AAMC Official Guide to the MCAT 2015 Chem/Phys - Page 76

[FlyMissMD93]

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Hey guys, I'm currently reading through the AAMC Official Guide paperback (4th edition) and was wondering if anyone could help me reason through how to get the right answer for the Chem/Phys problem on page 76?

 

[FlyMissMD93]

The question states:

"The radius of the aorta is about 1.0cm and blood passes through it at a velocity of 30 cm/s. A typical capillary has a radius of about 4 x 10^-4 cm with blood passing through it at a velocity of 5 x 10^-2 cm/s. Using this data, what is the approximate number of capillaries in a human body?

A. 1 x 10^4
B. 2 x 10^7
C. 4 x 10^9
D. 7 x 10^12"

 

[TheLongGame]

To start with, we need to understand that the volumetric flow rate of the system (the circulatory system in this case) will be conserved. This means that if there are X[ml/s] flowing in the aorta, then when you add up all of the flow rates for the capillaries their sum total will equal X [ml/s] as well.

Since the question gives you the radius and velocities of blood flowing through both the aorta and the capillaries, the calculation will be straight forward.

Hope this helps!


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