The ductus venosus is an important vessel within the fetal circulation. It is formed on the posteroinferior aspect of the liver by the union of the left umbilical vein and the left branch of the portal vein. It ends at its junction with the inferior vena cava. Just before this point, it receives the left hepatic vein. It is ensheathed by layers of the lesser omentum.
The ductus venosus provides a means for oxygenated umbilical vein blood to bypass the sinusoids of the liver. The amount of blood passing through the vessel seems to be regulated by a sphincter mechanism. For example, to prevent excessive venous return to the right side of the heart during uterine contractions, there is relative contraction of the ductus venosus.
After birth, the blood within the umbilical vein clots and this extends to the ductus venosus. Both become converted to fibrous cords that usually are in continuity: the ligamentum teres and the ligamentum venosum respectively.