The red cell for several days after it loses its nucleus in the marrow and for the first day of its life span in the blood contains leftover material that can be stained In this way, the proportion of cells that are newly delivered into the blood from the marrow can be identified Normally, a low proportion of reticulocytes (about 1% of red cells) are present in the blood An increase in numbers indicates an increased production in the marrow, for example following blood loss, to replace lost blood cells