Bone marrow is the blood-forming organ of the body and its main role is to produce various blood cells. The bone marrow contains hematopoietic stem cells and various stromal cells, which are like seeds that can mature and differentiate into various types of blood cells, such as red blood cells, white blood cells, platelets, and so on. The stromal cells of bone marrow provide the hematopoietic microenvironment and nutritional support necessary for the proliferation and division of hematopoietic stem cells. Because the blood components of the human body are in a constant metabolic process, red blood cells, white blood cells and platelets will always age and be engulfed by mononuclear macrophages, and they have different lifespans. The lifespan of red blood cells is about 120 days, the lifespan of platelets is 5-7 days, and the lifespan of white blood cells is sometimes only a few hours. Therefore, only through the continuous production of blood by the bone marrow and the release of various blood cells into the peripheral blood can we maintain a stable number of blood cells in the body and maintain the normal functioning of the body.