The kidneys remove wastes and excess water (fluid) collected by, and carried in, the blood as it flows through the body.

About 190 liters (335 pints) of blood enter the kidneys every day via the renal arteries. Millions of tiny filters, called glomeruli, inside the kidneys separate wastes and water from the blood.

Most of these unwanted substances come from what we eat and drink. The kidneys automatically remove the right amount of salt and other minerals from the blood to leave just the small quantities the body needs.

The cleansed blood returns to the heart and recirculates through the body.

Excess wastes and fluid leave the kidneys in the form of urine. Urine is stored in the bladder until it is full and then leaves the body via the urethra. Most people pass about 2 liters (4 pints) of urine every day.