Question #463
The foramen ovale
The interatrial septum is formed by the septum primum that fuses with the septum
intermedium and the septum secundum that forms to the right of the septum primum. The first
hole to form, the ostium primum, closes off as the septum primum fuses with the septum
intermedium. The second hole to form remains between the superior wall of the atrium and the septum primum (the ostium secundum).
The foramen ovale is the oblique pathway past both septums connecting the right and left atria.
The septum primum is a relatively floppy structure and acts as a valve for the foramen ovale.
At birth changes in atrial pressures pushes the septum primum against the septum secundum,
closing the ostium secundum.
The most common cause of an atrial septal defect is a deficient septum secundum such that
when atrial pressures change the septum primum does not meet the septum secundum.
