. Iron on absorption from upper small intestine circulates in plasma bound to transferrin and is transported to the bone marrow for utilisation in haemoglobin synthesis. The mature red cells are released into circulation, which on completion of their lifespan of 120 days, die. They are then phagocytosed by RE cells and iron stored as ferritin and haemosiderin. Stored iron is mobilised in response to increased demand and used for haemoglobin synthesis, thus completing the cycle (M = males; F = females).