Osmoregulation

• Organs involved

  A) Pituitary Gland

  B) Hypothalamus

  C) Adrenal gland

  D) Kidney

  
• Diagram brain

  A) Supraoptic Nucleus

  B) Oxytocin

  C) Neurosecretory

  D) Paraventricular Nucleus

  E) ADH

  F) Hypothalamus

  G) Artery

  H) Axon terminals

  I) hormones

  J) Capillary

  K) Posterior pituitary

  L) vein

  M) Anterior pituitary

  
• 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
  • Neurosecretory cells
  • Make ADH in cell body
  • Transport down axons to terminal bulbs in posterior pituitary gland
  • Temporarily stored
• 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