<P> Erythropoiesis is the process by which new red blood cells are produced ; it lasts about 7 days . Through this process red blood cells are continuously produced in the red bone marrow of large bones . ( In the embryo , the liver is the main site of red blood cell production . ) The production can be stimulated by the hormone erythropoietin ( EPO ) , synthesised by the kidney . Just before and after leaving the bone marrow , the developing cells are known as reticulocytes ; these constitute about 1 % of circulating red blood cells . </P>
where are red blood cells made in adults
in the red bone marrow of large bones