It's only by working together and using both the bulk flow and simple diffusion of oxygen that they can make possible the process of cellular respiration.
When we breathe in, our blood takes oxygen out of the air through our lungs and carries it to our cells where it's used in the process of cellular respiration.
Oxygen does play a crucial role in one of the last steps of cellular respiration, but luckily our bodies have alternative pathways that help us survive when we temporarily lack oxygen.
Ultimately, this is how nutrients, like O2 get pushed out to the organs and tissues that need it; and how wastes, like CO2, or carbon dioxide, which is the main by-product of cellular respiration, gets removed.
And when you inhale, your red blood cells grab the O2 from your lungs, and pass it off to your other cells, which use it for cellular respiration -- the process of breaking down sugar to create chemical energy.