MCAT Physics Question — Blood Flow | Next Step Test Prep MCAT Physics Question — Blood Flow | Next Step Test Prep

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The flow of blood through the aorta can be approximated with the volumetric flow rate equation. When the smooth muscle of the aorta contracts, the radius can decrease by as much as 10%. If the blood flow is to remain unchanged, what must happen to the velocity of blood flow in the aorta?

 

A) Decrease by approximately 10%

B) Increase by approximately 10%

C) Increase by approximately 25%

D) Remain unchanged

 

Explanation

 

The volumetric flow rate equation is Q = A x v where A is the cross-sectional area of the pipe, Q is the flow rate, and v is the velocity of the fluid.

 

The problem states that Q is to remain unchanged. That allows us to set up the following equation:

 

Q = A1v1 = A2v2

 

Plugging in 1 as the original values, we get:

 

1 x 1 = 0.81 x v2

 

Because the radius was decreased to 90% of its original value, the cross-sectional area will be reduced to 81% of its original value (A is proportional to radius squared and 0.92 = 0.81)

 

 

r2 = 0.9r1

A2 = 0.81A1

 

Solving for v2 we get 1.23. Thus the velocity increased by approximately 25%, making choice (C) correct.

 

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