So because of the high partial pressure of oxygen within the Alveoli, our hemoglobin in the red blood cells will easily pick up our oxygen. And so that 98%, or about 98% of hemoglobin will be fully saturated with oxygen within the alveoli of our lungs. So in the alveoli of the lungs, the partial pressure of oxygen in the alveolar space is about one to 5 mercury. Therefore, most of the hemoglobin is saturated. Now, notice if we examine this curve, within this region of the curve, we have a relatively flat slope. Now, what exactly does that mean?