Vasodilation
Vasodilation is the widening of blood vessels, most importantly of the small arteries and arterioles, that accompanies almost every physiological response of the body to heat. In the sauna it is the mechanism that delivers warm blood to the skin, makes the bather visibly flushed, and sets in motion the changes in blood pressure and heart rate that characterise bathing. It is one of the most frequently used terms in sauna physiology, and understanding it explains much of what the body does during a session.
Mechanism
The calibre of an arteriole is set by the contraction state of the smooth muscle in its wall, which is regulated by the nervous system, by hormones and by substances produced in the vessel wall and in the surrounding tissue. Heat acts on this system at several levels. Warm blood reaching the skin vessels relaxes their muscle directly; local reflexes initiated by heated skin contribute; and the vessels respond to mediators released by the endothelium, the inner lining of the vessel wall, of which nitric oxide is the best known. Signals from the thermoregulatory centre of the brain also reduce the sympathetic constricting drive to the skin while leaving the constriction of other beds, such as the gut, largely intact. The opposite process, narrowing of vessels, is vasoconstriction (described in the glossary).
During strong heat exposure the skin vessels can open to the point that skin blood flow rises several-fold above resting values. The effect is most visible in the face and trunk, which become red, and it is the reason a bather "glows" after a session. At the same time blood is redistributed: more of the cardiac output goes to the skin, and some organs, including the kidneys, receive a smaller share during the acute heat load, which is one reason that Dehydration and Electrolyte balance deserve attention in long sessions.
Vasodilation in thermoregulation
The purpose of the response is transport rather than temperature sensing. Core body temperature is defended by moving heat from the interior to the surface, where evaporating sweat removes it; vasodilation is the transport half of that arrangement. When the air temperature approaches skin temperature, radiation and convection become useless as routes of heat loss, and nearly the whole cooling burden falls on the combination of skin blood flow and evaporation. In a sauna, where the air is much warmer than the skin, the same vasodilation has the opposite result at the surface — it delivers heat into the skin, raising Skin temperature — but it also keeps the interior supplied with blood that the sweat film can cool, and it is the main reason the core rises only slowly despite the fierce environment (see Thermoregulation).
Because vasodilation lowers the resistance of the whole peripheral circulation, the heart must work harder to maintain arterial pressure; the result is the raised heart rate and the characteristic pressure curve described in Heart rate in a sauna and Blood pressure in a sauna. In a healthy person this is ordinary physiology, similar in load to mild exercise. The load becomes clinically relevant when the cardiovascular system has little reserve — in heart failure, in arrhythmia, or with medication that interferes with vascular tone — which is why those groups are treated cautiously in the sauna (see Sauna contraindications and Medical advice before sauna).
Vasodilation and the sauna bath
The time course of vasodilation during bathing follows the rise in skin temperature: within the first minutes the flush appears, and it deepens as the session proceeds. Adding water to the stones ('löyly') raises humidity, which slows evaporation, so skin blood flow stays high or rises further while the sweat cannot cool effectively; the circulatory demand is therefore not lower in a humid session. Cooling reverses the process: on leaving the hot room, skin vessels constrict again as the skin cools, and in a cold plunge bath the constriction is strong and abrupt, which is the physiological basis of contrast bathing and of the alternating-hot-and-cold routines described there.
- Long-term view
Repeated sessions of heat-induced vasodilation are the natural explanation candidates for the cardiovascular findings reported in the sauna literature — the associations of frequent bathing with lower blood pressure incidence and lower cardiovascular mortality in Finnish cohort studies. The evidence is consistent with, but does not prove, the hypothesis that regular vasodilatory stress improves vascular function, for example through repeated effects on the endothelium. Reviews of the field state the position carefully: the acute physiology is well established, while the long-term effects rest on a smaller number of observational and mechanistic studies (see Clinical sauna studies and Sauna epidemiology). Treatments that dilate vessels for other purposes — hot baths, warm compresses, some injections — are therefore not interchangeable with sauna bathing, and neither is the sauna a substitute for medical treatment of vessel disease.
- Practical relevance
Vasodilation explains several everyday sauna observations. The reddened skin after bathing is harmless and fades as the vessels return to their resting tone. The dizziness felt on standing up quickly after a hot session is the consequence of dilated vessels racing to refill the legs against gravity while the blood pressure near the head drops — the remedy is to stand slowly (see Fainting in a sauna). And the flush that appears in a bather who is otherwise comfortable is a normal sign that the heat-loss machinery is working; it is not itself the cause of the heat load, which is set by the room (see Heat stress and Overheating prevention). Sauna and health discusses the overall balance of effects, and Sauna research the quality of the evidence.
See also
References
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- Laukkanen JA, Laukkanen T, Kunutsor SK. "Cardiovascular and other health benefits of sauna bathing: a review of the evidence." Mayo Clinic Proceedings 93(8):1111–1121, 2018. doi:10.1016/j.mayocp.2018.04.008
- Ketelhut S, Ketelhut RG. "The blood pressure and heart rate during sauna bath correspond to cardiac responses during submaximal dynamic exercise." Complementary Therapies in Medicine 44:218–222, 2019. doi:10.1016/j.ctim.2019.05.002
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- Hussain J, Cohen M. "Clinical effects of regular dry sauna bathing: a systematic review." Evidence-Based Complementary and Alternative Medicine 2018:1857413, 2018. doi:10.1155/2018/1857413