The foramen ovale is a small, flap-like opening within the heart. This temporary structure is a normal feature present in all individuals before birth. It serves a specific purpose in the fetal circulatory system, facilitating blood flow.
Precise Location
The foramen ovale is situated within the atrial septum, the muscular wall dividing the heart’s two upper chambers: the right and left atria. This opening, formed during embryonic development by the septum primum and septum secundum, creates a direct passageway between these chambers. The septum primum acts as a valve-like leaflet, positioned on the left atrial side.
This opening is located in the posterior interatrial septum, near where the inferior vena cava enters the right atrium. This allows oxygenated blood returning from the placenta to be preferentially directed towards the foramen ovale. When viewed from the right atrium, the foramen ovale is typically not directly visible because the septum primum covers the opening on the left atrial side, facilitating unidirectional blood flow.
Its Purpose in Fetal Life
During fetal development, the lungs are not yet functional for gas exchange, as the fetus receives oxygen and nutrients directly from the mother’s placenta. The foramen ovale allows blood to bypass these undeveloped lungs, which are filled with fluid and offer high resistance to blood flow. It acts as a shunt, directing oxygenated blood from the right atrium directly into the left atrium.
This direct pathway ensures oxygen-rich blood efficiently bypasses the pulmonary circulation. From the left atrium, this highly oxygenated blood moves into the left ventricle and is pumped to the fetal brain and upper extremities, which are developing rapidly and require a consistent oxygen supply. The foramen ovale is one of three temporary shunts in fetal circulation that optimize blood flow before birth, ensuring vital organs receive adequate oxygenation.
What Happens After Birth
Upon birth, significant physiological changes occur as the newborn transitions from placental to pulmonary respiration. The baby’s first breaths expand the lungs, leading to a decrease in pulmonary vascular resistance and increased blood flow. This increased blood flow results in a greater volume of blood returning to the left atrium, increasing pressure within this chamber.
Simultaneously, the umbilical cord is cut, eliminating placental blood flow and reducing right atrial pressure. This pressure reversal, with left atrial pressure now exceeding right, causes the septum primum, the flap covering the foramen ovale, to press firmly against the septum secundum. This action functionally closes the opening. Over weeks to months, or even a year, the two septa typically fuse permanently, forming the fossa ovalis.
When It Remains Open
A Patent Foramen Ovale (PFO) occurs when the foramen ovale does not completely seal after birth. This persistence is a common finding, present in approximately one in four adults. Most individuals with a PFO remain asymptomatic throughout their lives and are often unaware they have the condition.
While typically benign, a PFO can, in rare instances, be associated with certain clinical implications. These may include an increased risk of specific types of stroke, known as paradoxical embolisms, where small blood clots can pass from the right side of the heart to the left. Some research also suggests a potential link between PFO and migraine headaches. For the majority, however, PFO does not cause problems or require medical intervention.