Red blood cells are the oxygen carriers in the body. They contain the molecule haemoglobin that binds to oxygen in the lungs and releases it in tissues that need it. Red blood cells are created in the bone marrow and typically have a lifespan of 100-120 days, after which point they are recycled. Red blood cells have no nucleus in order to be able to fit more haemoglobin into their structure. During development they 'enucleate' or forcibly eject their nucleus. This means that they can no longer create their own proteins.