Hobozach,
Hemoglobin carries 97% of the oxygen. Only 3% is dissolved. Therefore, if you ain't got the RBC's to carry the oxygen, the oxygen ain't gonna get carried. Even if the RBC's are 100% saturated, if you only have one little cell running around (to be extreme in this example), you still have very low oxygen delivery. You have to use hyperbarics to dissolve enough oxygen in the plasma to keep a system alive.
For those who need an analogy, I always compared oxygen delivery to trucking: the lungs are the loading docks, the RBC's are the trucks, the arteries are the roads and cells are the customers. Normally the trucks are packed at 92-96% capacity. You can pack them higher if you want, but 100% is 100%. If you have less trucks, they are still packed to 100%, but you will still have less arriving to deliver the goods. That's why if yo infuse litre upon litre of fluids into a bleeding patient, you may still have good SpO2 levels, but lousy carrying capacity.
COPD issues deal with problems on the loading dock...you get the idea.