what does "average volume blood flow rate" mean?

[keikoblue2]

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The average volume blood flow rate through the capillaries is: equal to that in both the arteries and veins.

I'm confused because TBR's Bio book had a question that kind of contradicts the answer here. When the blood flows from the arteries to the capillaries, it branches off into the many capillaries so the area increases, resulting in a decrease in velocity of the blood in the capillaries.

If the velocity is greater in the capillaries, wouldn't the volume of blood per unit time (flow rate) also increase, or are the two unrelated? Is the flow rate always constant in the body since no blood is being lost from the arteries to the capillaries to the veins?

 

[FeinMS]

Volume flow rate= Area*Velocity.

 

[going2breakdown]

It just sounds confusing. They gave you a flow rate. Flow rate= area times velocity . We know that the law of continuity is true for ideal fluids. Cross sec area 1 times velocity fluid at point 1 = cross sec area 2 times the velocity at point 2. Av=A"v" Arteries are larger than capillaries, HOWEVER, the flow rate at any point in the system of "pipes" must follow Av=A"v" aka the law of continuity. Therefore the average flow rate is just the flow rate because flow rate equals Av. and the product of A and v MUST be the same at all points it doesn't matter if they are wide arteries or narrow capillaries.

Perhaps you are getting confused with the fact that PRESSURE must increase as TOTAL cross sectional area increases. This is, I think, why overweight people have hearts that need to pump harder. The fat tissue needs to be supplied with oxygen necessitating a longer and/or more numerous system of vessels. This increases the pressure needed to supply the TOTAL cross sectional area. Don't confuse this, they are just asking you what Av is since Av=A'v' in other words flow rate at point 1= flow rate at point 2
BUT WAIT!!!
TOTAL CROSS SECTIONAL AREA DOES COME INTO PLAY IN THE LAW OF CONTINUITY!
I found this online:
"If a pipe's diameter changes over its length, a fluid flows through narrower segments of the pipe faster than it flows through the wider segments. Since the volume of flow per second must be constant through the entire pipe, the fluid must flow faster as the cross-sectional area of the pipe narrows (think of the velocity of water squirted by a hose with and without a nozzle).
Based on the law of continuity, you might think that blood should travel faster through capillaries than through arteries, because the diameter of capillaries is very small. However, it is the total cross-sectional area of the capillaries that determines flow rate. Each artery conveys blood to such and enormous number of capillaries that the total cross-sectional area is much greater in capillary beds than in any other part of the circulatory system.

For this reason, the blood slows substantially as it enters the arterioles from arteries, and slows further in the capillary beds. As blood leaves the capillaries and enters the venules and veins, it speeds up again as a result of the reduction in total cross-sectional area."

 

[keikoblue2]

Thank you, Guys. Yeah I think I got the pressure confused with flow rate.