Ammonia is a by-product of protein degradation which is transported in the portal vein and metabolised to urea in the liver via the urea cycle. Some ammonia is also used in the synthesis of glutamine. Ammonia is generated in healthy animals through the following physiological processes:
- Hepatic catabolism of body proteins and of proteins absorbed from the gut, including those obtained after gastro-intestinal haemorrhage.
- Metabolism of glutamine by enterocytes of the small intestine; enterocytes obtain 25% of their energy by this process.
- Bacterial and intestinal degradation of urea by urease in the colon.
- Bacterial degradation of undigested amino acids and purines in the colon.
Even in the most severe forms of liver failure, the urea cycle usually continues to remove ammonia sufficiently from the portal blood to prevent clinical symptoms.
In animals where the portal blood is diverted directly into the systemic circulation, however, blood ammonia concentrations may rise because it is no longer being removed by the liver. This abnormal pattern of blood flow occurs with porto-systemic shunts and microvascular dysplasia. Affected animals also have low blood urea concentrations because ammonia is not being converted to urea.
Blood ammonia concentration may also be elevated in animals with:
- Congenital abnormalities of enzymes of the urea cycle (such as argininosuccinate synthetase).
- Deficiencies of the substrates of the urea cycle, such as with experimental arginine deficiency in cats.
High blood ammonia concentrations are the most common cause of hepatic encephalopathy (HE).