The loops of Henle are U-shaped tubes that extend from the proximal convoluted tubule to the distal convoluted tubule in the nephron of the kidney. Their main function is to create a concentration gradient in the medulla of the kidney. This gradient is crucial for the kidney's ability to reabsorb water and concentrate urine. The descending limb of the loop of Henle is permeable to water but not to salts, allowing water to move out and concentrate the filtrate. The ascending limb is impermeable to water but actively transports salts out, making the surrounding medulla more concentrated and the filtrate less concentrated.