ADH and the water content of the bloodSpec 2.78B
In short
ADH (antidiuretic hormone) is released into the blood by the pituitary gland and controls the water content of the blood. When the blood water content is too low, more ADH is released, making the collecting duct more permeable to water. More water is reabsorbed into the blood, so a small volume of concentrated urine is made.
ADH (antidiuretic hormone) is released into the blood by the pituitary gland. It controls how much water is reabsorbed in the kidney, which keeps the water content of the blood steady. ADH changes the permeability of the collecting duct to water.
| Blood water content too low | Blood water content too high | |
|---|---|---|
| ADH released | More | Less |
| Collecting duct | More permeable to water | Less permeable to water |
| Water reabsorbed into blood | More | Less |
| Urine | Small volume, concentrated | Large volume, dilute |
| Blood water content | Rises back to normal | Falls back to normal |
More ADH means less urine, not more. ADH makes the collecting duct more permeable, so more water goes back into the blood.
Quick check
Name the excretory product of the lungs.
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Carbon dioxide.
Name three substances lost from the skin in sweat.
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Water, salts (ions) and urea.
Biology only Which part of the nephron does ultrafiltration take place in?
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The Bowman's capsule (from the glomerulus).
Biology only Where is glucose reabsorbed, and by what process?
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The proximal convoluted tubule, by active transport.
Biology only What does ADH do to the collecting duct?
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It makes the collecting duct more permeable to water, so more water is reabsorbed.
Written and checked against the Edexcel IGCSE Biology (4BI1) specification · Updated October 2026