osmoregulation and ADH

the reabsorption of water is called osmoregulation - this is coordinated by the hypothalamus in the brain

the hypothalamus synthesises the hormone ADH, it is then transported to the posterior pituitary gland where it is stored

the hypothalamus contains cells called osmoreceptors which are sensitive to the water potential of the blood

• if a person is dehydrated water leaves the osmoreceptor cells causing them to shrink

• when the osmoreceptor cells shrink they trigger the posterior pituitary gland to release the hormone ADH into the bloodstream

• the walls of the collecting ducts have receptors specific to ADH

• these cells also have vesicles containing protein channels for water - these water channels are called aquaporins

• ADH molecules attach to the cell surface receptors which triggers and enzyme called adenylate cyclase to produce the molecule cyclic AMP (cAMP) - molecules like cAMP are called second messengers

• cAMP triggers the vesicles to move to and fuse with the cell surface membrane

• aquaporins insert into the membrane allowing water to move into the cell and enter the blood stream, increasing the water potential of the blood and increasing the concentration of urine

the hypothalamus and osmoreceptors also trigger a feeling of thirst

• when osmoreceptors detect a rise in water potential they trigger the posterior pituitary gland to release less ADH