What are the two major bypasses in a fetus' lungs?

In fetal circulation, the two major bypasses that allow blood to bypass the non-functioning lungs are the foramen ovale and the ductus arteriosus. The correct choice highlights these two important structures.

The foramen ovale is an opening between the right and left atria that allows blood to flow directly from the right atrium to the left atrium, bypassing the pulmonary circulation entirely. This is crucial because a fetus does not breathe air and therefore does not require the lungs for oxygenation.

The ductus arteriosus is a vessel that connects the pulmonary artery directly to the aorta. This pathway allows some blood to flow from the right ventricle into the aorta, rather than into the lungs, as the lungs are still developing and are not in use for oxygen exchange in the fetus.

Understanding these bypasses is essential in recognizing how fetal circulation is adapted to ensure that oxygen-rich blood from the placenta efficiently reaches the developing organs, avoiding the lungs which are filled with fluid and not yet functional for gas exchange.