Final answer:
The descending limb of the loop of Henle increases filtrate concentration by allowing water out, whereas the ascending limb decreases concentration by actively transporting out solutes.
Step-by-step explanation:
In the loop of Henle, filtrate osmolality becomes more concentrated in the descending limb due to water permeability and less concentrated, or more dilute, in the ascending limb due to the active transport of solutes.
The descending limb is water permeable, allowing water to flow out, which increases osmolality. Conversely, the ascending limb is impermeable to water but allows the transport of Na+ and Cl- ions out of the filtrate. Active transport further up the limb continues to reduce osmolality by moving Na+ out, which makes the filtrate dilute. The renal medulla's high salt concentration plays a role in creating an osmotic gradient that facilitates these processes.
In the loop of Henle, the permeability of the membrane changes. The descending limb is water permeable, so the water flows from the filtrate to the interstitial fluid, while the ascending limb is permeable to solutes. As a result, the osmolality increases as it moves deeper into the medulla. The loop of Henle acts as a countercurrent multiplier, with the vasa recta acting as the countercurrent exchanger.