The vein which carries to the LIVER, blood that has been circulating in many of the abdominal organs. It is peculiar among the veins of the body in that it ends by breaking up into a capillary network instead of carrying the blood directly to the heart – a peculiarity which it shares only with certain small vessels in the kidneys. The PORTAL SYSTEM begins below in the haemorrhoidal plexus of veins around the lower end of the rectum; from this point, along the whole length of the intestines, the blood is collected into an inferior mesenteric vein upon the left, and a superior mesenteric vein upon the right side. The inferior mesenteric vein empties into the splenic vein, and the latter, uniting with the superior mesenteric vein immediately above the pancreas, forms the portal vein. The portal vein is joined by veins from the stomach and gall-bladder, and finally divides into two branches which sink into the right and left lobes of the liver. (For their further course, see LIVER.)

The organs from which the portal vein collects the blood are the large and small intestines, the stomach, spleen, pancreas, and gall-bladder.