If we inhale air to get oxygen and exhale carbon dioxide, why is oxygen by itself deadly?

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And do they put carbon dioxide inside those oxygen tubes?

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The oxygen we breathe isn’t directly deadly because it’s bound to hemoglobin (iron containing protein that carries oxygenated blood from lungs to the tissues). However, breathing in pure oxygen can lead to formation of superoxide free radicals (oxygen with a free electron in its outer ring) can lead to damage to the fat that makes up our cell membrane leading to cell death

100% oxygen isn’t deadly in the short term (e.g. oxygen bars seem to be pointless but largely harmless.) That said, it’s not what you want long term, and does have serious toxicity.

Oxygen is very useful to us precisely because it loves to react chemically. Having oxygen lets us make WAY more ATP than we could without it. It’s also likely to react with all kinds of stuff, which is a real strain on the system. Oxidative stress can kill cells.

Air is about 21% oxygen, which is what we’re used to. It’s the level where our lungs are already getting what they need. You want “enough oxygen to make energy” without “so much that it reacts with everything.”

That’s not quite correct, you’re missing some context. Oxygen by itself is not “deady,” it becomes toxic at higher pressures. The air we breath is 21% oxygen. At sea level, air pressure is 1013.25 millibar. That means at sea level, the partial pressure of oxygen is is 213 millibar. You could breathe 100% pure oxygen at 213 millibar 24/7 for the rest of your life and be fine because that’s the same partial pressure of oxygen as you breathe in air. If you were to breathe 100% pure oxygen at a higher pressure, say sea level pressure of 1013.25 millibar, then it becomes toxic.

I’m not sure what you mean by “those oxygen tubes.” Are you talking about air cylinders? Nasal cannula? Regardless, the answer is no, why would there be added CO2? CO2 is toxic. that’s why we exhale it.