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The BBC News Universal Wonders short video Why water is one of the weirdest things in the universe says after 01:50:

Water molecules can float upwards against the force of gravity. That’s because they love to stick to each other. They’re so good at it that they can actually pull each other up through tiny channels, such as the tiny blood vessels in your body. That’s how oxygen and nutrients reach the edges of your brain.

The same process, called capillary action, allows plants to move water from deep below the ground to nourish the leaves and branches that grow in the sunshine.

We learn about the importance of capillary action in water transport in plants, but I'd never heard that it was important for blood transport in Humans.

Question: How much does capillary action contribute to maintaining blood flow to the human brain? Is it a small effect compared to the difference between arterial and venous blood pressures, is it a substantial contributor?

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None. I have no idea what that video is talking about.

Capillary action of water happens when you have a thin tube and water likes to stick to the "edges." Blood vessels are already full of water, there is nothing for them to capillary up against: there is already water there, so there is no surface tension (there is no surface at all). Blood flow through capillaries is caused by the heart which creates a pressure gradient between the arterial and venous circulation, in the brain and everywhere else.


Cipolla MJ. The Cerebral Circulation. San Rafael (CA): Morgan & Claypool Life Sciences; 2009. Chapter 2, Anatomy and Ultrastructure. Available from: https://www.ncbi.nlm.nih.gov/books/NBK53086/

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Capillaries are very fine vessels that exist everywhere blood is distributed to tissues including the brain, and capillary action is a physical phenomenon of fine spaces that absorb liquids allowing anti-gravity pull of water on top of trees.

Same word, but unrelated objects here. Both come from the same Latin origin meaning hair. Capillary action is not the way capillaries work (blood is pushed in because of pressure).

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  • $\begingroup$ While "Capillary action is not the way capillaries work (blood is pushed in because of pressure)" sounds like a common-sense statement, can you actually support it somehow? Can you exclude any influence of capillary action in addition to the effect of blood pressure? Good Stack Exchange answers need good supporting links or references. Thanks! $\endgroup$ – uhoh Aug 16 at 14:45
  • $\begingroup$ You are welcome. Look up capillary action definition and tell me how it could possibly take place in vessels with varying diameter already filled with blood, having no equivalent of evaporation or other depressurization at the other end you find on a tree leaves besides blood lower pressure. And importantly, there is almost no attraction between the walls and blood. Unless you are talking about a non physiological situation where you dry up a piece of brain and then puts it in a water bed to get it wet, I have no idea how you can mix up those two different concepts in the first place. $\endgroup$ – Exocytosis Aug 18 at 8:58
  • $\begingroup$ It's not my place to tell you how something could possibly happen. This is Stack Exchange and an answer should try to support assertions whenever possible. Rather than tell me to look up something for you, it's more appropriate for you to find something suitable and cite it or quote from it. Concepts and phenomenon are not exclusive, two or more can apply simultaneously with different weights. Thus my question begins: "How much..." If you are certain it is zero, then you should state zero, support that, and let people vote on your assertion. $\endgroup$ – uhoh Aug 18 at 10:33

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