Computer graphic representation of part of the haemoglobin molecule, the oxygen-carrying substance of human blood, showing its four polypeptide (protein) chains. The protein component of haemoglobin combines with four haem groups (iron-containing porphyrin substances), shown here in green & red. Haemoglobin has the ability to combine reversibly with oxygen, taking it up in the lungs and releasing it through the tissues. Red blood cells (erythrocytes) are literally packed with haemoglobin, containing only very small amounts of other substances.