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A-Level Biology OCR A

Water Blood Concentration Adh Tubules Short Supply Excess

In dehydration, increased blood ion concentration stimulates ADH release, enhancing water reabsorption in the kidneys for concentrated, low-volume urine. With excess water, diluted blood ions inhibit ADH, leading to reduced water reabsorption and the excretion of dilute, high-volume urine.

When water is in short supply, blood ion concentration rises, osmoreceptors trigger ADH release, increasing water reabsorption in the collecting ducts/DCT, leading to a small volume of concentrated urine. Conversely, excess water dilutes blood ions, inhibiting ADH release, resulting in impermeable tubules and the production of a large volume of dilute urine.

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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