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A2 · Practice questions · Osmoregulation and ADH

Open the water gates.

Six original Cambridge-style questions on osmoregulation, from naming the parts of the control loop to explaining how ADH changes the volume and concentration of urine.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Detect, release, insert.

01
Recall
[2 marks]

State what is meant by osmoregulation and name the receptors involved in mammals.

  • Osmoregulation is the control of the water potential of the blood / body fluids (keeping it within narrow limits). ✓
  • Osmoreceptors in the hypothalamus. ✓
02
Explain
[5 marks]

A runner loses a lot of water as sweat. Explain how the volume of urine produced is reduced.

  • The water potential of the blood falls, which is detected by osmoreceptors in the hypothalamus (they lose water and shrink). ✓
  • Neurosecretory cells release more ADH from the posterior pituitary gland into the blood. ✓
  • ADH binds to receptors on collecting duct cells; vesicles containing aquaporins fuse with the luminal membrane. ✓
  • The collecting duct becomes more permeable to water. ✓
  • More water moves by osmosis into the concentrated medulla / blood, so a small volume of concentrated urine is made. ✓
03
Explain
[3 marks]

Explain why the effect of ADH on the kidney is described as an example of negative feedback.

  • A fall in water potential leads to more ADH and more water reabsorption, which raises the water potential. ✓
  • As the water potential returns towards normal, osmoreceptors are stimulated less and less ADH is released. ✓
  • The response therefore reverses the original change and then switches itself off. ✓
04
Analysis
[3 marks]

A person drinks 1 dm3 of water. Their urine flow rises from 1 cm3 min−1 to a peak of 9 cm3 min−1 after 45 minutes, then returns to 1 cm3 min−1 by 150 minutes. Explain these changes.

  • The water is absorbed into the blood, raising its water potential; osmoreceptors are less stimulated and less ADH is released. ✓
  • Aquaporins are removed from collecting duct membranes, less water is reabsorbed and urine flow rises (a ninefold increase at the peak). ✓
  • As the excess water is excreted, blood water potential falls back to normal, ADH release increases again and urine flow returns to 1 cm3 min−1; the delay reflects the time for absorption and for ADH to be broken down. ✓
05
Explain
[2 marks]

Suggest why alcohol, which reduces the release of ADH, can lead to dehydration.

  • Less ADH means fewer aquaporins in the collecting duct membranes, so less water is reabsorbed. ✓
  • A large volume of dilute urine is produced, so the body loses more water than normal. ✓
06
Recall
[3 marks]

Describe the roles of the hypothalamus, the posterior pituitary gland and the collecting ducts in osmoregulation.

  • Hypothalamus: contains osmoreceptors that detect the water potential of the blood; neurosecretory cells there make ADH. ✓
  • Posterior pituitary gland: stores ADH and releases it into the blood. ✓
  • Collecting ducts: the effector; ADH increases the number of aquaporins in their membranes, so more water is reabsorbed by osmosis. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.