Final answer:
The foramen ovale is an opening in the interatrial septum of the fetal heart that closes after the baby is born and begins to breathe. This closure leaves a depression called the fossa ovalis.
Step-by-step explanation:
The foramen ovale is an opening in the interatrial septum of the fetal heart that allows blood to bypass the pulmonary circulation until the baby is born and starts breathing air. A valve associated with this opening prevents backflow of blood during the fetal period. As the newborn begins to breathe and blood pressure in the atria increases, this shunt closes, leaving a small depression called the fossa ovalis.