Physio Resistance
resistance blood vessel q?
In the tube in the
diagram to the right, the
inlet pressure is 75 mm
Hg and the outlet
pressure at A and B is 25
mm Hg. The resistance
to flow is
(A) 2 PRU
(B) 0.5 PRU
(C) 2 (mL/min)/mm Hg
(D) 0.75 mm
Hg/(mL/min)
(E) 0.5 (mL/min)/mm Hg
see the pic of the diagram from physio book at
http://img.photobucket.com/albums/v291/absolutjag9/rho.png
I think there is an equation that relates blood flow to resistance:
Q = ΔP / R. Q is blood flow.
I’m guessing ΔP is change in pressure between the pressure you start with and pressure you end up with. So ΔP = Pin – Pout = 75 – 25 = 50 mmHg.
At first I tried calculating the resistance for each branch (A and B) then added them up, but didn’t get the right answer.
But I looked at it again and here’s what I came up with:
Okay, instead of just calculating the blood resistance at both branches, why not just add up the blood flows first?
At the point where branch A and B split up, the blood flow should be the combined blood flows of A and B, right? When I took physio, I remember learning that if you have a big blood vessel that splits up into many branches, the blood flow of the big blood vessel should be the same as the combined blood flows of each individual branch.
So at the point where A and B split up, the blood flow should be 95 mL/min + 5 mL/min = 100 mL/min. That’s the value of Q.
Now just plug and chug:
Q = ΔP / R —-> 100 = 50 / R —> R = 2.
The units for R should be whatever the units for ΔP is divided by the units for Q, right? Because R = ΔP / Q.
ΔP units is mmHg, Q units is mL/min.
So the units for R would be mmHg/(mL/min).
The answer should be 2 mmHg/(mL/min). That’s close to the answer C, except their units are backwards. And I’m not sure what a PRU is – is that the same as the units I figured out? I’d go with answer A.
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