CO2 and O2 - are concentrations directly proportional to each other?

Question

So if your levels of O2 were say 75% does this mean that there is 25% CO2? Or does it just mean that there is 75% O2 and 25% of other gases, whatever they may be...?

Answer 1

No, they are independent.  

Those gases are typically measured in partial pressures, and that's what you'd see on blood gas analysis.

The level of CO2 is generally related to ventilation - how much air is moving in and out.  The level of O2 is related to oxygenation - how much O2 is getting into the blood, and is influenced by ventilation, diffusion of oxygen, the concentration of inspired oxygen and shunting, among other things.

Answer 2

There are many gases in the atmosphere. There is very little CO2, less than 1%. Most is nitrogen, about 80% I believe. They are not proportional.

Answer 3

Since you say "your" I guess you mean a person's CO2/O2 in the blood? Levels of CO2 in the blood typically aren't measured in percentages, but hemoglobin O2 saturation - which is displayed on hospital O2 sat monitors - is. And it is almost always 98-99% except for some real sick cookies. Generally O2/CO2 proportions do vary somewhat inversely in the blood, since hemoglobin has greater O2 affinity when CO2 has been unloaded, but the relationship is indirect and the two gases are not transported the same way in the blood.  

75% O2 sat isn't a common percentage- rather, it is usually very high at around 99-100%, as in arteries and the veins leaving the lungs, or very low (less than 50%), as in the veins returning to the heart or the arteries going to the lungs. Hemoglobing "wants" to be either fully saturated or unsaturated with O2. I compare it to a college student and alcohol - either completely sober or binge drinking because they are always either in class or partying. Anyway, if someone's O2 sat were 75%, I would expect their CO2 to be near 100% of its carrying capacity, since 75% is quite low for O2. It's hard to predict though. This person would be blue in the face and in terrible pain.

More appropriate questions would be what are CO2 levels whe O2 is:

Pulm Veins/systemic arteries: 100% HbO2 sat (~100mmHg pO2), %CO2 is probably at its lowest- 0% typical transport capacity (~42mmHg pCO2).

Pulm Art/systemic veins: 10% HbO2 sat (~10mmHg pO2), %CO2 probably highest-100% typical transport capacity (~46 mmHg pCO2)

As you can see CO2 doesn't fluctuate nearly as much as O2, nor is it ever as depleted to the same extent as O2.