By the end of this lesson
- Describe the ADH negative-feedback pathway.
- Predict urine after dehydration or excess drinking.
- Distinguish hormone signalling from water movement.
BIG QUESTION
How can the brain instruct kidneys to save water without moving the water itself?
Osmoreceptors alter ADH release; ADH changes collecting-duct aquaporins and therefore osmotic water recovery.
Osmoreceptors detect blood water status
Osmoreceptors in the hypothalamus detect changes in blood water potential. When blood is too concentrated, the hypothalamus signals increased release of ADHAntidiuretic hormone: made in the hypothalamus and released from the posterior pituitary, it makes the collecting ducts more permeable to water so more water is reabsorbed. from the posterior pituitary.
When blood is dilute, ADH release decreases.
ADH changes collecting-duct permeability
ADH binds to receptors on collecting-duct cells and promotes insertion of aquaporins into their membranes. More water then leaves filtrate by osmosis and returns to blood.
ADH is a signal; it neither contains water nor supplies the osmotic energy.
Negative feedback restores the variable
After dehydration: more ADH, more water reabsorption, low-volume concentrated urine. After excess water intake: less ADH, fewer aquaporins, less reabsorption, high-volume dilute urine.
As blood water content returns toward normal, the original ADH response is reduced.
ADH response to water status
Predict each link in the chain.
| Blood condition | ADH and collecting duct | Urine |
|---|---|---|
| Low water potential/dehydration | More ADH; more aquaporins. | Small volume, concentrated. |
| High water potential/excess water | Less ADH; fewer aquaporins. | Large volume, dilute. |
| Return toward normal | Signal reduces by negative feedback. | Output stabilises. |
Language that earns precision
- ADH
- Antidiuretic hormone that increases collecting-duct water permeability.
- osmoreceptor
- Receptor sensitive to water potential or solute concentration of body fluids.
- target cell
- Cell with receptors for a particular hormone.
- negative feedback
- Control in which a response opposes the original change.
Common misconceptions
Claim: The pituitary detects blood concentration alone.
Correction: Osmoreceptors are in the hypothalamus; ADH is released from the posterior pituitary.
Claim: More ADH makes more urine.
Correction: It increases water recovery, producing less urine.
Claim: ADH actively pumps water.
Correction: It increases aquaporin permeability; water moves by osmosis.
Match the answer to the command word
Stimulus → osmoreceptor → ADH → aquaporins → osmosis → negative feedback.
Always state both urine volume and concentration.
Recall, apply, then justify
Explain the ADH response to dehydration.
Hint
Begin with blood water potential.
Suggested marking points
- Blood water potential falls/blood becomes concentrated.
- Osmoreceptors in the hypothalamus detect the change.
- More ADH is released from the posterior pituitary.
- Collecting ducts insert more aquaporins/become more permeable.
- More water returns to blood by osmosis.
- A small volume of concentrated urine is produced.
Predict the response after drinking excess water.
Hint
Reverse the pathway.
Suggested marking points
- Blood water potential rises.
- Less ADH is released.
- Collecting ducts become less permeable and reabsorb less water.
- A larger volume of dilute urine is produced.
Explain why failure of collecting-duct cells to respond to ADH causes water loss despite high ADH concentration.
Planning hint
Signal versus target response.
What a strong design includes
- Dehydration stimulates high ADH release.
- ADH must bind receptors on collecting-duct cells.
- If target cells cannot respond, aquaporin insertion does not increase.
- Duct permeability remains low.
- Less water is reabsorbed by osmosis.
- Large volumes of dilute urine can continue, worsening dehydration.