Renal Acid-Base Handling

The kidney is the slow-but-permanent arm of acid-base defence: it reclaims filtered bicarbonate and regenerates the bicarbonate consumed in buffering the daily fixed-acid load. Mastering proximal HCO₃⁻ reabsorption, titratable acid, ammoniagenesis, and the renal tubular acidoses unlocks a dense cluster of NEET PG marks.

Why the kidney matters in acid-base balance

The lungs handle the volatile acid (CO₂, ~15,000 mmol/day) by ventilation. The kidney handles the non-volatile / fixed acid load (~1 mEq/kg/day, ~70 mEq in an adult) generated mainly from sulphur-containing amino acid metabolism (sulphuric acid) and phosphoproteins (phosphoric acid).

Two distinct renal tasks must be separated, because students constantly confuse them:

1. Reclamation of the ~4,500 mEq of HCO₃⁻ filtered daily — no new bicarbonate is made; filtered base is merely saved. This happens chiefly in the proximal convoluted tubule (PCT).
2. Regeneration of new bicarbonate to replace the ~70 mEq consumed in titrating fixed acid. This requires the kidney to excrete H⁺ bound to urinary buffers, occurring largely in the distal nephron / collecting duct.

High-yield: Bicarbonate reabsorption does NOT excrete net acid — it only prevents base loss. Net acid excretion (NAE) is achieved by titratable acid + ammonium, and equals the rate of new HCO₃⁻ generation.

Proximal tubule bicarbonate reabsorption

About 80–85% of filtered HCO₃⁻ is reabsorbed in the PCT (the rest: ~15% thick ascending limb, ~5% distal/collecting duct).

Stepwise mechanism (PCT):

1. H⁺ is secreted into the lumen mainly by the apical Na⁺/H⁺ exchanger (NHE3), with a smaller contribution from apical H⁺-ATPase.
2. Luminal H⁺ + filtered HCO₃⁻ → H₂CO₃, which is split into CO₂ + H₂O by brush-border carbonic anhydrase (CA type IV).
3. CO₂ diffuses into the cell; intracellular carbonic anhydrase (CA type II) regenerates H₂CO₃ → H⁺ + HCO₃⁻.
4. HCO₃⁻ exits the basolateral membrane via the Na⁺-HCO₃⁻ cotransporter (NBC1, electrogenic, 1Na:3HCO₃) back into blood.
5. The H⁺ is recycled apically. Net result: filtered HCO₃⁻ "disappears" from lumen and an equivalent HCO₃⁻ appears in blood.

Flow: Filtered HCO₃⁻ → combines with secreted H⁺ → CA IV makes CO₂ → CO₂ enters cell → CA II remakes HCO₃⁻ → NBC1 returns it to blood.

High-yield: Both luminal (CA IV) and cytoplasmic (CA II) carbonic anhydrase are essential. Acetazolamide inhibits CA, blocking PCT HCO₃⁻ reabsorption → bicarbonaturia → a drug-induced proximal (Type II) RTA picture and metabolic acidosis.

Factors regulating proximal reabsorption

Factor	Effect on HCO₃⁻ reabsorption	Mechanism
↑ Arterial PCO₂ (resp. acidosis)	Increased	More intracellular CO₂ → more H⁺ secretion
↑ Angiotensin II	Increased	Stimulates NHE3
↑ Filtered HCO₃⁻ load	Increased (to a Tm)	Substrate-driven
Volume expansion (ECF↑)	Decreased	Reduces NHE3 activity
Hypokalaemia	Increased	Cell acidification stimulates H⁺ secretion + ammoniagenesis
PTH	Decreased	Inhibits NHE3 (phosphaturic, bicarbonaturic)

Tubular maximum (Tm) for HCO₃⁻ ≈ 26–28 mEq/L plasma, which is why the normal renal "bicarbonate threshold" sits near the normal plasma HCO₃⁻; above it, bicarbonate spills into urine.

Titratable acid excretion (phosphate buffer)

After bicarbonate is reclaimed, the distal nephron must dispose of H⁺ on urinary buffers. Titratable acid is the amount of H⁺ excreted bound to filtered buffers — operationally, the amount of strong base (NaOH) needed to titrate urine back to plasma pH (7.4).

• The dominant buffer is phosphate (HPO₄²⁻ + H⁺ → H₂PO₄⁻); pKa ≈ 6.8, ideal for urine.
• Creatinine and urate contribute small amounts.
• Each H⁺ excreted as titratable acid → one new HCO₃⁻ regenerated and added to blood.

Limitation: titratable acid is essentially fixed by the filtered load of phosphate and cannot rise much in acidosis. Therefore it contributes only ~10–40 mEq/day and is the minor, relatively inflexible component of net acid excretion.

High-yield: In diabetic ketoacidosis, β-hydroxybutyrate and acetoacetate (pKa ~4.8) act as additional filtered buffers, so measured titratable acid rises — a classic exception to the "phosphate-only" idea.

Collecting duct: ammoniagenesis — the major mechanism

The kidney's capacity to ramp up acid excretion in chronic acidosis lies in ammonium (NH₄⁺) excretion, the adjustable and dominant component (can rise from ~40 mEq/day to >250 mEq/day over days).

Key concept — ammonium is made, not just trapped:

1. PCT generates NH₄⁺ from glutamine (glutamine → glutamate → α-ketoglutarate, releasing 2 NH₄⁺). Crucially, metabolism of α-ketoglutarate also yields 2 HCO₃⁻ (new bicarbonate).
2. NH₄⁺/NH₃ is secreted into the lumen and undergoes medullary recycling in the loop (NH₄⁺ reabsorbed in TAL via NKCC2 substituting for K⁺, building a medullary NH₃ gradient).
3. In the collecting duct, the α-intercalated cells secrete H⁺ via apical H⁺-ATPase and H⁺/K⁺-ATPase; NH₃ diffuses in and is "diffusion-trapped" as NH₄⁺ and excreted.

High-yield: It is the regeneration of bicarbonate from α-ketoglutarate metabolism in the proximal tubule — NOT the buffering of luminal H⁺ by NH₃ — that yields net new HCO₃⁻. Excreting the NH₄⁺ in urine prevents that nitrogen returning to the liver to consume HCO₃⁻ via ureagenesis. This is a favourite conceptual MCQ.

Stimuli that increase ammoniagenesis: chronic metabolic acidosis (most important), hypokalaemia (intracellular acidosis stimulates it), and cortisol. Hyperkalaemia inhibits ammoniagenesis — the central lesion in Type IV RTA.

Net acid excretion (the master equation)

$$ NAE = (U_{NH_4^+} \times V) + (Titratable\ acid \times V) - (U_{HCO_3^-} \times V) $$

In words: NAE = Ammonium + Titratable acid − Urinary bicarbonate.

• In health NAE ≈ 70 mEq/day = fixed acid produced = new HCO₃⁻ generated.
• In chronic acidosis NAE rises mainly via NH₄⁺ (titratable acid is relatively fixed).
• Any urinary HCO₃⁻ loss is subtracted because it represents base wasted.

High-yield: The single most exam-relevant statement: ammonium excretion is the principal mechanism by which the kidney increases net acid excretion in chronic metabolic acidosis.

Renal tubular acidosis (RTA) — the NEET PG goldmine

RTA is a normal anion gap (hyperchloraemic) metabolic acidosis caused by impaired renal acidification, with normal or near-normal GFR (distinguishing it from uraemic acidosis, which has a high anion gap). Always think RTA when there is a hyperchloraemic metabolic acidosis and no diarrhoea.

Urine anion gap (UAG) — the discriminator

$$ UAG = (Na^+ + K^+) - Cl^- $$

UAG is an indirect index of urinary NH₄⁺ (the major unmeasured cation):

• Negative UAG ("neGUTive") → high urinary NH₄⁺ → appropriate renal response → suggests GI bicarbonate loss (diarrhoea) or proximal RTA with intact distal acidification.
• Positive UAG → low urinary NH₄⁺ → kidney failing to excrete acid → distal (Type I) or Type IV RTA.

Mnemonic — "neGUTive": a neGative urine anion gap points to the GUT (diarrhoea / extrarenal cause). A positive UAG points to the kidney (distal RTA).

The three classic types

Feature	Type I (Distal)	Type II (Proximal)	Type IV (Hyperkalaemic)
Primary defect	Impaired distal H⁺ secretion (α-intercalated cell)	Impaired PCT HCO₃⁻ reabsorption	Aldosterone deficiency / resistance
Serum K⁺	Low (hypokalaemia)	Low (hypokalaemia)	High (hyperkalaemia)
Urine pH	> 5.5 (cannot acidify)	Variable; < 5.5 once below threshold	Usually < 5.5
Serum HCO₃⁻	Can be very low (<10)	Moderate (12–18, "self-limited")	Mildly low (>17)
Urine anion gap	Positive	Negative (with bicarbonaturia)	Positive
Nephrocalcinosis / stones	Yes (Ca phosphate)	No	No
Fanconi syndrome	No	Yes (glycosuria, phosphaturia, aminoaciduria)	No
Bicarbonate to correct	Low dose (1–2 mEq/kg/day)	High dose (10–15 mEq/kg/day) + K⁺	Low; treat hyperkalaemia

High-yield (Type I): Cannot lower urine pH below 5.5 even with systemic acidosis → ↑ urine pH promotes calcium phosphate stones and nephrocalcinosis, plus rickets/osteomalacia. Causes: Sjögren syndrome, SLE, amphotericin B, lithium, hereditary. Treatment relieves hypokalaemia first to avoid worsening it with alkali.

High-yield (Type II): Bicarbonate threshold is reset low; once plasma HCO₃⁻ falls below the new threshold, distal acidification is intact, so urine can acidify (pH < 5.5) and the acidosis is self-limiting/non-progressive. Giving HCO₃⁻ worsens kaliuresis → hypokalaemia. Associations: Fanconi syndrome, multiple myeloma, acetazolamide, tenofovir, ifosfamide, cystinosis, Wilson disease.

High-yield (Type IV): The only RTA with hyperkalaemia. Hyperkalaemia suppresses ammoniagenesis, reducing NH₄⁺ buffer and hence acid excretion. Causes: diabetic nephropathy (commonest), hyporeninaemic hypoaldosteronism, Addison disease, ACE inhibitors/ARBs, spironolactone, NSAIDs, trimethoprim, heparin. Manage with low-K⁺ diet, loop diuretic, and fludrocortisone if hypoaldosterone.

Why no "Type III"? Type III is a rare combined proximal+distal defect (carbonic anhydrase II deficiency — osteopetrosis, cerebral calcification, RTA) and is generally not counted separately.

Quick diagnostic flow for hyperchloraemic metabolic acidosis

Confirm normal anion gap acidosis → check serum K⁺ → if HIGH think Type IV → if LOW/normal calculate urine anion gap → negative UAG = diarrhoea (or proximal RTA) → positive UAG with urine pH > 5.5 = distal (Type I) RTA.

Clinical and physiological correlations

• Acetazolamide: CA inhibitor → blocks PCT HCO₃⁻ reabsorption → mild metabolic acidosis; used for glaucoma, altitude sickness (counters respiratory alkalosis), and idiopathic intracranial hypertension.
• Hypokalaemia ↔ alkalosis loop: hypokalaemia drives intracellular acidosis → stimulates H⁺ secretion and ammoniagenesis → paradoxical aciduria in the face of metabolic alkalosis (classic in vomiting/contraction alkalosis).
• Potassium–acid reciprocity: acidosis tends to cause hyperkalaemia (H⁺ in, K⁺ out of cells) and vice versa — but organic acidoses (lactic, keto) shift K⁺ less than mineral acidoses.
• Chronic acidosis and bone: bone carbonate buffers chronic acid loads, contributing to osteopenia (relevant in CKD).

Complications of disordered renal acidification

• Distal RTA: nephrocalcinosis, recurrent calcium phosphate stones, hypokalaemic paralysis, rickets/osteomalacia, growth retardation in children.
• Proximal RTA: features of Fanconi syndrome — phosphaturia (rickets), glycosuria, aminoaciduria, uricosuria, proximal tubular proteinuria.
• Type IV: persistent hyperkalaemia (arrhythmia risk), mild acidosis.
• Untreated chronic acidosis: muscle wasting (protein catabolism), impaired growth, worsening bone disease.

Key differentials

Presentation	Differentials to separate
Normal anion gap (hyperchloraemic) acidosis	RTA (I/II/IV), diarrhoea, ureteral diversion, early renal failure, post-hypocapnia, acetazolamide, TPN — mnemonic HARDASS / USED CARP
High anion gap acidosis	MUDPILES (Methanol, Uraemia, DKA, Propylene glycol, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, Salicylates)
Metabolic alkalosis with aciduria	Vomiting/contraction alkalosis, hypokalaemia, mineralocorticoid excess

Mnemonic — USED CARP (normal anion gap acidosis): Ureterosigmoidostomy, Saline excess, Endocrine (hypoaldo/Type IV), Diarrhoea, Carbonic anhydrase inhibitors, Addison, RTA, Pancreatic fistula.

Recently asked / exam angle

• A single-best-answer linking UAG to type of RTA is among the most repeated renal-physiology MCQs: positive UAG + urine pH > 5.5 + hypokalaemia = distal (Type I) RTA.
• "Which is the major adjustable mechanism of renal acid excretion?" → Ammonium (NH₄⁺) excretion, not titratable acid.
• "Where is the majority of filtered bicarbonate reabsorbed?" → Proximal convoluted tubule (~80–85%), via NHE3 + carbonic anhydrase.
• "Which RTA has hyperkalaemia?" → Type IV; commonest cause diabetic nephropathy.
• "Drug causing proximal RTA / bicarbonaturia" → Acetazolamide (also tenofovir for Fanconi).
• Concept question: "New bicarbonate generation depends on metabolism of …" → glutamine/α-ketoglutarate (ammoniagenesis).
• Calculation-style: given urine Na⁺, K⁺, Cl⁻, compute UAG and interpret.
• "Cell responsible for distal H⁺ secretion" → α-intercalated cell (H⁺-ATPase, H⁺/K⁺-ATPase); β-intercalated cell secretes HCO₃⁻ (via pendrin) in alkalosis.

Rapid revision

1. PCT reabsorbs 80–85% of filtered HCO₃⁻ via NHE3 + carbonic anhydrase (CA IV luminal, CA II intracellular); exits basolaterally on NBC1.
2. No new bicarbonate is made during reabsorption — it only prevents base loss.
3. Net acid excretion = NH₄⁺ + titratable acid − urinary HCO₃⁻; equals new HCO₃⁻ generated (~70 mEq/day).
4. Titratable acid uses phosphate buffer (pKa 6.8) — minor and relatively fixed.
5. Ammonium excretion is the major adjustable mechanism; rises hugely in chronic acidosis.
6. New HCO₃⁻ comes from glutamine → α-ketoglutarate metabolism in the PCT; NH₄⁺ is diffusion-trapped in the collecting duct.
7. Hyperkalaemia inhibits ammoniagenesis → underlies Type IV RTA; hypokalaemia stimulates it.
8. α-intercalated cells secrete H⁺ (H⁺-ATPase, H⁺/K⁺-ATPase); β-intercalated cells secrete HCO₃⁻ via pendrin.
9. RTA = normal anion gap acidosis with normal GFR; uraemic acidosis = high anion gap.
10. Urine anion gap = (Na + K) − Cl; neGUTive = gut/diarrhoea, positive = distal/Type IV RTA.
11. Type I: hypokalaemia, urine pH > 5.5, nephrocalcinosis/stones. Type II: hypokalaemia, Fanconi, self-limited, high-dose alkali. Type IV: hyperkalaemia, diabetic nephropathy.
12. Acetazolamide = CA inhibitor → bicarbonaturia → proximal-RTA-like metabolic acidosis; Tm for HCO₃⁻ ≈ 26–28 mEq/L.