Explain how the body responds to a rise in blood osmolarity, such as that caused by excessive sweating during prolonged exercise.

IB DP Biology Standard Level (2023 syllabus) — D3.3 Homeostasis · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Osmoreceptors in the hypothalamus detect the rise in blood osmolarity.
The posterior pituitary gland releases ADH (antidiuretic hormone) into the blood.
ADH increases the permeability of the collecting ducts (and distal convoluted tubule) to water.
More water is reabsorbed by osmosis into the blood, producing a smaller volume of more concentrated urine, which restores blood osmolarity to normal (negative feedback).

Examiner tips

  • Use the exact terms: osmoreceptors, hypothalamus, posterior pituitary, ADH, collecting ducts, distal convoluted tubule, osmosis, negative feedback.
  • Show the sequence of events in order, linking each step to the next.

Common mistakes

  • Confusing ADH with vasopressin or using the wrong hormone name.
  • Omitting the role of the collecting ducts or distal convoluted tubule.
  • Failing to mention that the urine becomes more concentrated and the volume decreases.

Mark scheme (4 marks)

  1. Osmoreceptors in the hypothalamus detect the rise in blood osmolarity
  2. The posterior pituitary gland releases ADH (antidiuretic hormone) into the blood
  3. ADH increases the permeability of the collecting ducts (and distal convoluted tubule) to water
  4. More water is reabsorbed by osmosis into the blood, producing a smaller volume of more concentrated urine, which restores blood osmolarity to normal (negative feedback)

Key terms in this question

osmolarity

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