In the countercurrent multiplier system of the loop of Henle, which segment is responsible for active transport of sodium and chloride without water reabsorption?

Explanation

## Countercurrent Multiplier System — Segmental Function **Key Point:** The thick ascending limb (TAL) of the loop of Henle is the primary site of active Na⁺–K⁺–2Cl⁻ cotransport (via NKCC2 transporter), which pumps solutes into the interstitium while remaining impermeable to water. ### Functional Segments of Loop of Henle | Segment | Primary Function | Water Permeability | Active Transport | Osmolality Effect | | --- | --- | --- | --- | --- | | **Descending limb** | Passive water reabsorption | High | None | Equilibrates with medullary interstitium | | **Thin ascending limb** | Passive NaCl reabsorption | Low | None | Dilutes tubular fluid | | **Thick ascending limb** | Active NaCl reabsorption | Impermeable | Na⁺–K⁺–2Cl⁻ pump | Creates osmotic gradient | | **Collecting duct** | Water reabsorption (ADH-dependent) | Variable | None for water | Final concentration | **High-Yield:** The thick ascending limb is the "diluting segment" — it reabsorbs solute without water, progressively lowering intratubular osmolality while building the medullary osmotic gradient that drives the countercurrent multiplier. **Mnemonic:** **TAL = The Active Limb** — thick ascending limb does the work (active transport); descending limb is passive (permeable to water); thin ascending limb is also passive (permeable to solute). **Clinical Pearl:** Loop diuretics (furosemide, bumetanide) block the NKCC2 transporter in the thick ascending limb, abolishing the countercurrent gradient and causing profound natriuresis and dilute urine — even in the presence of ADH.