What is the function of the foramen ovale in fetal circulation and what occurs after birth?

• A. It shunts blood from the right atrium to the left atrium, bypassing the fetal lungs; after birth it closes to form the fossa ovalis
• B. It connects the pulmonary artery to the aorta
• C. It remains open after birth to allow mixing
• D. It shunts blood from left atrium to right atrium

Answer: (A)

In fetal life the foramen ovale allows blood to move directly from the right atrium to the left atrium, bypassing the nonfunctional fetal lungs so highly oxygenated blood from the placenta can reach the systemic circulation efficiently. After birth, when the lungs fill with air and pulmonary resistance falls, the pressure in the left atrium rises and the opening is pushed closed, converting into the fossa ovalis as the interatrial septum seals. The shunt is no longer needed because the lungs now oxygenate blood before it travels to the rest of the body.