The interatrial septum forms during the first and second months of
fetal development. Formation of the septum occurs in several stages. The first is the development of the
septum primum, a crescent-shaped piece of tissue forming the initial divider between the right and left atria. Because of its crescent shape, the septum primum does not fully occlude the space between the left and right atria; the opening that remains is called the
ostium primum. During fetal development, this opening allows blood to be shunted from the right atrium to the left. 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. 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. 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.
At birth 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 failure of closure of the foramen ovale results in
patent foramen ovale. ==Clinical significance==