So, the Y axis is the percent saturation of hemoglobin, and we range from zero to 100%. The x axis is the partial pressure of our oxygen inside our tissue, exercising our tissue, and that is given to us in millimeters of mercury. And so we begin at zero mmhg, and we end up at 100 hhmg. So notice that this partial pressure corresponds to the partial pressure inside our lungs. And this partial pressure of 40 mercury corresponds to the partial pressure inside the tissue of our body. Now, the blue curve is the curve that describes our relationship when we don't have any two three BPG present inside our blood plasma.