The kidneys perform several important functions. They:

• Remove waste. The kidneys filter waste products and excess fluid out of the blood and remove them from the body in the form of urine. Waste products and excess fluid are produced by the normal breakdown of food and the chemical reactions that occur in cells throughout the body (metabolism). Filtering waste products is the most important function of the kidneys.
• Keep the right balance of chemicals in the body. The kidneys keep the right balance of chemicals, such as sodium, potassium, calcium, magnesium, and other substances your body needs to function properly. They filter excess amounts of these chemicals from the blood and get rid of them in the urine.
• Make hormones. The kidneys produce three important hormones: renin, erythropoietin, and the active form of vitamin D. Renin helps regulate blood pressure. Erythropoietin is needed to make red blood cells. Your body needs the active form of vitamin D to absorb calcium from food. Together, vitamin D and calcium help build healthy bones and maintain normal muscle function.