At birth, the transition from fetal to neonatal circulation requires closure of several shunts. The mechanism driving functional closure of the foramen ovale immediately after birth is:

• A Prostaglandin-mediated contraction of the valve tissue
• B Oxygen-induced ductal constriction reducing right-sided pressure
• C Increased left atrial pressure (due to increased pulmonary venous return) exceeding right atrial pressure, pressing the septum primum against the secundum, mechanically closing the valve ✓
• D Active muscular constriction of the foramen ovale by specialized myocardium

Explanation

In fetal circulation, pulmonary vascular resistance (PVR) is high (lungs are fluid-filled) and systemic resistance is low (large placental circulation), so right atrial pressure exceeds left atrial pressure, keeping the foramen ovale open (right-to-left shunt). At birth, lung expansion and increased PO2 cause pulmonary vasodilation—PVR falls dramatically, pulmonary blood flow increases 8–10 fold, and pulmonary venous return to the left atrium rises. Simultaneously, removal of the placental circulation increases systemic resistance. Left atrial pressure now exceeds right atrial pressure, pushing the valve-like septum primum against septum secundum, functionally closing the foramen ovale. Anatomical closure (fusion) takes weeks to months. The ductus arteriosus closes by a separate oxygen-sensitive prostaglandin mechanism.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.