So the blue curve is the oxygen hemoglobin dissociation curve. So this S shaped sigmoidal curve. Now, the Y axis is, as always, the percent saturation of hemoglobin for this blue curve. And the X axis is the partial pressure of oxygen given to us in millimeters of mercury. Now, when the hemoglobin actually takes the oxygen from the lungs and brings it to the muscle tissue of our body, it has to drop that oxygen off. Now, the question is, what exactly picks up that oxygen?