The process of making blood cells is called hematopoiesis. Blood cells are made in the bone marrow. That's a spongy tissue located inside some bones. It contains young parent cells called stem cells. These blood-forming stem cells can grow into all 3 types of blood cells – red cells, white cells and platelets. These blood-forming stem cells make copies of themselves, and they also produce mature blood cells. When blood cells are fully mature and functional, they leave the bone marrow and enter the blood. Healthy people have enough stem cells to keep making all the blood cells they need every day.