The cardiovascular system is the first organ system to become functional in the fetus. Remember that the placenta, not the lung, is the organ of gas exchange. The right and left ventricles operate in parallel, not in series. Output from the right heart is approximately double that of the left heart.
About one-half of the blood coming from the placenta passes through the ductus venosus, bypassing the liver. Blood of somewhat higher oxygen content and PO2 passes through the foramen ovale in the interatrial septum, and into the left heart. This blood preferentially perfuses the head, upper body, and heart.
Blood leaving the right ventricle, passes into the pulmonary artery, but because of high vascular resistance in the pulmonary circuit, is shunted to the aorta through the ductus arteriosus.
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