Fetal blood and maternal blood exchange oxygen, carbon dioxide, nutrients and waste materials in the placenta, but do not mix. Fetal blood reaches the placenta through the paired umbilical arteries, which are branches of the internal iliac arteries. While the blood is circulating through the capillaries of the chorionic villi within the placenta , the exchange process takes place. Blood rich in oxygen and nutrients then leaves the placenta to return to the fetus through a single umbilical vein. The umbilical vein is the only fetal vessel that carries this enriched blood. The remaining vessels of the fetus carry a mixture of enriched and depleted blood.
Blood returning to the fetus via the umbilical vein is 80% saturated with oxygen. Traveling from the placenta to the organs of the fetus, the highly oxygenated blood in the umbilical vein slowly loses its oxygen concentration by mixing with desaturated blood in several places: 1) in the liver by mixing with a small amount of blood from the portal system, 2) in the inferior vena cava which carries deoxygenated blood returning from the lower extremities, pelvis and kidneys, 3) in the right atrium by mixing with returning blood from the head and extremities, 4) at the entrance of the ductus arteriosus into the descending aorta.
The blood returning from the placenta is first routed through the ductus venosus causing most of it to bypass the liver and empty directly into the inferior vena cava. Here it mixes with deoxygenated blood from the lower limbs. It then enters the right atrium. A sphincter mechanism in the ductus venosus near the entrance of the umbilical vein regulates the flow of umbilical blood through the liver. A small portion of the blood enters the liver sinusoids and mixes with blood from the portal circulation. This sphincter closes when venous return is high, in order to prevent an overloading of the heart. A large amount of blood is directed from the right atrium through the foramen ovale and into the left atrium. A smaller amount is prevented from entering the foramen ovale by the septum secundum or the crista dividens and remains in the right atrium to mix with the desaturated blood returning from the head and arms via the superior vena cava.
The blood in the left atrium mixes with a small amount of blood returning from the lungs and then enters the left ventricle and ascending aorta. The myocardium and brain are thereby supplied with well oxygenated blood because the coronary and carotid arteries are the first to branch from the ascending aorta. The desaturated blood from the superior vena cava is directed into the pulmonary trunk from the right ventricle. The high resistance in the pulmonary vessels causes a large portion of this blood to flow through the ductus arteriosus and into the descending aorta to mix with the blood from the proximal aorta. The two umbilical arteries, arising from the internal iliac arteries, facilitate the flow of blood toward the placenta. The oxygen saturation in the umbilical arteries is approximately 58%.