Fossa ovalis

The fossa ovalis is a feature found in the right atrium of the human heart. It is a small, oval-shaped depression in the interatrial septum, which is the wall that separates the right atrium from the left atrium. The fossa ovalis is a remnant of a structure called the foramen ovale, which plays a crucial role in fetal circulation.

During fetal development, the foramen ovale is a hole or opening in the interatrial septum that allows blood to bypass the lungs. This is necessary because the lungs are not used for oxygen exchange in the developing fetus; instead, oxygenated blood is supplied directly from the placenta through the umbilical vein. The foramen ovale allows oxygenated blood to flow from the right atrium to the left atrium, effectively bypassing the right ventricle and the pulmonary circulation.

Shortly after birth, when the baby takes its first breath and the lungs begin to function, the pressure in the left atrium increases, pushing the septum primum against the septum secundum (two layers of the interatrial septum). This closure of the foramen ovale typically occurs within the first few weeks of life.

Once the foramen ovale closes, it leaves behind the fossa ovalis, which is a small depression in the interatrial septum. In most individuals, the fossa ovalis is a permanent feature of the heart, with no significant physiological function after birth. However, in some cases, the fossa ovalis may not completely seal, leading to a patent foramen ovale (PFO), which is a small opening that can allow blood to flow between the atria. While PFOs are often harmless, they can be associated with certain medical conditions and may require treatment in some cases.