It's kind of difficult to understand hypoxia. The causes are different than what we would expect sometimes.
I had acute hypoxia due to hyperventilation recently. One would think that breathing a lot would cause more oxygen to reach the cells but it's not so. Breathing too rapidly actually makes it difficult for cells to absorb the oxygen, eventually leading to hypoxia.
I fainted as a result of hyperventilation. I remember being dizzy and confused and then I couldn't feel my legs. When I woke up, I was in the ambulance. Once my breathing slowed down and my cells could get oxygen again, I got better.