The answer to this question is choice C.
Before we rationalize this question, let us try to establish the terms that are related to valves:
- Semi-lunar valves: These are the valves that are found at the base of the heart, and concomitantly separate the ventricles from the great vessels. The 2 valves that can be named as “semi-lunar valves” are the aortic and pulmonic valve. The aortic valve separates that left ventricle and the aorta, while the pulmonic valve separates the right ventricle and the pulmonary artery.
- Atrioventricular valves on the other hand are the valves that separate the atria and the ventricles. The mitral valve separates the left atrium and the left ventricle, whereas the tricuspid valve separates the right atrium and the right ventricle.
It is also important to be reminded of an important physiological concept that concerns heart pressures and valves: To keep it simple, assuming that you are fairly knowledgeable on the different valves and the different chambers of the heart, valves open and close depending on the pressure differences among either side of the valve.
With that being said, let us then rationalize the choices:
A. the blood pressure in the aorta is greater than in the vena cava: this is incorrect since the aorta and vena cava are not on opposite sides of a semilunar valve, thus their pressure differences will not matter in terms of valve opening and closure.
B. the oxygen level in blood decreases at the respiring tissues: oxygen content of the blood does not play a direct role in terms of valve closures, since this is not related to pressure that can be exerted on the valves.
C. the blood pressure in the right ventricle is less than in the pulmonary artery: this is the correct answer. One clue will be that the right ventricle and the pulmonary artery are on either side of the pulmonic valve. Another reason why this choice is correct is because of the pressure difference. Given the structure of the valve, for it to open, the pressure of right ventricle has to be greater than the pressure of the pulmonary artery. For it to close on the other hand, the pressure has to be greater on the side of the pulmonary artery, thus making this the correct choice.
D. the blood pressure is greater in the left ventricle than in the aorta: This is similar to the explanation in choice C, however, the pressure is greater in the left ventricle than in the aorta, which will make the valve open, and not close, which makes this an incorrect consideration.
E. another set of semi-lunar valves opens: This is not possible, since the semilunar valves open and close at the same time, in accordance to whether the heart is in systole or in diastole.
Thus, the only correct choice for this question is choice C.