For any doctors out there, Im studying for MCAT and I have a quick question.?

Question

Why does fetal circulation need to bypass the liver?

Answer 1

The fetal liver is not mature enough to handle detoxification of the infants blood stream - this is performed by the mothers liver.  The addition of the hepatic portal system is one of the many changes that occur after birth - it would be a waste of fetal energy (which should be used for growth) to duplicate something the mother is already doing well.

Answer 2

It doesn't bypass the liver entirely.  About half of the blood that enters the fetus's body through the umbilical vein does go directly to the liver--the other half bypasses it via the ductus venosum.  The liver needs a lot of blood to develop.   But the shunt is there so that the oxygenated blood that is entering through the umbilical vein can travel more directly to the heart and from there to the brain.  If it didn't, those vital areas would run the risk of not receiving enough oxygen.  (It should be noted, as well, that when we talk about blood bypassing the lungs, we mean only the pulmonary circuit that will eventually be responsible for oxygenation.  The lungs themselves need oxygen and nutrients to develop and that circulation is already present in the fetus.)

It is also less important that blood travel through the liver once it's already circulating in the fetus, because the mother's liver is doing the much of the detoxification work and making clotting elements and such that the baby's liver is not yet equipped to handle--just as the mother's lungs are providing the oxygen that crosses the placenta.