The oval fossa, which can be seen on dissection of the heart chambers is an indentation in the posterior wall of the right atrium. It is an embryologic remnant of the oval foramen, which allows for a right to left shunt (i.e., bypassing the fetal lungs.)
In approximately 15 to 25% of the population there is incomplete closure of the oval foramen, although this is usually not clinically significant. Of note, although the patent oval foramen in the fetus allows for a right to left shunt, an incompletely closed oval foramen after birth if clinically significant causes a left to right shunt.
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