In the fetal heart, fetal blood at the atrial level is shunted right to left via the

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The main idea is how fetal circulation bypasses the nonfunctional lungs by moving blood from the right atrium to the left atrium. In the fetus, the right atrial pressure is higher than the left because the lungs offer high resistance and most oxygenated blood from the placenta returns to the heart via the inferior and superior venae cavae. This pressure gradient drives blood through the foramen ovale, a flap-like opening in the interatrial septum, allowing right-to-left flow. This shunt directs oxygen-rich blood toward the left heart and out to the systemic circulation. After birth, lung resistance falls and the left atrial pressure rises, causing the foramen ovale to functionally close and eventually become the fossa ovalis.

The other structures aren’t at the atrial level: the ductus arteriosus connects the pulmonary artery to the aorta to bypass lungs; the ductus venosus shunts umbilical venous blood to the inferior vena cava; ligaments like the ligamentum venosum are postnatal remnants.