Final answer:
ADH increases the reabsorption of water by stimulating the insertion of aquaporin channels into the cells of the collecting duct, leading to more concentrated urine and regulating blood osmolarity.
Step-by-step explanation:
Vasopressin, also known as antidiuretic hormone (ADH), quickly increases the reabsorption of water by the collecting duct in the kidney. This occurs because ADH stimulates the insertion of aquaporin channels into the apical membrane of the principal cells lining the collecting ducts. When these aquaporin channels are present, water is efficiently reabsorbed from the collecting duct into the surrounding interstitial space and then into the peritubular capillaries. This process is crucial for regulating the body's water balance, ensuring that the final urine is more concentrated, and maintaining stable blood osmolarity and blood pressure.