Haemoglobin molecule artwork. Haemoglobin is an oxygen-carrying molecule found in red blood cells (erythrocytes, red, upper frame). It contains four globin polypeptide chains (purple and blue); two alpha chains and two beta chains. Each chain carries a haem component (yellow disc) capable of reversibly binding oxygen. The disc contains iron (black sphere) which binds an oxygen molecule (red spheres). Oxygen is bound in the lungs, where its concentration is high, and released in the tissues, where the concentration is low. Haemoglobin also carries carbon dioxide to the lungs for exhalation. There are around 350 million haemoglobin molecules in each erythrocyte.