The kidneys help to keep the water content of the blood steady. They do this by changing how much water is reabsorbed from the urine as it passes along the collecting duct. The control comes from a hormone called ADH (anti-diuretic hormone), which is released into the blood by the pituitary gland.
The brain monitors the water content of the blood. If you are dehydrated, the blood is too concentrated, and the brain detects this. The pituitary gland releases more ADH. ADH makes the walls of the collecting duct more permeable to water. More water is reabsorbed by osmosis back into the blood, so the urine is a small volume and concentrated (dark).
If you have drunk a lot of water, the blood becomes too dilute. The brain detects this and less ADH is released. The collecting duct becomes less permeable, less water is reabsorbed, and a large volume of dilute (pale) urine is produced.
This is an example of negative feedback. Any change in the water content is detected and a response reverses the change, bringing the water content back to normal.