The foramen ovale () forms in the late fourth week of
gestation, as a small passageway between the septum secundum and the ostium secundum. Initially the atria are separated from one another by the
septum primum except for a small opening below the septum, the
ostium primum. As the septum primum grows, the ostium primum narrows and eventually closes. Before it does so, bloodflow from the
inferior vena cava wears down a portion of the septum primum, forming the
ostium secundum. Some embryologists postulate that the ostium secundum may be formed through
programmed cell death. The ostium secundum provides communication between the atria after the ostium primum closes completely. Subsequently, a second wall of tissue, the
septum secundum, grows over the ostium secundum in the right atrium. Blood then passes from the right to left atrium only by way of a small passageway in the septum secundum and then through the ostium secundum. This passageway is called the foramen ovale.
Closure The foramen ovale often closes at birth. At birth, when the lungs become functional, the pulmonary vascular pressure decreases and the left atrial pressure exceeds that of the right. This forces the
septum primum against the
septum secundum, functionally closing the foramen ovale. In time the septa eventually fuse, leaving a remnant of the foramen ovale, the
fossa ovalis. == Function ==