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The Thin Descending Limb of the Loop of Henle

figureThe thin descending limb of Henle’s loop is highly permeable to water, moderately permeable to urea and poorly permeable to other solutes. There is no significant active solute transport in the thin descending limb. As noted earlier there is a corticomedullary osmotic gradient into which the loops of Henle dip. The mechanism for establishment and the conditions necessary to maintain this gradient will be discussed later.

As water moves out of the thin descending limb into the surrounding hyperosmotic ISF the essentially impermeant solutes in the tubular fluid increase in concentration. Because the osmolarity of the peritubular fluid of the renal medulla increases with distance from the cortex the osmolarity reached in the tubular fluid is much higher at the bend of long-looped nephrons than at the bend of short-looped nephrons.