The Kidneys and Osmoregulation — AQA GCSE Biology (Separate)
The kidneys filter the blood, removing waste and controlling the water and ion content of the body.
What the kidneys do
The kidneys carry out osmoregulation (controlling water balance) and excretion (removing waste). They produce urine by:
- Filtration — small molecules (water, glucose, ions, urea) are filtered out of the blood under high pressure.
- Selective reabsorption — useful substances are reabsorbed back into the blood: all the glucose, the right amount of ions, and as much water as the body needs.
- The remaining water, ions and urea form urine, which passes to the bladder.
Removing urea
Excess amino acids cannot be stored. In the liver they are broken down (deamination) to form ammonia, which is toxic and is immediately converted to urea. Urea is carried in the blood to the kidneys and removed in urine.
Water balance
Water leaves the body via the lungs (breathing), skin (sweating) and kidneys (urine). The body cannot control losses from the lungs and skin, so the kidneys adjust the water content of the blood to keep it balanced.
Exam tips
- Learn the three stages: filtration → selective reabsorption → urine.
- All glucose is reabsorbed; water and ions are reabsorbed as needed.
- Urea comes from breaking down excess amino acids in the liver (deamination of ammonia).
- The kidneys control water balance because lung and skin losses can't be regulated.