In the developing heart, the atria are initially open to each other, with the opening known as the primary interatrial foramen or ostium primum (or interatrial foramen primum).

The foramen lies beneath the edge of septum primum and the endocardial cushions.

The foramen lies beneath the edge of septum primum and the endocardial cushions.

Children born with this condition may be asymptomatic, however, over time pulmonary hypertension and the resulting hypertrophy of the right side of the heart will lead to a reversal of this shunt.

This article incorporates text in the public domain from page 512 of the 20th edition of Gray's Anatomy (1918)