Portal vein

The portal vein is a major vein in the human body draining blood from the digestive system and its associated glands. It is formed by the union of the splenic and superior mesentric veins. It divides into a right and left branches, before entering the liver.

Note that the portal vein drains blood into the liver not from the liver. The blood entering the liver from the portal vein will then leave to the inferior vena cava through the hepatic veins.

Tributeries

The portal vein drains:

• The posterior superior pancreaticoduodenal vein
• The right gastric vein
• The left gastric vein

Significance

Almost all of the blood coming from the digestive system drains into a special venous circulation called the portal circulation. This is because it contains all the nutrients and toxins that have been absorbed along the digestive tract from ingested food. Before these absorbed substances can go into the systemic circulation (the main blood circulation in the body), it must be filtered first to remove or "detoxify" toxins first. This filtering and detoxification is one of the functions of the liver.