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The normal pressure in the venous portal system is 7-14 mm Hg. In portal hypertension, levels may rise to 20-50 mm Hg. In the UK, the most common cause is liver cirrhosis; worldwide, schistosomiasis is more likely.
Raised pressure stimulates expansion of collaterals between the portal and the systemic venous systems. Varices at the gastro-oesophageal junction are most significant; they are superficial and drain into the azygos veins. Under increased pressure, they may rupture causing haematemesis or melaena. Rectal varices are found in 30% of cases. They may be differentiated from haemorrhoids which are lower in the anal canal.