GS-1 Biological Sciences Question 50

[kevink6813]

If the pressure in the aortic artery is higher than the pressure in the pulmonary artery, why would we expect the flow deoxygenated blood to flow against this pressure gradient (thus decreasing pO2 in the systemic arteries?).

In any case, how can a decrease in oxygen content in the systemic arteries, not be matched by an increase in oxygen content in the pulmonary arteries?

The answer justifies D because it assumes fetal circulation applies to a newborn, but immediately following birth and the inflation of the lungs, pulmonary pressure drops and thus the shunt is reversed, is it not ? I.e. during fetal circulation the pulmonary artery shunts to the aorta, but following birth, pressure drops in the pulmonary circulation and the flow then proceeds in the opposite direction --> aorta to pulmonary artery.

Where am I going wrong? I chose A.

[mcat_premed3832]

Your reasoning is great. The exam Explanation is overly simplistic. Unfortunately, you are only mistaken for missing the big picture.

Forget about all the details for which you are mostly correct and consider the question: what would you say (guess without having seen any answers) would be the dominant feature of a newborn whose ductus arteriosis had failed to close?

I think that if you had not seen any answers, given your knowledge, you would have said that the dominant feature would be a blue baby ("cyanosis" which you don't need to know!). Then if asked, why? You would probably say because of lack of oxygen in systemic circulation and, of course, you would be right.

There are so many other factors: how "newborn" is the baby, what primary lung problems are affecting the degree of shunt, diffusion being dynamic, how patent is the ductus, etc. etc. But the bottom line in any case is that the question refers to the "dominant feature of the newborn" and that would be worth highlighting with that onscreen highlighter!