Descending Thin Limb

The descending thin limb of Henle's loop begins at the end of the proximal straight tubule and continues past the hairpin bend in Henle's loop to the start of the thick ascending limb. Descending thin limbs are virtually devoid of Na+-K+-ATPase and therefore do not participate in active sodium reabsorption. Moreover, the descending thin limb is highly impermeable to sodium and urea. Although the descending thin limb is not a site of diuretic action per se, its permeability contributes importantly to the action of osmotic agents because of its high water permeability. The presence of unabsorbable solute in the lumen retards water absorption and thereby contributes to the osmotic diuresis. Furthermore, drugs and other compounds in the tubular fluid are concentrated as a result of the removal of water as the descending thin limb of long-looped nephrons passes through the hypertonic renal medulla. The elevation of drug concentrations for agents working at downstream segments may aid in raising the drug concentrations to the levels necessary for diuretic action. These elevated concentrations would not be achieved in the systemic circulation. The selective increase in the concentration of these drugs within the tubular fluid may account for the relatively selective action of these compounds on the kidney, even though the same sodium transport proteins are present in other tissues.

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