i'm going to show how the kidney responds to dehydration. I'm not sure if its right so please correct me of any errors.
Ok suppose u eat something salty like patato chips. The Na+ gets absorbed in the bloodstream causing the normal water concentration to go down. The lowering of the water concentration signals ADH which causes reabsorption of water from collecting tubule to the bloodstream. This reabsorpation then causes the water concentration the the bloodstream to go back up again.