In the fetus, the ductus venosus (Arantius' duct after Julius Caesar Aranzi) shunts a portion of the left umbilical vein blood flow directly to the inferior vena cava. Thus, it allows oxygenated blood from the placenta to bypass the liver. Compared to the 50% shunting of umbilical blood through the ductus venosus found in animal experiments, the degree of shunting in the human fetus under physiological conditions is considerably less, 30% at 20 weeks, which decreases to 18% at 32 weeks, suggesting a higher priority of the fetal liver than previously realized. In conjunction with the other fetal shunts, the foramen ovale and ductus arteriosus, it plays a critical role in preferentially shunting oxygenated blood to the fetal brain. It is a part of fetal circulation.
The ductus venosus is open at the time of the birth and is the reason why umbilical vein catheterization works. Ductus venosus naturally closes during the first week of life in most full-term neonates; however, it may take much longer to close in pre-term neonates. Functional closure occurs within minutes of birth. Structural closure in term babies occurs within 3 to 7 days.
If the ductus venosus fails to occlude after birth, the individual is said to have an intrahepatic portosystemic shunt (PSS). This condition is hereditary in some dog breeds (e.g. Irish Wolfhound). The ductus venosus shows a delayed closure in preterm infants, with no significant correlation to the closure of the ductus arteriosus or the condition of the infant. Possibly, increased levels of dilating prostaglandins leads to a delayed occlusion of the vessel.
^Kiserud, T.; Rasmussen, S.; Skulstad, S. (2000). "Blood flow and the degree of shunting through the ductus venosus in the human fetus". American journal of obstetrics and gynecology182 (1 Pt 1): 147–153. doi:10.1016/S0002-9378(00)70504-7. PMID10649170.edit