Left branch

The portal vein divides into a right branch and a left branch just before or at the level of the porta hepatis.

The left branch of the portal vein (ramus sinister venae portae) is typically larger than the right in most domestic mammals, reflecting the proportionally larger left lobe of the liver. It proceeds in a cranial and slightly dorsal course, entering the left hepatic lobe and often issuing several segmental branches.

The left branch of portal vein (Ramus sinister) directly extends the portal vein toward the left lobe. It consists of two successive segments named according to their orientation in the human liver: the transverse part (Pars transversa) and the umbilical part (Pars umbilicalis). The latter, which curves toward the fissure of the round ligament, is so named because in the fetus it directly prolonged the umbilical vein (V. umbilicalis), whose extrahepatic portion, obliterated after birth, becomes the round ligament of the liver (Lig. teres hepatis). The boundary between the two parts of the left branch is marked by the fibrous remnant of the ductus venosus (Ductus venosus), which in the fetus connected the umbilical vein to the caudal vena cava. The distinction between the two parts is difficult in species where this fibrous remnant is itself barely distinguishable (e.g., Equidae).