Practice Questions MCAT 2015 Official guide

[RSK25]

Hi everyone, I am starting a thread for official guide to mcat 2015 and the questions the book includes.

my first question (on page 76)
The radius of the aorta is about 1.0 cm and blood passes through it at a velocity of 30 cm/s. A typical capillary has a radius of about 4*10^-4 cm with blood passing through at a velocity of 5*10^-2 cm/s. using this data, waht is the approximate number of capillaries in a human body.

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

no idea how to do this

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[justadream]

@RSK25

Is the answer C?

 

[RSK25]

the correct answer is c. how did you do that?

 

[justadream]

@RSK25
Total volume flow is conserved.

Total volume flow = Cross-Sectional Area * Velocity

In the aorta, total volume flow = pi*(.01)^2 * .3 = 9.42 * 10^-5


After you leave the aorta, the blood splits up into tons of capillaries. The total volume in all these capillaries is equal to the number I calculated above.

Calculate the volume each capillary can hold: pi *(4*10^-6)^2 * 5*10^-4 = 2.5*10^-14

Ok so to find the total # of capillaries you need:

(9.42*10^-5) / (2.5*10*10^-14)

= 3.75*10^9

 

[Cawolf]

@RSK25
Total volume flow is conserved.

 

[MurderMysteryDr]

@justadream
Hi, I am tackling the office guide at this time and Im glad this was posted because I had no idea how to do this, so thank you!

After reading over this, I now see the equation that was needed (didn't know it) and the subsequent steps that need to be taken--it makes so much sense, but I still cannot figure out the actual ( math (I am terrible at math, I admit).

So did you convert all the numbers to meters? Is that the proper thing to do? I ask because the answers don't have any units and the math seems simpler (whole numbers, yay!) but it's not computing right either.

Thanks for any help--I know I need some serious arithmetic classes anyways.

 

[TheLongGame]

So conceptually it's easier to explain in terms of something a little easier to visualize...

Imagine you have a gallon of water and you want to figure out how many shot glasses you can fill up using that gallon...the question above uses this same exact concept.

Figure out how much volume you have (or volumetric flow) and divide it by the volume (or volumetric flow) of the vessel of interest. This will give you the total # of smaller vessels/larger vessel.




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