What is the role of the kidneys in osmoregulation?

Osmoregulation is the homeostatic control of the water potential of the blood. The kidneys are involved in filtering the blood and deciding which substances to reabsorb and which to excrete as waste. Tiny tubular structures known as tubules carry out this filtration.
There are five main parts to the kidney tubules: the Bowman's capsule, the proximal convoluted tubule, the loop of Henle, the distal convoluted tubule and the collecting duct. The Bowman's capsule is used for ultrafiltration as the fluid is forced out of the blood and into the tubule. Then, the proximal convoluted tubule is used for the selective reabsorption of all glucose and some salts and water. The loop of Henle then maintains a sodium gradient, before the distal convoluted tubule makes final adjustments to the amount of water and salts that are reabsorbed. The collecting duct is the final piece of the puzzle that allows water to move out by osmosis to decrease the water content of urine.