An iron-containing protein in the blood of many animals that, in vertebrates, carries oxygen from the lungs to the tissues of the body and carries carbon dioxide from the tissues to the lungs. Hemoglobin is contained in the red blood cells of vertebrates and gives these cells their characteristic color.he protein in the red blood cells of vertebrates that carries oxygen from the lungs to tissues and that consists of four polypeptide subunits, each of which is bound to an iron-containing heme molecule. [Ultimately short for.