How does antidiuretic hormone (ADH) influence urine concentration?

• A. It decreases water reabsorption in collecting ducts
• B. It promotes water reabsorption in collecting ducts, increasing urine osmolality and reducing volume
• C. It increases secretion of water into filtrate
• D. It has no effect on urine concentration

Answer: (B)

Antidiuretic hormone controls how permeable the kidney’s collecting ducts are to water, directly shaping urine concentration. When ADH is present, it binds to receptors on collecting-duct cells and causes the insertion of aquaporin-2 water channels into the apical membrane. This dramatically increases water permeability, so more water is reabsorbed from the filtrate back into the bloodstream along the medullary gradient. The result is urine that is more concentrated (higher osmolality) and in a smaller volume. If ADH is low or absent, the collecting ducts stay relatively water-impermeable, producing larger amounts of dilute urine. That’s why the best description is that ADH promotes water reabsorption in collecting ducts, increasing urine osmolality and reducing volume.