The body will not get oxygen if what system fails

I would say the respiratory system (the lungs) fail. The lungs are to receive oxygen and it sends it to the heart.

I am not much of a biology nerd but I am taking an educational guess.

All I can say that the lungs need oxygen. Without oxygen, the heart will stop pumping blood and you will be suffocating with this essential. That is why you should never smoke and actually take great care of your precious body.