Final answer:
The countercurrent multiplier system in the kidney primarily involves the exchange of water and ions between the renal interstitium and the filtrate in the loop of Henle, where the descending limb allows water out and the ascending limb pumps ions out, while the vasa recta acts as a countercurrent exchanger.
Step-by-step explanation:
The countercurrent multiplier system in the kidney involves the exchange of water and ions primarily between the renal interstitium and (D) the filtrate in the loop of Henle. The loop of Henle acts as a countercurrent multiplier, utilizing energy to establish an osmotic gradient within the renal medulla. The descending limb of the loop is permeable to water but not to solutes, allowing water to flow out into the interstitial space and increasing the osmolarity of the filtrate as it descends. Conversely, the ascending limb is permeable to ions but not to water, facilitating the active transport and passive diffusion of Na+ and Cl- out of the filtrate. This interplay between the two limbs of the loop of Henle creates a high osmolar environment in the renal medulla. The vasa recta, surrounding the loop of Henle, acts as a countercurrent exchanger, assisting in maintaining the concentration gradient while returning reabsorbed substances to circulation.