Section: Chemical and Physical Foundations of Biological Systems

36) Assuming the circulatory system in humans obeys Bernoulli’s principle of fluid dynamics, which of the statements most accurately compares the blood pressure in a capillary of the neck to a capillary with an equal cross-sectional area in the right knee?

1. The pressure in the neck is greater than the pressure in the knee because of the increase in pressure head
2. The pressure in the neck is equal to the pressure in the knee because of the equal dynamic pressure according to the continuity equation
3. The pressure in the knee is greater than the pressure in the neck because of the increase in pressure head
4. An accurate comparison cannot be given without knowledge of the fluid’s density and viscosity

This question is asking the examinee to compare the blood pressure in the capillary beds in two different regions of the body. To answer this question, you must apply Bernoulli’s equation, P + 1/2pv² + pgh, at a point in the neck and a point in the knee. Since the cross sectional area is equal, the fluid velocity will also be equal. Thus, Bernoulli’s equation can be simplified as shown:

P_neck + pgh_neck = P_knee + pgh_knee

Since hneck is greater than the hknee, the pressure in the neck is lower than the pressure in the knee. Thus, the correct answer is choice C.