The portal vein and the hepatic vein are two different venous systems. The main trunk of the portal vein is formed by the confluence of the superior mesenteric vein, the inferior mesenteric vein, and the splenic vein. The left and right trunks of the portal vein enter the left and right hepatic halves respectively, and then gradually branch out, with their branches and small branches of the hepatic artery eventually merging in the hepatic sinusoids within the hepatic lobules, which then merge into the inferior vena cava via the central vein, the inferior lobular vein, and the hepatic vein. Thus, the portal venous system is located between two capillary networks, one of which is the capillary network of the stomach, spleen, pancreas, and intestine, and the other is the hepatic blood sinusoids within the hepatic lobules. In addition, there are many small traffic branches between the arterioles in the interlobular confluent area of the liver, which allow the portal vein and the hepatic artery to communicate with each other. Due to the characteristics of the portal vein system, portal hypertension can occur when the hepatic vein or the inferior vena cava above its opening is obstructed. When the portal vein pressure rises, the portal vein system and the vena cava system also have obvious traffic branches, among which the varices of the lower esophageal traffic branch of the fundus are the most obvious and can cause bleeding in the upper esophagus, which is also a common serious complication of portal hypertension.