The fetus has several adaptations that serve to bypass the lungs, which are not needed to oxygenate the blood. When blood coming from the placenta enters the right atrium, the foramen ovale, a small hole in the septum between the atria, allows some of the blood to go directly into the left atrium, thus bypassing the pulmonary artery. Further, blood pumped out of the right ventricle can shunt directly into the aorta through a short vessel, the ductus arteriosus, which connects the pulmonary artery with the descending aorta. Both of these passages close off at birth when the pulmonary circuit is established. Their failure to close hampers the work of the heart and may require medical attention.

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