In sauna the human body is subjected for a certain time to higher temperature than no matter where in natural environment. Many temporary physiological changes might be brought on in consequence of the high temperature.
Skin temperature rises in a few minutes to over 40 °C, although subsequent to incipient sweating - normally in three to five minutes - this temperature declines and increase bit by bit over again. After a twenty minute bath the temperature of skin amounts just about 40 °C. Rather lesser is the temperature of internal of the body. For example, in rectum and esophagus the temperature is around 37.5-38 °C after a twenty minute sauna session.
Due to the increased temperature dermal capillary vessels widen and the cardiac output increases 2-3 times as much to maintain ample blood pressure. In consequence we have extensive redistribution of cardiac output: the skin blood flow can increase to 50-70 % in the sauna, whereas normally amounts 5-10 % of the cardiac output. On the contrary, reduces the blood flow of internal and muscles. The changes of blood pressure are moderate. It is a small drop of diastolic and systolic blood pressure take place most often. Blood pressure rapidly rises in the cooling period, principally being swimming in ice-cold water or taken a cold shower. The increase of cardiac load in the sauna is close to that seen during fast walking.
On average sauna visitor loses about 0.5 kg of fluid during a session, which corresponds to less than 1 % weight loss. Sweat contains less salt than blood does, therefore moderate sauna-induced increments of blood potassium and sodium have occurred in some (but not in all) studies. The concentration of hemoglobin may rise to some extent as well. Altogether, fluid loss is relatively small during a common sauna bath, and it can be easily balanced after gulping a couple of glasses of water. There are no significant changes in blood coagulation attributable to the sauna.
In many ways sauna exerts influence on the endocrine system. As said by a good number of studies sauna stimulates the generation of norepinephrine, lactotropin, cardiac natriuretic peptide and growth somatotropic hormone, and activates renin-angiotensin-aldosterone system. The findings on the behavior of adrenocorticotropic hormone and hydrocortisone are somehow contradictory. So, it seems that no changes come to pass in adrenaline, follicle-stimulating hormone, testosterone, luteinizing hormone or thyroid hormone. The hormonal changes observed in the sauna are not unlike those seen in other bodily stress situations.
 Child in sauna
Children have limited ability to endure heat, since the ratio between their skin area and body mass is rather high, fat layer is thin and sudoriferous system is immature. However children can take sauna baths absolutely securely, on conditions that some precautions are remembered. Sauna bathing must take place on basis of child’s own accord only. Thermal stress should be limited in accordance with the age (10 min at 70 °C is too much for many children at 2-5 years) and the child must be aged enough to express discomfort in a way understandable to parents. Finnish children are among the healthiest in the world; 12 % of Finnish children have their first sauna experience before the age of one month, 70 % before the age of one year and 95 % before the age of two years.
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