Now, the x axis is the partial pressure of oxygen inside our tissues given to us in millimeters of mercury. So it ranges from zero to 100. Now, the blue curve describes the normal oxygen hemoglobin dissociation curve. That is, it describes a situation when we have no carbon monoxide present inside our blood plasma. On the other hand, the red curve describes a situation when we do have carbon monoxide present inside the blood plasma. In fact, it describes a particular situation where 50% of the hemoglobin is occupied by carbon monoxide.