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A D V E R T I S E M E N T
A D V E R T I S E M E N T
The portal vein is formed by the convergence of branches from foregut, midgut and hindgut drainage basins. The foregut drainage consists of collecting channels along the lesser and greater curves of the stomach and from the spleen and pancreas. The right gastric vein lies along the lesser curve and drains into the beginning of the portal vein. At the esophagus, it communicates with the left gastric vein which runs diagonally downward beneath the posterior peritoneum of the lesser sac to drain into the portal vein near the right gastric vein orifice. These veins together form a crown-like circuit and the left gastric vein is also called the coronary vein for that reason. The splenic vein eceives drainage from the fundus and cardia of the stomach via the gastrosplenic ligament and the left end of the gastroepiploic arcade also feeds into the distal splenic. The gastroepiploic arcade drains both the omentum and the greater curve of the stomach. The right end of the gastroepiploic drains into the superior mesenteric vein, the final common pathway from the midgut drainage.
The midgut from the duodenum to the distal transverse colon drains into the superior mesenteric system of veins. The superior mesenteric joins the splenic vein to form the portal vein. The junction lies anterior and to the right of the superior mesenteric artery origin and beneath the neck of the pancreas.
A D V E R T I S E M E N T
The hindgut from the distal transverse colon to the mid-rectum drains into the inferior mesenteric vein. The inferior mesenteric vein usually lies to the left of the fourth portion of the duodenum and joins the splenic vein beneath the body of the pancreas but may form a trifurcation or occasionally drain into the superior mesenteric vein.
The portal vein rises from its origin under the neck of the pancreas and enters the hepatoduodenal ligament where it is the most posterior of the three portal structures. It lies diagonally across the infrahepatic portion of the inferior vena cava, separated by the cleft of the epiploic foramen (of Winslow). In the liver hilum it divides into right and left branches and distributes its blood flow to the hepatic segments in a pattern which parallels the bile ducts and hepatic arteries.
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A D V E R T I S E M E N T
The portal blood flow accounts for 60% of the hepatic blood supply. The defining feature of a portal system of veins is that it passes through an intermediate vascular bed, in this case the liver, before the blood passes into systemic veins (hepatic veins). When the normal architecture of the liver is disrupted by disease (scarring from infection or toxins), the increased resistance to portal blood flow increases the pressure in the portal system of veins. The increased pressure seeks out connections between the portal system and the lower pressure systemic veins. Such connections occur in the liver itself between the left branch of the portal vein and the umbilical vein, which may recanalize, from the gastric drainage to the azygous and hemiazygous veins via lower esophageal veins, and from the superior rectal veins to the middle and inferior rectal veins. The varicose veins in the submucosa of the lower esophagus can and often do bleed massively when eroded and the hemorrhoidal veins at the other end of the GI tract also bleed, but are usually not as life threatening.
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This page was last modified on 3/1/1999.
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