The hepatic and portal veins are still relatively distinct. The portal vein enters the liver mainly through the first hepatic portal and receives blood from the superior mesenteric vein, inferior mesenteric vein and splenic vein, and is the main component of the blood flow into the liver, carrying oxygen and a large amount of nutrients. After the portal vein enters the liver, it gradually branches into the hepatic sinusoids with the Glisson. And the blood in the hepatic blood sinusoids eventually passes through the central vein of the hepatic lobules, the inferior lobular vein, and finally converges into the hepatic vein via the middle hepatic vein, the right hepatic vein, and the left hepatic vein, injects into the posterior inferior hepatic vena cava, and exits the liver via the second hepatic hilar, which is the main way for blood to exit the liver. However, although the portal venous system and the hepatic venous system are two relatively independent systems, if there is an obstructive lesion in the hepatic veins, such as a thrombosis or stenosis, it can lead to the formation of portal hypertension. This not only leads to widening of the diameter of the portal and splenic veins, but also causes a series of clinical symptoms, such as esophagogastric fundic varices and hypersplenism.