Which is the most common site of potassium reabsorption in the nephron?

Explanation

## Potassium Reabsorption Along the Nephron **Key Point:** The **proximal convoluted tubule (PCT)** is the most common site of potassium reabsorption in the nephron, accounting for approximately **65–70% of filtered K⁺** reabsorption (Guyton & Hall, Textbook of Medical Physiology; Costanzo, Physiology). ### Mechanism in the Proximal Convoluted Tubule 1. **Paracellular Reabsorption (Primary Route)** - As Na⁺ and water are reabsorbed, the luminal K⁺ concentration rises slightly, creating a favorable concentration gradient - K⁺ follows water movement via solvent drag through tight junctions (paracellular pathway) - This is the dominant mechanism in the early PCT 2. **Transcellular Reabsorption** - Minor contribution via apical K⁺ channels and basolateral Na⁺-K⁺-ATPase - Constitutively active; not subject to major hormonal regulation ### Distribution of K⁺ Reabsorption Across the Nephron | Segment | % of Filtered K⁺ Reabsorbed | Mechanism | Regulation | | --- | --- | --- | --- | | **Proximal Convoluted Tubule** | **~65–70%** | Paracellular (solvent drag) + transcellular | Constitutive | | **Thick Ascending Limb (TAL)** | **~25–30%** | NKCC2 cotransporter + paracellular | Loop diuretics inhibit | | **Distal Convoluted Tubule** | Variable | ROMK channels, Na⁺-K⁺-ATPase | Aldosterone-regulated | | **Collecting Duct** | Variable (secretion > reabsorption) | ROMK, H⁺-K⁺-ATPase | Aldosterone, K⁺ intake | **Why not the TAL?** Although the NKCC2 transporter in the TAL is physiologically important and is the *primary regulated* site for K⁺ handling, the TAL reabsorbs only ~25–30% of filtered K⁺ in absolute terms — far less than the PCT. The question asks for the "most common site," which refers to the segment reabsorbing the greatest *quantity* of filtered K⁺. **High-Yield:** Loop diuretics (furosemide) inhibit NKCC2 in the TAL, reducing K⁺ reabsorption there and causing hypokalemia — but the PCT remains the quantitatively dominant reabsorptive site overall. **Clinical Pearl:** Because PCT reabsorption is constitutive and bulk-flow driven, conditions that reduce proximal reabsorption (e.g., Fanconi syndrome, carbonic anhydrase inhibitors) can significantly increase K⁺ delivery to distal segments, potentially causing hypokalemia. **Mnemonic:** **PCT = 65%** — The **P**roximal **C**onvoluted **T**ubule handles the lion's share (~65%) of filtered K⁺, making it the most common reabsorption site.