Water Balance and ADH: Controlling Urine Concentration
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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
ADH (antidiuretic hormone) is released by the pituitary gland when blood water concentration is low. ADH makes the collecting ducts of the kidney more permeable to water, so more water is reabsorbed into the blood, producing a small volume of concentrated urine. When blood water is high, less ADH is released, less water is reabsorbed, and a large volume of dilute urine is produced.
ADH and water balance is a Supplement topic that combines homeostasis with kidney function. The examiner tests whether you can explain the negative feedback loop: stimulus → hormone release → effect on kidney → restoration of normal water concentration.
How ADH controls water reabsorption
When the body loses water (through sweating, breathing, or not drinking enough), the water concentration of the blood decreases. Receptor cells in the brain detect this change. The pituitary gland responds by releasing more ADH (antidiuretic hormone) into the blood.
ADH travels to the kidneys and increases the permeability of the collecting duct walls to water. More water is reabsorbed by osmosis from the collecting ducts back into the blood. The result is a small volume of concentrated urine and conservation of body water.
When blood water concentration is high
After drinking a large volume of water, the water concentration of the blood increases. The pituitary gland releases less ADH. The collecting duct walls become less permeable to water, so less water is reabsorbed. More water passes into the urine, producing a large volume of dilute urine.
This is negative feedback: the response (changing ADH levels and urine volume) opposes the original change and restores blood water concentration toward normal.
Summary of ADH action
Condition
ADH level
Collecting duct permeability
Water reabsorbed
Urine produced
Dehydrated (low blood water)
More ADH released
More permeable
More water reabsorbed
Small volume, concentrated
Overhydrated (high blood water)
Less ADH released
Less permeable
Less water reabsorbed
Large volume, dilute
ADH acts on the collecting duct, not on the glomerulus or the loop of Henle. The filtration rate in the glomerulus remains essentially constant.
Try a focused question (4 marks)
On a hot day, a person sweats a lot but does not drink much water. Explain how the body produces a small volume of concentrated urine.
Show the answer and marking guidance
Sweating causes water loss, so the water concentration of the blood decreases.
The pituitary gland detects the change and releases more ADH into the blood.
ADH increases the permeability of the collecting duct walls in the kidneys.
More water is reabsorbed by osmosis from the collecting ducts into the blood, so urine volume is small and concentrated.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Saying ADH is produced by the kidneys: it is produced by the pituitary gland.
Describing ADH as increasing water filtration: ADH affects reabsorption in the collecting duct, not filtration.
Getting the relationship backwards: more ADH means more water reabsorbed (more concentrated urine), not more urine.
Forgetting to mention osmosis as the mechanism of water reabsorption.