All normal blood cells have a limited survival in circulation and need to be replaced on a continual basis.
This means that the bone marrow remains a very active tissue throughout your life. Natural chemicals in your blood called growth factors or cytokines control the process of blood cell formation. Different growth factors stimulate the stem cells in the bone marrow to produce different types of blood cells.
The growth factor that stimulates the production and maturation of red blood cells is called erythropoietin. This is mainly produced in the kidneys.
The growth factor that stimulates the production of platelets is called thrombopoietin and it is mainly produced in the liver and the kidneys.
Various cytokines stimulate the production of the white blood cells.
These days some growth factors can be made in the laboratory (synthesised) and are available for use in people with blood disorders. For example, granulocyte-colony stimulating factor(G-CSF) stimulates the production of white cells called neutrophils while erythropoeitin (EPO) stimulates the production of red cells.