What is portal hypertension?

To understand portal hypertension, one must first have a working knowledge of the normal blood flow into and out of the liver (Figure 5). Blood leaves the intestines and flows upward through the mesenteric veins to the portal vein. The blood in these vessels carries all

the nutrients and byproducts from digestion of food to the liver. Blood from the spleen also flows toward the portal vein, where it joins the blood from the mesenteric veins. The portal vein carries the blood into the liver and then splits into right and left branches. These branches then divide over and over again into small capillary vessels. A good analogy for this system is a tree: The tree trunk is the portal vein, and the large branches are the right and left portal veins. The twigs are the capillaries. A network of vessels also carries blood out of the liver to the heart. These vessels are called the hepatic veins.

To visualize how this system comes together, imagine two trees: one growing out of the ground (the portal vein system) and one hanging upside down from the sky (the hepatic vein system). The trees are connected together by their twigs. Blood flows through the liver by moving up the portal vein, traveling to the right or left branch, continuing on to the portal capillaries (twigs) and then into the hepatic vein capillaries (also twigs), up into the larger hepatic vein branches and ultimately into the inferior vena cava (the trunk of the upside-down tree). From there blood is carried to the heart.

When the liver is damaged due to cirrhosis, the blood finds it difficult to pass from the portal vein system into the hepatic vein system because the capillaries (twigs) are narrowed, twisted, and scarred. Nonetheless, blood continues to flow from the intestinal vessels and spleen toward the portal vein. This results in a “backup” of blood in the portal vein, causing high pressure there. High pressure in the portal vein is also known as portal hypertension.