Answer: Option A
:
A
Countercurrent exchanger would not exist if there were not some mechanism to initially make the vasa recta more concentrated at the bottom of the loop. This is accomplished by the loop of Henle. The ascending limb of Henle and the early distal tubule are impermeable to water as indicated by their thick wall. These regions actively transport sodium chloride (NaCl) out of the filtrate and into the surroundings. The NaCl diffuses into the descending limb of the vasa recta. It will not diffuse into the ascending vasa recta because that fluid is already highly concentrated. This is the mechanism that 'multiplies' the concentration of NaCl in the descending vasa recta making the countercurrent exchanger possible