ADH and the collecting ductSpec 7.20B
In short
ADH is a hormone released by the pituitary gland that controls how much water the kidney reabsorbs. It makes the collecting duct more permeable to water. When the blood water content is too low, more ADH is released, more water is reabsorbed, and a small volume of concentrated urine is made. Less ADH gives a large volume of dilute urine.
ADH is released by the pituitary gland. It controls how much water is reabsorbed in the kidney, which keeps the water content of the blood steady (osmoregulation). ADH changes the permeability of the collecting duct.
| Blood water content too low | Blood water content too high |
|---|
| ADH released | More | Less |
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| Collecting duct | More permeable to water | Less permeable to water |
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| Water reabsorbed into blood | More | Less |
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| Urine | Small volume, concentrated | Large volume, dilute |
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| Blood water content | Rises back to normal | Falls back to normal |
|---|
Common mistake:ADH changes how permeable the collecting duct is to water. It does not change the amount of water the person drinks.
Written and checked against the Edexcel GCSE Biology (1BI0) specification · Updated October 2026
Frequently asked questions
How does the kidney filter blood?
The kidney filters blood in its many nephrons. In each nephron, blood in the glomerulus is at high pressure, so small molecules such as water, glucose, urea and ions are forced out into the Bowman's capsule. Proteins and blood cells are too large and stay in the blood. Useful substances are then reabsorbed.
Why is there no glucose in urine?
There is no glucose in the urine of a healthy person because it is all reabsorbed in the nephron. Glucose is small, so it is filtered out of the blood at the glomerulus into the Bowman's capsule. In the first part of the tubule, all of it is reabsorbed into the blood by active transport, which uses energy from respiration.
What does ADH do in the kidneys?
Higher tier ADH makes the collecting ducts of the kidneys more permeable to water, so more water is reabsorbed into the blood. The pituitary gland releases more ADH when the blood water content is too low, so a small volume of concentrated urine is made. When the water content is too high, less ADH is released and the urine is dilute.
All 4 questions on The urinary system and the kidneys
