15.6 the kidney and osmoregulation

what does ADH do

increases the permeability of the distal convoluted tubule and collecting duct to water

where is ADH released from

the pituitary gland

How does ADH cause an affect

• it binds to receptors on the cell membrane and triggers the formation of cyclic AMP as a secondary messenger inside of the cell

• the second messenger is a molecule which relays signals received at cell surface receptors to molecules inside the cell

the events caused by cAMP which is relased from the actions of ADH

• vesicles in the cell lining the collecting duct fuse with the cell surface membrane on the side of the cell in contact with the tissue fluid of the medulla

• the membranes of these vesicles contain protein-based water channels and when they are inserted into the cell surface membrane, they make it permeable to water

• this provides a route for water to move out of the tubule cells into the tissue fluid of the medulla and the blood capillaries by osmosis

what happens when ADH levels fall

• levels of cAMP fall

• the water channels are removed from the tubule cell membranes are enclosed in vesicles again

• the collecting duct becomes impermeable to water once more, so no water can leave

• this results in the production of large amounts of very dilute urine, and maintains the water potential of the blood and the tissue fluid

where are osmosreceptors

in the hypothalamus

what happens when water is in short supply

• concentration of inorganic ions in blood increases

• water potential of the blood becomes more negative

• this is detected by the osmoreceptors in the hypothalamus

• they send nerve impulses to the posterior pituitary which in turn releases stored ADH into the blood

• the ADH is detected by receptors in the cells of the collecting duct and increases the permeability of the tubules to water

• water leaves the filtrate in the tubules and passes into the blood in the surrounding capillary network

• a small volume of concentrated urine is produced

what happens when there is an excess of water

• blood becomes more dilute and its water potential becomes less negative

• the change is detected by the osmoreceptors in the hypothalamus

• nerve impulses to the posterior pituitary are reduced or stopped and so the release of ADH by the pituitary is inhibited

• very little reabsorption of water can take place because the walls of the collecting duct remain impermeable to water

• large amounts of dilute urine are produced