The portal vein carries about 1500 ml/min of blood from the small and large bowel, spleen, and stomach to the liver at a pressure of 5-10 mm Hg. Any obstruction or increased resistance to flow or, rarely, pathological increases in portal blood flow may lead to portal hypertension with portal pressures over 12 mm Hg. Although the differential diagnosis is extensive, alcoholic and viral cirrhosis are the leading causes of portal hypertension in Western countries, whereas liver disease due to schistosomiasis is the main cause in other areas of the world. Portal vein thrombosis is the commonest cause in children.

Increases in portal pressure cause development of a portosystemic collateral circulation with resultant compensatory portosystemic shunting and disturbed intrahepatic circulation. These factors are partly responsible for the important complications of chronic liver disease, including variceal bleeding, hepatic encephalopathy, ascites, hepatorenal syndrome, recurrent infection, and abnormalities in coagulation. Variceal bleeding is the most serious complication and is an important cause of death in patients with cirrhotic liver disease.