Figure 51.10 Concentrating the Urine A countercurrent multiplier mechanism enables the mammalian kidney to produce urine that is far more concentrated than the blood plasma. The composition—but not the concentration—of the filtrate is changed by the proximal convoluted tubule, which reabsorbs valuable molecules (including NaCl). Bulk reabsorption of water follows osmotically. The urine concentration process begins in the thick ascending limb of the loop of Henle, which reabsorbs NaCl but is impermeable to H₂O. This reabsorbed NaCl creates an osmotic differential that pulls water out of the thin descending limb; NaCl also can diffuse into the thin descending limb, making the fluid in that limb more concentrated. Through this continuous process, a concentration gradient in the interstitial fluid of the renal medulla is created as indicated by the gradient of color in the medulla. As urine in the collecting duct passes through this concentration gradient, it can lose water osmotically, becomes more concentrated as indicated by the shading of color, and can become almost as concentrated as the interstitial fluid at the bottom of the renal medulla.