The Vascular System (Blood) and Physics

[byeh2004]

When we're relating the vascular system to physics what do we use in fluid dynamics? Do we use anything else?

For blood flow I know Q=Av, but I'm not sure if that's the right pressure. Because capillaries are way smaller in diameter, shouldn't the velocity be faster (considering if we are dealing with the same flow rate)?

How do we relate the Bernoulli equation to the vascular system?
Atm P is the same so we only have (1/2)*rho*v^2 + rho*g*h = (1/2)*rho*v^2 + rho*g*h

Does P=F/A come into any of this? How about rho*g*h? Is hydrostatic pressure related to blood pressure or blood flow?

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[futuredoctor10]

When we're relating the vascular system to physics what do we use in fluid dynamics?

I think all the equations you have listed are valid for the vascular system, if the question stem states to assume blood has "ideal fluid flow." I believe blood & our blood vessels do not strictly follow these equations but I really doubt we'd have to know deviations from the "ideal" - so I'd say for PS, yes just these equations are what one would use.

For blood flow I know Q=Av. Because capillaries are way smaller in diameter, shouldn't the velocity be faster (considering if we are dealing with the same flow rate)?

Yep capillaries have smallest area and thereby largest velocity, I guess. Assuming blood = ideal fluid

How do we relate the Bernoulli equation to the vascular system?
Atm P is the same so we only have (1/2)*rho*v^2 + rho*g*h = (1/2)*rho*v^2 + rho*g*h

I don't think this equation would apply really. If it does I am not sure how

Does P=F/A come into any of this? How about rho*g*h? Is hydrostatic pressure related to blood pressure or blood flow?

Yes, if they asked what is the pressure on the walls of the vessels you could calculate P= F/A.

If they asked for the difference in pressure from the heart to the toes, a distance of 2 meters, then you could calculate rho*g*h as that pressure difference.

Hydrostatic pressure is the pressure that causes flow out of the capillaries during capillary exchange (when gases/nutrients exchange with the blood). Not sure on its relation to blood pressure directly though? As far as I know it just has effect during capillary exchange, where osmotic pressure opposes it (pulling fluid back into the capillary)

Take it with a grain of salt (what I've said) because I'm just kind of predicting/guessing!

 

[jisu23]

Hmm..according to EK, in terms of Q=Av, the capillaries have the largest total cross-sectional area compared to arteries/arterioles/venules/veins and thus have the lowest velocity. No idea about the other questions though

 

[FutureInternist]

Hmm..according to EK, in terms of Q=Av, the capillaries have the largest total cross-sectional area compared to arteries/arterioles/venules/veins and thus have the lowest velocity. No idea about the other questions though

Correct. While individual arteries have a much smaller CSA than the aorta, the total CSA after adding up each artery is way higher --> dec velocity which makes sense since you would want the blood to be slowest in the gas/nutrient exchange portion of the vascular system.

 

[FutureInternist]

http://forums.studentdoctor.net/showthread.php?t=560007

 

[Vihsadas]

Guys, I don't have the links handy, but search this forum for a few threads that have already discussed bernoulli's principle and blood flow in depth. To summarize:


Blood flow does NOT follow Bernoulli's principle. Bernoulli's prinicple does not apply!