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How do the foramen ovale and ductus arteriosus help fetal circulation before birth?

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Illustration of a human heart cross-section showing four chambers-left and right atria and ventricles-with labeled major blood vessels. An inset highlights cardiac muscle tissue, showing myocytes and intercalated disks. — human cardiovascular system The human heart is both a pump and a muscle. Its four chambers move blood efficiently, while cardiac muscle cells joined by intercalated disks synchronize each contraction.

In the fetus, the foramen ovale and ductus arteriosus serve as vital shunts that allow blood to bypass the nonfunctional fetal lungs, which do not yet participate in gas exchange.123

Foramen Ovale: This opening in the septum between the two atria allows oxygenated blood arriving from the placenta via the inferior vena cava to pass directly from the right atrium to the left atrium. This enables the most highly oxygenated blood to reach the left ventricle and be pumped out through the aorta to the head and upper extremities.142
Ductus Arteriosus: This vessel connects the pulmonary artery to the aorta. It shunts the majority of blood exiting the right ventricle away from the lungs and directly into the aorta, where it can be circulated to the rest of the body and returned to the placenta for re-oxygenation.5312

These structures close shortly after birth as the lungs inflate and begin to function, establishing the separate pulmonary and systemic circulations required for independent life.512