Aquaporin-2 is a protein that functions as a water channel in the kidneys, specifically in the collecting ducts. It plays a crucial role in regulating water balance in the body by facilitating the reabsorption of water from urine back into the bloodstream. This process is essential for maintaining proper hydration and electrolyte levels. The activity of Aquaporin-2 is influenced by the hormone vasopressin, also known as antidiuretic hormone (ADH). When vasopressin is released, it binds to receptors on kidney cells, prompting the insertion of Aquaporin-2 into the cell membranes. This action increases water permeability, allowing more water to be reabsorbed, which helps concentrate urine and reduce water loss.