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### AAMC FL3 (C/P Q52)

Posted: **Mon Jul 29, 2019 4:22 pm**

by **931287m**

Hello!

I'm not understanding how this answer was derived. I know Q = Av, so I was able to eliminate A & C allowing me to at least make an educated guess.

How do we arrive at this answer?

Thanks!

### Re: AAMC FL3 (C/P Q52)

Posted: **Mon Jul 29, 2019 5:01 pm**

by **NS_Tutor_Mathias**

It's like the famous children's book says: What goes in one end must come out the other.

You know that total flow through the capillary bed has to be equal to total flow through the incoming blood vessel. This has to be true outside of major injury, otherwise there are blood-wizards conjuring extra fluid somewhere.

So we can say Q_cap = Q_vessel

Q_vessel = v_vessel * A

Q_cap = v_cap * a * n

(Since you only have ONE artery carrying things into these capillaries, but you have n number of capillaries)

So, since we want to solve for v_cap:

v_cap= v_vessel * A / (a * n)

In a pinch, you can also logic this one out: For a fixed amount of flow, the flow velocity has to be inversely proportional to how many vessels you've got, and inversely proportional to how wide those vessels are.

### Re: AAMC FL3 (C/P Q52)

Posted: **Mon Jul 29, 2019 5:26 pm**

by **931287m**

If only the AAMC explanations were this concise.. haha

Thanks a lot!