Interatrial Septum

In adults, the main function of the interatrial septum is to separate the two atrial chambers so that there is no shunting (movement) of blood between them. During fetal development, however, its function is just the opposite. 

The interatrial septum forms during the first weeks of fetal development, when a strip of tissue called the septum primum begins to grow downward between the left and right atria. Initially, the septum primum does not fully occlude the space between the two cavities, leaving an opening called the ostium primum (i.e., first opening). As the septum primum continues to grow downward, the ostium primum progressively narrows. Before the ostium primum is completely closed, another gap called the ostium secundum (i.e., second opening) appears in the upper area of the septum. Next, a septum secundum begins to form adjacent to the septum primum, covering the ostium secundum and leaving a small opening called the foramen ovale. Since the lungs aren’t fully developed during fetal development, the foramen ovale allows oxygenated blood from the placenta to be shunted directly from the right atria into the left atria, bypassing the pulmonary circulation. The oxygenated blood then goes to the left ventricle to be pumped through the aorta to the rest of the body. After birth, the septum secundum and septum primum slap shut due to the elevated pressures in the left heart, and then fuse to close the foramen ovale, so that the individual can rely on their own lungs for oxygen. If the foramen ovale fails to fuse correctly, this is known as a patent foramen ovale (PFO).