The portal vein is formed by the confluence of the splenic vein with the superior mesenteric vein and its formation mostly occurs behind the pancreas in the retroperitoneum. It transports the blood mainly from the gastro-intestinal tract and the spleen to the liver. Seventy percent of the total blood supply to the liver is contributed by the portal vein while the hepatic artery contributes to the remaining thirty percent. The portal venous system is the only venous system in our body, which begins with capillaries and ends with capillaries. The intrahepatic branches of the portal vein terminate in small vessels that supply the hepatic sinusoids. Embryologically, the systemic veins of our body develop from the intra-embryonic anterior and posterior cardinal veins while the portal system develops from the extra-embryonic vitelline and umbilical veins, which drain from the yolk sac and the placenta.

Portal hypertension could be defined as an increase in the intravascular pressure within the portal vein of over 11 mm of mercury as measured directly or a splenic pulp pressure of over 16 mm of mercury. A rise in the portal pressure leads to splenomegaly and the development of natural porto-systemic shunts at the following sites:

• Lower end of the oesophagus and cardia through the gastro-oesophageal veins
  
  
• The anal canal via the haemorrhoidal veins
  
  
• In the falciform ligament via the umbilical veins
  
  
• In the abdominal wall and retroperitoneum
  
  

The diagnosis of portal hypertension should be suspected in a child after the occurrence of any large gastro-intestinal bleed. In this age group, oesophageal varices are the most likely cause for such an event. Variceal bleeding is associated with a mortality rate of 5 - 9 percent in children with portal vein obstruction but there is a higher risk of death of those with cirrhosis.