Kidney cancer starts in the kidney cells, or renal cells. When certain DNA mutations occur, it causes an overproduction of kidney cells that form a tumor on the kidney. This tumor can cause several problems within the kidney and throughout the body. Since the kidneys are responsible for filtering the blood, kidney cancer may lead to an imbalance of certain substances in the blood stream. Sometimes, kidney cancer can cause an increased red blood cell count. Other times, the tumor can cause bleeding through the urine, which may lead to a decreased red blood cell count. Some people also experience high calcium levels. All of these imbalances may lead to other issues throughout the body.