|With an increase in the synthesis, secretion, and blood concentration of vasopressin (anti-diuretic hormone, ADH), the number of water channels and water reabsorption increase. These decrease plasma osmolarity and urine volume, and also increase urine osmolarity.|
Prior to vasopressin (ADH) increasing (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) increases.
Step 2: The water channel-increasing effect of vasopressin (ADH) becomes stronger, which increases the number of water channels. This increases the water reabsorption from the tubule.
Step 3: With the increase in water reabsorption, diluted (hypotonic) solution enters the plasma. This decreases plasma osmolarity (diluted, hypotonic). Because diluted (hypotonic) solution leaves the tubule due to reabsorption, the fluid remaining in the tubule has an increased osmolarity (concentrated, hypertonic). Also, with the increase in water reabsorption, there is less water remaining in the tubule. Overall, the urine becomes concentrated and decreases in volume.