The simple answer to this question is that we need oxygen. But I have a feeling you are looking for a more complex explanation. So here goes: All the processes in your body require energy. This energy is made inside your cells and is in the form of a molecule called ATP. Just about everything takes ATP. It is used to break food down into nutrients, for growth, for brain function, for damage repair – all the things that keep you alive. Your cells are constantly making and using ATP. The way that it is made is through a long chemical pathway in which electrons get passed around. Oxygen is the last electron acceptor! So without oxygen, your body cannot make any ATP. Amazingly, it also takes ATP to UNDO some things in your body. Your muscles cannot relax unless they use ATP. The lack of it is what causes “rigor mortis,” stiff muscles after death.