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The overall physiological effects of sauna exposure have been presented previously .The cardiovascular system combats thethermal stress by cutaneous vasodilatation,and increased skin blood flow, heart rate andsweating. The heart rate accelerates up to twice the resting rate and even more.

The cardiac output is estimated to increaseby about 70 % over the resting state, the total peripheral resistance of the vessels decreases by about 40 % , the stroke volume is unaffected,and the diastolic and mean arterial pressures decrease, with practically no change in the systolic pressure. The overall work of the heart, as estimated from the heart rate and systolic blood pressure, indicating myocardial oxygen demand, does not increase greatly.

The strain on the heart and the cardiovascular system depends not only on the temperature in the hot room, its air humidity and the duration of stay in the heat, but also on the type of cooling employed during the cool-off session. The skin temperature fluctuates from up to 40°C in the hot room, down to 33°C upon icewater immersion. The core temperature, as
measured from the esophagus, is more stable, rising in the hot room at an average rate of 0.07°C × min-1 up to 38°C, then accelerating to 0.4°C × min-1 up to 39°C, and returning to initial values rapidly after the exposure.The rate of reduction depends on the type of cool-off.

Sweating results in a weight loss of about 0.5 to 1 kg. The water should be replenished by drinking during, or after the sauna bath. Athletes in sports performed in weight classes often use prolonged sauna bathing to decrease their weight rapidly. However, this can impair sports performance.

The sympathetic nervous system and the hypothalamus-pituitary-adrenal hormonalaxis are activated to maintain thermal balance. The overall effect of the hormonal changes is water retention and a fight-orflight response of the organism, manifested as reduced perception of pain, elated mood and alertness. The thermal challenge of the bath
cycle is reflected in increased plasma concentrations of noradrenaline. The adrenaline concentrations have been reported to remain unchanged, or to increase. The discrepancy is due to different bathing patterns, especially the type of cool-off, and to different blood sampling techniques. When blood samples were taken in the hot room and the subjects were moved in wheelchairs for cooloffs, plasma adrenaline concentrations were elevated during bathing in young women.

Sauna with cool-offs at room temperature did not elicit increased circulating adrenaline levels, whereas a sauna alternated with icewater immersion resulted in increased adrenaline in persons used to winter swimming. The bath cycles elicit secretions of ACTH in some studies and, especially with ice-water immersion, of cortisol. Plasma growth hormone, prolactin and ?-endorphin have also been reported to increase. The reduced plasma volume  and loss of sodium in the sweat activate the renin-angiotens in aldosterone system. The plasma arginine vasopressin levels and the plasma atrial natriuretic peptide concentrations rise during a bath. In men, the plasma testosterone concentrations do not change.

Evidence of regularsauna bathing reducing male fertility was not found. A two-week sauna exposure altered sperm movement characteristics, but they were reversed within one week after the end of bathing sessions. A comfortable relative humidity of 15 to 30% keeps the mucous membranes of the upper airways moist. Provided that the sauna air is hot enough, about 80 to 90°C, there is no concern about viable micro-organisms being inhaled. The respiration becomes more frequent and shallow, while the total respiratory minute volume increases. The vital capacity (VC), the peak expiratory flow rate (PEF), and the forced expiratory volume in the first second (FEV1) all increase, implying improved pulmonary ventilation. All changes are minor, roughly 10 %, and initial levels are reached quickly after the sauna. The diffusing capacity did not change in healthy subjects.

Courtesy of: Health Effects And Risks Of Sauna Bathing

Katriina Kukkonen-Harjula, D. Med. Sci., Adjunct Prof., Senior Researcher

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