Describe the role of the Loop of Henle

The Loop of Henle is an important part of the kidney nephron in forming concentrated urine. It is a long loop, which forms something known as a hairpin countercurrent mechanism. It is called countercurrent because the direction of flow in the descending loop is in the opposite direction to the flow in the ascending loop. It concentrates urine in a number of steps.

1. The ascending loop is impermeable to water, but actively pumps ions out of the tubule fluid

2. This creates a hyperosmolar state in the surrounding renal medulla

3. The descending limb is permeable to water, and because the medulla is hyperosmolar because of the active pumping from the ascending limb, it moves out passively by osmosis and can be reabsorbed