In a mammalian nephron, the osmolarity of the filtrate changes from 1200 mOsmL ⁻¹ at the hairpin loop to approximately 200 mOsmL ⁻¹ by the time it reaches the distal convoluted tubule. Which specific permeability characteristic of the intervening segment allows this significant dilution of the filtrate?

Step-by-step solution

The segment connecting the hairpin loop to the distal convoluted tubule is the ascending limb of the loop of Henle. As the filtrate moves upward through the ascending limb, its osmolarity decreases from 1200 mOsmL ⁻¹ to 200 mOsmL ⁻¹ . This dilution occurs because the ascending limb is impermeable to water but allows the transport of electrolytes (actively or passively) into the medullary fluid. Thus, solutes are removed while water is retained in the tubule. The descending limb (which concentrates the filtrate) is