Final answer:
The portal vein receives blood through the superior mesenteric vein, splenic vein, inferior mesenteric vein, gastric veins, and cystic veins. This mixture of blood is then transported to the liver for processing. After liver processing, blood exits through hepatic veins to the inferior vena cava.
Step-by-step explanation:
The blood that drains into the portal vein comes from several sources related to the gastrointestinal tract and associated organs. The primary vessels contributing to the formation of the hepatic portal vein include the superior mesenteric vein, the splenic vein, and the inferior mesenteric vein. The superior mesenteric vein collects blood from the small intestine, two-thirds of the large intestine, and the stomach. Blood from the spleen, along with the pancreas and portions of the stomach, is collected by the splenic vein. Furthermore, the inferior mesenteric vein drains the distal third of the large intestine encompassing the descending colon, the sigmoid colon, and the rectum. Additional contributions come from the gastric veins of the stomach and the cystic veins from the gall bladder.
Once blood is processed in the liver, removing various toxins and excess nutrients, it exits through the hepatic veins before rejoining the systemic circulation via the inferior vena cava. This dual blood supply to the liver and the systemic removal of wastes and delivery of nutrients are crucial components of the hepatic portal circulation system, which is significantly affected by conditions such as cirrhosis and portal hypertension that lead to complications like esophageal varices.