The hepatic portal vein is a vessel that moves blood from the spleen and gastrointestinal tract to the liver.

It is approximately three to four inches in length and is usually formed by the merging of the superior mesenteric and splenic veins behind the upper edge of the head of the pancreas. In some individuals, the inferior mesenteric vein may enter this intersection instead. 

In most people, the portal vein splits into left and right veins before entering the liver. The right vein then branches off into anterior and superior veins.

The portal vein supplies approximately 75 percent of blood flow to the liver. The portal vein is not a true vein, which means it does not drain into the heart. Instead, it brings nutrient-rich blood to the liver from the gastrointestinal tract and spleen. Once there, the liver can process the nutrients from the blood and filter out any toxic substances it contains before the blood goes back into general circulation.

Abnormally high blood pressure in the portal vein is known as portal hypertension. The condition may cause the growth of new blood vessels that bypass the liver, which can result in the circulation of unfiltered blood throughout the body. Portal hypertension is one of the potential serious complications of liver cirrhosis, which is a condition where normal liver tissue is replaced with scar tissue.