15.6 - Osmoregulation & the Kidney

Question 1

Define osmoregulation

Answer 1

Osmoregulation is the maintenance of water potential in the blood (it can be altered and needs to be returned to a ‘normal’ level

Question 2

What is the ADH action mechanism?

Answer 2

• Secreted from the pituitary gland and carried in the bloodstream to the collecting duct
• ADH made in the hypothalamus
• Hormone does not cross the tubule cell membranes
• It binds to receptors of the cell membrane and triggers the formation of cyclic AMP (cAMP) – it is a secondary messenger (molecule that relays received signal to molecules inside the cell) inside the cell
• cAMP causes
  
  • The vesicles in the cell lining of the collecting duct fuse with the plasma membranes of the cells in contact with tissue fluid in the medulla
  • Vesicle membranes have aquaporins (protein channels for water), they make membranes permeable to water
  • Provide a route for water to move out of tubule cells and into the tissue fluid of the medulla by osmosis
• More ADH = more aquaporins
  
  • Easier for water to leave the tubules by osmosis, makes small amount of conc. urine
• As ADH falls, cAMP falls, aquaporins removed, enclosed vesicles form again
  
  • Forms dilute urine

Question 3

How does the body respond to water in short supply?

Answer 3

• Water potential is more negative
• Detected by osmoreceptors in hypothalamus
  
  • Water moves out of the osmoreceptros causing them to shrink
• Nerve impulses are sent to the posterior pituitary gland to release ADH

Question 4

How does body respond to the water levels in the body being too high?

Answer 4

Blood is more dilute , less negative water potential

• Increase in blood water potential, causes blood to pass into osmoreceptors, they do not shrink
• This does not stimulate neurosecretory cells from producing ADH
  
  • It inhibits ADH secretion
• ADH mechanism is reversed