Portal Venous System
The hepatic portal venous system drains blood from most of the alimentary tract (including the abdominal oesophagus, stomach, small and large intestines), pancreas and spleen. Blood from these organs is drained by the superior mesenteric and splenic veins which unite to form the portal vein (Fig. 4.77). This large vein approaches the porta hepatis with the proper hepatic artery, and branches of both vessels enter the liver and further subdivide. The liver is a common site of secondary infection or cancer because blood is conveyed to it from so many other organs. After traversing the liver, blood from both sources is drained by the hepatic veins, which enter the inferior vena cava (Fig. 4.92).
The superior mesenteric vein (Fig. 4.78) receives blood from most of the small intestine and the proximal half of the large intestine. It drains the duodenum distal to the major papilla, the jejunum and ileum, the caecum and appendix, and the ascending and most of the transverse colon. The vein accompanies the superior mesenteric artery within the root of the mesentery and its tributaries generally correspond to the branches of the artery.
The splenic vein (Fig. 4.78) drains the spleen and parts of the pancreas and stomach. It arises at the hilum of the spleen and runs to the right in the splenorenal ligament behind the tail of the pancreas. The vein continues behind the body of the pancreas and usually receives the inferior mesenteric vein and other tributaries corresponding to the branches of the splenic artery.
The inferior mesenteric vein drains the upper part of the anal canal, the rectum, the sigmoid and descending parts of the colon and the distal part of the transverse colon. The vein initially accompanies the corresponding artery but in the latter part of its course, it ascends the posterior abdominal wall independently, passing to the left of the duodenojejunal junction. The vessel usually terminates by joining the splenic vein behind the body of the pancreas (Fig. 4.77), but it may enter the superior mesenteric vein (Fig. 4.78).
The portal vein is formed behind the neck of the pancreas by the union of the superior mesenteric and splenic veins (Fig. 4.77). It runs upwards and to the right behind the gastroduodenal artery and the first part of the duodenum. The vein then enters the free border of the lesser omentum accompanied anteriorly by the bile duct and the proper hepatic artery (Fig. 4.44). Near the porta hepatis, the vein divides into left and right branches, which enter the liver. Several small veins enter the portal vein directly, including the left and right gastric veins and the cystic vein.
There are numerous anastomoses between the tributaries of the portal venous system and those of the systemic venous system. In health, these portacaval anastomoses are microscopic; but if there is obstruction of blood flow through the portal vein or liver, they may dilate and give rise to characteristic clinical features. For example, in the wall of the lowest part of the oesophagus, veins draining into the left gastric vein communicate with tributaries of the azygos venous system. Portal vein obstruction can cause gross swelling of these oesophageal veins (oesophageal varices), which may bleed profusely.
Similarly, paraumbilical veins accompanying the round ligament of the liver communicate with the portal vein at the porta hepatis and with veins of the anterior abdominal wall around the umbilicus. Portal vein obstruction may result in dilatation of the subcutaneous veins of the abdominal wall. Other sites of portacaval anastomoses include the anal canal and retroperitoneal parts of the intestines.