The process of blood cell development in the marrow. The most undeveloped cells in the marrow are stem cells. They start the process of blood cell development. The stem cells begin to develop into young or immature blood cells such as red cells or white cells of various types. This process is called "differentiation." The young or immature blood cells then further develop into fully functional blood cells. This process is called "maturation." The mature cells leave the marrow, enter the blood and circulate throughout the body. Hematopoiesis is a continuous process that is active normally throughout life. The reason for this activity is that most blood cells live for short periods and must be continually replaced. Red cells die in four months, platelets in 10 days and most neutrophils in one to three days. About 500 billion blood cells are made each day. When the marrow is invaded with cancer cells, it cannot produce enough normal blood cells to meet the constant demand for them, and blood cell counts become severely depleted.