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Blood in the inferior vena cava of the fetus originates from several sources but it is relatively oxygenated compared to other points within the circulation:
It enters the right atrium where the valve of the inferior vena cava deflects the majority of the stream towards the foramen ovale. Hence, most blood passes into the left atrium. A small amount of blood from the inferior vena cava passes through the right atrioventricular orifice with blood from the superior vena cava. Generally, the flows of blood from the inferior and superior vena cavae are kept separate across the right atrium.
Within the left atrium, blood that has traversed the foramen ovale mixes with a small amount of deoxygenated blood returning from the lungs within the pulmonary veins. Contraction of the heart then conveys this mixed blood, still predominantly oxygenated, to the left ventricle and then to the aorta. The first branches of the aorta, the coronary and carotid arteries, tend to receive the bulk of this oxygentated supply in accordance with their functional importance.
A small amount of the blood ejected by the left ventricle passes into the descending aorta. Of this, the minority passes distally to peripheral beds such as the lower limbs and abdomen. The majority is passed within the umbilical arteries to the placenta for reoxygenation.