Figure 2.

Model of the urea nitrogen salvaging process. Urea is produced in the liver via the ornithine-urea cycle and passed into the blood. Urea can then pass (a) into the kidney, where it is freely filtered and either reabsorbed or excreted, or (b) via UT-B urea transporters into specific regions of the gastrointestinal tract that contain large bacterial populations (e.g. rumen, caecum, colon). Within these regions urea is broken down by the bacterial enzyme urease into ammonia and carbon dioxide. The ammonia can either be reabsorbed directly back into the blood or be utilized as a nitrogen source by the bacteria to produce amino acids and peptides, which themselves can then be reabsorbed. The return of the nitrogen to the mammalian host in these different forms represents the ‘salvaging’ of the original urea nitrogen.