Explain how the urinary system and the cardiovascular system are integrated to maintain water balance in the body.

IB DP Biology Standard Level (2023 syllabus) — C3.1 Integration of body systems · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

The kidneys filter blood that arrives via the renal arteries, a branch of the cardiovascular system.
Osmoreceptors in the hypothalamus sense an increase in blood osmolarity and signal the posterior pituitary to release ADH.
ADH travels through the bloodstream to the kidneys, where it binds to receptors on the collecting ducts.
This action increases the ducts’ permeability to water, so more water is re‑absorbed back into the blood, lowering urine output and restoring water balance.

Examiner tips

  • Show the flow from cardiovascular to urinary system, then back via ADH; use the term ‘collecting ducts’ and ‘permeability’
  • Include the role of osmoreceptors and the posterior pituitary to hit all four points

Common mistakes

  • Forgetting to mention the renal arteries or the collecting ducts
  • Mixing up ADH with vasopressin or omitting the role of osmoreceptors

Mark scheme (4 marks)

  1. The kidneys (as part of the urinary system) filter blood delivered by the cardiovascular system, receiving blood via the renal arteries.
  2. Specialised receptors (osmoreceptors) in the hypothalamus detect changes in blood osmolarity / water potential of the blood.
  3. ADH (antidiuretic hormone) is released from the posterior pituitary and travels via the bloodstream (cardiovascular system) to the kidneys.
  4. ADH increases the permeability of the collecting ducts to water, so more water is reabsorbed into the blood, reducing urine output and restoring blood osmolarity / water balance.

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