The thin ascending limb of the loop of Henle is characterized by which combination of properties?

Explanation

## Thin Ascending Limb of the Loop of Henle — Structure and Function **Key Point:** The thin ascending limb (TAL) is impermeable to water but permeable to NaCl; it reabsorbs Na⁺, K⁺, and Cl⁻ passively via the Na⁺-K⁺-2Cl⁻ cotransporter (NKCC2), driven by the concentration gradient established in the descending limb. ### Structural and Functional Characteristics | Property | Thin Descending Limb | Thin Ascending Limb | Thick Ascending Limb | |----------|---------------------|-------------------|---------------------| | **Water permeability** | High (aquaporin-1) | Low/absent | Low/absent | | **Solute permeability** | Low | High (NaCl) | High (NaCl, K⁺) | | **Na⁺ reabsorption mechanism** | None | Passive (NKCC2) | Active (NKCC2 + Na⁺-K⁺-ATPase) | | **ATP requirement** | None | None | High | | **Osmolarity of fluid leaving** | Hypertonic | Hypotonic | Hypotonic | | **Epithelial type** | Simple squamous | Simple squamous | Simple cuboidal | **High-Yield:** The thin ascending limb is the only segment that reabsorbs solute WITHOUT reabsorbing water, making the tubular fluid progressively more dilute (hypotonic). This is the key to the countercurrent multiplier mechanism. ### Why the Thin Ascending Limb is Permeable to NaCl but Not Water - **NKCC2 transporter** (Na⁺-K⁺-2Cl⁻ cotransporter) is expressed on the apical membrane - **Passive transport** driven by the concentration gradient created by water reabsorption in the descending limb - **Aquaporin-1 is absent** → water cannot follow the osmotic gradient → tubular fluid becomes hypotonic - **Tight epithelium** → prevents paracellular water reabsorption **Mnemonic:** **"TAL = Tight, Ascending, Low water"** — The thin ascending limb is a tight epithelium that allows ascending fluid to remain hypotonic despite NaCl reabsorption. **Clinical Pearl:** Loop diuretics (furosemide, bumetanide, torsemide) block NKCC2 on the thick ascending limb, preventing Na⁺-K⁺-2Cl⁻ reabsorption and disrupting the countercurrent multiplier — this is why they are the most potent diuretics. ### Countercurrent Multiplier Mechanism ```mermaid flowchart TD A["Fluid enters descending limb<br/>Osmolarity = 300 mOsm/L"]:::outcome B["Descending limb: Water leaves<br/>via aquaporin-1<br/>Osmolarity ↑ to 600 mOsm/L"]:::action C["Thin ascending limb:<br/>NaCl leaves via NKCC2<br/>Water CANNOT leave<br/>Osmolarity ↓ to 100 mOsm/L"]:::action D["Thick ascending limb:<br/>Active NaCl reabsorption<br/>Water impermeable<br/>Osmolarity ↓ to 100 mOsm/L"]:::action E["Collecting duct:<br/>Water reabsorption via ADH<br/>Final urine concentration"]:::action A --> B --> C --> D --> E ``` [cite:Guyton and Hall Textbook of Medical Physiology Ch 26]