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A-Level Biology OCR A

Water Blood Concentration Adh Tubules Short Supply Excess

Front Compare the conditions, and events that occur, when water is in short supply and when there is an excess of water.
Back
Short supply
  • Concentration of inorganic ions rises, water potential of blood and tissue fluid is more -ve
  • This detected by osmoreceptors in hypothalamus
  • Nerve impulses sent to posterior pituitary, releasing ADH
  • ADH binds to receptors in cells of collecting duct/DCT, increasing permeability of tubules to water
  • Water leaves filtrate in tubules & passes into blood in capillary network
  • Small volume of concentrated urine produced
Excess of water
  • Concentration of inorganic ions drops, water potential of blood and tissue fluid is less -ve
  • This detected by osmoreceptors in hypothalamus
  • Nerve impulses sent to posterior pituitary, inhibiting ADH release
  • Walls of collecting duct and DCT remain impermeable to water, concentration of blood is maintained
  • Water does not leave filtrate in tubules & does not pass into blood in capillary network
  • Large volume of diltute urine produced

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