Portal venous system, illustration. The portal vein itself is short entering the liver. Its main tributaries are the superior mesenteric vein (draining the small bowel, ascending, transverse colon, pancreas and part of the stomach), the splenic vein, and the inferior mesenteric vein (drains the descending colon and rectum). The latter usually enters the splenic vein. From Testut, L. (1931) Traite d'anatomie humaine. Vol. 4, Doin and Co, Paris.