Medicine & Health
How do the kidneys work?
The kidneys work as the body's filtration system: they continuously clean the blood, removing waste and excess water to make urine while keeping the right balance of water, salts, and minerals. Two fist-sized kidneys filter all your blood about 30 times a day.
See it in motion.
Watch a 2-minute animated lesson that shows exactly how the kidney works.
Step by step
- 1Each kidney contains about a million tiny filters called nephrons.
- 2Nephrons filter waste and excess fluid from the blood, returning useful substances to it.
- 3The filtered waste and water leave as urine, which drains to the bladder.
- 4Kidneys also balance salts and minerals, help regulate blood pressure, and release hormones.
- 5They filter roughly 180 litres of blood a day, concentrating waste into about 1–2 litres of urine.
Frequently asked questions
- What do the kidneys actually remove?
- Metabolic waste like urea, excess water, and surplus salts and minerals — keeping the blood's composition balanced and removing toxins.
- Can you live with one kidney?
- Yes. A single healthy kidney can do the work of two, which is why living kidney donation is possible.
- Besides filtering, what do kidneys do?
- They help control blood pressure, balance the body's water and minerals, keep blood chemistry steady, and release hormones that signal the body to make red blood cells.