As fibrosis increases, a point is reached at which blood flow in hindered by distortion of the usual flow of blood and the portal pressure must increase to preserve flow. Inevitably, both occur. Acute portal hypertension may cause mild to moderate enlargement of the spleen, but marked enlargement requires sustained portal hypertension over a long period of time. Sustained elevations in pressure occur when cirrhosis is present. The blood volume is expanded in patients with chronic liver disease.
Visceral congestion causes a larger percent of blood to be pooled in the portal system.
This occurs slowly as liver disease develops, allowing the body to compensate by formation of more blood. When a stream is dammed up, water accumulates until the dam overflows or streams develop around the dam. The rate at which this occurs depends on the speed with which the water builds up. The portal pressure builds up in response to fibrosis. As the portal pressure increases, more blood flows into the liver, but some tries to find another route to the lower-pressure vena cava system.