Figure 3. Summary of ammonia metabolism in the kidney.

The proximal tubule produces ammonia, as NH₄⁺, from glutamine. NH₄⁺ is secreted preferentially into the luminal fluid, primarily by NHE-3, and is reabsorbed in the TAL, primarily by NKCC2. Ammonia delivery to the distal nephron accounts for approximately 20% of urinary ammonia; approximately 80% is secreted in the collecting duct through parallel NH₃ and H⁺ transport. Numbers indicate proportion of total urinary ammonia present at the indicated sites under basal states.