The hepatic portal vein is a large blood vessel, which carries nutrient-enriched blood to the liver. The vein (in adults) is around 8cm in length, beginning behind the neck of the pancreas and ending in several divisions at the liver.
This vessel is alternatively known as the splenic-mesenteric confluence, because it comes into being when the splenic vein (which collects blood from the spleen) and the superior mesenteric vein (which collects blood from the small intestine). Later, the vein also combines with the gastric and cystic veins, which take in the blood from the stomach and gall-bladder, respectively.
When the hepatic portal vein reaches the liver, it divides to make two portal veins, left and right. These smaller veins further divide until they become portal venules, which deliver blood into the liver sinusoids (permeable capillaries).
The purpose of the hepatic portal system is to supply the liver with 70% of its blood and 50% of its oxygen. By taking nutrient-rich blood from the spleen, stomach, gall bladder and small intestine, then transporting it to the liver, the hepatic portal vein allows the liver to process nutrients from all around the body and to filter out ingested toxins.
Sources: en.wikipedia.org/wiki/Hepatic_portal_vein; www.merriam-webster.com; encyclopedia brittannica; radiopedia.org; www.imageradiology.blogspot.co.uk