The Loop of Henle concentrates urine through a process called countercurrent multiplication. 1. Descending Limb: As filtrate flows down the descending limb, water leaves the tubule by osmosis because the surrounding interstitial fluid is hypertonic. The filtrate becomes more concentrated. 2. Ascending Limb: The ascending limb is impermeable to water but actively transports salts (like NaCl) out into the interstitial fluid. This makes the interstitial fluid in the medulla increasingly hypertonic. The filtrate inside the ascending limb becomes less concentrated. 3. Countercurrent Flow: The descending and ascending limbs flow in opposite directions, creating a countercurrent exchange mechanism. This continuous outflow of salt from the ascending limb maintains the high solute concentration in the interstitial fluid of the medulla. 4. Collecting Duct: As the concentrated filtrate from the Loop of Henle enters the collecting duct, it passes through the hypertonic medullary interstitial fluid. Under the influence of ADH (antidiuretic hormone), water moves out of the collecting duct by osmosis, further concentrating the urine before it is excreted.