How is water reabsorption carried out in the kidney?

When water levels are low in the blood, osmoreceptors in the hypothalamus detect this change and trigger the posterior pituitary gland to release a hormone called ADH. ADH binds to receptors on the cell membrane of the collecting ducts which then triggers the formation of cyclic AMP (a secondary messenger). The cyclic AMP causes vesicles containing aquaporins (protein based water channels) to bind to the cell surface membrane, increasing its permeability and allowing water to move out of the tubule cells by osmosis.