The loop of Henle is a hairpin-shaped section of the nephron that dips from the cortex down into the medulla and back up again. Its job is to create a region of high solute concentration (low water potential) in the tissue fluid of the medulla, so that water can later be reabsorbed from the collecting duct and concentrated urine can be produced.
The two limbs have different properties.The descending limb is narrow, thin-walled and permeable to water, but relatively impermeable to sodium and chloride ions. As the filtrate flows down, water leaves by osmosis into the medulla, which has a low water potential. The filtrate becomes more concentrated, reaching its lowest water potential at the tip of the loop.The ascending limb is impermeable to water. In its thin lower part, Na+ and Cl− ions diffuse out. In the thick upper part, the cells actively transport Na+ and Cl− out of the filtrate into the medulla, using ATP from their many mitochondria. Because water cannot follow, the filtrate becomes more dilute as it rises.The ions removed from the ascending limb lower the water potential of the medulla, which draws more water out of the descending limb. The water that leaves is carried away by nearby blood capillaries (the vasa recta), so it does not dilute the medulla.Results:• the filtrate leaving the loop and entering the distal convoluted tubule is dilute, with a higher water potential than blood
• the medulla has a gradient of water potential, highest near the cortex and lowest (most concentrated) deep in the medulla, near the tips of the loops
The two limbs have different properties.The descending limb is narrow, thin-walled and permeable to water, but relatively impermeable to sodium and chloride ions. As the filtrate flows down, water leaves by osmosis into the medulla, which has a low water potential. The filtrate becomes more concentrated, reaching its lowest water potential at the tip of the loop.The ascending limb is impermeable to water. In its thin lower part, Na+ and Cl− ions diffuse out. In the thick upper part, the cells actively transport Na+ and Cl− out of the filtrate into the medulla, using ATP from their many mitochondria. Because water cannot follow, the filtrate becomes more dilute as it rises.The ions removed from the ascending limb lower the water potential of the medulla, which draws more water out of the descending limb. The water that leaves is carried away by nearby blood capillaries (the vasa recta), so it does not dilute the medulla.Results:• the filtrate leaving the loop and entering the distal convoluted tubule is dilute, with a higher water potential than blood• the medulla has a gradient of water potential, highest near the cortex and lowest (most concentrated) deep in the medulla, near the tips of the loops