Kidneys
If your body is dehydrated and your blood osmolarity is too high, you'll want to reabsorb lots of water in the collecting duct. If you just drank a lot of water, though, you won't need to reabsorb any from the collecting duct.
What hormone do you think controls the amount of water you reabsorb from the collecting duct?
Choose your answer from the links at the right.