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The fetus receives oxygenated blood from the placenta through the umbilical vein. Part of the received blood passes through the hepaticsinusoids, whereas most of the incoming blood passes through the ductus venosus directly into the inferior vena cava. At the inferior vena cava, the oxygen rich blood from the placenta mixes with the blood from the caudal portions of the fetus. The mixed stream of blood enters the right atrium and crosses the interatrial membrane through the foramen ovale into in the left atrium. At the left atrium, the blood is mixed again with poorly oxygenated blood from the pulmonary veins and then passes through the left ventricle to the aorta. The blood from the superior vena cava and a small amount of blood from the inferior vena cava is diverted into the pulmonary artery, where the blood is shunted into the descending thoracic aorta through the ductus arteriosus. The resultant mixed blood goes into the abdominal aorta, to the circulation of the viscera and the lower extremities, eventually reaching the placenta through the umbilical arteries, for oxygenation.
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