osmoregulation

• osmoregulation


  control of water potential in the body.

  • involves controlling levels of both water and salt in the body to prevent water entering cells and causing lysis or leaving cells and causing crenation
• body gains water from:

  • food
  • drink
  • respiration
• water lost from body by:

  • urine
  • sweat
  • water vapour in exhaled air
  • faeces
• • on a cold day, when you have drunk a lot of fluid, the kidneys will produce a larger volume of dilute urine.
  • on a hot day, when you have drunk very little, kidneys will produce smaller volumes of more concentrated urine
• mechanism of osmoregulation 1


  • kidneys alter volume of urine produced by altering the permeability of collecting ducts. Walls of collecting ducts can be made more or less permeable according to needs of the body
• mechanism of osmoregulation 2


  • if you need to conserve less water (on a cool day when you have drunk a lot of fluid), walls of collecting ducts become less permeable. This means less water is reabsorbed and greater volume of urine produced
• mechanism of osmoregulation 3


  • If you need to conserve more water (on a hot day or when you have drunk very little), collecting duct walls are made more permeable so more water can be reabsorbed into blood. Smaller volume of urine produced
• altering permeability of collecting duct 1

  • cells in walls of collecting duct respond to level of ADH in blood
  • these cells have receptors for ADH
  • ADH binds to receptors and causes chain of enzyme controlled reactions inside the cell
  • end result of these reactions is to cause vesicles containing water-permeable channels (aquaporins) to fuse with cell surface membrane. This makes walls more permeable to water
• Altering permeability of collecting duct 2


  • when level of ADH in blood rises, more water-permeable channels are inserted
  • This allows more water to be reabsorbed by osmosis, into the blood. Less urine produced and urine has lower water potential
• Altering permeability of collecting duct 3 


  • If level of ADH in blood falls, cell surface membrane folds inwards to create new vesicles that remove water-permeable channels from membrane
  • This makes walls less permeable and less water is reabsorbed by osmosis into blood
  • More water passes down collecting duct to form greater volume of urine which is more dilute (higher water potential)
• Adjusting concentration of ADH in blood 1


  • Hypothalamus in brain contains specialised cells called osmoreceptors
  • these are sensory receptors that detect the stimulus - monitor water potential of blood.
  • these cells respond to effects of osmosis.
  • when water potential of blood is low, osmoreceptor cells lose water by osmosis and shrink. as a result they stimulate neurosecretory cells in hypothalamus
• Adjusting concentration of ADH in blood 2


  • neurosecretory cells are specialised neurones that produce and release ADH
  • ADH is manufactured in cell body, which lies in the hypothalamus.
  • ADH moves down axon to terminal bulb in posterior pituitary gland where its stored in vesicles
  • when neurosecretory cells are stimulated by osmoreceptors, they carry action potentials down their axons and cause release of ADH by exocytosis.
• Adjusting concentration of ADH in blood 


  • ADH enters blood capillaries running through posterior pituitary gland.
  • It's transported around the body and acts on cells of collecting ducts (target cells)
  • once water potential of blood rises again, less ADH is released.
  • ADH is slowly broken down.
  • ADH present in blood is broken down and collecting ducts will receive less stimulation
• alcohol


  alcohol inhibits release of ADH. so drinking alcohol reduces reabsorption from collecting ducts and makes you need to go toilet
• ADH


  water reabsorption in collecting duct