Low pressure system carrying 75 per cent of hepatic blood flow
Expansion
- Carries about 1100 ml/min, three-quarters of hepatic blood flow, at a pressure of only 5 to 10 mmHg
- Drains the gut, spleen and pancreas, so it delivers absorbed nutrients, gut hormones and bacteria to the liver first
- Has no valves, so raised pressure is transmitted backwards freely
The hepatic artery buffer response maintains total hepatic flow: when portal flow falls, adenosine accumulates and dilates the hepatic artery, compensating. The reverse does not occur, so the artery cannot be compensated by the portal vein.
Portal hypertension is defined as a hepatic venous pressure gradient above 5 mmHg, with clinically significant disease above 10 and varices bleeding above 12.
Consequences follow the anatomy: varices at portosystemic anastomoses, splenomegaly, ascites (raised sinusoidal pressure plus hypoalbuminaemia and splanchnic vasodilatation), encephalopathy from shunted ammonia, and reduced first pass metabolism of drugs.
Non-selective beta blockers reduce portal pressure by lowering cardiac output and allowing unopposed alpha-mediated splanchnic vasoconstriction.