A patient has been in chronic metabolic acidosis for 5 days. Which renal mechanism is most responsible for the large increase in net acid excretion observed over this time period?
AIncreased bicarbonate filtration at the glomerulus, which provides more substrate for H⁺ buffering in the tubule
BUpregulated ammonium (NH₄⁺) production and excretion — proximal tubule cells metabolize glutamine to generate NH₃, which buffers secreted H⁺ and is excreted as NH₄⁺
CIncreased respiratory rate, which reduces PCO₂ and allows the kidney to excrete more CO₂
DIncreased distal tubule bicarbonate reabsorption, which frees up H⁺ for direct excretion
The ammonia pathway is the most flexible and upregulatable mechanism for net acid excretion during sustained acidosis. During chronic acidosis, proximal tubule cells dramatically increase glutamine uptake and metabolism, stripping amino groups as NH₃. NH₃ diffuses into the lumen and combines with secreted H⁺ to form NH₄⁺, which is trapped (charged, membrane-impermeant) and excreted. This mechanism can increase many-fold over days, providing the large, sustained acid excretion capacity needed to correct chronic acidosis.
Question 2 Multiple Choice
In the proximal tubule, H⁺ is actively secreted into the tubular lumen via Na⁺/H⁺ exchangers. What is the primary fate of this secreted H⁺?
AIt accumulates in the tubular fluid, progressively acidifying the urine along the proximal tubule
BIt combines with filtered bicarbonate to form H₂CO₃, which is converted to CO₂ and water — effectively reabsorbing bicarbonate rather than excreting acid
CIt directly neutralizes metabolic acids (like lactic acid) filtered at the glomerulus
DIt is secreted into the peritubular capillaries to buffer venous blood returning to the heart
This is the key distinction between bicarbonate reabsorption and net acid secretion. In the proximal tubule, the secreted H⁺ does not accumulate as free acid — it immediately combines with filtered HCO₃⁻ to form carbonic acid (H₂CO₃), which carbonic anhydrase converts to CO₂ and water. The CO₂ diffuses into the tubular cell, is rehydrated back to H₂CO₃, and the resulting HCO₃⁻ is transported to the blood. The net result: filtered bicarbonate is reclaimed, but no acid is actually excreted. This process does not acidify the urine.
Question 3 True / False
Proximal tubule H⁺ secretion directly contributes to net acid excretion by lowering urinary pH in the proximal nephron.
TTrue
FFalse
Answer: False
Proximal tubule H⁺ secretion does NOT acidify the urine or constitute net acid excretion. Every H⁺ secreted in the proximal tubule is consumed in the process of reclaiming filtered bicarbonate — the secreted H⁺ combines with HCO₃⁻, forming CO₂ that diffuses back into the cell. The urine remains near-neutral pH throughout the proximal tubule. Net acid secretion — actual elimination of H⁺ from the body — occurs in the distal tubule and collecting duct, where H⁺ is secreted against a steep gradient and buffered by titratable acids and ammonia rather than bicarbonate.
Question 4 True / False
The kidneys respond more rapidly to acid-base disturbances than the lungs, but the renal correction is less complete.
TTrue
FFalse
Answer: False
The temporal relationship is reversed. Lungs respond within seconds to minutes by adjusting ventilation rate (increasing or decreasing CO₂ elimination). Kidneys respond over hours to days, requiring time to upregulate transporters, enzymes (carbonic anhydrase), and ammonia production pathways. However, the renal correction is more complete and durable: lungs can only partially compensate (they cannot fully normalize pH in metabolic disorders without overshooting respiratory control), while the kidney can precisely adjust bicarbonate reabsorption and acid secretion to fully correct the disturbance over time.
Question 5 Short Answer
Distinguish between bicarbonate reabsorption and net acid excretion: where does each occur in the nephron, what is the mechanism, and why does only one of them actually correct an acid-base imbalance?
Think about your answer, then reveal below.
Model answer: Bicarbonate reabsorption occurs mainly in the proximal tubule: H⁺ is secreted into the lumen, combines with filtered HCO₃⁻, and the resulting CO₂ re-enters the cell to regenerate HCO₃⁻ for the blood. No acid is eliminated — the secreted H⁺ is consumed reclaiming bicarbonate, not added to the urine. Net acid excretion occurs in the distal tubule and collecting duct: alpha-intercalated cells pump H⁺ against a steep gradient, buffered by titratable acids (phosphate) and ammonia (NH₄⁺). This H⁺ leaves the body in the urine, and each excreted H⁺ is matched by a new HCO₃⁻ added to the blood. Only net acid excretion corrects an acid-base imbalance — bicarbonate reabsorption merely prevents bicarbonate loss, maintaining balance, but does not generate new bicarbonate to replace that consumed by buffering acids.
This distinction is the most commonly confused aspect of renal acid-base physiology. Both processes involve H⁺ secretion, but the fate of that H⁺ is completely different: in the proximal tubule it is recycled back into bicarbonate; in the distal nephron it exits the body as buffered acid. Only the distal process actually removes an acid load.