So the sigmoidal curve is produced as a result of hemoglobin's ability to bind oxygen in a cooperative fashion. So earlier we said that myoglobin binds oxygen in a non cooperative fashion, while hemoglobin binds it cooperatively. So what do we mean by that? 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.