Haemoglobin. Computer artwork of haemoglobin molecules on a background of red blood cells (red). Haemoglobin is the oxygen-carrying pigment in red blood cells. Haemoglobin molecules are comprised of four globin polypeptide chains (two are brown, two are grey), each carrying an iron- containing haem group (red disc) which binds to oxygen. Haemoglobin transports oxygen around the body by binding reversibly to it when the local concentration is high (in the lungs) and releasing it when the local concentration falls (in other body tissues). There are about 350 million haemoglobin molecules in each red blood cell.