A portocaval anastomosis is a specific type of anastomosis that occurs between the veins of the portal circulation and those of the systemic circulation.
The inferior end of the esophagus and the superior part of the rectum are potential sites of a harmful portacaval anastomosis.In portal hypertension, as in the case of cirrhosis of the liver, the anastomoses become congested and form venous dilatations. [Source: Wikipedia 
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