What is the function hemoglobin?

Haemoglobin, the oxygen-carrying metalloprotein found in the red blood cells of mammals and some other animals, including some invertebrates, is responsible for the transport of oxygen from the lungs or gills to the tissues and the transport of carbon dioxide from the tissues to the lungs or gills. Haemoglobin is composed of a protein component, globin, and a non-protein component, the iron-containing haem group. The haem group is what binds to oxygen molecules and allows them to be transported through the bloodstream.