The liver receives nutrient-rich blood from the gastrointestinal tract via the portal hepatic system. The major vessel of this system is the portal vein, which is formed in the retroperitonium by the union of the superior mesenteric and splenic veins. Posterior to the neck of the pancreas at the portal's splenic confluence. It passes obliquely to the right, posterior to the hepatic artery within the lesser omentum, and enters the liver at the porta-hepades. At the porta-hepades, the portal vein branches into right and left main portal veins. That then follow the course of the right and left hepatic arteries. The right main portal vein first sends branches to the caudate lobe, segment 1, and then divides into anterior and posterior branches. That subdivide into superior and inferior branches to supply the right lobe of the liver. Segment 5, 6, 7, and 8. The left main portal vein initially courses to the left, then turns immediately toward the ligamentum tiers. The branches to supply the lateral segments, segments 2 and 3 of the left lobe, and the superior and inferior sections of segment ramen 4.