Septum Formation in the Atrium
By the end of the 4th week of development, Septum primum descends from the roof of the atrium toward the endocardial cushions and begins to divide the atrium into two parts. Ostium primum is a temporary space left between the lower limb of septum primum and the endocardial cushions. Fusion of septum primum with endocardial cushions obliterates ostium primum. Ostium secundum is an opening formed in septum primum by cell death process.
Septum secundum is a new crescent-shaped fold appears on the right side of the septum primum and overlaps the ostium secundum. Septum secundum leaves an opening on its posterior-inferior surface is called oval foramen (foramen ovale). This foramen keeps flow of blood from right atrium to the left atrium during prenatal life. The remaining part of the septum primum becomes the valve of the oval foramen. Due to increase pressure in the left atrium after birth, the valve of the oval foramen is pressed against the septum secundum and closes the oval foramen.
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** Pulmonary vein develops at the posterior wall of the left atrium and forms the smooth walled part of the adult atrium.