You have two kidneys on either side of your spine that are each approximately the size of a human fist. They’re located posterior to your abdomen and below your rib cage. Your kidneys play several vital roles in maintaining your health. One of their most important jobs is to filter waste materials from the blood and expel them from the body as urine. The kidneys also help control the levels of water and various essential minerals in the body. In addition, they’re critical to the production of: vitamin D, red blood cells, hormones that regulate blood pressure, If your doctor thinks your kidneys may not be working properly, you may need kidney function tests. These are simple blood and urine tests that can identify problems with your kidneys.