Now, let's begin by recalling some information about hemoglobin that we learned previously. So we said the fact that the oxygen binding curve for hemoglobin is sigmoidal basically implies that hemoglobin behaves in a cooperative fashion. And what that means is the different heme groups actually interact with one another. So as one of the heme group becomes occupied with oxygen, it causes the other heme groups to increase their affinity for oxygen. And likewise, when one of the heme groups releases the oxygen, the other occupied heme groups become much more likely to unload and release that oxygen. So this is what we mean by the cooperative nature and the cooperative behavior of hemoglobin molecules.