Final answer:
In the juxtamedullary nephron, a single NaCl pump in the thick ascending limb of the loop of Henle actively transports ions to create a significant osmolarity difference that helps concentrate urine.
Step-by-step explanation:
Considering a juxtamedullary nephron, a single NaCl pump in the thick ascending limb of the loop of Henle is able to produce a concentration difference. This concentration difference is key to the countercurrent multiplication mechanism that the kidney uses to concentrate urine. In the ascending limb, Na+ and Cl- ions are actively reabsorbed by a cotransport system. This system is capable of producing a difference in osmolarity between the inside and outside of the tubule, which contributes to the generation of a hyperosmotic environment in the kidney medulla and a hypoosomotic filtrate by the time it reaches the distal convoluted tubule (DCT).