Urinary system
Kidneys
The kidneys are two bean-shaped organs that filter waste and excess water from the blood to make urine. They also balance the body's salts, control blood pressure and help make red blood cells.
What it is
The kidneys are a pair of fist-sized, bean-shaped organs in the back of the abdomen. Each contains around a million tiny filters called nephrons.
Where it is
Towards the back of the abdomen, one on each side of the spine, just below the rib cage.
What it does
Filter the blood to remove waste and excess fluid as urine, balance salts and minerals, help regulate blood pressure, and produce hormones controlling red blood cell production and bone health.
How it works
Blood flows through millions of tiny filters that remove waste and surplus water while keeping useful substances. The filtered waste becomes urine, which drains to the bladder, while the cleaned blood returns to the body.
When things go wrong
Common conditions affecting the kidneys
- Chronic kidney disease
- Kidney stones
- Urine infections spreading to the kidney
- Acute kidney injury during illness
Education and reference only. This explains the anatomy in plain terms and is not a diagnosis. Sudden severe symptoms — such as severe chest pain, breathlessness or collapse — are an emergency; call 999.
Looking after it
Keeping your kidneys healthy
Staying well hydrated, keeping blood pressure and diabetes well controlled, using anti-inflammatory painkillers sparingly, and not smoking all protect the kidneys.
Did you know?
An interesting fact
The kidneys filter the body's entire blood supply around 40 times a day — roughly 180 litres of fluid, almost all of which is reabsorbed.
Answers
Kidneys: frequently asked questions
What do the kidneys do?
The kidneys filter waste and excess water from the blood to make urine, balance the body's salts, help control blood pressure, and support red blood cell production.
Can you live with one kidney?
Yes. One healthy kidney can do the work of two, which is why people can donate a kidney and live normally. Those with a single kidney are advised to protect it.
The urinary system
Related organs
Sources
Where this is drawn from
- NHS — Anatomy and body systems
- Gray's Anatomy for Students
- TeachMeAnatomy / TeachMePhysiology
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