To better understand the portal–caval anastomoses, recall that veins in the abdomen return blood to the heart via two routes (Figure 10-4B):
Portal–caval anastomoses occur at regions of the gastrointestinal tract that are drained by both the portal and systemic (-caval) systems. The principal portal–caval anastomoses are as follows:

When hepatocytes are damaged (e.g., due to disease,
alcohol, or drugs), the liver cells are replaced by fibrous tissue, which impedes the flow of blood through the liver (cirrhosis). When the hepatic portal system is blocked, the return of blood from the intestines and spleen through the liver is impeded, resulting in
portal hypertension. Therefore, veins that usually flow into the liver are blocked. Consequently, blood pressure in the blocked veins increases, causing them to dilate and gradually reopen previously closed connections with the caval system. Veins in the distal portion of the esophagus begin to enlarge (
esophageal varices); veins in the rectum begin to enlarge (
internal hemorrhoids); and in chronic cases, the veins of the paraumbilical region enlarge (
caput medusa).
