Final answer:
In the fetal heart, the foramen ovale closes after birth and becomes the fossa ovalis, which helps isolate deoxygenated and oxygenated blood in the adult heart.
Step-by-step explanation:
In the fetal heart, there is a structure known as the foramen ovale, which is an opening in the interatrial septum allowing blood to flow from the right atrium to the left atrium, effectively bypassing the non-functioning fetal lungs. Upon birth and the initiation of breathing, the foramen ovale closes as blood pressure in the atria changes, leaving behind a small depression called the fossa ovalis. This closure isolates deoxygenated and oxygenated blood within the adult heart, ensuring proper circulation.