Final answer:
The portal venous system drains blood from digestive organs and the spleen into the liver for processing, involving the hepatic portal vein and its tributaries like the superior mesenteric, splenic, and inferior mesenteric veins.
Step-by-step explanation:
The portal venous system drains blood primarily from the digestive organs and spleen to the liver. This system includes the hepatic portal vein and its tributaries, such as the superior mesenteric vein, the splenic vein, and the inferior mesenteric vein. The superior mesenteric vein carries blood from the small intestine, the majority of the large intestine, and the stomach.
The inferior mesenteric vein drains the remaining third of the large intestine, which includes the descending colon, sigmoid colon, and rectum. Meanwhile, the splenic vein forms from branches originating in the spleen, pancreas, and parts of the stomach. Once the blood has been collected by these veins, it is delivered directly to the liver where it is processed to remove wastes and store excess nutrients. The processed blood is then returned to general circulation via the hepatic veins, flowing into the inferior vena cava.