The main function of the portal vein is to move blood from the spleen and gastrointestinal tract to the liver, according to Healthline. The portal vein supplies 75 percent of blood flow to the liver.
The portal vein is a vessel 3 to 4 inches long, formed by the fusing of the superior mesenteric and splenic veins, explains Healthline. When an issue arises in the portal vein, a medical issue known as portal hypertension can take place. It occurs when there is an increase of pressure in the portal vein due to a blockage of the liver's blood flow. Portal hypertension is usually caused by a blood clot in the portal veins, known as thrombosis, notes Cleveland Clinic.