ABSTRACT
Fetal circulation Postnatally, oxygen uptake and carbon dioxide removal from the body occur in the lungs. Energy substrates are absorbed in the gastrointestinal tract and delivered to the liver through the portal venous system before entering the general circulation. In the mammalian fetus, oxygen uptake and carbon dioxide removal are accomplished in the placenta through the umbilical circulation. Energy substrates diffuse or are actively transported from the maternal circulation across the placental membrane and are transferred to the fetal body via the umbilical vein. Although a proportion of the substrates enter the hepatic circulation, a variable amount bypasses the liver to enter the general fetal circulation via the ductus venosus.
