Blood circulation

Blood circulation does more work during a sauna session than during almost any comparable period of rest, because the transport of heat is laid on the cardiovascular system: blood carries heat from the interior of the body to the skin, where it can be given off, and in a hot room the rate of that transport must increase several-fold. Almost everything that happens in the body during bathing — the flushed skin, the fast pulse, the redistribution of blood between organs — is an expression of the circulation responding to heat stress.

Circulation and heat transport

The body transfers heat between its interior and its surface almost entirely through blood flow. Arterial blood warmed by the deep organs flows through the skin vessels, where dilated capillaries allow part of the heat to move to the surface and be lost by radiation, convection or, above a certain skin temperature, by the evaporation of sweat. When the environment is warmer than the body, as in a sauna, this same mechanism turns partially into a liability: the dilated skin receives heat from the air as well as from the interior, and the circulation's job becomes the delivery of blood to the surface where evaporating sweat can cool it. The result is a large increase in skin blood flow — several times the resting value — at the cost of a heavy load on the heart.

Heart rate in a sauna rises accordingly, and cardiac output (the volume of blood pumped per minute) increases by roughly 50 to 70 per cent in an ordinary session, driven mainly by the faster heart rate. The redistribution of this increased output is characteristic of heat exposure: the skin receives a growing share, while the blood supply to the abdominal organs and, in particular, to the kidneys is relatively reduced during the acute heat load. These shifts are normal physiology, but they explain two practical cautions: fluid losses must be replaced (see Dehydration and Electrolyte balance), and prolonged, very hot sessions put stress on organs whose circulation is temporarily curtailed, which matters for people with kidney disease (see Kidney disease and sauna).

Pressure, pulse and recovery

Because the skin vessels open wide, the resistance of the peripheral circulation falls, and the body compensates by raising cardiac output while blood pressure follows a characteristic curve: a moderate systolic rise early in the session, then a gradual settling, and a dip below the resting level in the minutes after the room is left. The sauna thus behaves, in circulatory terms, like mild-to-moderate dynamic exercise — a comparison made directly by studies that recorded heart rate and blood pressure during bathing and during submaximal cycling.

Recovery after the session is as much a part of the bath as the heating: as the skin cools, its vessels constrict, blood returns from the surface, and the pulse slows. A swim or plunge in cold water accelerates this by causing strong vasoconstriction of the skin vessels; the alternation of vasodilation and vasoconstriction is the physiological content of Contrast bathing and of the cold plunge routines that follow the sauna in many traditions. Some facilities offer a warm foot bath beforehand; immersion of the feet in warm water dilates the vessels of the legs locally, so that the circulation begins its session already adjusted to the seated posture (see Foot bath). Gentle movement, such as walking, and massage after the bath (see Banya massage) are used in several traditions to help the circulation return to its ordinary pattern; the general advice is to allow the cooling phase to happen gradually and to avoid standing abruptly while the vessels are still wide (see Fainting in a sauna).

Circulatory load and its limits

The heat-induced rise in circulatory work is well tolerated by healthy people and is part of the reason regular bathing is associated with cardiovascular adaptations: repeated sessions are accompanied by a degree of Heat acclimation, higher heart-rate reserves at rest, and, in the epidemiological literature on Finnish men, associations between frequent bathing and lower rates of cardiovascular disease. The association should not be read as a guarantee of safety: the acute load itself is considerable, and persons whose circulation has little reserve — those with high or low resting blood pressure, heart failure, cardiac arrhythmia or coronary disease, and those on medication that affects the vessels or the heart — may find the same session dangerous (see Sauna contraindications and Medical advice before sauna).

Measuring one's own pulse during a session is possible with the devices described under Heart rate monitoring in a sauna; measuring blood pressure validly is harder in the heat (see Blood pressure measurement in a sauna). Symptoms of circulatory distress — dizziness, chest discomfort, palpitations, unusual breathlessness or a sense of weakness — are reasons to cool down promptly (see Overheating prevention); trained staff at a wellness facility, where one can be observed, are part of the safety net (see Sauna safety).

Further reading

• Vasodilation
• Cardiovascular effects of sauna
• Heart rate in a sauna
• Blood pressure in a sauna
• Thermoregulation

References

• Kukkonen-Harjula K, Kauppinen K. "Health effects and risks of sauna bathing." International Journal of Circumpolar Health 65(3):195–205, 2006. doi:10.3402/ijch.v65i3.18102
• Heinonen I, Laukkanen JA. "Effects of heat and cold on health, with special reference to Finnish sauna bathing." American Journal of Physiology – Regulatory, Integrative and Comparative Physiology 314(5):R629–R638, 2018. doi:10.1152/ajpregu.00115.2017
• 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
• Tansey EA, Johnson CD. "Recent advances in thermoregulation." Advances in Physiology Education 39(3):139–148, 2015. doi:10.1152/advan.00126.2014
• 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