Part of the urinary tract, your kidneys are a pair of organs in the upper abdomen, one on each side of your spine. They have several functions in the body including removing wastes and extra water from the blood, producing urine, and making substances that help control blood pressure and the production of red blood cells.
As your kidneys filter your blood, the urine collects in the renal pelvis before leaving the kidneys through tubes called ureters that lead to the bladder. When you empty your bladder, the urine leaves your body through another tube called the urethra.
Each kidney is slightly larger than a fist, including the adrenal glands attached at the top. Your adrenal glands produce certain hormones, including adrenaline. A mass of fatty tissue and an outer layer of fibrous tissue (Gerota's fascia) enclose the kidney and adrenal gland together.
Cancer occurs when the genes of a cell become damaged, called a mutation, which causes the cell to multiply out of control. As these damaged cells continue to multiply, they form a mass, called a tumor. In the kidneys, the cells most likely to become damaged are the renal cells that line the small tubes that filter the blood. Less commonly, cancer may begin in cells of the renal pelvis.