The septum primum and septum secundum divide the primitive atrium. At birth, which event causes the functional closure of the foramen ovale?

• A Fall in right atrial pressure due to cessation of umbilical venous return
• B Active muscular contraction of the septum primum triggered by increased arterial oxygen tension
• C Rise in left atrial pressure following lung expansion and closure of the ductus arteriosus, pressing the septum primum against septum secundum ✓
• D Anatomical fusion of foramen ovale immediately at the moment of birth due to prostaglandin release

Explanation

At birth, with the first breath, pulmonary vascular resistance falls dramatically, pulmonary blood flow increases, and left atrial pressure rises above right atrial pressure. This pressure reversal pushes the flexible septum primum (which forms the valve of the foramen ovale) against the rigid septum secundum, functionally closing the foramen ovale. This functional closure typically becomes anatomical fusion within the first year (permanent closure). If this fails to close, a patent foramen ovale (PFO) persists in approximately 25–30% of adults. The ductus arteriosus closes functionally within hours due to increased oxygen tension causing smooth muscle contraction.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.