How is a concentrated filtrate produced in the kidney to remove harmful toxins from the body?

To begin with, sodium ions are actively (using ATP) pumped out of the ascending limb of the loop of henle, however, as the ascending limb is impermeable, water remains . This creates a lower water potential in the medulla. This lower water potential causes water to diffuse out of the descending limb of the loop of henle via osmosis. Water is then reabsorbed into the surrounding capillary network, making the filtrate in the loop of henle highly concentrated. At the bottom of the ascending limb, sodium ions diffuse out into the medulla, lowering the water potential further. Water moves out of the distal convoluted tubule and out of the collecting duct via osmosis. This water is then reabsorbed into the surrounding capillary network, leaving a highly concentrated filtrate in the collecting duct.