|With a decrease in the water reabsorption from the tubule, plasma osmolarity and urine volume increase, and urine osmolarity decreases.|
Prior to the water reabsorption from the tubule decreasing (at baseline), we will make the assumption that the subject is in homeostasis with normal plasma and urine osmolarities (isotonic) and urine volume.
Step 1: Suppose that the water reabsorption from the tubule decreases.
Step 2: With the decrease in water reabsorption, concentrated (hypertonic) solution enters the plasma. This increases plasma osmolarity (concentrated, hypertonic). Because concentrated (hypertonic) solution leaves the tubule due to reabsorption, the fluid remaining in the tubule has an decreased osmolarity (diluted, hypotonic). Also, with the decrease in water reabsorption, there is more water remaining in the tubule. Overall, the urine becomes diluted and increases in volume.