MCAT Physics Question 52: Answer and Explanation

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Question: 52

7. The speed of blood in the aorta is much higher than the speed of blood through a capillary bed. How can this fact be explained using the continuity equation, assuming that we are interested in average flow and that there is no net fluid loss?

  • A. The aorta is located higher than the capillary bed.
  • B. The pressure in the aorta is the same as the pressure in the capillary bed.
  • C. The cross-sectional area of all the capillaries added together is much greater than the cross-sectional area of the aorta.
  • D. The cross-sectional area of a capillary is much smaller than the cross-sectional area of the aorta.

Correct Answer: C

Explanation:

The continuity equation states that the flow rate of a fluid must remain constant from one cross-section to another. In other words, when an ideal fluid flows from a pipe with a large cross-sectional area to one that is narrower, its speed decreases. This can be illustrated through the equation A1ν1 = A2ν2. If blood flows much more slowly through the capillaries, we can infer that the cross-sectional area is larger. This might seem surprising at first glance, but given that each blood vessel divides into thousands of little capillaries, it is not hard to imagine that adding the cross-sectional areas of each capillary from an entire capillary bed results in an area that is larger than the cross-sectional area of the aorta.

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