This is known as a Sigmoidal shape. And the reason it is a Sigmoidal shape is because of this positive cooperative behavior of hemoglobin. And we'll see why that's the case in just a moment. So let's examine and let's try to describe and understand what this curve actually tells us. So let's begin with the alveoli of our lungs. We know that in the alveoli of our lungs, in the Alveolar space, the partial pressure of oxygen is about 105 mercury.