Descending Thin Limb. The descending thin limb reabsorbs an additional fraction of the filtered water. There is a large gradient for water reabsorption from this segment even though it reabsorbs only a small amount of solute. This gradient reflects the high rates of reabsorption from the thick ascending limb, which is adjacent to the ascending thin limb and adds solute to its surrounding interstitium. As in the proximal tubule, water crosses the tubular epithelium through AQP-1 channels.

As described on Plate 1-24, the descending thin limbs of short-looped (cortical) and long-looped (juxtamedullary) nephrons differ not only in length but also in cellular composition. In short-looped nephrons, the descending thin limb consists of type I cells, whereas in long-looped nephrons, it consists of type II cells in the outer medulla and type III cells in the inner medulla. Type I and II cells are more permeable to water than type III cells. Thus, in long-looped nephrons, water reabsorption from the descending thin limb decreases near the inner medulla.

Because water reabsorption exceeds solute reabsorption in the descending thin limb, tubular fluid becomes more concentrated. This process, however, is not under tight control. As a result, the descending thin limb does not have a major role in determining the final concentration of excreted urine.

Ascending Thin Limb and Thick Ascending Limb. The ascending thin limb (found only in long-looped nephrons) and thick ascending limb do not contain aquaporin channels and are therefore impermeable to water. The extensive reabsorption of solutes from these segments, however, dilutes tubular fluid and establishes a concentration gradient for water reabsorption from adjacent segments, such as the descending thin limb and collecting duct.