Because remember, as we increase the H plus ions, we decrease our PH. So these two things shift the curve to the right, thereby decreasing the affinity of hemoglobin for oxygen and allowing the hemoglobin to unload or release more of those oxygen molecules into the exercising tissues, the cells of our body. Now, how does this actually take place? So, let's begin with the PH effect. And let's focus on this blue curve here. So as of now, we're not focusing on increasing or decreasing the CO2 concentration.