<P> As the septum primum grows, the ostium primum progressively narrows . Before the ostium primum is completely occluded, a second opening called the ostium secundum begins to form in the septum primum . The ostium secundum allows continued shunting of blood from the right atrium to the left . </P> <P> To the right of the septum primum, the septum secundum begins to form . This thick, muscular structure initially takes on the same crescent shape as the septum primum, except that it originates anteriorly, whereas the septum primum originates posteriorly . As the septum secundum grows, it leaves a small opening called the foramen ovale . The foramen ovale is continuous with the ostium secundum, again providing for continued shunting of blood . </P> <P> The ostium secundum progressively enlarges and the size of the septum primum diminishes . Eventually, the septum primum is nothing more than a small flap that covers the foramen ovale on its left side . This flap of tissue is called the valve of the foramen ovale . It opens and closes in response to pressure gradients between the left and right atria . When the pressure is greater in the right atrium, the valve opens; when the pressure is greater in the left atrium, the valve closes . Because the lungs are nonfunctional in fetal life, pressure in the pulmonary circulation is greater than that of the systemic circulation . Consequently, the right atrium is generally under higher pressures than the left atrium, and the valve of the foramen ovale is normally open . </P> <P> At birth, there is a reversal in the pressure gradient between the atria, resulting in functional closure of the valve of the foramen ovale . Permanent anatomical closure of the foramen ovale occurs with time in normal infants . Inappropriate closure of the foramen ovale results in patent foramen ovale . </P>

The flaplike opening in the interatrial septum of the fetus