Final answer:
The filtrate in the loop of Henle has become concentrated as the descending limb is water permeable and the ascending limb allows ionic passage, working together as a countercurrent system to establish an osmotic gradient in the renal medulla.
Step-by-step explanation:
By the time the filtrate reaches the loop of Henle in the kidney, it has undergone significant concentration changes due to the specialized functions of the loop's segments. The descending limb of the loop is permeable to water but not to solutes, enabling water to exit the filtrate through aquaporins and enter the interstitial fluid, which increases the osmolality of the filtrate.
Meanwhile, the ascending limb is impermeable to water but allows the passage of ions such as Na+ and Cl- out of the filtrate, partially through passive transport in the thin segment and then active transport in the thick segment, which decreases the osmolality. Additionally, the loop of Henle functions as a countercurrent multiplier and works closely with the surrounding vasa recta, which serves as a countercurrent exchanger, to recover Na+ and water and establish a steep osmotic gradient in the renal medulla.