Fig. 4.

Basic cellular mechanism of generation of “new” HCO₃⁻ . A: Titratable acid formation. At various sites along the nephron, the tubule cells secrete H⁺ into the lumen. One result is the titration of weak bases (A⁻), filtered from the blood in the glomerulus, to form their conjugate weak acids (HA). In parallel, a new HCO₃⁻ appears in the interstitial space and then in the blood. B: Ammonium excretion. In the proximal tubule, cells generate ammonia (NH₃) de novo from the amino acids glutamine and glutamate. In the lumen, secreted H⁺ titrates this NH₃ to form ammonium (NH₄⁺). In parallel, a new HCO₃⁻ appears in the interstitial space and then in the blood. Because the maximum free [H⁺] of the urine is only ~4 × 10⁵ M (pH ~4.4), the urinary excretion of H⁺ in the form of HA and NH₄⁺–which are typically present at mM level–allows the kidney to eliminate far more acid than it could if all of the secreted protons were excreted in the form of free H⁺. Modified from Fig. 39-2B and C in Medical Physiology, 2nd Edition Updated Edition, edited by WF Boron and EL Boulpaep, Philadelphia: Elsevier, 2012.