That is a tough one! Basicaly, just try to think of WHY the circulation is re-routed...because the mother is oxygenat8ing the blood. If you think of it that way, kind of makes sense that the circ bypassess the baby's lungs, b/c he doesn't need to oxygenate it, mom is doing that, so that is why you have your PDA, and foramen ovale. Beyond that, I really can't be of much help to you, sorry. It's just one of those have to memorize things unfortnately!!
Jan 31, '03
Remember that any blood vessel foing to the heart is a vein, away is an artery--that remains true for the fetus. Extra features are functional--foramen ovale and ductus arteriosis are to keep most (~95%, I think) of the blood from going to the lungs, which don't need it. The umbilical vein brings blood from the placenta through the liver to the heart, the umbilical arteries take blood from the baby to the placenta.
Oxygen/CO2 and nutrient exchange takes place in the placenta, but the circulations do not mix. Within the baby, cellular nourishment is the same as post birth.
Use the diagrams and trace the circulation with your finger or a pen and think about each difference/similarity and why/what the function is at that point. If you have one of those "coloring" books, I think that would help you make sense out of it.
Jan 31, '03
And the ductus venosus is the passageway the blood takes after it enters the baby from the cord. It bypasses the baby's liver because it's already detoxified from the mother. The oxgenated blood enters the right atrium and most of it passes through the foramen ovale to the left atrium and then left ventricle to go to the body.
The changes that close these extra passageways after birth have to do with pressure changes, systemic and pulmonary vascular resistances and PaO2 levels. Do you need to be specific about this? It gets more complicated... Basically removing the placenta, initiating respirations and increasing the PaO2 levels transitions the baby to normal circulation.