The closure of the foramen ovale after birth is primarily due to which change?

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After birth, the lungs aerate and pulmonary resistance falls, boosting pulmonary venous return to the left atrium. This raises left atrial pressure relative to right atrial pressure. The higher left atrial pressure pushes the foramen ovale flap against the septum secundum, creating functional closure of the opening. Over time, the septa may fuse, giving permanent closure. The other options don’t explain this pressure-driven seal: increasing systemic vascular resistance mainly affects the ductus arteriosus, while higher right atrial pressure or lower left atrial pressure would favor keeping the opening open.