Oxygen first, and probably a lot of other things. The less blood you get in your brain, the less oxygen and nutrients you get and you get worse. This is just a simple case where quantity matters. Low quantity (insufficient blood volume) is just as bad or worse than low quality (not enough oxygen). What you are saying is that as long as you have enough oxygen in your blood you are going to be ok even though you may not get enough blood. When you are losing blood from an injury or a cut, your body makes your blood vessels smaller to try to keep your blood pressure up. Do you think people who have lost a good amount of blood feel tired or dizzy or they still feel fine because they have enough oxygen in their blood?