Loop of Henle

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  • Loop of Henle
    • Two-limbed (U-shaped) structure that extends into medulla
      • Different parts of the loop have different permeabilities. 
      • Descending Limb
        • Extends from cortex down into the relatively salty medulla 
        • Highly permeable to water but impermeable to NaCl
          • Reabsorption of water only by facilitated diffusion
      • Ascending Limb
        • Ascends from medulla back to the cortex
        • Permeable to NaCl (salt) but impermeable to water
          • Reabsorption of ions only by passive + active transport
      • Vasa recta is a capillary system running parallel to loop that returns water and solutes to the bloodstream
    • Opposite flows (countercurrents) interact to increase (multiply) interstitial concentration gradient, known as countercurrent multiplication
      • Active reabsorption of NaCl in ascending limb increases passive water reabsorption in descending limb 
      • Structure causes gradient where solute is most concentrated at the bottom of the loop
      • The longer the loop, the more the gradient “multiplies”, causing more water to be reabsorbed by the body.