interatrial septum

The interatrial septum is the thin wall between the heart's two upper chambers. Think of it as the partition that keeps the right-side and left-side waiting rooms apart so their blood stays unmixed.

This wall divides the right atrium from the left atrium. It is thinner than the wall between the ventricles, because the atria are low-pressure chambers that do not need a heavy barrier.

Near the center of this septum is a small oval depression, the fossa ovalis. Before birth, an opening here (the foramen ovale) lets blood bypass the non-working fetal lungs; it normally seals after birth. If it stays partly open, the result is a patent foramen ovale, and a true hole in the wall is called an atrial septal defect — both can allow abnormal blood flow between the atria.