So, the oxygen binding curve of hemoglobin inside red blood cells differs considerably from the pure hemoglobin curve, as we can see from the following graph. So, in the graph, the y axis is the fractional saturation of hemoglobin. It ranges from zero to one. 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.