Kidney drug development, conceptual composite image. The kidneys filter waste from the blood and excrete it as urine. At centre is a sectioned kidney, which consists of an outer cortex and inner medulla. The medulla is divided into regions called renal pyramids (7 seen here). Each renal pyramid drains into a renal papilla (cream), which merge to form the ureter (bottom centre) that carries the urine to the bladder. Blood enters the kidney through the renal artery (red) and leaves it through the renal vein (blue). Common kidney diseases, include nephritis, kidney stones, cystic kidney disease and renal cancers.