How is water reabsorbed in the loop of Henle?

To understand how water is reabsorbed in the loop of Henle, it helps to first consider what happens in the ascending limb. The ascending limb is permeable to ions but impermeable to water. Ions are pumped out of the limb into the peritubular space. Because this transport of ions occurs all the way up the ascending limb, the osmolarity of the urine decreases as it ascends. The ions from the ascending limb create an area of high concentration in the around the LoH, which increases deeper into the medulla (there is a concentration gradient down the LoH into the medulla) The descending limb is impermeable to ions but permeable to water. Water is drawn out of the tubule by osmosis due to the high concentration of ions in the surrounding tissue. So, the osmolarity of the urine increases as it descends the LoH. The concentration gradient created by the loop of Henle is maintained by the vasa recta producing a counter-current exchange mechanism. Blood in these capillaries flows in the opposite direction to urine in the LoH so doesn’t ‘wash away’ the osmotic gradient in the renal medulla.