Tubes with narrow diameter are osmotic pumps, and drive concentration of urine in the kidney

“Counter-current multiplication” is nonsensical because the solutes in the medulla would prevent water from leaving the kidney. The actual cause of the hyperosmolar, or, dehydrated medulla, is the osmotic effect in the “surface water” at the thin descending limb, that inherently removes water from the kidney. The water that is removed from the loop of Henle and from the kidney, is replaced with water from the collecting duct. The result is a net increase in solutes. Since this water enters the thin ascending limb (Horster and Thurau, 1968; Marsh and Solomon, 1965; Thurau and Henne, 1963), where the inner diameter increases continuously (Koepsell and Scnherman, 1972), the concentration of solutes will decrease, giving the appearance that they are pumped out at the thick ascending limb. It is understandable that the scientific community agreed on “counter current multiplication” given that they were not aware that osmosis is caused by the fourth phase of water.