Trace ADH control of water reabsorption
Explain how the hypothalamus, posterior pituitary, ADH, aquaporins and collecting ducts regulate blood water potential.
A Level; Cambridge International Biology 9700, 2028–2030. Written answers are self-checked; this is not a practical skills certification.
Before you begin
- A control system detects a change, communicates information and changes an effector response.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which gland releases ADH into the blood?
What you will learn
- Describe ADH release in response to blood water-potential change
- Explain how aquaporins alter collecting-duct water permeability
Build the explanation
Osmoreceptors in the hypothalamus detect changes in blood water potential. When it falls, the hypothalamus signals the posterior pituitary to release more antidiuretic hormone (ADH). ADH binds to collecting-duct cells and increases insertion of aquaporin water channels in their membranes. More water is reabsorbed into the blood, producing a smaller volume of more concentrated urine. When water potential rises, less ADH is released and the duct becomes less permeable to water.
Apply the limits
ADH is released from the posterior pituitary, while the hypothalamus detects the change and synthesises ADH. Aquaporins are channels; they do not pump water. Keep blood water potential and urine concentration directions distinct.
Pause and explain
After drinking a large volume of water, predict the ADH response and urine change.
Worked example
What happens to collecting-duct water permeability when ADH concentration rises?
Show the worked solution
- More aquaporins are inserted in duct-cell membranes.
- Water moves from filtrate to blood down a water-potential gradient.
Answer Permeability rises, so more water is reabsorbed and urine volume falls.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A dehydrated person has lower blood water potential. Trace the response to the urine.
Give me a hint
Name the relevant structure or signal, then state its effect.
Compare my reasoning
- More ADH is released from the posterior pituitary.
- More aquaporins make collecting ducts more permeable; water reabsorption rises and urine volume falls.
Low blood water potential stimulates greater ADH release. More aquaporins increase collecting-duct water permeability, so more water returns to blood and a smaller volume of concentrated urine is produced.
Look for these in your work
- Correct mechanism
- Direction of response explained
A student says aquaporins use ATP to pump water into the blood. Correct the statement.
Give me a hint
Separate the observed change from the body's response.
Compare my reasoning
- Aquaporins are channel proteins.
- Water moves by osmosis down a water-potential gradient.
Aquaporins provide channels that increase water permeability. Water moves by osmosis down its water-potential gradient; the channels do not pump water using ATP.
Look for these in your work
- Evidence interpreted
- Control step identified
After drinking a large volume of water, predict the ADH response and urine change.
Give me a hint
Apply the mechanism to the new context without claiming more than the evidence shows.
Compare my reasoning
- Blood water potential rises, reducing the stimulus for ADH release.
- Collecting ducts become less water-permeable and more dilute urine is produced.
Higher blood water potential leads to less ADH release. Fewer aquaporins are present in collecting ducts, so less water is reabsorbed and urine volume increases while its concentration decreases.
Look for these in your work
- New context applied
- Limits preserved
Common mistakes
- Saying ADH is produced by the posterior pituitary
- Claiming aquaporins actively pump water
What can you explain now?
Which gland releases ADH into the blood?
Compare with the explanation
The posterior pituitary gland
The hypothalamus detects water-potential changes and signals release.
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Make it stick
How does ADH change collecting-duct permeability and urine volume?
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