More turbulant than expected
Uneven blood flow promotes the development of arteriosclerosis
Can we really assume that our heart pumps the blood through our arteries so slowly that a steady, turbulence-free blood flow is created? In the publication that has just been released in the Proceedings of the National Academy of Sciences of the USA (PNAS), an international team of researchers reveals that our bloodstream is often more turbulent than would be beneficial for the human body. Irregularities in the blood flow have been shown to promote inflammation and dysfunction of the inner layer of blood vessels, the endothelium. Inflammation in the endothelial cell layer can in turn lead to the development of arteriosclerosis, the disease of civilization, which is considered the most common cause of death worldwide. The research work was submitted by Duo Xu, who has been researching fluid dynamics at the Centre of Applied Space Technology and Microgravity (ZARM) at the University of Bremen for four years.
"Pulsating flows through tubular geometries are laminar provided that velocities are moderate." Thus begins the summary of the scientific work, which has now been published in one of the world's most renowned scientific journals. It means that no turbulence is created in a liquid if it is pumped sufficiently slowly through a pipe. In general, pulsating flows are more susceptible to turbulence than continuously flowing flows, but it has been assumed until now that no turbulence is created in the human circulatory system due to the low speed and high viscosity of the blood. Duo Xu, on the other hand, has now proven that only in ideal circumstances a turbulence-free flow behavior is always achieved. The central finding of the research team is that pulsating flows react very sensitively to geometric disturbances and thus become turbulent even at a lower flow velocity than would be the case with a non-pulsating, constant mass flow. Applied to the human blood stream, this means that turbulence occurs much more frequently than would be expected on the basis of classical fluid mechanics theory, since curves, unevenness or even stenoses - e.g. due to arteriosclerotic lesions - often occur in the human bloodstream.
The experimental evidence
The research team has proven both theoretically, on the basis of simulations, and experimentally that blood vessels with geometric irregularities cause turbulence. In the experiments, it is clearly visible how, during the phase in which the pulsating blood flow slows down, vortices are created at these critical areas, which quickly break down into turbulence. Only when the flow is accelerated with the next heartbeat it calms down again and becomes laminar. This means that in blood vessels, which are not ideally shaped, a disturbance of the blood flow can occur in each individual pulse cycle.
Why is turbulence dangerous to health?
The inner wall of the blood vessels, the endothelium, reacts very sensitively to shear stress. In this case, shear stress refers to the friction created by the flow of blood on the inside of the blood vessels. Normally, the endothelial cells are adjusted to a steady flow in one direction. If in each pulse cycle a turbulence with corresponding shear stress fluctuations and a reversal of flow occurs, it can trigger cellular dysfunctions which can lead to inflammation of the endothelium and, in the long term, to arteriosclerosis. For people with previous cardiovascular diseases, the research findings mean that there is an increased risk of either developing or worsening of atherosclerosis due to the occurrence of turbulence at existing irregularities or stenoses in the blood vessels. However, turbulence can also occur in the bloodstream of healthy people, which clearly shows us the high complexity and sensitivity of our blood circulation system - and also that research in this area is not yet completed.
The research was carried out by scientists from the Center of Applied Space Technology and Microgravity (ZARM) at the University of Bremen, the Institute of Science and Technology Austria (IST), the Friedrich-Alexander University Erlangen-Nuremberg and the Center for Applied Mathematics at Tianjin University. The experiments were carried out at the IST under Björn Hof, the simulations in the research group of Marc Avila at ZARM (FOR 2688 funded by the German Research Foundation DFG).
Link to the PNAS publication: www.pnas.org/content/early/2020/05/08/1913716117
Contact person for scientific questions:
Dr. Duo Xu
Prof. Dr. Marc Avila
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