Blood returns from the placenta via the umbilical vein. Most is shunted through the liver to the inferior vena cava via the ductus venosus. As it enters the right atrium, this relatively well-oxygenated blood is preferentially shunted through the foramen ovale to the left atrium where it is mixed with a small amount of blood coming from the pulmonary veins. After this still relatively well oxygenated blood is ejected into the aorta it supplies the heart and the brain as well as the upper extremities. Only a small amount goes through the isthmus of the aorta into the descending aorta. The relatively less-well-oxygenated blood entering the right atrium from the superior vena cava preferentially flows across the tricuspid valve into the right ventricle. From there it is ejected into the pulmonary trunk. Only a small portion flows to the lungs via the pulmonary arteries; the bulk goes into the descending aorta via the ductus arteriosus. Blood flow in the descending aorta supplies the abdominal organs and lower extremities and returns blood to the placenta via the umbilical arteries, thus completing the circuit.