# Hemodynamics questions

1. Sep 19, 2008

### coolia

Hi, I'm having trouble understanding how P, F, and R are related in the human circulation.

I know Pressure gradient = Flow x Resistance. But is the flow constant throughout the different vessels. If not, how can we use this equation.

Why is there more blood pooled in the veins?, is it due to a slower flow rate or because the veins are more compliant? I'm trying to create an anology between the human circulation and an electrical circuit, if someone has one that would be greatly appreciated.

Why is blood in the arteries more pulsatile than blood in the veins?

Thank you.

2. Sep 20, 2008

### Staff: Mentor

Hi coolia, welcome to PF,

Pressure and flow are the directly measurable quantites, we simply take their ratio and call that resistance (i.e. there is no direct way to measure resistance). The resistance of a blood vessel is not constant, but changes with diameter according to Poiseuille's equation.

There is more blood in the veins than the arteries simply because they have a larger volume.

If the vasculature were perfectly rigid then the pulse wave would propagate from the heart through the entire system without loss, but because the vessels are compliant some of the energy is lost to stretching the vessel walls. Because the walls are elastic this energy is largely recovered at a later time, resulting in an overall "smoothing" of the pulsatility.

3. Sep 20, 2008

### coolia

thank you for your answer. I did some research and come to some similar answers that I'd like to share please tell me what you think. The greatest resistance in the human circulation occurs in the arterioles and this is where pressure is lost. Because of this, two things happen. 1) the volume flow rate (L/s) decreases due to the pressure loss and 2) because of this the pulsatile nature of blood flow that exists in the arteries is decreased (the 2nd part is what I have a tough time understanding, if someone could explain it better that would be great). You are right about there being more blood in the veins because of their larger volume, however veins also have more blood because they have less volume flow rate. And secondly they are more distensible allowing them to expand and carry more blood.

4. Sep 20, 2008

### coolia

After giving some more thought. I think volume flow rate does not change throughout the vasculature because the difference in pressure from the the aorta to cappilaries is greater than than the pressure from the capillaries to the vena cava, however the resistance will be less in the latter area because the diamater of the veins are larger. So, if Flow rate = delta P/ Resistance, flow rate will be constant because in when delta P increases while in the other resistance decreases. This must be the case because the flow rate at the arterial end must match the venous end other wise the heart will be pumping more or less blood each time. I know this will be case in a rigid tube with continues flow, but I'm sure about it in elastic vessels with pulsatile flow. Please help.

Last edited: Sep 20, 2008
5. Sep 20, 2008

### Staff: Mentor

Excellent thoughts. You are correct, the volume flow rate does not change, this is primarily because blood is incompressible. That means that the same volume of blood that enters a vessel must either leave it or the vessel itself must change volume.

Did you look at the link I posted above. If you did, you may have noticed that resistance is inversely proportional to the 4th power of the radius of the vessel. In other words, a bigger vessel has less resistance. Now, given that information, do you see a possible connection between there being less pressure drop on the venous circulation and there being more volume in the venous circulation?

6. Sep 21, 2008

### coolia

ok, so the pressure drop is not as great because the area of the vessels are large, so there will be less resistance (to power of 4 to be exact). I don't see how there being a less pressure drop could cause there too be more volume in the venous circulation though. I'm I missing something.

7. Sep 22, 2008

### Staff: Mentor

Think about it. The veins must have the same length as the arteries since they have to go all the way back to the heart. In order to have a lower resistance they must have a larger radius and therefore a larger area. A larger area times the same length is a larger volume.

8. Sep 22, 2008

### coolia

ok I see your point. You are indirectly measuring the radius of veins by taking into consideration the pressure drop. Thanks for the help.