Final answer:
The constriction of the ductus arteriosus as oxygen levels increase leads to a reduction of blood flow to the lungs, redirecting blood to the now functioning lungs for oxygenation. The correct option is A.
Step-by-step explanation:
As blood levels of oxygen increase, the ductus arteriosus begins to constrict and the correct answer is that it reduces blood flow to the lungs. This structure is a vital part of fetal circulation, which allows blood to bypass the non-functioning fetal lungs. It connects the pulmonary artery with the aorta, diverting blood directly from the right ventricle into the systemic circulation.
When a newborn begins breathing air, the increased oxygen levels cause the smooth muscles in the ductus arteriosus to constrict, thereby redirecting the blood flow to the now-functioning lungs. The blood is then oxygenated in the lungs, rather than being shunted away from them.
Hence, Option A is correct.