The Loop of Henle
serves to create a concentration gradient throughout the nephron which helps increase the reabsorption of water and certain ions. In the thin descending limb, water is very permeable and is reabsorbed due to the existing concentration gradient in the medulla. Sodium, chloride, and potassium are reabsorbed readily in the thick ascending limb while being completely impermeable to water. This creates a gradient to draw more water back into circulation from the thin descending limb. Approximately 25 percent of filtered sodium will be absorbed here.