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).
Tributaries
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).
Portal vein
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.
Portacaval anastomoses
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.
