The Loop of Henle is a U-shaped tube within the nephron of the kidney that plays a critical role in concentrating urine. It has two main parts: the descending limb and the ascending limb. Function of the Descending Limb: This part is permeable to water* but not very permeable to solutes (like salts). As the filtrate flows down into the medulla (the inner part of the kidney), the surrounding interstitial fluid becomes increasingly salty. Because of this saltiness, water is drawn out of the descending limb by osmosis* and reabsorbed into the blood. This process makes the filtrate inside the loop more concentrated. Function of the Ascending Limb: This part is impermeable to water* but actively transports solutes (salts) out of the filtrate. As the filtrate flows up, salts are pumped out into the surrounding interstitial fluid. This makes the interstitial fluid in the medulla very salty, creating an osmotic gradient*. It also makes the filtrate inside the ascending limb less concentrated (more dilute). Overall Contribution: The Loop of Henle's ability to create and maintain this salty environment in the medulla is essential. It allows the kidney to reabsorb a large amount of water from the filtrate in the collecting ducts, producing concentrated urine and conserving water for the body.