Fetal and Post Transition Circulation
In the fetal circulatory system, oxygenated blood is delivered via the umbilical vein, entering the inferior vena cava via the ductus venosus. The majority of this oxygenated blood passes through the right atrium and into the left atrium through the foramen ovale to enter the systemic circulation.
Meanwhile, high pulmonary pulmonary vascular resistance (due to hypoxic vasoconstriction in fluid-filled alveoli) means that most of the deoxygenated right ventricular output is routed through the ductus arteriosus and enters into the systemic circulation – mixing with oxygenated blood distal to the highest priority end-organs (brain and heart), to be reoxygenated at the placenta.
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