Renal Regulation of Acid-Base Balance

Core Idea

The kidney compensates for respiratory acid-base disorders and corrects metabolic disorders through three mechanisms: reabsorption of filtered bicarbonate, secretion of hydrogen ions, and excretion of ammonia (which buffers acid). Proximal tubule reclaims filtered bicarbonate through H⁺ secretion. Distal tubule and collecting duct secrete hydrogen ions to establish larger gradients. Chronic acid-base disorders are primarily corrected by renal mechanisms.

Explainer

From your study of acid-base homeostasis, you know that blood pH is maintained near 7.4 by the carbonic acid–bicarbonate buffer system, with the lungs controlling CO₂ and the kidneys controlling bicarbonate (HCO₃⁻). The lungs respond to pH changes within seconds to minutes by adjusting ventilation; the kidneys respond over hours to days but produce lasting corrections. Understanding how the kidney does this requires tracking protons (H⁺) through the nephron.

The first and quantitatively largest mechanism is bicarbonate reabsorption in the proximal tubule. About 180 liters of plasma are filtered daily, containing roughly 4,500 mEq of bicarbonate that must be reclaimed or it would be lost in urine. Tubular cells secrete H⁺ into the lumen (via Na⁺/H⁺ exchangers), where it combines with filtered HCO₃⁻ to form H₂CO₃, which carbonic anhydrase rapidly converts to CO₂ and water. The CO₂ diffuses into the tubular cell, where it is re-converted to HCO₃⁻ and transported back to blood. This process does not acidify the urine — the secreted H⁺ is consumed reclaiming bicarbonate rather than accumulating as free acid.

Net acid secretion — the actual elimination of acid from the body — occurs in the distal tubule and collecting duct. Here, specialized alpha-intercalated cells pump H⁺ against a steep gradient (urine can reach pH 4.5, meaning H⁺ concentration 1,000× higher than blood). This H⁺ is buffered in the tubular lumen primarily by titratable acids (especially phosphate) and by ammonium (NH₄⁺). The ammonium pathway is particularly important during chronic acidosis: glutamine released from muscle is taken up by proximal tubule cells, which strip off amino groups as ammonia (NH₃). NH₃ diffuses into the lumen and accepts a proton to become NH₄⁺, which is trapped in the urine and excreted. This mechanism can be upregulated many-fold during sustained acidosis, providing a large and flexible acid-excretion capacity.

The renal response to respiratory disorders illustrates the compensation principle you learned earlier. In respiratory acidosis (high PCO₂, low pH), the kidneys compensate by increasing H⁺ secretion and HCO₃⁻ retention — raising plasma bicarbonate to restore the ratio [HCO₃⁻]/[CO₂] and partially normalize pH. In respiratory alkalosis (low PCO₂, high pH), the kidneys reduce HCO₃⁻ reabsorption, letting more bicarbonate escape in urine. These compensations develop over 2–5 days. In metabolic disorders, the kidney is not compensating for something else — it is the primary site of pathology or correction. Metabolic acidosis (low bicarbonate) prompts maximum acid excretion and bicarbonate regeneration; metabolic alkalosis (high bicarbonate) prompts bicarbonate excretion. The kidney's power over long-term acid-base balance is unmatched: respiratory compensation is faster, but renal correction is more complete and more durable.