All subjects · Biology · WBI15 (Unit 5)

ADH and control of water balance

The water potential of the blood is kept within narrow limits by antidiuretic hormone (ADH), which changes how much water the collecting ducts reabsorb.

Detecting a change

Osmoreceptors in the hypothalamus detect changes in the water potential (solute concentration) of the blood.

When you are dehydrated (for example after sweating or eating salty food), blood water potential falls. Osmoreceptor cells lose water by osmosis and shrink, which stimulates neurosecretory cells in the hypothalamus.

These cells make ADH, which passes down their axons to the posterior pituitary gland, where it is released into the blood. A fall in blood volume, detected by receptors in the heart and blood vessels, also increases ADH release.

How ADH acts

1. ADH binds to receptors on cells of the collecting duct (and distal convoluted tubule).

2. This activates an enzyme that produces a second messenger (cyclic AMP) inside the cell.

3. Vesicles containing aquaporins (water channel proteins) fuse with the cell membrane facing the filtrate.

4. The walls become more permeable to water, so more water leaves the filtrate by osmosis into the concentrated medulla and returns to the blood.

Result: a small volume of concentrated urine, and blood water potential rises back towards normal.

Negative feedback

When blood water potential is too high (after drinking a lot), osmoreceptors are less stimulated, less ADH is released, aquaporins are removed from the membrane, less water is reabsorbed, and a large volume of dilute urine is produced.

Because the response reverses the change, this is negative feedback. Alcohol inhibits ADH release, which is why it increases urine output.

Worked example

Describe what happens to ADH and urine after a long run on a hot day without drinking.

  1. Sweating loses water, so blood water potential falls.
  2. Osmoreceptors in the hypothalamus detect this; more ADH is released from the posterior pituitary.
  3. Collecting ducts insert aquaporins and become more permeable to water.
  4. More water is reabsorbed.

Answer: More ADH, more water reabsorbed, small volume of concentrated urine.

Key idea: Osmoreceptors in the hypothalamus detect low blood water potential; ADH (made in the hypothalamus) is released from the posterior pituitary; it causes aquaporins to be inserted into collecting duct membranes, increasing water reabsorption. Negative feedback.

Check yourself

  1. Where are the osmoreceptors that control ADH release?
    • Collecting duct
    • Hypothalamus
    • Posterior pituitary
    • Bowman's capsule
    Show answer

    Hypothalamus. Osmoreceptors in the hypothalamus monitor blood water potential.

  2. From which gland is ADH released into the blood?
    • Anterior pituitary
    • Adrenal gland
    • Thyroid
    • Posterior pituitary
    Show answer

    Posterior pituitary. ADH is made in the hypothalamus and released from the posterior pituitary.

  3. How does ADH increase water reabsorption?
    • It actively pumps water molecules
    • It causes aquaporins to be inserted into collecting duct cell membranes
    • It makes the loop of Henle shorter
    • It increases glomerular filtration
    Show answer

    It causes aquaporins to be inserted into collecting duct cell membranes. More water channels mean more osmosis.

  4. A person is dehydrated. What happens to their urine?
    • Large volume, dilute
    • Large volume, concentrated
    • Small volume, concentrated
    • No change
    Show answer

    Small volume, concentrated. More ADH means more water reabsorbed.

  5. After drinking a large volume of water, what happens to ADH secretion?
    • It decreases
    • It increases
    • It stays the same
    • It stops the kidneys filtering
    Show answer

    It decreases. High blood water potential reduces ADH release.

  6. What is the role of cyclic AMP in ADH action?
    • It is the hormone itself
    • It forms the water channels
    • It is a second messenger inside the collecting duct cell
    • It is made in the pituitary
    Show answer

    It is a second messenger inside the collecting duct cell. ADH binds outside; cAMP carries the signal inside.

  7. Why does drinking alcohol increase urine volume?
    • Alcohol inhibits ADH release
    • Alcohol increases ADH release
    • Alcohol blocks glomerular filtration
    • Alcohol is a source of aquaporins
    Show answer

    Alcohol inhibits ADH release. Less ADH, fewer aquaporins, less water reabsorbed.

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Last updated 7 November 2026. Spotted a mistake? Tell us.