The interatrial septum is the wall of cardiac tissue that separates the right atrium from the left atrium. Due to its obliquity, right atrium mainly lies anterior to left atrium. The interatrial septum begins to take shape during the first and second months of fetal development and its complete formation occurs in several stages. The first is the development of the septum primum, which is 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 close the space between left and right atria and 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. When the ostium primum occlude, another opening appear in septum primum, allowing continued shunting of blood from the right atrium to the left.
The second stage is the formation of the septum secundum, which is a thick, muscular structure that initially takes on the same crescent shape as the septum primum. 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, 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.