The loop of Henle comprises: A descending thin limb, which is highly permeable to water and urea, causing it to be in osmotic equilibrium with the interstitial fluid. It is impermeable to sodium.
An ascending thin limb, which is permeable to sodium but not to water.
An ascending thick limb, which is involved in the active transport of sodium out of the tubule into the interstitial fluid, mediated by the Na+K+-ATPase. It is permeable to sodium, this being mediated by the apical Na+/K+/2Cl~ apical carrier. Active sodium and chloride transport generates an osmotic difference between the tubular fluid and the surrounding local interstitium.
Maximum operation of the loop of Henle requires: Ability of the ascending limb to secrete solute into the interstitium via a sodium pump mechanism Impermeability of the ascending limb to water Permeability of the descending limb to water and solute Overall, 25% of sodium and 15% of water is absorbed in the loop of Henle.
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