In the renal regulation of acid-base balance, which of the following is the most common site of hydrogen ion secretion in the nephron?

Explanation

## Renal H⁺ Secretion Along the Nephron **Key Point:** The proximal convoluted tubule (PCT) is responsible for the majority (~90%) of total renal hydrogen ion secretion and is the primary site of acid-base regulation. ### Distribution of H⁺ Secretion by Segment | Nephron Segment | % of Total H⁺ Secretion | Mechanism | Function | |-----------------|-------------------------|-----------|----------| | Proximal convoluted tubule | ~90% | Na⁺/H⁺ antiporter (NHE3); H⁺-ATPase | Bulk reabsorption of filtered HCO₃⁻; titratable acid formation | | Loop of Henle | ~5% | Minimal H⁺ secretion | Not a major site | | Distal convoluted tubule | ~3% | H⁺-ATPase; limited NHE | Fine-tuning of acid-base balance | | Collecting duct | ~2% | H⁺-ATPase (Type A intercalated cells) | Ammonia secretion; final acid excretion | **High-Yield:** Remember the **90-5-3-2 rule**: PCT handles ~90% of H⁺ secretion, making it the workhorse of renal acid-base regulation. ### Mechanisms in the Proximal Convoluted Tubule 1. **Sodium-hydrogen exchanger (NHE3)**: Exchanges luminal Na⁺ for intracellular H⁺ (energy from Na⁺ gradient). 2. **Vacuolar H⁺-ATPase**: Active transport of H⁺ into the tubular lumen. 3. **Carbonic anhydrase**: Catalyzes H₂CO₃ ↔ CO₂ + H₂O, facilitating HCO₃⁻ reabsorption. ### Why the PCT Is the Primary Site - **High capacity**: The large surface area and numerous mitochondria (ATP-dependent processes) enable bulk H⁺ secretion. - **HCO₃⁻ reabsorption**: ~99% of filtered bicarbonate is reabsorbed in the PCT, making it the dominant site for acid-base regulation. - **Titratable acid formation**: The PCT converts filtered phosphate (HPO₄²⁻) and other buffers to their acid forms. **Clinical Pearl:** In metabolic acidosis, the PCT increases H⁺ secretion and HCO₃⁻ reabsorption to restore pH. In metabolic alkalosis, H⁺ secretion decreases, allowing HCO₃⁻ to be excreted. **Mnemonic: PCT-FIRST** — The proximal convoluted tubule is the FIRST and FOREMOST site of H⁺ secretion in the nephron.