The Role of ADH
- Water reabsorption occurs along the nephron tubules in the kidneys
- Water reabsorption is important for osmoregulation
- The control of water reabsorption by the tubules is an example of negative feedback
- If the water content of the blood is too high then less water is reabsorbed, if it is too low then more water is reabsorbed
- This is controlled by the hormone ADH
- Any change to the water level of the blood is detected by the hypothalamus, which then sends a signal to the pituitary gland
- The pituitary gland in the brain constantly releases a hormone called ADH
- How much ADH is released depends on how much water the kidneys need to reabsorb from the filtrate
- ADH affects the permeability of the tubules to water
- If the water content of the blood is too high:
- The pituitary gland releases less ADH which leads to less water being reabsorbed in the collecting ducts of the kidney by osmosis (the collecting ducts become less permeable to water)
- As a result, the kidneys produce a large volume of dilute urine
- If the water content of the blood is too low:
- The pituitary gland releases more ADH which leads to more water being reabsorbed in the collecting ducts of the kidney by osmosis (the collecting ducts become more permeable to water)
- As a result, the kidneys produce a small volume of concentrated urine
The negative feedback loop of ADH controlling the level of water in the blood
Examiner Tip
You must remember the key phrase "ADH increases the permeability of the collecting duct to water" for your exams. Mark schemes will expect you to use this exact terminology.