Haemoglobin. Computer artwork of part of a haemoglobin molecule, the oxygen-carrying pigment found in red blood cells. The haem groups are represented by the two red discs. Haem is the pigment that gives blood its red colour. Each haem group contains an iron atom which can reversibly bind to an oxygen molecule. In a complete haemogl- obin molecule there are four haem groups. Each haem group is attached to a polypeptide chain known as globin (mauve tubing). There are some 350 million haemoglobin molecules in each red blood cell. Haemoglobin, as well transporting oxygen from the lungs to the body, also carries carbon dioxide from the body back to the lungs.