Describe the role of the loops of Henle in the process of concentrating urine.

The loop of Henle is made up first of a descending limb and then an ascending limb. The descending limb is permeable to both water and sodium and chloride ions, whereas the ascending limb is impermeable to water. Sodium and chloride ions are actively transported out of the filtrate in the ascending limb, into the interstitial space, lowering the water potential of the tissue fluid. Water leaves the filtrate in the descending limb via osmosis as the water potential in the tissue fluid is lower than in the descending limb. At the same time, sodium and chloride ions in the tissue fluid diffuse into the descending limb, causing the fluid to become progressively concentrated, therefore the filtrate is most concentrated at the bottom of the hairpin. With the two limbs running in opposite directions next to one another, the maximum concentration of filtrate can be achieved, this mechanism is called the counter-current multiplier.