A negative feedback system, in which tissue oxygenation controls Epo production and Epo controls red blood cell (RBC) production, provides homeostasis in oxygen delivery to body tissues. The stages for the erythrocyte are rubriblast, prorubriblast, rubricyte and metarubricye. Finally the stages can also be named according to the development of the normoblast stage. This gives the stages pronormoblast, early normoblast, intermediate normoblast, late normoblast, polychromatic cell