A portal system is a pattern of circulation in which the vessels that drain one group of capillaries transport blood to a second group of capillaries, which in turn are drained by more common systemic veins that take blood to the right atrium of the heart. Therefore, there are two capillary beds in series. The hepatic portal system consists of veins, by which blood is drained from capillaries in the intestines, pancreas, spleen, stomach, and gallbladder into capillaries in the liver named sinusoids and of the right and left hepatic veins that drain the liver and empty into the inferior vena cava. The absorbed products of digestion must first pass through hepatic sinusoids in the liver before entering the normal circulation as a result of the hepatic portal system.
Blood is received by the large vessel, the hepatic portal vein, from the digestive organs. It is produced by a union of the superior mesenteric vein, by which the nutrient-rich blood is drained from the small intestine, and the splenic vein. The spleen is drained by the splenic vein, which is enlarged for the reason of a convergence of the following 3 tributaries:
- The inferior mesenteric vein from the large intestine;
- The pancreatic vein from the pancreas;
- The left gastroepiploic vein from the stomach.
From the stomach as well, the right gastroepiploic vein drains directly into the superior mesenteric vein.