Iron in the blood. Computer artwork of iron molecules on a background of red blood cells (red). Iron atoms within the haemoglobin molecules of red blood cells bind to oxygen. Haemoglobin molecules are comprised of four globin polypeptide chains each carrying a haem group. The haem group itself consists of an iron atom bonded to an organic molecule known as a porphyrin. 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.