Portal venous system

appreciated especially well on sagittal and coronal imaging. Poor differentiation between the normal spleen and tumour may be improved by the use of reticuloendothelial contrast agents.

Scintigraphy Activity may be seen in splenunculi in addition to the normal spleen.

Portography (See also the section on portal system.) This may be performed by splenic puncture via the ninth or tenth interspace posteriorly, remembering that the costodiaphragmatic recess of the pleura always extends to the level of the lower pole of the spleen.

THE PORTAL VENOUS SYSTEM (Figs 5.44 and 5.45) Blood from the gastrointestinal tract (not including the anus), the spleen, pancreas and gallbladder drains to the liver via the portal venous system. This consists of the superior and inferior mesenteric and the splenic veins, which unite to form the portal vein. There are valves in this system in the fetus and young infant but not in the adult.

Superior mesenteric vein

This lies to the right of the superior mesenteric artery and drains the area supplied by this artery. Its branches are similar except proximally and are as follows: • Right gastroepiploic and inferior pancreaticoduodenal proximally; • Jejunal and ileal branches to its left side; • Ileocolic, right colic and midcolic branches to its right side.

Inferior mesenteric vein

This lies to the left of the inferior mesenteric artery and drains its area of supply in the colon via the: • Left colic, sigmoid and superior rectal veins; and • Some pancreatic branches.

The splenic vein

Four or five branches leave the splenic hilum and unite to form the splenic vein. Occasionally, a vein from the upper pole of the spleen joins the splenic vein more proximally (the superior polar vein). Proximally the splenic vein receives the short gastric veins and the left gastroepiploic

Fig. 5.44 Portal venous system.

vein from the stomach. It receives multiple pancreatic branches and the inferior mesenteric vein as it passes posterior to the tail and body of the pancreas. The splenic vein then unites w i th the superior mesenteric vein to form the portal vein behind the neck of the pancreas.

The portal vein

The portal vein is formed by the union of the superior mesenteric and splenic veins behind the neck of the pancreas at the level of the L1/L2 lumbar vertebra. It then runs to the right behind the neck of the pancreas and receives the superior pancreaticoduodenal vein. It passes behind the first part of the duodenum and receives the right gastric and left gastric (coronary) veins. It passes in the free edge of the lesser omentum posterior to the common bile duct and the hepatic artery. Here it is separated from the IVC by the epiploic foramen. Its proximity to the IVC at this point is utilized at surgery for portosystemic venous shunting in portal hypertension.

At the porta hepatis the portal vein divides into right and left branches. The right branch receives the cystic vein if one is formed, and the left receives the paraumbilical veins. Attached to the left portal vein is the ligamentum teres and its continuation, the ligamentum venosum. Within the liver the portal vein is distributed w i th the branches of the hepatic artery.

Portosystemic anastomoses (Fig. 5.45) Portal venous pressure is raised when there is obstruction to blood flow in the portal vein, the liver or the hepatic veins. Portosystemic collateral pathways then open where these two systems communicate (see Table 5.4).

Fig. 5.45 Porto-systemic communications.