As anatomically described, the descending limb of the loop of Henle begins with the proximal straight tubule. In the outer medulla, the epithelium of the proximal straight tubule transforms into the thin descending limb of the loop of Henle, which is named for its very thin epithelium. Functionally, the proximal straight tubule can be regarded as a continuation of the proximal tubule, whereas the thin descending limb is functionally quite different from the proximal tubule. The length of the thin descending limb varies among nephrons. Most superficial nephrons have relatively short loops of Henle, whereas many juxtamedullary nephrons have long ones that may extend to the tip of the papilla. In this discussion, both long and short thin descending limbs are considered collectively.
A unique feature of the loop of Henle is that it resides in an interstitial fluid having a composition completely different from any other tissue—one that, under typical conditions, is markedly hyperosmotic to the plasma. The osmolality of the medullary interstitial fluid rises from approximately isosmotic at the border between the cortex and the medulla and increases progressively to a maximum of about 1200 mOsm/kg H2O at the papillary tip. The interstitial fluid at the papillary tip has a urea concentration of about 600 mmol/L and a NaCl concentration of about 300 mmol/L. Thus, approximately 50% of the osmolality is due to the osmotic effect of NaCl and 50% is due to urea.
The fluid flowing into the thin descending limb of the loop of Henle from the proximal tubule has an osmo-lality that is approximately the same as plasma—about 280 mOsm/kg H2O. The concentrations of Na+ and K+ are approximately the same as in plasma, but Cl— is the primary anion because of the reabsorption of most of the filtered HCO— by the proximal tubule. The urea concentration is about 6 mmol/L. As this fluid flows through the thin descending limb, an osmotic pressure gradient occurs between this initially isosmotic tubular fluid and the hyperosmotic medullary interstitium.
Figure 1 illustrates schematically the transepithelial transport processes that occur as fluid flows from the outer medullary to the papillary regions of a thin descending limb. There is no active transepithelial transport in the thin descending limb of the loop of Henle. The cells that constitute this epithelium are thin and contain few mitochondria. The most important transport characteristic of the thin descending limb is that it is highly permeable to water because aquaporin-1 water channels are present in the luminal and basolat-eral membranes of thin descending limb cells as they are in the proximal tubule. The epithelium of the thin descending limb is also somewhat permeable to urea and to a lesser extent NaCl, which would allow the diffusion of these solutes from the interstitium to the tubular fluid, but these permeabilities are significantly less than that of water. Therefore, because of the hyperosmolality of the medulla, there is an osmotic water flow from the tubular fluid into the interstitium with a relatively small diffusional entry of urea and NaCl from the interstitial fluid. The net result is that the tubular fluid becomes concentrated as it flows down into the medulla, and this increase in osmolality develops primarily because of the water loss rather than solute gain.
Because of the loss of water, and to a much lesser extent the entry of urea and NaCl, the osmolality of the tubular fluid rises along the thin descending limb from
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