FIGURE 1 Reabsorption of water but not solute in the thin descending limb of the loop of Henle. The interstitial fluid in the medulla has an osmolality that rises from 280 mOsm/kg H2O to 1200 mOsm/kg H2O at the tip of the papilla, as shown by the shading in the figure. As fluid flows from the proximal tubule toward the papillary tip, it is concentrated by the movement of water out of the lumen into the hyperosmotic medullary interstitium. This causes the NaCl and urea concentrations to rise in the tubular fluid so that the total osmolality equals that of the adjacent interstitium. Although the osmotic equilibration occurs primarily by the loss of water to the interstitium, some urea also diffuses into the tubular fluid.
280 mOsm/kg H2O at its proximal end to 1200 at the tip of the loop of Henle. In other words, osmotic equilibration of the tubular fluid with the surrounding medullary interstitium occurs continuously along the thin descending limb because of its high water permeability. Therefore, at the tip of the loop of Henle for the long-looped nephrons, only approximately 10% of the filtered volume remains, and its osmolality has risen to 1200 mOsm/kg H2O. At this point, most of the total 1200 mOsm/kg H2O osmolality of the tubular fluid is due to NaCl, and 50-100 mOsm/kg H2O is due to urea. In the short-looped nephrons, about 25-30% of the filtered fluid volume remains, and the osmolality has increased to a lesser extent, depending on the level within the medulla at which the tip of the loop resides.
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