The kidneys remove urea from the blood through tiny filtering units called nephrons. What are the major organs of the urinary system? Their function is to remove liquid waste from the blood in the form of urine; keep a stable balance of salts and other substances in the blood; and produce erythropoietin, a hormone that aids the formation of red blood cells. The kidneys remove urea from the blood through tiny filtering units called nephrons. How does the kidney form urine? Blood from the heart comes into your kidneys from the renal arteries where it enters about 2-3 million nephrons per kidney.