Portal venous-systemic venous shunting of blood impairs delivery of portal substances to the liver. Shunting of this sort can occur as a result of congenital vascular anomalies, or secondary to chronic inflammatory disease with associated fibrosis. The decreased availability of normal hepatotrophic factors in portal blood leads to hepatic atrophy. Certain substances such as ammonia and bile acids are normally absorbed by the intestine and removed efficiently by hepatocytes after delivery from portal blood.
Blood levels increase if portal blood bypasses the liver via shunts.
The liver is essential for production of many substances in the body, including urea, coagulation factors, cholesterol and albumin. Each of these, however, can also be greatly affected by abnormalities outside the liver, rendering them unreliable as primary indicators of hepatic function. Therefore, hepatic function is assessed primarily by determination of ammonia and bile acids, especially the latter as it the most accessable test for the veterinary practitioner.