# B Why is blood flow rate constant?

1. Jan 2, 2017

### hongiddong

I am having trouble understanding why blood flow rate is constant between the aorta and the sum of capillaries/sum of the arterioles. I keep thinking that in the arterioles or capillaries there is a decrease in velocity and somehow this decrease in velocity will decrease the total amount of blood or time that passes through the total amount of capillaries?

Thank you physicsforums!

2. Jan 2, 2017

### Orodruin

Staff Emeritus
If there was a difference blood would accumulate somewhere.

The velocity may very well be different but the flow rate is velocity multiplied by area.

3. Jan 2, 2017

### A.T.

Or leave the body. Both is possible and dangerous.

4. Jan 2, 2017

### Orodruin

Staff Emeritus
Or life saving in the case of blood donations.

5. Jan 2, 2017

### hongiddong

From this discussion, I have another question that relates to this problem.

Say in bernoulies equation we go from a bigger pipe to a smaller pipe, the velocity increases in the smaller pipe to maintain the flow rate.

1. In the problem above, Why does the flow rate stay the same? It seems as if it is due to the ideal condition(no resistance, friction, and incompressibility.

2. also, for the example of a human body, it seems weird that the cardiac output would remain the same when it is not in an ideal condition?

Thank you Orodruin and A.T.!

6. Jan 2, 2017

### Staff: Mentor

I'm not following: are you sure you aren't still confusing flow velocity and [volumetric] flow rate? Again, in a steady and closed system, the flow rate has to stay the same to satisfy continuity. The flow has nowhere else to go.
It isn't. The human circulatory system is very, very non-ideal, but continuity must apply (given the noted constraints against flow into or out of the system).
I don't understand this either. The same as what? The human body is non-ideal and certainly does behave differently from a similarly constructed but ideal system.

7. Jan 2, 2017

### hongiddong

Hey Watters,

Ahh I see now the difference between velocity which is the speed of flow vs flow(the volume that passes by per time.

I guess my only question now is what forces would keep the continuity of flow(Liters/min)to keep on moving without dissipation in the closed circuit of the heart.

Thought experiment: as the heart pushes the blood, the initial blood then pushes the blood in front of it, so perhaps the heart has enough strength to push out the initial blood against the all the blood in front of it to maintain the flow rate.

Thank you Watter.

8. Jan 2, 2017

### Staff: Mentor

Fluid flow is all about velocity versus pressure. They are related by Bernoulli's principle.
Yes.

One thing I think is important to understand about non-ideal flow situations that may be relevant here is that when you have a pump (or fan) in a steady flow system, all of the input energy is lost. All of the input energy provided by the pump goes toward countering friction and viscous losses in order to maintain the steady flow.