Ayurveda Insights on Physiology of Sweating and Thermoregulation

Physiology of sweating and thermoregulation was well conceived by Ancient Ayurveda scholars. The detailed description of Svedavaha Srotas (channels carrying sweat), Sveda (sweat) as Mala (waste), its mechanism of formation on exposure to heat, therapeutic application of heat for inducing sweating under the concept of Svedan Karma (sudation) for purification and balancing the Dosha for management of disorders reflects the deep understanding of Ayurveda scholars about the physiology of sweating and temperature regulation. Keeping these facts in mind a detailed review has been done as an attempt to understand the physiology of sweating and temperature regulation mechanism described in Ayurveda in view of contemporary sciences and recent researches. DOI:10.18311/jnr/2019/23761


Introduction
The fundamental concept of human physiology in Ayurveda revolves around the concept of Dosha, Dhatu and Mala. These three have been considered as root for the sustenance and homeostasis of the body 1 since the balanced functional state of these three components is important for various functions as well as origin, development and progression of life.
Among these, Dosha are the biological entity governing and regulating all the physiological functions of the body, Dhatu are the major tissues of the body while Mala refers to the excretory products of the digestion and metabolism. But Acharya Charak has given more comprehensive understanding about Mala i.e. any factor which produces hindrance in the normal physiological functions or causes deterioration or pain in the body is considered as Mala. It includes aggravated Dosha i.e. Vata, Pitta and Kapha, Dhatu (tissues), or Mala (the waste products of metabolism, which get stuck to the orifices of the body, of various forms and are in process of being removed from the body) 2 . All the vitiated Dosha, Dhatu are said to be Mala and considered as the primary factor in causation of all diseases 3 . All these Mala can be classified into two types i.e. Aahara Mala and Dhatu Mala. Aahara Mala are Mutra (urine), Purisha (feces) and Sveda (sweat) 4 while Dhatu Mala are Kapha, Pitta, Khamala, Sveda, Keshaloma, Sneha of eyes and skin for Rasadi Dhatu respectively 5 . Detailed descriptions are available regarding the physiological functions and features of hypo and hyper functional state of Mutra (urine), Purisha (feces) and Sveda (sweat). Sveda is carried away via Svedavaha Srotas, plays an important role in excretion of waste metabolites produced during digestion and metabolism and temperature regulation along with Pitta.
of sweat secretion, causes of vitiation of Svedavaha Srotas, features of vitiated Svedavaha Srotas and their management. Relevant matters were also collected from online resources like Google scholar, Medline plus, PubMed health and central etc.

Sveda (Sweat)
Sweat is a fraction of Udaka (water) that comes out of Lomakoop (skin pores/hair follicles) on exposure to heat 6 . Sveda has been considered as Mala of both Aahara and Meda Dhatu 7 , as it is formed during digestion and metabolism. It has been also considered as Upa Dhatu of Meda Dhatu 8 . Sweat is carried by Svedavaha Srotas and the Moola of Svedavaha Srotas is Lomakoopa and Meda 9 .

Formation of Sveda and Temperature Regulation on Exposure to Heat
Sveda is formed of Udaka which comes out through Lomakupa (hair follicles) which is the Moola of Svedavaha Srotas, when it gets combined with Ushma (heat) 6 . This description regarding Sveda by the great physician Charak shows his understanding about the formation of sweat, that when body gets exposed to heat, in adaptive response there is sweating to lower down the body temperature. Sveda shares Aashraya Aashrayi relationship with Pitta Dosha 10 and has been considered as one of the important dwelling site of Pitta Dosha along with Rakta (blood) 11  Moreover, it is well understood that quantity of sweat secreted, depends upon the level of hydration and metabolism of the body. It is also well understood from the contemporary literature that sweat loss is one of the main mechanism regulating the temperature of the body 15 . Sweat controls body temperature, cools the body, purifies the blood etc. Thus, Sveda formation described by Ayurveda scholars physiologically simulates with sweating mechanism of contemporary literature.

Sweat Carrying Vessels: Description of Arterial and Venous Network of Skin in Relation to Sweat Formation
Sushruta has described about the arterial and venous network of skin in relation to sweat formation in skin under the description of Triyaga Dhamani 16 (blood vessels placed diagonally). Each one of four oblique or transverse Dhamani (blood vessels) gets further divided into hundreds and thousands of branches and finally becomes innumerable so that the entire body is covered like network, bound and pervaded; their openings are attached to the hair follicles 16 . These purvey Sveda (sweat to the exterior) and replenish Rasa both inside and outside. Through these vascular networks, only the active fraction of medicament applied in the form of massage, bath or paste enters into the body after being transformed in skin. They are also responsible for the perception of sensation of the touch both comfortable and uncomfortable 16 . Sushruta has described that Rakta (blood) is responsible for touch sensation or perception of stimulus along with Vata Dosha 17 . These are also the path for the manifestation of vitiated Dosha 18 . This description might be indicating the broad capillary network of the body which regulates permeation of fluid, secretion of sweat through vasodilatation and vasoconstriction and carrying touch impulses through regulating the proper blood flow ( Figure 1).

Temperature Regulation on Exposure to Cold
In Shishir Ritucharya (winter season regimen), they have discussed that the exposure of cold leads to Vatavriddhi which causes the closure of body pores (Romakoopavrodha) thus prevents the external heat loss and the heat gets internalized resulting the augmentation of Agni (power of digestion and metabolism) in winter season 19,20 . This description is similar to the contemporary sciences as exposure to cold leads to sympathetic stimulation causing increased metabolism, heat production and vasoconstriction in skin vascular bed that prevents the heat loss 21 (Figure 2).

Factors Affecting Sweat
• Psychosomatic constitution (Prakriti): Ayurveda has described that the quantity and odor of sweat depends upon psychosomatic constitution of an  Studies have reported that obesity is associated with increased heat production but the heat loss impedes due to excess deposition of fat in subcutaneous layer resulting in increased thickness of subcutaneous tissue and vasoconstriction in skin because of sympathetic over activity due to autonomic dysregulation 27 , so in obese the major site of heat loss is palm and sole due to presence of arteiovenous anastomoses. Degree of thermal insulation is positively correlated to degree of obesity 28 . Ayurveda has also given similar thought that in obesity there is Vata Vriddhi (~ autonomic dysfunction) and Srotoavrodha due to excess deposition of fat resulting in the augmentation of digestive fire and more appetite 29 , more intake of food and thus more formation of Aahara Mala i.e. sweat also. While the heat loss is hampered, results in increased core temperature and hence, more sweating. • Quantity of Sveda: Quantitative methods (Anjali Pramana) of assessment have been given for the measurement of fluid substances of the body. These measurements pertain to the ideal standards and are subjected to undergo variation in the form of increase or decrease in a normal individual and this can be ascertained by inference. Anjali Pramana of Udaka is 10 Anjali and as Sveda is formed from Udaka, its quantity will vary with the quantity of the Udaka 6 . The amount of secretion of Sveda will depend upon the state of Pitta Dosha and Prakriti of an individual. Hyper functioning of Pitta Dosha will lead to increased secretion of sweat, Pitta Prakriti individuals will have increased secretion of sweat 30 while Kapha Prakriti 31 individuals will have decreased quantity of sweat. • Composition of Sweat: Sweat mainly consists of water 32 , primary secretion formed by sweat gland is similar to plasma except, it does not contain proteins. It also contains Na + , K + , Cl + , urea and lactic acid. The concentration of K + in sweat is 1.2 times of plasma while urea is 2 times of plasma and lactic acid is 4 times of plasma. pH of sweat is 4.5 to 7.0; mineral composition of sweat depends upon various factors of an individual like, their acclimatization to heat, exercise, the particular stress source, the duration of sweating, and the composition of minerals in the body. Fukumoto et al, (1988) in their work found that the chloride concentration of sweat is well related to work intensity and appears to be dependent on thermal stress. Sodium and chloride concentration of sweat are higher after vigorous physical exercise than after exposure to thermal stress. It is not known that whether concentration would be similar in cases of other substances such as creatinine, urea, nitrogen and potassium 33

Conclusion
Review of literature elucidates that Ayurveda has deep insight about the physiology of sweating, sweat formation, its role in temperature regulation and excretory function of sweat as well as in body homeostasis. Ayurveda duly acknowledged the applied aspect of Svedavaha Srotas in maintenance of homeostasis of body and in elimination of the Mala (toxins), which has been successfully adopted and designed as the Svedana Chikitsa. Heat applied in Svedana produces heat stress and initiates the thermoregulatory mechanism and produces various haemodynamic changes helping in excretion of metabolic wastes with enhanced immune mechanism. Svedavaha Srotas has great role in temperature regulation, protection of body as well as excretory function.