The fetal circulatory system varies from the adult’s in a variety of ways.  The fetus’ source of oxygen and nutrients is from the umbilical vein, so named for the fact that it is returning to the fetus’ heart (typically arteries carry nutrients/O2, not veins).  The umbilical bypasses the liver’s portal vein using a unique fetal structure, the ductus venosus.  Problems can arise if this vein remains open in an adult, causing a shunt so that the liver receives very little blood.  However, this is normal in the fetus.
Once the blood reaches the heart, it bypasses the lungs by way of another unique fetal vessel, the ductus arteriosus.  Again, if this remains open in the adult, instead of closing a few days after birth, severe problems can arise and heart failure can result.  But in the fetus, there is no need for blood to go to the lungs for oxygen extraction, as it is already available thanks to the mother’s lungs.
Once the blood leaves the heart, it returns to the mother for filtration and oxygenation by way of the umbilical artery.  

The fetal circulatory system varies from the adult’s in a variety of ways.  The fetus’ source of oxygen and nutrients is from the umbilical vein, so named for the fact that it is returning to the fetus’ heart (typically arteries carry nutrients/O2, not veins).  The umbilical bypasses the liver’s portal vein using a unique fetal structure, the ductus venosus.  Problems can arise if this vein remains open in an adult, causing a shunt so that the liver receives very little blood.  However, this is normal in the fetus.

Once the blood reaches the heart, it bypasses the lungs by way of another unique fetal vessel, the ductus arteriosus.  Again, if this remains open in the adult, instead of closing a few days after birth, severe problems can arise and heart failure can result.  But in the fetus, there is no need for blood to go to the lungs for oxygen extraction, as it is already available thanks to the mother’s lungs.

Once the blood leaves the heart, it returns to the mother for filtration and oxygenation by way of the umbilical artery.  

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