<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>
the flaplike opening in the interatrial septum of the fetus
the valve of the foramen ovale