Final answer:
The hepatic portal vein is formed by the confluence of the superior mesenteric, splenic, and inferior mesenteric veins, which then carry blood to the liver for processing.
Step-by-step explanation:
The three veins that come together to form the hepatic portal vein are the superior mesenteric vein, the splenic vein, and the inferior mesenteric vein. The superior mesenteric vein receives blood from the small intestine, most of the large intestine, and the stomach. The inferior mesenteric vein drains blood from the latter part of the large intestine, including the descending colon, sigmoid colon, and rectum.
The splenic vein carries blood from the spleen, part of the pancreas, and portions of the stomach. These veins merge at the level of the second lumbar vertebra (L2), forming the hepatic portal vein which then delivers nutrients and other substances directly to the liver for processing.