Myeloid (blood producing) tissue is found in the red bone marrow located in the spongy bone. As a person ages much of this marrow becomes fatty and ceases production. But it retains stem cells and can be called on to regenerate and produce blood cells later in an emergency. RBCs enter the blood at a rate of about 2 million cells per second. The stimulus for erythropoiesis is the hormone erythropoietin, secreted mostly by the kidney. This hormone triggers more of the pleuripotential stem cells (hemocytoblasts) to follow the pathway to red blood cells and to divide more rapidly.
	It takes from 3 to 5 days for development of a reticulocyte from a hemocytoblast. Reticulocytes, immature rbc, move into the circulation and develop over a 1 to 2 day period into mature erythrocytes. About 1 to 2 % of rbc in the circulation are reticulocytes, and the exact percentage is a measure of the rate of erythropoiesis.