What does hemoglobin do in the body? Hemoglobin consists of four polypeptide chains each containing a heme group with an iron atom that binds to oxygen 1O22 in the lungs. From the lungs, hemoglobin transports oxygen to the tissues of the body where it is used to provide energy. Once the oxygen is released, hemoglobin binds to carbon dioxide 1CO22 for transport to the lungs where it is released.