What is the role of the kidney in water homeostasis?

The kidney has several functions, but in this short paragraph I am going to explain about it's principal function- controlling water levels in the body. Imagine you are in a desert and you have not had a drink for 12 hours- you will be very dehydrated. Your blood contains lots of different things, and the osmolarity of your blood depends on the concentrations of salt and water in it. When you are very dehydrated, your blood will have low levels of water in it because you have used up your limited water supply e.g. sweating. This means the concentrations of salts/ions (like sodium and potassium) increase as the levels of water in the blood reduce. Therefore, your blood has an increased osmolarity.  This is detected by osmoreceptors, which activates the water homeostasis pathway. The activated osmoreceptors then activate the hypothalamus in the brain, which then causes the pituitary gland (also in the brain) to release a hormone called Anti-Diuretic Hormone (ADH). This hormone causes increased water reabsorption in the collecting duct of the nephron, meaning the water which has been filtered out of the blood into the nephron can be taken back up into the blood stream rather than being excreted as urine. So, a small amount of concentrated urine is produced when you are dehydrated in an effort to keep water in your blood stream due to the release of ADH. Conversely, alcohol inhibits ADH release which is why when someone has had lots of alcohol, they produce larger amounts of clear urine and dehydration is experienced in a hangover the following day.