They filter out waste products and excess water, balance chemicals in the blood such as potassium and sodium, and also remove excess acid. They also produce a hormone to help bone marrow make red blood cells.
Most people have two kidneys, each the size of a fist.
These are located on either side of the backbone, just above the small of the back. Each day, the kidneys process roughly 1600 liters of blood. Acute kidney failure is a sudden loss of kidney function that may reverse within a few weeks. It usually occurs in response to an injury or poison. A person with acute kidney failure needs treatment very quickly to stay alive.