system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If we breathe out carbon dioxide why does CPR work?
gpt
Our lungs exchange O2 with CO2, but not all of it. 

Going in, the air is: 78% Nitrogen, 21% oxygen and .04% CO2. 

Coming out its: still 78% Nitrogen, but now ~15% oxygen and 5% CO2. So there's still oxygen in it. 

If you're performing assisted breathing on someone its because they can't breath on their own. So the 15% O2 you're _forcing_ into their lungs is better than whatever little they're getting on their own account.