Figure 3.

Processes involved in N balance in the gastrointestinal tract.

Notes: There are three distinct processes producing NH₃. Urea diffuses from the blood to the intestinal lumen, where it is hydrolyzed to NH₃ and CO₂ by bacterial urease. As indicated by the dashed arrow, mucosal permeability is very low in the large intestine, limiting the rate at which urea can enter the lumen. The second process is the metabolism of glutamine by the intestinal mucosa. This produces varying amounts of NH₃ and alanine. The third process is bacterial deamination of the malabsorbed protein in the large intestine.