Edexcel GCSE Biology (1BI0) — Animal Coordination, Control and Homeostasis
Understanding how hormones physically alter kidney cells explains why our urine can change from almost clear to dark yellow in just a few hours.
Once released into the blood, ADH travels to its target organs: the kidneys.
Within the kidney nephrons, ADH specifically targets the walls of the collecting duct.
ADH acts by increasing the permeability of the collecting duct walls to water.
At a cellular level, it causes more water channels (pores) to be inserted into the membrane of the duct.
Because the walls are now highly permeable, more water moves out of the kidney filtrate and back into the blood capillaries via osmosis.
This crucial process of returning useful substances from the filtrate to the bloodstream is known as .
A central heating thermostat constantly turns a radiator on and off to keep a room's temperature steady, and your brain uses exactly the same principle with the water levels in your blood.
This continuous cycle of reversing a change to return conditions to an optimum set point is called negative feedback.
When your blood water potential drops, the system triggers a response to conserve water, counteracting the initial change.
How negative feedback responds to dehydration:
Why do certain drinks, like alcohol, make you need the toilet much more frequently?
Alcohol physically inhibits the production of ADH, meaning the collecting ducts remain less permeable and large amounts of water are lost, often leading to dehydration.
Depending on the levels of ADH present, the kidneys will produce either concentrated urine (high solute, low water) or dilute urine (low solute, high water).
Compare: Urine Production based on Hydration Levels
| Condition | ADH Level | Permeability of Collecting Duct | Urine Volume | Urine Concentration |
|---|---|---|---|---|
| Dehydrated (e.g., after exercise) | High | High (more water reabsorbed) | Low (small) | High (concentrated) |
| Over-hydrated (e.g., high water intake) | Low | Low (less water reabsorbed) | High (large) | Low (dilute) |
Students frequently write that the hypothalamus releases ADH; you must specify that the hypothalamus DETECTS the change while the pituitary gland RELEASES the hormone.
In 4-mark or 6-mark explanation questions, examiners actively look for the exact phrase 'increases the permeability of the collecting duct'. Vague answers like 'makes it let more water through' will not be credited.
Always explicitly mention that water is reabsorbed back into the blood via 'osmosis' to secure top marks in mechanism questions.
Remember the mnemonic: Anti-Diuretic Hormone works 'against diuresis' (against making lots of urine), meaning more ADH always results in less urine volume.
Osmoregulation
The control of water and salt concentrations in the body fluids to maintain a constant water potential as part of homeostasis.
Homeostasis
The maintenance of a constant internal environment in the body.
Hypothalamus
The part of the brain containing osmoreceptors that monitors blood water potential and coordinates the homeostatic response.
Pituitary gland
An endocrine gland in the brain that releases ADH into the bloodstream when stimulated by the hypothalamus.
ADH (Anti-diuretic Hormone)
A hormone released by the pituitary gland that regulates water reabsorption in the kidneys by changing the permeability of the collecting duct.
Collecting duct
The final section of the kidney nephron targeted by ADH, where water is reabsorbed into the blood.
Permeability
The state of a membrane or wall that determines how easily substances (such as water) can pass through it.
Osmosis
The movement of water molecules from a region of higher water potential to a region of lower water potential across a partially permeable membrane.
Reabsorption
The process of taking back useful substances, like water, from the kidney filtrate into the blood.
Negative feedback
A regulatory mechanism in which a change in a physiological condition triggers a response that reverses the initial change to restore the optimum set point.
Concentrated urine
Urine containing a high amount of solutes and a low volume of water, typically produced when ADH levels are high.
Dilute urine
Urine containing a low amount of solutes and a high volume of water, typically produced when ADH levels are low.
Put your knowledge into practice — try past paper questions for Biology
Osmoregulation
The control of water and salt concentrations in the body fluids to maintain a constant water potential as part of homeostasis.
Homeostasis
The maintenance of a constant internal environment in the body.
Hypothalamus
The part of the brain containing osmoreceptors that monitors blood water potential and coordinates the homeostatic response.
Pituitary gland
An endocrine gland in the brain that releases ADH into the bloodstream when stimulated by the hypothalamus.
ADH (Anti-diuretic Hormone)
A hormone released by the pituitary gland that regulates water reabsorption in the kidneys by changing the permeability of the collecting duct.
Collecting duct
The final section of the kidney nephron targeted by ADH, where water is reabsorbed into the blood.
Permeability
The state of a membrane or wall that determines how easily substances (such as water) can pass through it.
Osmosis
The movement of water molecules from a region of higher water potential to a region of lower water potential across a partially permeable membrane.
Reabsorption
The process of taking back useful substances, like water, from the kidney filtrate into the blood.
Negative feedback
A regulatory mechanism in which a change in a physiological condition triggers a response that reverses the initial change to restore the optimum set point.
Concentrated urine
Urine containing a high amount of solutes and a low volume of water, typically produced when ADH levels are high.
Dilute urine
Urine containing a low amount of solutes and a high volume of water, typically produced when ADH levels are low.
