Figure 2.

Summary of renal ammonia metabolism. The proximal tubule produces ammonia, as NH₄⁺, from glutamine. NH₄⁺ is then secreted preferentially into the luminal fluid, primarily by NHE-3, and, in addition, there is a component of NH₃ secretion. Ammonia is then reabsorbed in the thick ascending limb, resulting in ammonia delivery to the distal nephron accounting for approximately 20% to 40% of final urinary ammonia. The remaining approximately 60% to 80% of urinary ammonia is secreted in the collecting duct through parallel NH₃ and H⁺ transport. Numbers in red indicate the proportion of total urinary ammonia present at the indicated sites under baseline conditions. Figure modified from reference (180) with permission.