Which hormone signals the kidney to conserve water when you are dehydrated?

• A. Insulin
• B. Antidiuretic hormone
• C. Melatonin
• D. Adrenaline

Answer: (B)

When you’re dehydrated, the body needs to hold onto water, and that goal is driven by antidiuretic hormone. Osmoreceptors in the brain detect higher blood osmolality and signal the posterior pituitary to release this hormone. In the kidneys, antidiuretic hormone binds to receptors on collecting duct cells and triggers a cascade that increases the number of aquaporin-2 water channels on their membranes. With more channels, water is reabsorbed from the filtrate back into the bloodstream, producing a smaller amount of concentrated urine and helping conserve body water. If you’re hydrated, ADH levels drop and the kidneys excrete more water. Other hormones listed don’t regulate this water-reabsorbing process in the kidney.