Mechanism of ADH

Question 1

How does ADH trigger a signal?

Answer 1

• does not cross the membrane of tubule cells
  
  • binds to receptors on the cell membrane

Question 2

What does ADH trigger?

Answer 2

• the formation of cyclic AMP (cAMP)
  
  • second messenger
• second messenger is a molecule which relays signals received at cell sufrace receptors to molecules inside the cell

Question 3

What does cAMP do?

Answer 3

• vesicles in the cells lining the collecting duct fuse with the cell surface membrane on the side of the cell in contract with the tissue fluid of the medulla
• the membranes of these vesicles contain protein-based water channels (aquaporins)
  
  • 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

Question 4

How is the system a negative feedback control system?

Answer 4

• permeability is match by the water requirement
• osmoreceptors in the hypothalmus are sensistive to the conc of inorganic ion in the blood and are linked with the release of ADH
• therefore the changes brough about by the ADH trigger the release of ADH from not occuring

Question 5

What happens when water is in short supply?

Answer 5

• conc of inorganic ions in the blood rises and the water potential of the blood and tissue fluid becomes more negative
• this is detected by the osmosreceptors in the hypothalmus
• they send nerve impulse to the posterior pituitary which in turn release ADH in the blood
• ADH is picked up by receptors in the cells of the collecting duct and increase the permeability of the tubules to water
• water leaves the filtrate and passes into blood in the surrounding capillary network

Question 6

What happens when there is an excess of water?

Answer 6

• blood becomes more dilute
  
  • water potential becomes less negative
• change is detected by the osmoreceptors of the hypothalmus
• nerve impulses to the poserior pituitary is inhibited
• very little reaborbtion of water can take place because the walls of the collecting duct remain impermeable to water
• conc of blood is maintained
• large amount of dilute urine are produced

Question 7

What else prevents the release of ADH?

Answer 7

• stimulated or inhibited by changes in blood pressure
  
  • detected by baroreceptors in the aortic and carotoid arteries
  • these baroreceptors are also involved in the heart rate
• increase in pressure can be caused by a rise in blood volume
  
  • increase in pressure prevent the release of ADH
  • increases vol of water lost in urine, reducing the blood volume so pressure falls

Question 8

What happens when you are dehydrated?

Answer 8

• water content of blood drops so its water potential drops
• detected by osmoreceptors in the hypothalmus
• posterior pituitary gland stimulated to release more ADH into the blood
• more ADH means that DCT and collecting duct are more permeable
  
  • so more water is reabsorbed into the blood by osmossi
• a small amount of high conc urine is produced
  
  • less water lost