Active Sodium Chloride Reabsorption in IheThickAscending Limb and the Distal ConvoIutedTubuIe Dilutes the Tubule Fluid

The thick ascending limb and the distal convoluted tubule actively reabsorb Na*, which drives CΓ reabsorption. Because these segments are impermeable to water, active solute reabsorption causes a progressive decline in the tubule fluid osmolality. Thus the thick ascending limb and distal con­voluted tubule are often called the diluting segments. The result is that the tubule fluid delivered to the collecting duct is hypo­tonic, regardless of the physiological state of the animal.