Renal Acid-Base Regulation
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Direct answer
The kidney defends pH on two fronts: it reclaims about 4,300 mmol of filtered bicarbonate daily (180 L × 24 mmol/L, 85-90 per cent of it in the proximal tubule via NHE3 and carbonic anhydrase) and it generates new bicarbonate by excreting acid — as titratable acid (mainly phosphate buffer, fixed capacity near 30 mEq/day) and as ammonium (adaptable over days, two-thirds or more of the acid load). Hydrogen ion secretion by alpha-intercalated cells can acidify urine to a minimum pH of 4.4, roughly a 1000-fold concentration gradient against plasma. Net acid excretion equals titratable acid plus ammonium minus bicarbonate, and matches the 50-100 mEq (about 1 mEq/kg) of fixed acid produced daily by protein metabolism.
What you must remember
- Bicarbonate reclamation: 4,300 mmol/day filtered, virtually all reabsorbed — secreted H+ combines with filtered HCO3 in the lumen, and carbonic anhydrase on both sides keeps the shuttle running; acetazolamide breaks it, causing bicarbonaturia and metabolic acidosis.
- Generation of new base: every ammonium and every titratable acid molecule excreted leaves behind a new bicarbonate in the blood — reclamation alone adds nothing to the body's alkali.
- Ammoniagenesis: glutamine metabolism in proximal cells yields two NH4+ and two new bicarbonates; NH4+ is trapped in the acidic tubular lumen, recycled through the medulla, and excreted — capacity can rise five to ten-fold in chronic acidosis.
- Titratable acid: mostly phosphate (pKa 6.8) — about 30 mEq/day, fixed by the filtered phosphate load; creatinine and urate contribute little.
- Alpha-intercalated cells: H+-ATPase and H-K-ATPase secrete acid (potassium retention in acidosis); beta-intercalated cells reverse and secrete bicarbonate in alkalosis.
- Minimum urine pH 4.4: a hydrogen ion gradient of about 800-1000 to 1 — the limiting step, so beyond it the load must be buffered as NH4+ and phosphate.
- Net acid excretion formula: titratable acid + NH4+ − HCO3−; in acidosis it can exceed 300 mEq/day.
- Respiratory companion: Winter's formula — expected PaCO2 = 1.5 × HCO3 + 8 (± 2) — checks whether respiratory compensation for a metabolic acidosis is appropriate.
A worked case: diabetic ketoacidosis
A young woman arrives with ketoacidosis: pH 7.08, bicarbonate 6 mmol/L, Kussmaul breathing. Buffers have been consumed neutralising ketone acids, so nearly all her bicarbonate is spent; her fixed-acid load is hundreds of milliequivalents. The kidney responds on both fronts immediately and over days. At once, proximal glutaminase activity rises and alpha-intercalated H+ secretion pushes urine pH to its 4.4 floor, with phosphate buffer carrying what it can. Over the next several days, ammoniagenesis escalates five to ten-fold, and it is this ammonium arm — not the urine pH, which cannot go lower — that regenerates the bicarbonate she will keep. That delay explains the clinical observation that serum bicarbonate lags clinical improvement: new-base generation is an inducible enzymatic programme, not a tap. Throughout, Winter's formula audits her breathing: expected PaCO2 = 1.5 × 6 + 8 = 17 mmHg — if measured higher, a coexisting respiratory problem is hiding.
How the exam frames it
The examiner's sharpest question is conceptual: "Does reabsorbing filtered bicarbonate correct an acidosis?" It does not — it merely prevents loss; only excretion of ammonium and titratable acid, each paired with new bicarbonate generation, repairs the deficit. The second angle is urine pH as a diagnostic: in distal renal tubular acidosis, urine pH stays above 5.5 despite systemic acidosis because the alpha-intercalated H+-ATPase is defective — the kidney cannot reach its own floor. Third, the acetazolamide chain is asked end to end: carbonic anhydrase inhibition, proximal bicarbonate wasting, hyperchloraemic metabolic acidosis with alkaline urine, and the side-effect of calcium phosphate stones from the alkaline, citrate-poor urine.
Frequently asked questions
How much filtered bicarbonate is reabsorbed daily, and where?
About 4,300 mmol (180 L × 24 mmol/L), with 85-90 per cent reclaimed in the proximal tubule via sodium-hydrogen exchange and carbonic anhydrase.
Why is ammonium excretion more important than titratable acid in chronic acidosis?
Phosphate buffers are fixed by the filtered load (about 30 mEq/day), whereas ammoniagenesis from glutamine can be upregulated five to ten-fold over days, making NH4+ the adaptable arm of acid excretion.
What is the minimum urine pH and what gradient does it represent?
About 4.4 — a hydrogen ion concentration roughly 800-1000 times plasma, the secretory limit of the alpha-intercalated cell.
Why does acetazolamide cause metabolic acidosis?
It inhibits proximal carbonic anhydrase, so filtered bicarbonate escapes reabsorption and is lost in an alkaline urine, depleting the body's alkali stores.
How is net acid excretion calculated?
Net acid excretion = titratable acid + ammonium − bicarbonate excreted; normally 50-100 mEq/day, matching daily fixed-acid production from protein metabolism.