|With a decrease in the synthesis, secretion, and blood concentration of vasopressin (anti-diuretic hormone, ADH), the number of water channels and water reabsorption decrease. These increase plasma osmolarity and urine volume, and also decrease urine osmolarity.|
narrated video explanation
Prior to vasopressin (ADH) 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 synthesis, secretion, and blood concentration of vasopressin (ADH) decreases.
Step 2: The water channel-increasing effect of vasopressin (ADH) becomes weaker, which decreases the number of water channels. This decreases the water reabsorption from the tubule.
Step 3: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 water reabsorption, the fluid remaining in the tubule has 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.