1. Water regulation in the kidney

2. The loop of Henle
The descending limb of the loop of Henle is permeable to water
The ascending limb is impermeable.
Na+ and Cl- ions are actively transported out of the ascending limb and passively move into the descending limb.
This creates a concentration gradient across the medulla.
There is a lower ion concentration gradient at the top of the ascending limb and a much higher ion concentration gradient at the bottom.

4. The Role of ADH
The water potential of the tissue fluid in the medulla is always more negative than that of the filtrate in the collecting duct.
Whether the water actually leaves the collecting duct (by osmosis) is determined by the hormone ADH (anti-diuretic hormone)
Osmoreceptors in the hypothalamus detect the low levels of water,so the hypothalamus sends an impulse to the pituitary gland which releases ADH into the bloodstream.
ADH makes the wall of the collecting duct more permeable to water.
Therefore, when ADH is present more water is reabsorbed and less is excreted.