Thin descending limb of the nephron loop
Tubulus attenuatus, pars descendens
Also known as: descending limb of Henle, tDL
A thin, water-permeable segment of the loop of Henle lined by squamous epithelium that concentrates the tubular fluid by losing water to the hypertonic medullary interstitium.
Overview
The thin descending limb follows the straight part of the PCT at the junction of the outer and inner stripes of the outer medulla. Its flat epithelium is highly permeable to water via constitutive aquaporin-1, with low permeability to Na⁺ and Cl⁻ (and modest, transporter-mediated urea permeability). As the limb descends through progressively hypertonic interstitium, water leaves osmotically, and tubular fluid osmolality rises from ≈300 to ≈1200 mOsm/kg at the hairpin of the longest loops. Because water leaves without solute, the fluid becomes rich in NaCl, which sets up the passive NaCl exit in the thin ascending limb.
Water reabsorption here (≈15–20% of the filtered load) is not regulated by ADH.
- Location
Outer and inner medulla.
Anatomy
- Histology
Simple squamous epithelium (cells ≈1–2 µm thick) resembling capillary endothelium but without blood cells in the lumen.
Function
- Reabsorb water (≈15–20% of filtered load) passively via AQP1
- Concentrate tubular fluid toward the hairpin
- Physiology
Tubular fluid osmolality ≈300 → ≈1200 mOsm/kg (long loops). Osmotic diuretics (mannitol) reduce water reabsorption here.
Compiled from standard anatomy and physiology references — see the Urinary System sources. For education only; not medical advice.