Figure 1.

(A) HCO₃⁻ is freely filterable (~4.5 moles/day); (B) Proximal tubule cell bicarbonate transport processes. In patients, CAII and NBCe1 mutations can inhibit transepithelial HCO₃⁻ absorption causing Type II (proximal) RTA. (C) Dietary net metabolic production of H⁺ depletes whole body HCO₃⁻ generating CO₂ which is excreted by the lungs; (D) Schematic depiction of the equality of dietary net metabolic production of H⁺ and new HCO₃⁻ produced by the kidney that had not been consumed in the urea cycle. Approximately 60% of total new HCO₃⁻ generation is utilized to match metabolic production of H⁺ production and ~40% is consumed in the urea cycle with NH₄⁺ generated by the kidney.