The fossa ovalis is a depression in the right atrium of the heart, the remnant of a thin fibrous sheet that covered the foramen ovale during fetal development.
During fetal development, the foramen ovale allows blood to pass from the right atrium to the left atrium, bypassing the nonfunctional fetal lungs while the fetus obtains its oxygen from the placenta. A thin flap of tissue called the septum primum acts as a valve over a hole in the interatrial septum called the foramen ovale.
After birth, the introduction of air into the lungs causes the pressure in the pulmonary circulatory system to drop. This change in pressure pushes the septum primum against the interatrial septum, closing the foramen. The septum primum and interatrial septum eventually fuse together to form a complete (non-perforated) interatrial septum, leaving a depression called the fossa ovalis where the foramen ovale was.
By age two, about 75% of people have a complete interatrial septum. If the septum primum and interatrial septum do not fuse, this condition is called a patent foramen ovale. Depending on the circumstances, a patent foramen ovale may be completely asymptomatic or it may require surgery.