the wall between two upper heart chambers?

The wall between the two upper heart chambers, the atria, is called the interatrial septum. It divides the left atrium from the right atrium and prevents the mixing of oxygenated and deoxygenated blood. The interatrial septum is a muscular wall with an opening called the foramen ovale, which allows blood to flow from the right atrium to the left atrium in fetal development. After birth, the foramen ovale closes, and the two atria are completely separated.