The fossa ovalis is a shallow, oval-shaped depression located in the interatrial septum, the muscular wall separating the right and left upper chambers (atria) of the adult heart. It is the remnant of a temporary opening that played a major role in circulation before birth. In a fully developed heart, the fossa ovalis is normally a sealed patch of tissue.
Anatomical Structure and Location
The fossa ovalis is a thin, fibrous sheet of tissue found on the right side of the interatrial septum. It appears as a shallow depression on the right atrial wall. Its boundaries are clearly defined by a slightly raised rim of tissue known as the limbus fossae ovalis, or annulus ovalis. This rim is particularly pronounced along the superior and lateral edges of the depression. It is the final anatomical result of a closure process that occurs shortly after birth, representing the fusion of two developmental septa.
The Fetal Role in Circulation
Before birth, this structure is an open passageway called the foramen ovale. Since the fetal lungs are not used for gas exchange, the circulatory system must bypass the non-functional pulmonary circuit. The foramen ovale acts as a shunt, allowing oxygenated blood from the placenta to move directly from the right atrium to the left atrium.
Mechanism of Fetal Shunting
This blood flow occurs because the pressure is significantly higher in the right atrium than in the left atrium during fetal life. The high resistance in the pulmonary blood vessels causes the pressure on the right side of the heart to rise. This pressure gradient pushes a flap of tissue, the septum primum, aside like a one-way valve, allowing blood to stream into the left atrium. From the left atrium, this blood is pumped to the upper body, including the developing brain.
Transition at Birth
When a newborn takes its first breath, the circulatory system undergoes a rapid physiological transformation. Lung expansion causes a significant drop in pulmonary vascular resistance, allowing blood to flow easily into the lungs and decreasing pressure in the right atrium. Simultaneously, the large volume of blood returning from the lungs sharply increases pressure in the left atrium. This pressure reversal, where the left atrial pressure now exceeds the right atrial pressure, forces the septum primum flap against the interatrial septum. This functional closure seals the opening, preventing blood from shunting between the atria, and the sealed remnant structure is then referred to as the fossa ovalis.
When Closure is Incomplete (Patent Foramen Ovale)
If the flap of tissue fails to permanently fuse with the interatrial septum, the result is a Patent Foramen Ovale (PFO). The term “patent” means open, though the small opening is often functionally sealed by the normal adult pressure difference. PFO is common, found in an estimated 17% to 35% of the population. For the vast majority of individuals, a PFO is entirely asymptomatic and poses no health risk.
However, the opening creates a potential pathway for paradoxical embolism, the most significant clinical concern. This occurs when a blood clot, typically formed in the leg veins, travels to the right side of the heart and passes through the PFO to the left side. Once on the left side, the clot enters the systemic circulation and can travel to the brain, potentially causing a cryptogenic stroke, which is a stroke of unknown cause. PFO has also been linked to an increased risk of decompression sickness in deep-sea divers due to the potential passage of gas bubbles into the arterial circulation.