Why was the ductus venosus important during fetal circulation?

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The key idea is that the ductus venosus acts as a fetal shunt that directs highly oxygenated blood from the placenta toward the heart, while bypassing hepatic circulation. Blood travels from the umbilical vein into the liver, but the ductus venosus provides a direct path into the inferior vena cava, mixing with hepatic venous blood and delivering well-oxygenated blood to the right and then left sides of the heart. From there, most of this blood goes to the brain and systemic circulation, which is crucial for fetal development.

This bypass is why the ductus venosus is not directing blood to the lungs or supplying the liver with blood. The lungs are nonfunctional in the fetus, so most blood avoids them anyway, through other shunts. It also isn’t connecting hepatic arteries to the heart—the ductus venosus links the umbilical vein to the systemic circulation, bypassing the liver. And the liver’s oxygen supply comes mainly from the hepatic artery and portal system, not from this shunt.