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. They can cooperate with one another to basically either release or bind that oxygen manner, that oxygen molecule in a cooperative fashion. Now, what this means physiologically, as I said earlier, it makes the hemoglobin a great carrier of oxygen. So what that means is at the lungs, when our hemoglobin is in the lungs, it can easily bind that oxygen.