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MCAT Question A Day - 6/15/13 - Answer!

The fetal heart has an opening between the two atria, called the foramen ovale, which allows blood to flow directly from the right atrium to the left atrium. What results from blood flowing through this opening?

A.  It bypasses the fetal lungs.
B.  It ensures that the fetal lungs receive extra blood.
C.  It bypasses both ventricles.
D.  It provides additional time for the blood to pick up extra oxygen.

The correct answer is (A). In the adult, blood leaving the right atrium enters the right ventricle, which pumps it to the lungs for gas exchange. This blood then leaves the lungs in the pulmonary veins and returns to the left atrium freshly oxygenated. Since the fetal lungs are submerged in amniotic fluid and are not yet functioning (fetal capillaries obtain oxygen by diffusion from maternal capillaries in the placenta), this temporary opening (foramen ovale) provides an efficient detour that bypasses the nonfunctional lungs.