Red blood cell formation, or erythropoiesis, occurs in the red bone marrow located in the vertebrae, sternum, ribs, skull, scapulae, pelvis, and proximal limb bones. Red blood cells begin as large, immature cells (proeythroblasts), and over a seven-day period they change into a much smaller, mature, red blood cell that then enters the blood stream.
Several types of blood cells, including platelets, white and red cells, serve functions such as immunity, healing, and the transport of oxygen. (From Willis, M. C. Medical Terminology: A Programmed Learning Approach to the Language of Health Care. Baltimore: Lippincott, Williams & Wilkins, 2002.)