The fetal heart receives the superior vena cava at the right atrium. The superior vena cava carries deoxygenated blood from the head and upper limbs. From the right atrium, blood flows inferiorly into the right ventricle across the stream of relatively oxygenated blood passing to the foramen ovale from the inferior vena cava. There is relatively little mixing of streams; only that part of the inferior vena caval stream which intercepts the inferior margin of the septum secundum remains in the chamber to join the superior vena caval stream.
From the left ventricle, blood passes along the route of least resistance. Before birth, the resistance in the pulmonary vessels is high. Consequently, blood tends to flow through the ductus arteriosus into the descending aorta. It receives a lesser stream from the proximal aorta that originates from the left ventricle during systole.
The minority of blood passing into the descending aorta may pass distally to peripheral beds such as the lower limbs and abdomen. The majority is passed within the umbilical arteries to the placenta for reoxygenation.
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