Oxygen Shunting: Before birth, the fetal lungs are non-functional. Oxygen exchange occurs through the placenta, and oxygenated blood returns to the fetus via the umbilical vein. The foramen ovale acts as a shunt, allowing most of this oxygenated blood to bypass the non-functional lungs and be directed to the systemic circulation.
Pressure Regulation: The foramen ovale also helps regulate blood pressure in the fetus. The pressure in the fetal pulmonary arteries is generally lower than in the fetal systemic circulation. By allowing blood to flow directly from the right atrium to the left atrium, the foramen ovale helps maintain higher systemic blood pressure needed to perfuse the fetal organs and tissues.
Deoxygenated Blood Mixing: A small amount of deoxygenated blood returning from the fetal body (via the superior vena cava) does mix with the oxygenated blood in the right atrium. Because most of the oxygenated blood is directed through the foramen ovale, the overall oxygen saturation of blood pumped out to the fetal body remains high.
Overall, the foramen ovale plays a vital role in fetal circulation by enabling the efficient delivery of oxygenated blood to the body and facilitating proper fetal development. After birth, once the lungs begin to function, the foramen ovale gradually closes, and the pulmonary circulation takes over the primary role of oxygenating the blood.