What is a countercurrent multiplier, and what is its role in the loop of Henle? Briefly explain how it works. Use a diagram if necessary

Question

What will be an ideal response?

Answer 1

Answer: A countercurrent multiplier uses the principle of fluids flowing in opposite directions (i.e., countercurrent flow), to assist in concentrating one solution while making the other one more dilute. A countercurrent multiplier is somewhat different than a countercurrent exchanger because it requires the input of energy to help establish concentration gradients to get the system started. Countercurrent multipliers are found in the salt glands of birds and reptiles, and also in the loop of Henle in the mammalian kidney. In the loop of Henle, the countercurrent multiplier is formed by the descending and ascending paths, and along with differences in tubule permeability, it helps to establish a concentration gradient within the medulla of the kidney. This concentration gradient allows the urine to be modified in its solute concentration and volume. The loop of Henle is a part of the nephron that is folded back on itself, in hairpin fashion, so that the descending loop and the ascending loop are running countercurrent to one another. In this way, the fluid entering the nephron is continually being modified from start to finish, with the result being appropriate water and salt retention. The loop of Henle descends into the medulla of the kidney, where the osmotic gradient continues to increase deeper into the medulla. This osmotic gradient is favorable for the movement of water, and because the descending limb is selectively permeable to water (via aquaporins), water moves out of the lumen and into the interstitial space. At the bottom of the loop of Henle, the urine reaches its maximum osmolarity. The tubule then ascends toward the cortex containing highly concentrated urine, moving through interstitial fluid that is less so than before. Along with passive diffusion of salts out of the lumen, the ascending limb (which is adjacent to the descending one) ACTIVELY pumps solutes from the lumen to the interstitial fluid, increasing the osmolarity of the interstitial fluid. The input of energy allows for excessive salt pumping by the ascending limb. All together, this process creates a gradient within the medulla.