Valve of foramen ovale

After birth, the septum primum is a small flap that covers the foramen ovale on its left side. This flap of tissue is called the valve of the foramen ovale.

The valve of the foramen ovale is defined as the thin, flap-like portion of the septum primum that overlies the foramen ovale within the atrial septum. During fetal life, this structure allows right-to-left shunting of oxygenated blood from the right atrium to the left atrium, bypassing the nonfunctional fetal lungs. After birth, increased left atrial pressure pushes the septum primum against the septum secundum, functionally closing the foramen ovale; the septum primum thus acts as a one-way valve that prevents left-to-right shunting under normal postnatal conditions. 

Anatomically, the valve is located on the left atrial side of the septum secundum and forms the floor of the fossa ovalis. Its movement and closure are determined by the interatrial pressure gradient, and it is composed of endocardial tissue with underlying connective tissue and myocyte bundles.

The American Society of Echocardiography describes the valve of the foramen ovale as the flap formed by the septum primum, which attaches to the left atrial aspect of the septum secundum and is integral to the normal anatomy of the atrial septum.

In some patients, 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.