Blood plasma water potential decreases

• If blood plasma ψ decreases, it’s detected by osmoreceptors (they lose water and shrink) in hypothalamus (also initiates feelings of thirst), which stimulates neurosecretory cells
• Neurosecretory cells, specialised neurones, produce ADH in their cell body and transport it down axons to terminal bulbs in posterior pituitary gland, where it is temporarily stored in terminal bulbs
• When the neurosecretory cells in hypothalamus are stimulated, they transmit impulses down axons to their terminal bulbs in the posterior pituitary gland, which then secretes more ADH into the blood plasma
• ADH acts on collecting duct wall cells and DCT; binds to ADH receptors on basal and lateral plasma membranes of target cells, it simulates a sequence of enzyme (e.g. phosphorylase enzyme) controlled reactions
• Blood plasma water potential then increases, returning to a normal level, urine becomes more concentrated and lower in volume; ADH is slowly broken down (20 min half life) to reduce stimulation of collecting ducts
• Flow Chart

  A) concentrated

  B) water potential

  C) osmoreceptors

  D) hypothalamus

  E) water potential

  F) Neurosecretory

  G) hypothalamus

  H) ADH

  I) Posterior pituitary

  J) collecting duct

  K) collecting duct

  L) permeable

  M) osmosis

  N) medulla

  O) collecting ducts

  P) tissue fluid

  Q) Medulla

  R) decrease

  S) thirst

  T) Hypothalamus

  U) Hypothalamus