And the x axis is the concentration of oxygen in the surroundings given to us in partial pressures. So basically, we have millimeters of mercury, and the range is from zero to about 100. Now, the black curve describes the oxygen binding curve for pure hemoglobin when we essentially isolate it out of the red blood cells, while the blue curve describes the binding curve of hemoglobin as it is present inside red blood cells. And notice that the blue curve is shifted to the right with respect to that black curve. The question is why? So let's talk about how much of the hemoglobin actually unloads when it goes from the lungs to the exercising tissue in two cases, to see the difference between these two cases.