Osmoregulation and ADH
Laura Armstrong
Teacher
Contents
Recall Questions
This topic requires prior knowledge of the kidney structure and negative feedback. You can test your knowledge below.
What is the function of the kidney in homeostasis?
To regulate blood water potential and remove waste products such as urea.
What is negative feedback?
A corrective mechanism that restores a condition to its normal level when it deviates from the set point.
Which hormone controls water reabsorption in the kidney?
ADH (antidiuretic hormone).
Topic Explainer Video
What Is Osmoregulation?
- Osmoregulation is the homeostatic control of water potential in the blood.
- It ensures that cells are not damaged by osmosis (swelling or shrinking).
- Controlled by the hypothalamus, posterior pituitary, and kidneys via negative feedback.
Key Players in Osmoregulation
| Compnent | Function |
| Hypothalamus | Detects changes in blood water potential via osmoreceptors. |
| Posterior Pituitary | Releases ADH in response to signals from hypothalamus. |
| ADH | Increases permeability of kidney tubules to water. |
| Kidney | Reabsorbs water to adjust urine concentration. |
When Water Potential Decreases (e.g., sweating, eating a salty meal)
- Osmoreceptors in the hypothalamus detect a fall in blood water potential.
2. ADH is released from the posterior pituitary into the blood.
3. ADH binds to receptors on the cells of the distal convoluted tubule (DCT) and collecting duct.
4. Causes aquaporins (water channels) to be inserted into membranes of the epithelial cells.
5. More water is reabsorbed by osmosis into the blood.
6. Urine becomes more concentrated, and is released in smaller volumes, water potential is restored.
When Water Potential Increases (e.g., drinking lots of water)
- Osmoreceptors detect the rise in water potential.
- Less ADH is released/ no ADH is released from the posterior pituitary gland.
- Fewer aquaporins inserted into membranes.
- Less water reabsorbed by osmosis, more water stays in the urine.
- Urine is more dilute, and is released in larger volumes, excess water is excreted.
Negative Feedback Loop Summary
| Stimulus | Response Triggered | Result |
| ↓ Water potential | ↑ ADH → ↑ water reabsorption. | Water potential restored. |
| ↑ Water potential | ↓ ADH → ↓ water reabsorption. | Excess water lost in urine. |
Role of the Nephron in ADH Action
- DCT and collecting duct = target sites for ADH action.
- ADH increases permeability to water by stimulating insertion of aquaporins.
- Reabsorbed water passes back into blood capillaries.
- Urine volume ↓, concentration ↑ when ADH is present.
Key Terms
- Osmoregulation: The homeostatic control of water potential in the blood.
- ADH (antidiuretic hormone): A hormone that increases water reabsorption in the kidneys.
- Aquaporins: Channel proteins that allow water to pass through membranes.
- Hypothalamus: Part of the brain that contains osmoreceptors that detect changes in blood water potential.
- Posterior Pituitary: Gland that releases ADH into the blood.
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Exam Tips
Be precise with your language – ADH increases permeability of the collecting duct and distal convoluted tubule, so more water is reabsorbed by osmosis.
Always link hormone → receptor → response clearly.
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Describe how the body responds when the water potential of the blood decreases. (6 marks)
- Osmoreceptors in the hypothalamus detect a drop in blood water potential.
- The posterior pituitary releases ADH into the bloodstream.
- ADH binds to receptors on cells of the DCT and collecting duct in the nephron.
- This stimulates the insertion of aquaporins into the membrane.
- More water is reabsorbed by osmosis into the blood.
- Making urine more concentrated / smaller volumes of urine produced, restoring water potential.
Practice Question Videos
Try to answer the practice questions from the TikToks on your own, then watch the videos to see how well you did!