Ostium secundum
| Foramen secundum | |
|---|---|
| Details | |
| Days | 33 |
| Identifiers | |
| Latin | foramen secundum |
| Code | TE E5.11.1.5.2.1.2 |
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Anatomical terminology
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The foramen secundum, or ostium secundum, is a foramen (opening) in the septum primum, a precursor to the interatrial septum of the embryological heart.
It should not be confused with the foramen ovale, which is a foramen in the septum secundum.
The discovery of the formation of the foramen secundum and septum secundum was published by P. N. B. Odgers at the University of Oxford in the Journal of Anatomy in 1935.
During development, the atria initially share a common chamber. As the two begin to separate, the septum is not fully formed to allow blood shunting from the right to left atrium in order for blood to bypass the lungs. The septum primum and spina vestibuli grow together to divide the right and left atria during the fifth week of development. This results in the formation of the foramen primum. The septum primum continues to grow until the sixth week when the dorsal and ventral cushions fuse to form the atrioventricular septum. The fusion process closes the foramen primum via controlled cell death (apoptosis), and several smaller perforations come together to form the foramen secundum. The perforations form before the foramen primum closes completely, ensuring the shunting process continues at all stages.
The septum primum is on the left side of the heart in the left atrium while the septum secundum is much thicker and is located 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 atrium and then out through the left ventricle and the aorta. The position of the foramen secundum and the size of the septum primum are crucial to ensuring that blood 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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