Please explain why no changes in partial pressure of oxygen in the case of anemia hypoxia?

Hemoglobin: Hemoglobin has four ligands to carry o2, i believe usually 2 used.You give o2, has dissolubility, and membrane transport to enter the cells to the hemoglobin, bind and in tissues where there is acid and low o2 tension and other factors are released to those tissues for energy processing. You give more o2 and the o2 in the plasma increases but not in the saturation of the hemoglobin due to above.