I know this seems like a retarded question (I hope nobody takes offence to that for it was not intended), but how much oxygen do we breathe in compared to the exhale of carbon dioxide? Also, how many oxygen atoms do we breathe in to exhale an amount CO2? And finally, if the oxygen in the CO2 equals the amount of oxygen intake, why can’t we split the carbon from the oxygen and reuse it? (I’m sure it can’t be that simple, that’s why I’m asking.)
While some of the oxygen is absorbed and used for a variety of functions in the body, a 1:1 ratio of O2 used during oxidative metabolism to CO2 out isn’t a bad rule of thumb. But for any given breath we only ‘take up’ about 20% of the O2 available in the air.
As for turning the CO2 into O2, CO2 has double bonds which are energetically expensive to break. Plants and bacteria turn CO2 into O2 because they want the carbon molecule, the O2 is a by-product. Since we get our carbon from other sources, we don’t need to spend the energy to fix carbon directly.
I was just curious. I was thinking about a space suit when I asked this. We use pure oxygen in space. Would that change anything?
Not really, no. It is more an issue of weight. It takes a lot of money to get into space so even things like air become really expensive. Why would we ship air that is 70% dead-weight (nitrogen) when we could ship what we actually need?
I don’t think I fully understand what your saying. I was saying that the gas they use in space suits are almost 100% oxygen. I see how it could be costly, but wouldn’t breathing air (Oxygen) last longer if the oxygen could be extracted from the CO2 and then reused? I’m not sure what you meant by 70% nitrogen (We don’t use that in space).
With respect to your second post: true, I over-simplified to keep the discussion streamlined. Sorry for the confusion.
With respect to your first post: it’s a question of energy. Splitting CO2 into some form of carbon and oxygen is very energetically expensive. So why don’t we do that with space suits? Because it isn’t financially worth it.
I understand the cost, but what I asked still remains unanswered (I’m not pissed if it sounds like it, just trying to learn.). I asked would our breatheing air last longer. Now that I think about it, that is more like common sense. Ok, another question. How do we or can we split CO2 into C and O2? This is not for the space suit no longer, it is for the question of how we would extract oxygen from the martian atmosphere.
Probably the best way to do it is to use plants and cyanobacteria. They are really, really good at it 
In terms of Mars, lichen would probably be a good start because, while slow growing, those things are incredibly hardy.