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Capillaries, velocity, and pressure

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David513

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Hello all,

I know that when you consider the total cross-sectional area of different types of blood vessels, capillaries are said to have a lower velocity of blood moving through them than arteries. Got it.

But directly after a graph depicting the above, Examkrackers has a graph showing pressure being extremely low in capillaries, lower than arteries. I assume that this is now considering the individual capillary and not the total pressure in all capillaries in aggregate? Otherwise, from a physics standpoint, it doesn't make sense to me, since by Poiseulle's Law pressure and velocity are inversely related.

Just want to confirm my suspicion that they changed the reference point from all capillaries in aggregate for the velocity graph to a single capillary for the pressure graph without informing the reader. It seems like if you consider all the capillaries, together, they would have a lower velocity and a higher pressure than the arteries, but an individual capillary has a higher velocity and lower pressure than an individual artery. Right?

Thanks in advance!
 

NextStepTutor_2

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Hello all,

I know that when you consider the total cross-sectional area of different types of blood vessels, capillaries are said to have a lower velocity of blood moving through them than arteries. Got it.

But directly after a graph depicting the above, Examkrackers has a graph showing pressure being extremely low in capillaries, lower than arteries. I assume that this is now considering the individual capillary and not the total pressure in all capillaries in aggregate? Otherwise, from a physics standpoint, it doesn't make sense to me, since by Poiseulle's Law pressure and velocity are inversely related.

Just want to confirm my suspicion that they changed the reference point from all capillaries in aggregate for the velocity graph to a single capillary for the pressure graph without informing the reader. It seems like if you consider all the capillaries, together, they would have a lower velocity and a higher pressure than the arteries, but an individual capillary has a higher velocity and lower pressure than an individual artery. Right?

Thanks in advance!

Overall blood flow must be equal across all segments of the circulatory system, otherwise we would have blood accumulation in certain areas and low perfusion in others. You are correct in stating that yes, while that an individual capillary is smaller than an artery that feeds it, the extensive branching of capillary beds means that total area the blood is now "seeing" is increasing and the velocity through each capillary is much smaller than it is in the artery. This makes sense as the capillaries are where all fluid/nutrient/waste exchange takes place so we want the blood to have sufficient time to complete these processes.

Thus, an individual artery would expect to have a fluid velocity greater than that seen in an individual capillary. this is due to the connection between fluid velocity and total area of the system.

biol-121-chp-21-the-cardiovascular-system-blood-vessels-and-hemodynamics-59-638.jpg

However, the blood pressure will drop the further from the heart you go. By that process, we would also expect the pressure in an average capillary to be much lower than the pressure in a given artery. This does not violate any fluid dynamics laws as again, we need to consider total energy of the fluid (Bernoulli equation) and the fact that these vessel walls are compliant. The continuity equation is only a part of the picture.

2109_Systemic_Blood_Pressure.jpg

hope this helps, Good luck!
 
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aldol16

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But directly after a graph depicting the above, Examkrackers has a graph showing pressure being extremely low in capillaries, lower than arteries. I assume that this is now considering the individual capillary and not the total pressure in all capillaries in aggregate? Otherwise, from a physics standpoint, it doesn't make sense to me, since by Poiseulle's Law pressure and velocity are inversely related.

The pressure of an incompressible fluid in a closed system is equal at each and every point of the system. That's how hydraulic lifts work.

Edit: Disregard. Blood vessels are modeled terribly by rigid tubes.
 

David513

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Thank you very much!

So basically because of capillaries always being part of capillary beds, even if you're looking at an individual capillary (which would have a very small cross-sectional area), the blood flow velocity would still be lower than that of the artery that feeds it.

I'm always grateful to @NextStepTutor_2 and the NextStep team (and you too, @aldol16 !).
 

NextStepTutor_2

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The pressure of an incompressible fluid in a closed system is equal at each and every point of the system. That's how hydraulic lifts work.

Edit: Disregard. Blood vessels are modeled terribly by rigid tubes.

Yes they are. For the most part, you can get away with these simple models on the MCAT, but the AAMC now expects a more thorough understanding of fluid dynamics in compliant vessels like our circulatory system.

Good luck!
 
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