The foramen ovale (FO) is a temporary, specialized opening located in the heart of a developing fetus. This structure acts as a shunt, allowing blood to detour around the fetal lungs, which are not yet capable of gas exchange. It is an adaptation that ensures the most oxygen-rich blood bypasses the pulmonary circuit, which is not functional before birth.
Location and Structure of the Foramen Ovale
This interatrial communication is situated within the wall separating the two upper chambers of the heart, the right and left atria. The opening is a unique flap-valve structure formed by two overlapping septal tissues. The rim of the opening is created by a crescent-shaped partition called the septum secundum, while the actual flap is a thin membrane known as the septum primum. The septum secundum acts as a protective boundary, preventing the septum primum from opening toward the right atrium.
The Shunting of Blood During Fetal Life
The primary substance that passes through the foramen ovale is highly oxygenated blood returning from the placenta. Since the fetus receives its oxygen supply from the mother via the umbilical cord, the lungs are inactive and offer high resistance to blood flow. This specialized blood enters the right atrium primarily through the inferior vena cava.
In the fetal heart, the pressure in the right atrium is naturally higher than the pressure in the left atrium. This pressure differential forces the thin septum primum flap to open, creating the shunt. Approximately 40% of the oxygenated blood entering the right atrium is immediately directed through the foramen ovale into the left atrium. This shunted blood then moves to the left ventricle, which pumps it into the systemic circulation, preferentially supplying the head, neck, and upper extremities, including the developing brain.
The blood that does not cross the foramen ovale flows into the right ventricle, which pumps it toward the lungs. However, most of this blood is diverted away from the lungs through another fetal shunt, the ductus arteriosus, to the descending aorta. The pressure gradient across the foramen ovale is maintained by the high resistance of the fetal pulmonary blood vessels, ensuring the continual right-to-left shunting.
Physiological Changes and Closure at Birth
When a newborn takes its first breath, physiological changes immediately transform the circulatory system. The lungs inflate, which causes a dramatic and rapid drop in pulmonary vascular resistance. Simultaneously, the clamping of the umbilical cord increases systemic vascular resistance, reducing the flow of blood returning to the right side of the heart.
This shift in resistance reverses the pressure gradient between the heart’s atria. As a large volume of blood now returns from the newly functioning lungs to the left atrium, the pressure in the left atrium rises significantly. The left atrial pressure quickly exceeds the right atrial pressure, forcing the septum primum flap against the septum secundum, functionally closing the foramen ovale.
This initial functional closure occurs almost immediately after birth, separating the pulmonary and systemic circulations. Over the following weeks and months, the septal tissues typically fuse together permanently. The fibrous remnant of the closed foramen ovale in the adult heart is known as the fossa ovalis. Anatomical fusion occurs in about 80% of individuals by the time they reach one year of age.
Understanding Patent Foramen Ovale (PFO)
In about 20% to 25% of the general population, the septal tissues fail to fuse completely, leaving a small, flap-like opening that persists into adulthood. This condition is called a Patent Foramen Ovale (PFO), meaning the opening remains “patent” or open. A PFO is not considered a true heart defect because it is an incomplete sealing of a normal fetal structure, not a deficiency in the septal tissue.
For most individuals, a PFO remains asymptomatic because the pressure in the left atrium remains higher than the right, keeping the flap pressed shut. However, under certain conditions, such as a strong cough or strenuous straining (Valsalva maneuver), the pressure in the right atrium can momentarily exceed the left, allowing a temporary right-to-left shunt.
PFO is often investigated in patients who have experienced an unexplained stroke, known as a cryptogenic stroke, or in divers who develop decompression sickness. In these cases, a blood clot forming in the venous system, which would normally be filtered by the lungs, may pass through the PFO into the arterial circulation and travel to the brain. Diagnosis of a PFO is typically made using a bubble study, a specialized form of echocardiogram where agitated saline containing tiny micro-bubbles is injected into a vein.