The kidneys maintain the delicate balance of chemicals and water that your body needs. When working well, they hold on to what your body needs, and they get rid of what it doesn't need. They send the extra water and other waste as urine (pee) through pencil-sized tubes called ureters. The kidneys also help to regulate your blood pressure, red blood cell production, and your body's calcium and other mineral levels.
When a person's kidneys stop working altogether, it's called kidney failure. Someone who has kidney failure can develop several health problems because the body is unable to get rid of excess water and waste products.