Osmoregulation

• Blood Plasma WP decreases


  • Osmoreceptors detect blood WP (decrease: lose water and shrink) in hypothalamus
  • Stimulate neurosecretory cells (specialised neurones)
  • Transmit impulse down axon to terminal bulbs in posterior pituitary gland
  • Secretes more ADH into blood plasma
  • ADH
  • Act on collecting duct wall + DCT
  • Bind to receptors on basal + lateral membranes
  • Simulates enzyme sequence
  • More aquaporin insertion in luminal membranes
  • Collecting duct more water permeable
  • Blood plasma WP increases to normal level
  • Urine more concentrated + lower volume
  • Slowly broken: 20 min 1/2 life
• Blood Plasma WP increases

  • Blood plasma water increases excess fluid taken in body
  • Events reverse occur, detected by osmoreceptors in hypothalamus
  • Less/no stimulation of neurosecretory cells + ADH secretion
  • Less/no ADH binds to receptors on collecting duct membranes
  • Aquaporins taken back to cytoplasm by endocytosis at luminal plasma membrane
  • Collecting duct cells become less water permeable/impermeable
  • Less/no water re-absorbed to medulla
  • Then to blood plasma so WP returns to normal (urine volume increases and concentration decreases)
• Urea Re-absorption

  • As filtrate moves down collecting duct, more water is reabsorbed
  • Urea concentration in filtrate increases
  • Creates steep urea diffusion gradient
  • So some reabsorbed to medulla by diffusion
  • Urea re-absorption into medulla helps osmoregulation
  • As urea concentration increases
  • Medulla’s WP lowers
  • Helping water re-absorption from collecting duct
• Osmoregulation

  • Maintains constant osmotic pressure in organism’s fluids
  • Control water + salt concentrations with negative feedback
  • Blood plasma ψ changes detected, body corrects this, return it to normal
  • Organs:
  • Hypothalamus (in forebrain)
  • Detects blood WP changes
  • Produces ADH (antidiuretic hormone)
  • Pituitary gland
  • Stores and secretes ADH in kidney
  • Particularly collecting ducts respond to blood ADH concentration
  • Adrenal gland
  • Kidney
  • Neurosecretory cells
  • Make ADH in cell body
  • Transport down axons to terminal bulbs in posterior pituitary gland
  • Temporarily stored