ADH and Water Balance — AQA GCSE Biology (Separate / Higher)
The amount of water reabsorbed by the kidneys is controlled by the hormone ADH through negative feedback.
Controlling water content
The water level of the blood is monitored by the brain. The hormone ADH (anti-diuretic hormone), released by the pituitary gland, controls how much water the kidney tubules reabsorb back into the blood.
When the blood is too concentrated (too little water)
- The brain detects the low water content.
- More ADH is released.
- The kidney tubules become more permeable, so more water is reabsorbed into the blood.
- A small volume of concentrated urine is produced.
When the blood is too dilute (too much water)
- The brain detects the high water content.
- Less ADH is released.
- The kidney tubules reabsorb less water.
- A large volume of dilute urine is produced.
Negative feedback
This is controlled by negative feedback: a change in blood water content triggers a change in ADH that reverses it, restoring the normal level.
Exam tips
- ADH is released by the pituitary and controls water reabsorption in the kidneys.
- Too little water → more ADH → more reabsorbed → concentrated urine.
- Too much water → less ADH → less reabsorbed → dilute urine.
- Explain it as negative feedback.