Primary Interatrial Foramen - Closing of Ostium Primum

Closing of Ostium Primum

This opening is closed by the union of the septum primum with the septum intermedium, and as the ostium primum closes, the communication between the atria is preserved with the formation of an opening in the upper part of the septum primum; this opening is known as the ostium secundum as it is chronologically the second opening that occurs in the septum prium.

A second entity, the septum secundum, develops to the right of the septum primum and the opening between the upper and lower limbs of the septum secundum is known as the foramen ovale of the heart. The part of the septum primum that remains to the left of the septum secundum acts as a one way flow valve due to the greater pressures in the right atrium compared to the left atrium. At birth the neonate begins breathing and the associated decrease in pulmonary vascular resistance leads to a reversal of the pressure differential between the atria. The left atrium now has a greater pressure compared to the right, effectively closing the one way flow valve formed by the two septae. The two septae fuse later in life, to complete the formation of the atrial septum.

Persistence of the ostium secundum is the most common atrial septal defect. Additionally, in a subset of the population, the foramen ovale is not overtly patent, however the two septae have not fused. In normal physiologic circumstances the septum primum acts as a one way valve preventing blood flow as described above, however if pathologic conditions cause right atrial pressure to exceed left atrial pressure, blood may flow through the foramen ovale from right to left.