (2): 70–5. PMID 2264363.Nadel ER, Bullard RW, Stolwijk JA (July 1971). 'Importance of skin temperature in the regulation of sweating'. Journal of Applied Physiology. 31 (1): 80–7. doi:10.1152/jappl.1971.31.1.80. PMID 5556967.Sato K, Kang WH, Saga K, Sato KT (April 1989). 'Biology of sweat glands and their disorders. I. Normal sweat gland function'. Journal of the American Academy of Dermatology. 20 (4): 537–63. doi:10.1016/S0190-9622(89)70063-3. PMID 2654204.
- Media related to Perspiration at Wikimedia Commons