Effects of gravity on blood pressure

[coolia]

When you stand up, blood pressure in your feet rise the most and become appx. 180 mmHg. How then does blood flow throughout the vascular when aortic pressure is 120 mmHg? difference in P=FlowX Resistance

 

[Dale]

Hi coolia,

Sorry I missed this earlier. First, I think your 180 mmHg is a little too high, but you are correct that the pressure can often be greater than could be hydrostatically pulled up without some other mechanisms. There are two of which I am aware. First, due to breathing the pressure in the chest is, on average, about 5 mmHg lower than atmospheric. Second, the larger veins have valves in them that allow only one-way flow, when the leg muscles contract these valves help push the blood back up. Vericose veins are what result when these valves no longer function.

 

[coolia]

Ok, I agree with that in the circulatory system. However, I was reading Barnes and Levy Cardiovascular book and it shows a U-shaped rigid tube. The inflow pressure was 100 and the outflow was 0. The pressure at the bottom is greatest at appx. 130. Yet, fluid will still flow from one side of the tube to the other given the above conditions. If we use the equation delta P = Flow X Resistance. Delta P using the two ends of the U-shaped tube is 100-0 so fluid will flow in and come out. Yet, if we use delta P from the Top of the side of tube with the greater pressure and the bottom of the tube delta P will be 100-130=-30 based on this flow will be from bottom to top? I would like to know why this is the cause in this situation this will help me understand the human body better.

 

[Dale]

Ahh, I misunderstood your question. OK, that is fairly easy to answer.

Consider the bottom of the U at a moment when there is no flow. The weight of the column of fluid is equal to the weight of a 30 mm column of Hg, so the pressure due to the weight of the fluid is 30 mmHg. In addition there is the externally applied pressure of 100 mmHg on one side and 0 mmHg on the other side. So, in the static case, down at the bottom of the U there is a pressure of 130 mmHg on one side and 30 mmHg on the other side. Therefore the fluid at the bottom will begin to move (and similarly with the rest of the fluid).

Have you ever used a siphon? That is a similar concept, but in reverse.

 

[coolia]

So I'm assuming pouiselle's abreviated equation work does not hold for this case?, change in P= F x R.

 

[coolia]

is there a similar phenomenon in the reverse of a siphon (circulation of inferior extremity) where the maximum height of the siphon is limited due to cavitation?

 

[coolia]

I'm guessing the pouiselles equation doesn't take into consideration kinetic and gravitational potential energy. Bernoulli's equation does however. So constant, = v^2/2 +gh +P/p. gh will be higher at top, but at bottom will be turned into a greater P. v does not change throughout the tube because fluid is incompressible. What does not make sense with this however is if we compare the right side of the tube (P=0) to the left (P=100), gh will be the same, and so will the v term yet P will be different and so the constant is no longer constant. Help!

Oh yeah I forgot to take into consideration the frictional forces or resistance which is responsible for the loss of pressure on the left side, if we add that factor into the equation the constant will be a constant. I think I answered my own question, please tell me if this is the right logic. Thank you.