A continuous process of blood cell development from stem cells in the marrow. Stem cells differentiate into immature blood cells of various types. The immature blood cells develop into mature fully functional blood cells which enter the blood and circulate throughout the body. Most blood cells live for short periods and must be steadily replaced. Red cells die within four months, platelets within 10 days and most neutrophils in one to three days. About 100 billion blood cells are made each day.