A necessary mineral for the proper function of hemoglobin, the protein in red blood cells that carries oxygen. It is also needed for proper muscle and organ function. About three-fourths of the body's iron is bound to hemoglobin in red blood cells, while the rest is either bound to other proteins (transferrin in blood or ferritin in bone marrow) or stored in other body tissues. When red blood cells die, their iron is released and carried by transferrin to the bone marrow. In the bone marrow, iron is stored and used as needed to make new red blood cells.