Final answer:
The hepatic vein is the structure that does not drain into the hepatic portal system; instead, it carries blood away from the liver to the inferior vena cava.
Step-by-step explanation:
The structure that would not drain into the hepatic portal system is any blood vessel that transports blood from the liver, such as the hepatic vein. The hepatic portal system is essential for transporting absorbed nutrients and certain wastes directly to the liver. The structures that do contribute to this system include the superior mesenteric vein, the inferior mesenteric vein, the splenic vein, and the gastric veins.
Blood from the small intestine, two-thirds of the large intestine, stomach, spleen, and pancreas flows through these veins on its way to the liver. However, the hepatic vein carries blood away from the liver, merging it into the inferior vena cava, which in turn carries it back to systemic circulation.