The foramen ovale closes after birth. Initially, this is due to a functional equalization of pressures within the atria that apposes the septum primum and the septum secundum. Equalization of pressure occurs because of:
Functional closure can be reversed during the immediate neonatal period; this occurs during bouts of crying when cyanosis may be seen and there is a re-emergent right-to-left shunt.
Anatomical fusion of the septum primum and secundum occurs by one year of age in 80% of the population. Fibrosis results in the fossa ovalis of the interatrial septum. The remaining 20% have a foramen which is only potentially patent. The normal pressures within the atria ensure that it remains closed; the septum primum acts as a flap-valve against the septum secundum due to the greater pressures within the left atrium compared to the right atrium.
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