Anatomical Position of The Foramen Secundum
The septum primum is found on the left side of the heart in the left atrium while the septum secundum is much thicker and found on the right side in the right atrium. During development, blood shunts from the floor of the right atrium through the foramen ovale in the septum secundum then up through the foramen secundum in the septum primum. The foramen secundum is positioned so that blood exits in the ceiling of the left atria and then out through the left ventricle and the aorta. The position of the foramen secundum, as well as the size of the septum primum, are crucial to ensuring blood does not flow backwards from the left atrium to the right atrium. The septum primum, being much thinner, is easily pressed against the septum secundum if blood attempts to flow in the reverse direction, effectively sealing off both the foramen secundum and the foramen ovale.
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