How much oxygen does an animal need in the air? CO2 for plants?

Question

I'm doing a personal research project, mostly focused on different types of planets, their compositions, atmospheres, etc; It would be very helpful to this pet project if I knew a few bits of information-

1- What is the minimum density of oxygen nessisary in the air for us to breathe and survive? Since different planets have different atmospheric pressures, I'm hoping to get this in g/cm2, but even a percentage by our own atmospheric pressure would be helpful (i can calculate it myself)

2- What about other animals? Considering whichever multicell air breathing animal requires the least oxygen, what minimum 02 concentration do they need?

3- What about sea animals and O2 in the water?

4- Lastly, what about plants and CO2? Same as all the above.

Note that I am interested in the needed O2 and CO2 only, not air pressure or other factors.

I know this might be a tall order but this info would be VERY helpful, and I have no clue where to look for it. Thanks and God bless!

Also, any sources that can be cited would be very helpful.

Answer 1

Your complex question is even more complicated than it seems. That is because survival is not always survival. Allow me to elaborate:

The amount of oxygen you 'need' depends a lot on what you are doing. If you are unconscious, then your need is a lot lower than if you are running a marathon. The point being that it is quite possible to SURVIVE in an environment with very low oxygen, but survive without the ability to do more than sleep. And as we increase it, your ability to do stuff will increase up to a point.

A good example of this are some aboriginal socities which dwell in very high altitudes. I can think of one where children do all the farming and the like while parents stay at home because adults are too big to engage in that kind of activity at their oxygen level.

Which brings us to a second point - not everybody is the same. Children, having less mass, require less energy (and therefore less oxygen) to do the same things that adults might (link 3). The same applies to shorter people of all stripes. Different people also have different oxygen level tolerances from genetics, health, and other factors. Each person is going to be different.

There is also much to be said for adaptation. Mountain climbers, for example, often set up camps on particularly high mountains and rest there for a month to give their body a chance to produce extra hemoglobin for the lower oxygen content. Likewise, colonies established at high altitudes often show really low birth rates as the colonists have suppressed fertility (link 2), even though natives who have lived at those heights all their lives do just fine. So again there's more complexity - which people are you using for your oxygen calculations?

Having said all that, I can help point you in the direction of where to find some of this information, though again it will be far more broad then you may suspect. Link 1 below is a calculator for hemoglobin saturation at various altitudes. Other good places to look are 'high altitude agriculture' and 'hypoxia' (the medical condition of not having enough oxygen).