A student drinks a large volume of water on a hot day. Explain how the kidney responds to restore normal water balance in the body.

Eduqas A-Level Biology — 4.5 Homeostasis and the kidney · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

After drinking 1.5 litres of water quickly, the water concentration of the student's blood rises above the normal level.

Model answer (5 marks)

The hypothalamus senses the rise in blood water concentration and signals the posterior pituitary to reduce ADH secretion. With less ADH, the collecting ducts of the kidney become less permeable to water. Consequently, less water is reabsorbed from the tubules back into the blood and more remains in the filtrate. The result is a larger volume of dilute urine, which removes the excess water and restores normal blood water concentration.

Examiner tips

  • Mention the hypothalamus–pituitary axis first
  • Explain the effect of reduced ADH on duct permeability
  • Show the chain from less reabsorption to dilute urine
  • Use the term ‘dilute urine’ to hit the mark

Common mistakes

  • Confusing ADH release with ADH secretion
  • Saying the kidney ‘absorbs more water’ instead of ‘less’
  • Omitting the role of the hypothalamus or the pituitary

Mark scheme (5 marks)

  1. The hypothalamus detects the increase in blood water concentration
  2. Less ADH is released / secreted by the pituitary gland
  3. The kidney tubules become less permeable to water
  4. Less water is reabsorbed back into the blood / more water remains in the tubules
  5. A larger volume of dilute urine is produced, restoring normal blood water concentration

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