Regarding the renal mechanisms of acid-base balance, all of the following statements are correct EXCEPT:

Explanation

## Renal Acid-Base Regulation: Mechanisms and Sites ### Overview of Renal H+ Handling The kidney regulates acid-base balance through three main processes: bicarbonate reabsorption, H+ secretion, and ammonia excretion. Each segment of the nephron plays a distinct role. ### Proximal Convoluted Tubule (PCT) — Primary Bicarbonate Reabsorption **Key Point:** The PCT reabsorbs 80–90% of filtered HCO₃⁻, making it the dominant site for bicarbonate conservation. 1. H+ is secreted into the tubular lumen via the Na+/H+ exchanger (NHE3) — a secondary active transport mechanism coupled to Na+ reabsorption. 2. Secreted H+ combines with filtered HCO₃⁻ to form H₂CO₃, which is rapidly dehydrated to CO₂ and H₂O by carbonic anhydrase. 3. CO₂ diffuses back into the proximal tubule cell, where it is rehydrated to H₂CO₃ and dissociates to HCO₃⁻ and H+. 4. HCO₃⁻ is reabsorbed across the basolateral membrane via Na+/HCO₃⁻ cotransporter (NBC1). ### Ammonia Excretion — Acid Buffering **High-Yield:** Ammonia (NH₃) is produced from glutamine deamination in the proximal tubule mitochondria. This process is enhanced in chronic acidosis. - NH₃ diffuses into the tubular lumen and is protonated to NH₄⁺ (ammonium). - NH₄⁺ cannot be reabsorbed (trapped as the charged form) and is excreted in urine. - This mechanism allows the kidney to excrete titratable acid and is crucial for long-term acid-base balance. ### Distal Convoluted Tubule (DCT) and Collecting Duct — Fine-Tuning, NOT Primary Reabsorption **Key Point:** The DCT and collecting duct reabsorb only 10–20% of filtered HCO₃⁻ and are responsible for fine-tuning acid-base balance and responding to aldosterone and ADH. - **Intercalated cells** in the collecting duct contain two subtypes: - **Type A (α) intercalated cells:** Secrete H+ via H⁺-ATPase (vacuolar-type) and H+/K⁺-ATPase; reabsorb HCO₃⁻ via Cl⁻/HCO₃⁻ exchanger (AE1). These cells dominate in acidosis. - **Type B (β) intercalated cells:** Secrete HCO₃⁻ and reabsorb H+ (less common); dominate in alkalosis. ### Why Option 3 Is Incorrect The **distal convoluted tubule and collecting duct are NOT the primary sites of bicarbonate reabsorption**. They are responsible for: - Fine-tuning and regulation of acid-base balance (10–20% of HCO₃⁻ reabsorption). - Responding to hormonal signals (aldosterone, ADH). - Generating new HCO₃⁻ through H+ secretion and ammonia excretion. The **primary site of bicarbonate reabsorption is the proximal convoluted tubule** (80–90% of filtered load). ### Summary Table: Nephron Segments and Acid-Base Roles | Segment | HCO₃⁻ Reabsorption | H⁺ Secretion Mechanism | Primary Role | | --- | --- | --- | --- | | **PCT** | 80–90% | NHE3 (Na+/H+ exchanger) | Bulk HCO₃⁻ reclamation | | **Thick Ascending Limb (TAL)** | ~10% | NHE3 | Modest contribution | | **DCT & Collecting Duct** | ~10% | H⁺-ATPase, H⁺/K⁺-ATPase | Fine-tuning, regulation | | **Proximal Tubule (mitochondria)** | — | Glutamine → NH₃ → NH₄⁺ | Titratable acid excretion | **Clinical Pearl:** In acute acidosis, the kidney increases ammonia excretion within hours. In chronic acidosis, ammonia excretion can increase 2–3 fold, providing a major mechanism for acid elimination.