1 Cortex – the outer region of the kidney; extensions of the cortical tissue, contains about one million blood filtering nephrons. 2  Nephron – these are the filtration units in the kidneys. 3  Medulla – inner region of the kidney contains 8-12 renal pyramids.4  The pyramids empty into the calyx. Cortex – the outer region of the kidney; extensions of the cortical tissue, contains about one million blood filtering nephrons. 2  Nephron – these are the filtration units in the kidneys. 3  Medulla – inner region of the kidney contains 8-12 renal pyramids.