Porto-systemic anastomosis also known as portocaval anastomosis is the collateral communication between the portal and the systemic venous system.
Portal venous system
The portal venous system transmits deoxygenated blood from most of the gastrointestinal tract and gastrointestinal organs to the liver. Substance absorbed in the GIT are processed in the liver, hepatocytes receives oxygenated blood via hepatic artery. This mixture filters through the sinusoids and collects in a central vein and finally to the heart through the inferior vena cava by hepatic veins.
The portal vein is the most important vein in the portal venous system; it starts its formation close to the level of the second lumbar vertebrae (L2) and it is located in front (anterior) of the inferior vena cava and at the back (posterior) of the neck of the pancreas. It is about 8cm long. The portal vein is formed by joining of the superior mesenteric vein and the splenic vein. It runs upwards and lies behind the bile duct and hepatic artery and it also lies anterior to the inferior vena cava. It penetrates in the right border of the lesser omentum and continues upwards in front (anterior) of the epiploic foramen to reach the porta hepatis (transverse fissure on the liver). After it reaches the porta hepatis, it bifurcates into a right and left branch which penetrates the liver.
Various veins drain into the portal vein. These veins are:
- superior mesenteric vein: drains blood mainly from small intestine
- splenic vein: receives blood from short gastric, left gastroepiploic, inferior mesenteric, and pancreatic veins
- right and left gastric veins : drain blood from the stomach and oesophagus
- superior pancreaticoduodenal veins: drain blood from the pancreas and duodenum
- cystic veins: drain blood from the gallbladder and the paraumbilical vein
From the portal vein the blood is drained into the left and right branches of the portal vein into the left and right side of the liver. Inside the liver it passes through tiny capillary beds called venous sinusoids of the liver and finally into the hepatic vein which transmits the blood into the inferior vena cava (carries deoxygenated blood to the heart).
The importance of portosystemic anastomoses is to provide alternative routes of circulation when there is a blockage in the liver or portal vein. These routes ensure that venous blood from the gastrointestinal tract still reaches the heart through the inferior vena cava without going through the liver.
The various anastomoses and the sites in which they occur are described below:
- The anastomosis between the left gastric veins, which are portal veins, and the lower branches of oesophageal veins that drain into the azygos and hemiazygos veins, which are systemic veins. The site of this anastomosis is the lower oesophagus.
- The anastomosis between the superior rectal veins, which are portal veins, and the inferior and middle rectal veins, which are systemic veins. The site of this anastomosis is the upper part of the anal canal.
- The anastomosis between the paraumbilical veins, which run in the ligamentum teres as portal veins, and small epigastric veins, which are systemic veins. The site of this anastomosis is the umbilicus.
- The anastomosis between the intraparenchymal branches of the right division of the portal vein and retroperitoneal veins (systemic veins) that drain into the azygos, hemiazygos and lumbar veins (systemic veins). The site of this anastomosis is the bare area of the liver.
- The anastomosis between omental and colonic veins (portal veins) with the retroperitoneal veins (systemic veins) in the region of hepatic and splenic flexure.
Another anastomosis is between the ductus venosus (portal vein) and the inferior vena cava (systemic vein). This is very rare and at the site of patent ductus venosus.
Portal hypertension: This is increase in blood pressure in the veins of the portal system. It is caused by blockage in the veins of the liver due to pathological conditions such as liver cirrhosis and the inability of the blood to flow through. Signs and symptoms are varicose veins on the abdominal wall called caput medusae, oesophageal varices, enlargement of the spleen, accumulation of fluid in the peritoneal cavity and bleeding in the gastrointestinal tract.
Portosystemic shunts: This is an abnormal connection between the veins of the portal and systemic system. In portosystemic shunts, blood is shunted directly to the systemic circulation from the portal vein without reaching the liver. Porto systemic shunts occur naturally in the developing fetus because blood from the placenta flows through the ductus venosus into the system without going through the liver; the ductus venosus is meant to close on the first week after birth but persistence leads to a pathological condition called congenital portosystemic shunts. There is an extrahepatic congenital portosystemic shunt as well which is the developmental abnormality of the vitelline vein connecting the portal vein to the caudal vena cava. Signs and symptoms are tremors, epileptic seizures, weight loss, bladder stones and vomiting. Portosystemic shunts are also performed in the clinical setting to reduce the effects of portal hypertension and this can be done surgically by creating a link between the portal vein and the inferior vena cava or by creating a link between the splenic vein and left renal vein.