Key highlights
In the sauna, it takes approximately two to three minutes prior to feeling any type of heat stress response
When we go into the sauna and heat ourselves up, our nervous system alters our heart rate and blood flow. This leads to a redirection of blood flow, increasing skin blood flow.
This increase in skin blood flow leads to an increase in shear stress, which when repeated over time, has the potential to improve our vascular function!
I have personally been using the sauna for close to 20 years. Although that may not be as long as the average Nordic or Finnish user, I always make it a point to take notes of how I feel, and what my body is experiencing during my sauna exposures. Till this day, it remains very important for me to be able to describe the feelings and sensations of a sauna experience to someone whenever it is necessary (which is not very often in Finland).
The human body is a perfectly orchestrated organism engineered by nature. The majority of the processes occurring in it are tightly regulated to achieve and maintain stability. This is what the term homeostasis refers to.
Our breathing, digestion, heart rate, and other processes self-regulate according to the stresses we subject our bodies to.
This is an essential mechanism as it serves to sustain the body’s physical function and keep us alive. This is a vital point to understand because ultimately, we decide what we would like to subject our bodies to, and the body, being the brilliant piece of engineering that it is, simply adapts to these demands and continues functioning (thankfully!).
The body will react acutely or immediately to the stress that it is placed under as a result of the homeostatic mechanism mentioned earlier. With sufficient regularity however, the body starts to alter its baseline function. What the body does NOT do however, is to distinguish what is distress or eustress.
It simply adapts, in an attempt to overcome the stress to maintain stability.
For instance, the hearts of seasoned endurance athletes are often enlarged due to the regular and repetitive cardiovascular stress it is placed under. Similarly, the hearts of regular tobacco smokers are also enlarged. One of the main distinctions is that the positive stress (eustress) that endurance athletes subject their hearts to lead to improved cardiac function, whereas the negative stress (distress) that smokers subject their hearts to results in compromised function. It is also one of the main reasons why athletes or people who live at altitude are able to outperform their counterparts who live at sea-level due to the difference in oxygen levels.
With that in mind, we can begin to explore what subjecting our bodies to elevated temperatures experienced during whole body hyperthermia (or heat) does. In the sauna, it takes approximately two to three minutes prior to feeling any type of heat stress response. Try to go the sauna completely dry (no shower before) and you will see what I mean. After the initial couple of minutes, the signature physiological response will kick in. You will notice tiny beads of sweat starting to form, preceded by vasodilation of the blood vessels close to the surface of the skin.
Figure 1: Blood arteries showing a normal artery, vasoconstriction and vasodilation
This response enables our blood to flow to the surface of the skin so that convection can occur. A somewhat similar process happens when we boil a pot of water. The warmer molecules of water (blood from the core) travel to the upper surface of the water (skin surface) to help dissipate the overall heat load. Scientifically speaking, the process is much more complicated and intricate than that, as the chemicals in our body that govern vasodilation and vasoconstriction (Figure 1, above) alters according to the duration and magnitude of heat stress the body experiences (Cheng & MacDonald, 2019). We will get back to these chemicals shortly.
When we sit in the sauna for a prolonged period of time, skin blood flow can experience as much as a ten-fold increase. This is balanced by the reduction of blood flow to the brain and the core, which is one of the main reasons why many sauna users, especially first-timers, experience light-headedness or syncope if they had been in the sauna for a long continuous period of time. For apparently healthy people, this is not particularly harmful in and of itself, though it should still be duly noted. Perhaps not the best time for complicated conversations as well!
Figure 2: Forces exerted on the blood vessel via blood flow (Adapted from Jones, Noble & Eichmann, 2006)
What is responsible for the blood flow and sweat response changes during sauna exposure?
This is regulated by our nervous system, which incidentally also governs the changes to our heart rate and the maintenance of our blood pressure (Greaney, Kenney & Alexander, 2016).
This naturally leads us to the next question, how does all this result in improved health or cardiovascular function?
The increase in blood flow leads to more shear stress; a crucial physiological phenomenon responsible for vascular adaptations. Bear with me, as it is vital to understand each point clearly step-by-step in order to appreciate the entirety. You're doing great so far!
Remember those chemicals that control vasodilation and vasoconstriction of our blood vessels? Their production is generated via shear stress. Cells in our blood vessels detect shear stress through changes that occur in their fibers (Figure 2, above).
The increase in skin blood flow demand during sauna exposure leads to additional (vascular) shear stress, which has been postulated as one of the mechanisms that lead to improved function when repeated regularly (Tinken et al. 2009).
Conclusion
When we go into the sauna and heat ourselves up, our nervous system alters our heart rate and blood flow to help our bodies cool down in order to maintain homeostasis. This consequently leads to a redirection of blood flow, increasing skin blood flow. This increase in skin blood flow leads to an increase in shear stress, which when repeated over time, has the potential to improve our vascular function! Now how easy was that?
There are many other processes that happen as well, as nothing the body does is as simple as it seems. If that was not enough information for you, do give Pizzey and colleagues’ (2021) paper a read.
References
1. Cheng, J. L., & MacDonald, M. J. (2019). Effect of heat stress on vascular outcomes in humans. Journal of applied physiology (Bethesda, Md. : 1985), 126(3), 771–781. https://doi.org/10.1152/japplphysiol.00682.2018
2. Greaney, J. L., Kenney, W. L., & Alexander, L. M. (2016). Sympathetic regulation during thermal stress in human aging and disease. Autonomic neuroscience : basic & clinical, 196, 81–90. https://doi.org/10.1016/j.autneu.2015.11.002
3. Jones, E. A., le Noble, F., & Eichmann, A. (2006). What determines blood vessel structure? Genetic prespecification vs. hemodynamics. Physiology (Bethesda, Md.), 21, 388–395. https://doi.org/10.1152/physiol.00020.2006
4. Tinken, T. M., Thijssen, D. H., Hopkins, N., Black, M. A., Dawson, E. A., Minson, C. T., Newcomer, S. C., Laughlin, M. H., Cable, N. T., & Green, D. J. (2009). Impact of shear rate modulation on vascular function in humans. Hypertension (Dallas, Tex. : 1979), 54(2), 278–285. https://doi.org/10.1161/HYPERTENSIONAHA.109.134361
5. Crandall, C. G., & Wilson, T. E. (2015). Human cardiovascular responses to passive heat stress. Comprehensive Physiology, 5(1), 17–43. https://doi.org/10.1002/cphy.c140015
6. Pizzey, F. K., Smith, E. C., Ruediger, S. L., Keating, S. E., Askew, C. D., Coombes, J. S., & Bailey, T. G. (2021). The effect of heat therapy on blood pressure and peripheral vascular function: A systematic review and meta-analysis. Experimental physiology, 106(6), 1317–1334. https://doi.org/10.1113/EP089424
7. Ketelhut, S., & Ketelhut, R. G. (2019). The blood pressure and heart rate during sauna bath correspond to cardiac responses during submaximal dynamic exercise. Complementary therapies in medicine, 44, 218–222. https://doi.org/10.1016/j.ctim.2019.05.002
