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I recently learned that the velocity of blood moving in veins is faster than that in capillaries, but the blood pressure in veins is much lower than that of any other blood vessel. Since veins have a relatively high blood velocity (at least compared to capillaries), shouldn't they also have a higher blood pressure? Why don't velocity and pressure in fluids go hand in hand?
Question Date: 2015-10-26
Answer 1:

I am not sure that the velocity is faster in veins. It would actually vary more in veins. Blood in arteries is pumped by the heart, but it will slow down because of friction with the walls of the blood vessels (resistance). It is moving more slowly by the time it gets to the capillaries.

If you’re standing, blood would have a hard time getting from your feet to your heart. If you stood perfectly still for a long time, you might faint. But every time the muscles contract, they squeeze blood back to the heart. One-way valves keep the blood from being squeezed farther away from the heart. If you’re running a few miles, you are probably moving that blood back to the heart very fast. So the velocity of the blood in veins will vary due to muscle activity. So will the blood in arteries, but the difference may be greater in veins.

Higher velocity could mean higher pressure, but there are other variables to think about. A big one is the size of the vessels. Think about it this way, if you fill a balloon with air, then try to squash it, the pressure goes up. It’s not due to the speed of the air, it’s because the same amount of air is being forced into a smaller container. Likewise, if you want more pressure out of a hose, you put your thumb over some of the opening. This reduces the size of the tube diameter, so the pressure increases. Veins hold more blood than arteries, so the container is bigger. This would result in lower pressure.

Why do you think the walls of the veins and arteries are so different? This is a great physiology question. Thanks for asking, Becky Burton

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