osmoregulation

• Homeostatic control of osmoregulation (water potential) in the blood is controlled by the hormone anti-diuretic hormone. 
• ADH acts on the distal convoluted tubule and the collecting duct of the nephron in the kidney. 
• Collecting duct:

  • As fluid passes down the collecting duct and leaves the kidney it forms urine 
  • If water potential of blood is too high then more water is lost and less is reabsorbed from the collecting duct 
• Dehydration:

  • Water potential of blood is low 
  • osmoreceptor cells in hypothalamus shrink and lose water 
  • Hypothalamus produces ADH and stimulates neurosecretory cells 
  • This stimulates posterior pituitary gland which increases ADH secretion 
  • ADH is transported into blood 
  • ADH arrives at the collecting duct in the kidney 
• ADH regulates the permeability of the DCT and the collecting ducts. 

  • ADH binds to receptors on the membrane of target tissue
  • Activating G protein that activates phosphorylase which catalyses the synthesis of cAMP 
  • cAMP causes the synthesis of aquaporins 
  • Aquaporins are inserted into the membrane by exocytosis 
  • Cell surface membrane is more permeable to water 
  • More water leaves collecting duct by osmosis to re-enter blood 
  • Prevents water potential of blood decreasing further 
  • Osmoreceptors in hypothalamus send nerve impulses to thirst centre and the person drinks more 
• Hypothalamus:

  • Releases ADH 
  • Send impulses to thirst centre 
  • Contains osmoreceptor cells 
• Posterior pituitary gland:

  • Increases secretion of ADH into blood 
• ADH:

  • Increases permeability of cell membrane to water
  • Insertion of aquaporins into cell membrane by exocytosis 
  • Production of cAMP 
  • Increases water potential of blood as water moves from collecting duct to blood 
  • Binds to receptors on collecting duct membrane