Valve of foramen ovale

The valve of the foramen ovale is a a thin concave flap of tissue located on the left atrial side of the atrial septum in projection of the floor of the fossa ovalis (the fossa ovalis is only visible on the right atrial side of the atrial septum). In dogs, often an irregularity of the smooth septal surface is the only remnant of where this valve closed and sealed.

During fetal life, the valve of the foramen ovale 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.