In the countercurrent multiplier system of the loop of Henle, which segment is responsible for active sodium chloride reabsorption and is impermeable to water?

Explanation

## Countercurrent Multiplier: Thick Ascending Limb **Key Point:** The thick ascending limb (TAL) of the loop of Henle is the primary site of active NaCl reabsorption and is impermeable to water, making it the key engine of the countercurrent multiplier system. ### Mechanism of TAL Function 1. **Active Transport**: The Na-K-2Cl cotransporter (NKCC2) actively pumps NaCl from the tubular lumen into the interstitium 2. **Water Impermeability**: Absence of aquaporins in the TAL epithelium prevents water reabsorption despite high osmolarity in the interstitium 3. **Osmotic Gradient Generation**: This creates a positive osmotic gradient (up to 200 mOsm/L) between the tubular fluid and interstitium ### Comparison of Loop Segments | Segment | Active Transport | Water Permeability | Function | |---------|------------------|-------------------|----------| | **Thick Ascending Limb** | **High (NaCl)** | **Impermeable** | **Multiplier** | | Thin Ascending Limb | Low/Passive | Low | Passive NaCl reabsorption | | Thin Descending Limb | None | Highly permeable | Water reabsorption | | Collecting Duct | Variable | Regulated by ADH | Fine-tuning | **High-Yield:** The TAL reabsorbs ~25% of filtered NaCl and is the site of action for loop diuretics (furosemide, bumetanide), which block NKCC2 and are the most potent diuretics available. **Clinical Pearl:** Loop diuretics are more potent than thiazides because they block the TAL, which generates the osmotic gradient essential for urine concentration. Patients on loop diuretics cannot concentrate urine effectively. **Mnemonic:** **TAL = The Active Limb** — remember it as the workhorse of the countercurrent system, actively pumping salt while excluding water.