The main function of the loop of Henle is to decrease the water potential in the medulla via the movement of ions out of filtrate into surrounding tissues of the medulla.

This establishes a water potential gradient that allows water to be reabsorbed into the blood from filtrate in the collecting duct.

The descending limb:

1. Filtrate travels through here first
2. It is narrow
3. It has thin walls that are highly permeable to water but impermeable to ions

The ascending limb:

1 Filtrate travels through here after it passes through the descending limb

2 It is wider than the descending limb

3 It has thick walls that are impermeable to water but permeable to ions

1. As filtrate moves down the collecting duct, it loses water, decreasing its water potential.
2. However, due to the pumping of ions out of the ascending limb of the loop of Henle, especially deeper in the medulla, the water potential of the surrounding tissues in the medulla is even lower than in the collecting duct.
3. This allows water to continue to move out of filtrate down the whole length of the collecting duct.

The walls of the descending limb are permeable to water, so it passes out of the filtrate by osmosis into the tissues of the medulla that surround the loop of Henle.

The filtrate progressively loses water as it moves deeper into the medulla, lowering its water potential.

1. As filtrate moves up the ascending limb, sodium and chloride ions are actively pumped out of the filtrate into the surrounding tissues of the medulla.
2. This creates a low water potential in the medulla due to the high concentration of ions.
3. As the ascending limb is impermeable to water, no water leaves the filtrate.
4. Towards the top of the ascending limb, the water potential of the filtrate progressively increases as more ions are actively transported out.

There is a gradient of water potential with the highest water potential in the cortex and an increasingly lower water potential deeper into the medulla.

The countercurrent multiplier ensures that there is always a water potential gradient drawing water out of the collecting duct.

As the filtrate moves through the collecting duct, water passes out of it by osmosis into the surrounding blood vessels and is carried away in the bloodstream.

At the end of the collecting duct, the filtrate, which is now called urine, has lost most of its water and leaves the collecting duct.

ATP provides the energy for active transport of sodium ions out of the filtrate in the ascending limb of the loop of Henle.

This ATP is synthesised by the many mitochondria in the cells of the wall of the ascending limb.

This water enters the blood capillaries in this region by osmosis and is carried away.