Hemoglobin is the primary oxygen carrier in our blood. It delivers the oxygen from the lungs to the tissues of our body. Now, recall that hemoglobin consists of four individual polypeptide subunits. And each one of these polypeptide subunits contains a single Hen group that is capable of binding oxygen molecules. Now, because these four polypeptides can interact with one another our hemoglobin displays something called Cooperativity. And what that means is if we take a fully unsaturated deoxy hemoglobin protein and one of the heme groups accepts an oxygen molecule then that will create a conformational change in the entire structure and that will cause the other three unoccupied hein groups to become much more likely to accept additional oxygen molecules.