And why does that actually occur? Well, this means that the binding of oxygen at one heme group at one side on the hemoglobin makes the other unoccupied sites much more likely to bind oxygen. So what that means is the different heme groups inside our hemoglobin actually interact with one another. And so, for example, if we have a hemoglobin that contains fully unoccupied sites, when one of those heme groups binds oxygen, that will make the other three unoccupied sites much more likely to bind an oxygen. And conversely, the release of oxygen from one side on the hemoglobin molecule makes to other occupied sites much more likely to unload those oxygen. So, because hemoglobin consists of these four polypeptide chains, the four heme groups found on the four different chains can actually interact together.