What Is Portal Hypertension?
Increased portal vein pressure caused by intrinsic liver disease or obstruction in the extrahepatic portal vein or hepatic venous outflow tract.
The portal venous system carries all blood from the abdominal GI tract, spleen, pancreas, and gallbladder back to the heart through the liver. The portal vein is formed by the union of the superior mesenteric and splenic veins. At the porta hepatis it divides into the right and left branches, which are segmentally distributed intrahepatically; the terminal portal venules drain into the sinusoids. In the resting state, the portal vein carries about 1 to 1.2 L/min of blood (about 75% of total hepatic blood flow) and provides 2/3 of the liver’s O2 supply. The portal vein is valveless; thus, pressure in the portal system depends on the product of input from blood flow in the portal vein and total hepatic resistance to outflow. Normal pressure in the portal vein is between 1 and 4 mm Hg higher than the free pressure in the hepatic vein, or up to 6 mm Hg higher than right atrial pressure. Portal hypertension is defined as pressures above these limits.
Source: The Merck Manual