Blood pressure in circulatory system (Ch. 9)

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barlow_2
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Joined: Mon Jun 01, 2020 5:23 pm

Blood pressure in circulatory system (Ch. 9)

Post by barlow_2 » Thu Jul 23, 2020 7:42 pm

I am a little confused why blood pressure drops in the capillary compared to the artery. From what I have always been taught from physics is that the greater the area of moving fluid, the slower the flow, and the higher the pressure. If the combined area the capillary system is greater than the area of the arteries, wouldn't that mean that the pressure would be greater and not less?

Moreover, applying Bernoulli's equation confirms this general principle.

Thanks for the clarification!
NS_Tutor_Nancy
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Joined: Wed May 27, 2020 7:25 pm

Re: Blood pressure in circulatory system (Ch. 9)

Post by NS_Tutor_Nancy » Thu Jul 23, 2020 9:16 pm

Hey!

So a couple different things to unpack here. First, the cross-sectional area and velocity of blood flow can be related with the flow rate continuity equation where A1 x v1 = A2 x v2. Arteries have a much smaller area compared to the combined area of all the capillary beds, so blood flows much faster in the arteries. Looking at pressure, it is easiest to think of conceptually (the circulatory system is not the best model of Bernoulli's equation as blood is not an ideal fluid). The artery is closest to the heart pumping the blood, so it makes sense that the highest pressure would be in the artery. Arteries are also thick walled to withstand this pressure. By the time the blood reaches the capillaries, this pressure has decreased (which it would have to or the thin-walled capillaries would burst!) The pressure decreases the farther away from the heart you get. You could think of it like a water fall feeding into a river. Right at the bottom of the water fall the water is going to be choppiest and as you get farther away the water will start to calm down.

I hope this helps!
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