Portal–Systemic
Anastomoses
Under normal conditions, the portal venous blood traverses the
liver and drains into the inferior vena cava of the systemic venous circulation
by way of the hepatic veins. This is the direct route. However, other, smaller communications
exist between the portal and systemic systems, and they become important when
the direct route becomes blocked
These communications are as follows
:
:
■■ At
the lower third of the esophagus, the esophageal branches of the left gastric
vein (portal tributary) anastomose with the esophageal veins draining the
middle third of the esophagus into the azygos veins (systemic tributary).
■■
Halfway down the anal canal, the superior rectal veins (portal tributary)
draining the upper half of the anal canal anastomose with the middle and
inferior rectal veins (systemic tributaries), which are tributaries of the
internal iliac and internal pudendal veins, respectively.
■■ The
paraumbilical veins connect the left branch of the portal vein with the superficial
veins of the anterior abdominal wall (systemic tributaries). The paraumbilical
veins travel in the falciform ligament and accompany the ligamentum teres.
■■ The
veins of the ascending colon, descending colon, duodenum, pancreas, and liver
(portal tributary) anastomose with the renal, lumbar, and phrenic veins
(systemic tributaries).
Portal
Hypertension
Portal hypertension is a common clinical condition; thus,
the list of portal–systemic anastomoses should be remembered. Enlargement of
the portal–systemic connections is frequently accompanied by congestive
enlargement of the spleen. Portacaval shunts for the treatment of portal
hypertension may involve the anastomosis of the portal vein, because it lies
within the lesser omentum, to the anterior wall of the inferior vena cava
behind the entrance into the lesser sac. The splenic vein may be anastomosed to
the left renal vein after removing the spleen.
Blood
Flow in the Portal Vein and Malignant Disease
The portal vein conveys about 70% of the blood to the liver.
The remaining 30% is oxygenated blood, which passes to the liver via the
hepatic artery. The wide angle of union of the splenic vein with the superior
mesenteric vein to form the portal vein leads to streaming of the blood flow in
the portal vein. The right lobe of the liver receives blood mainly from the
intestine, whereas the left lobe plus the quadrate and caudate lobes receive
blood from the stomach and the spleen. This distribution of blood may explain
the distribution of secondary malignant deposits in the liver.
