velocity of blood in capillaries

[blah354]

I know there have been various other threads on this topic, but I'm still a little confused.
I know that pressure in the capillaries is relatively low and therefore should have a higher velocity. But then I've been reading that since capillaries have a large surface area, the velocity is very low. How do I make sense of both concepts?
Does this have something to do with blood pressure in the capillaries versus pressure on the walls?

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[Adamska]

The way I think of it is that basically from the heart to capillaries the blood is moving by way of the impulse from ventricular systole, but as you keep going farther from the heart the velocity is reduced, and the reason blood travels from venules back to the vena cavae is because of assistance from skeletal muscles. Then again, I haven't taken a college-level bio class in 4 years so I might be off.

 

[Charles_Carmichael]

First, think of it intuitively. Nutrient exchange occurs in the capillaries; do you want the blood to rush past very quickly (giving little time for nutrient exchange) or do you want blood to go through slowly (maximizing nutrient exchange)? You'd want it to go slowly so you get a good amount of nutrients for the tissues.

If you want to think of it more mathematically, just think of the continuity equation (area*velocity = area*velocity). Blood is coming in from the arterioles, which have a smaller total cross-sectional area (the key point here is the total, as in the sum of the cross-sectional areas of every single arteriole) than capillaries. By using the continuity equation, the velocity in the capillaries HAS to be slower than in the arterioles because you're dividing by a greater value of capillary cross-sectional area:

(Aarteriole * Varteriole)/(Acapillary) = Vcapillary

Edit: To make it even simpler, think about this equation (which is essentially the continuity equation): v = Q/A, where v is velocity, Q is blood flow, and A is total cross-sectional area. You know blood flow is the same everywhere in the CV system; it's the area that changes. Since capillaries have the highest total cross-sectional area (ie. the value in the denominator is the greatest for capillaries), the slowest blood velocity occurs in the capillaries.

Hope this helps.

 

[unDRdog]

First, think of it intuitively. Nutrient exchange occurs in the capillaries; do you want the blood to rush past very quickly (giving little time for nutrient exchange) or do you want blood to go through slowly (maximizing nutrient exchange)? You'd want it to go slowly so you get a good amount of nutrients for the tissues.

If you want to think of it more mathematically, just think of the continuity equation (area*velocity = area*velocity). Blood is coming in from the arterioles, which have a smaller total cross-sectional area (the key point here is the total, as in the sum of the cross-sectional areas of every single arteriole) than capillaries. By using the continuity equation, the velocity in the capillaries HAS to be slower than in the arterioles because you're dividing by a greater value of capillary cross-sectional area:

(Aarteriole * Varteriole)/(Acapillary) = Vcapillary

Edit: To make it even simpler, think about this equation (which is essentially the continuity equation): v = Q/A, where v is velocity, Q is blood flow, and A is total cross-sectional area. You know blood flow is the same everywhere in the CV system; it's the area that changes. Since capillaries have the highest total cross-sectional area (ie. the value in the denominator is the greatest for capillaries), the slowest blood velocity occurs in the capillaries.

Hope this helps.

well said..this is the way i think..i understand the math stuff but at the MCAT lvl i believe the intutive part of your brain should be doing most of the work...but thats just me!

 

[blah354]

thank you so much!!

 

[Deekay]

First, think of it intuitively. Nutrient exchange occurs in the capillaries; do you want the blood to rush past very quickly (giving little time for nutrient exchange) or do you want blood to go through slowly (maximizing nutrient exchange)? You'd want it to go slowly so you get a good amount of nutrients for the tissues.

If you want to think of it more mathematically, just think of the continuity equation (area*velocity = area*velocity). Blood is coming in from the arterioles, which have a smaller total cross-sectional area (the key point here is the total, as in the sum of the cross-sectional areas of every single arteriole) than capillaries. By using the continuity equation, the velocity in the capillaries HAS to be slower than in the arterioles because you're dividing by a greater value of capillary cross-sectional area:

(Aarteriole * Varteriole)/(Acapillary) = Vcapillary

Edit: To make it even simpler, think about this equation (which is essentially the continuity equation): v = Q/A, where v is velocity, Q is blood flow, and A is total cross-sectional area. You know blood flow is the same everywhere in the CV system; it's the area that changes. Since capillaries have the highest total cross-sectional area (ie. the value in the denominator is the greatest for capillaries), the slowest blood velocity occurs in the capillaries.

Hope this helps.

That's all very well but Bernoulli's principle states that flow mass rate is conserved and so therefore velocity increases while pressure decreases (constant energy) when the diameter of the vessel is reduced. As you say, if the total cross sectional area is greater then the flow mass velocity will be slower and/but frictional resistance is reduced in larger diameter vessels. This is because of lower velocity and relative reduction of fluid in contact with the vessel walls.This might suggest that there is less resistance to blood flow in capillaries when summed together and yet the literature states that flow is greatly decreased in the capillaries because of increased friction as the vessel diameter reduces. There appears to be a contradiction!?
However in this case there is a much greater area of fluid to vessel wall contact and so friction resistance is greatly increased and so flow rate is impeded.
Therefore we should end up with large cross sectional area and low velocity flow with high resistance to flow.
However this is not confirmed by the parallel resistance sum 1/Rt=1/R1+1/R2+1/R3 -> +1Rn. As you add the reciprocals of the individual capillary resistance values then you end up with a total resistance (Rt) that is less than the lowest individual resistance and approaches 1 or less than 1, where 1 is equal to the resistance in the large arterial vessel.

Regards Dave

 

[487806]

That's all very well but Bernoulli's principle states that flow mass rate is conserved and so therefore velocity increases while pressure decreases (constant energy) when the diameter of the vessel is reduced. As you say, if the total cross sectional area is greater then the flow mass velocity will be slower and/but frictional resistance is reduced in larger diameter vessels. This is because of lower velocity and relative reduction of fluid in contact with the vessel walls.This might suggest that there is less resistance to blood flow in capillaries when summed together and yet the literature states that flow is greatly decreased in the capillaries because of increased friction as the vessel diameter reduces. There appears to be a contradiction!?
However in this case there is a much greater area of fluid to vessel wall contact and so friction resistance is greatly increased and so flow rate is impeded.
Therefore we should end up with large cross sectional area and low velocity flow with high resistance to flow.
However this is not confirmed by the parallel resistance sum 1/Rt=1/R1+1/R2+1/R3 -> +1Rn. As you add the reciprocals of the individual capillary resistance values then you end up with a total resistance (Rt) that is less than the lowest individual resistance and approaches 1 or less than 1, where 1 is equal to the resistance in the large arterial vessel.

Regards Dave

Nice feedback. But please don't bump a 3-year-old thread just to join the conversation. The original members won't care about it.

 

[Deekay]

Oh yeah sorry I didn't notice the date of the last post. I was researching blood flow and surfing the net and came across this thread which reflected some of the contradictions of maths and intuition that I have had.

Regards Dave