Regarding the countercurrent multiplier system in the loop of Henle, all of the following statements are correct EXCEPT:

## Countercurrent Multiplier System: Structural and Functional Principles ### Understanding the Loop of Henle Architecture **Key Point:** The countercurrent multiplier in the loop of Henle creates and maintains an osmotic gradient through differential permeability and active transport in each limb. ### Limb-Specific Characteristics | Feature | Descending Limb | Ascending Limb | |---------|-----------------|----------------| | Water permeability | High (aquaporin-1) | Impermeable | | Solute permeability | Low | High | | Active transport | Absent | Present (Na⁺-K⁺-2Cl⁻ cotransporter) | | Osmolality gradient | Increases toward bend | Decreases toward cortex | | Fluid movement | Water exits passively | NaCl exits actively, water stays | ### Why Option 3 Is Incorrect **High-Yield:** The osmolality is **highest in the collecting duct** (up to 1200 mOsm/kg in deep medulla), NOT in the descending limb at the bend. The descending limb osmolality increases as fluid moves downward due to passive water loss and solute influx from the interstitium, but it never exceeds the osmolality of the ascending limb at the same level. The ascending limb actively pumps out NaCl without water reabsorption, creating a *relative* hypoosmolality within the tubular fluid, while the interstitium becomes progressively more hypertonic. **Warning:** A common misconception is that the descending limb fluid becomes the most concentrated; in reality, the **interstitium** (not the tubular fluid in the descending limb) reaches maximum osmolality at the papillary tip. ### Why Options 1, 2, and 4 Are Correct **Option 1 (Correct):** The descending limb epithelium is highly permeable to water via aquaporin-1 channels but has low permeability to ions. Water equilibrates osmotically with the hypertonic medullary interstitium. **Option 2 (Correct):** The thick ascending limb actively transports NaCl via the Na⁺-K⁺-2Cl⁻ cotransporter (NKCC2) and is impermeable to water, making it the "diluting segment." This is the engine of the countercurrent multiplier. **Option 4 (Correct):** The vasa recta (peritubular capillaries) function as a countercurrent exchanger: solutes diffuse out as blood descends and diffuse back in as blood ascends, minimizing solute washout and preserving the medullary gradient. ### Clinical Pearl Loop diuretics (furosemide) block NKCC2 in the thick ascending limb, abolishing the countercurrent multiplier and preventing urine concentration — explaining why loop diuretics cause polyuric nephrogenic diabetes insipidus–like states. ### Mnemonic **DALI:** **D**escending limb — **A**quaporin (water permeable); **L**oop bend — **L**owest osmolality in ascending limb tubular fluid; **I**nterstitium — **I**ncreasing osmolality (highest at papilla).