Embryology
In early life, when a fetus is in the womb, the right atrium has a hole within its septum through to the left atrium; this makes them continuous with each other, which is essential for fetal circulation. This junction is called the Foramen ovale. Once born (usually within 9 months' time) the foramen ovale seals over. This occurs when the first breath is taken; fetal blood flow is reversed and now travels through the lungs, no longer requiring the foramen ovale. Therefore it closes and is renamed as the Fossa ovalis. The fossa ovalis is considered an embryonic remnant.
In some cases, the foramen ovale fails to close and is present in 20% of the general population; however, it does not cause problems in the vast majority. This is known as a patent foramen ovale.
Within the fetal right atrium, blood from the inferior vena cava and the superior vena cava flow in separate streams to different locations in the heart, and this has been reported to occur through the Coanda effect.
Read more about this topic: Right Atrium