At the realization that David was gone, all of the oxygen in her seemed to switch functions with her blood -- coursing through her veins creating the very definition of a hollow feeling.
Ok thought I'd just add my own opinion in about it. Two parts immediately jumped out at me while reading it at-a-glance, so to speak.
The first has to do with connotation. The word "oxygen" immediately brings to mind science, and you're left wondering if you're talking about the respiration process happening. The comment pops into people's heads, "why is this here at such a moment? I want to friggin be sad that he's gone, not concerned with the science of a potential metaphor!" The word "functions" calls up the same connotation, bringing to mind the dread days of learning how f(x) = mx + b (I happen to like math, but that's beside the point of your story). My advice here is to be careful of your metaphors (if you removed "seemed to" that is), in general, and make sure they make sense and really do achieve the greater depth of feeling you're going for. More often than not, I find I achieve this if I (1) know the scene and world of my story in and out and (2) I am in the moment, feeling the feeling, looking for what I can write down for people to attach themselves to with firmness, not turn them away in a feeling of isolation or cliche.
The second part that stuck out was "the very definition of a hollow feeling". First off, you assume by putting that in there that people know the very definition of a hollow feeling. Putting in "the very definition" only detracts from the read because it seems to imply to the reader "if you looked this up, you could get it because you don't already". If it was some weird word that no one hardly ever uses, I could see that phrasing working.. but it still wouldn't fit into the story. Also, you want to get to the heart of what she is feeling at this moment, correct? I know you are trying to "show" what a hollow feeling is through the oxygen phrase, but you are instead coming accross as "telling" because you state that it was "a hollow feeling". The question that comes from that is - "How hollow?! Why didn't you just tell me?" Then they realize that you were being clever in your exploration of the word "hollow" at an emotionally sensitive moment, and all their confusion about science and functions and definitions comes to a head and they detach from the story. They perhaps read on, but not within your world.. and that's a danger. Also, although they might enjoy your interpretation of feeling hollow, you have to be aware of the emotions that your readers are employing when reading your story and are fully invested in it.
Aside from these reactions... if your metaphor/simile works for you and the moment, it might be wise to switch the order, especially because the connotated meaning of oxygen (which I would replace with air, to be honest) should not overtake what the passage says it means. It will, however, be less effective if the oxygen is thrown at the reader before you've manipulated them into thinking of the oxygen-blood exchange as a metaphor for the hollowness of loss.
In my opinion, such a serious topic shouldn't have this metaphor attached with it, as it both borders on the lighthearted, and seems to connotate ideas dissociated with the effect you're trying to reach with the situation in the story. That is, of course, from all I know of your story, and those decisions are up to you.
Hope that's helpful!
Don't ever hesitate to ask for help like that - good on you.
EDIT - also thought of the word "flooded". It seems that this would work better than "switched places with" and we can simply infer that the blood left the building. It connotates invasion and distruction, especially when the thing it is invading is blood (which allows life). If you want this metaphor, "flooded" is an excellent word to choose.
