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. And whenever carbon monoxide bind onto hemoglobin, we call the hemoglobin carboxy hemoglobin. Now, the question is, what exactly can we tell from this curve? Well, first thing we can see is if we examine this side of the curve, the red curve is bound to the left of the normal blue curve.