Most people have two kidneys, each about 10 centimetres (4 inches) long. They are positioned in the back of the body behind the lower ribs.
A healthy kidney is basically a filtering system. In fact, each kidney is made up of about a million filtering units called nephrons.
As blood passes through the kidneys, it is "cleaned" - the body's waste products and excess fluid are removed. These waste products, along with the excess fluid, are then sent through the ureters to the bladder. Then they are passed out of the body as urine.
A quick summary of the functions of the kidney:
- Acts as a filter system
- Gets rid of waste products
- Balances the body's fluid content
- Produces hormones that control blood pressure
- Produces the hormone Erythropoietin to help make red blood cells
- Activates vitamin D to maintain healthy bones