The foramen ovale is the fetal communication between the right and left atria. It is formed by the incomplete descent of the septum secundum to the left of the septum primum during septation of the primitive atrium. It lies within the interatrial septum.
During fetal life, oxygenated blood is deflected by the valve of the inferior vena cava across the right atrium and through the foramen ovale. Actually, this entails:
At birth, the elevation of pressure within the left atrium forces the septum primum and secundum together; usually, they have fused by the time the child is one year of age. In 20% of the population, fusion does not occur and there is exists the potential for an interatrial shunt - patent foramen ovale.
The remnant of the foramen ovale after fusion is the fossa ovalis.
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