system
Given the following post, respond as if you're commenting on Reddit:
human
Why can I breathe the same air longer in and out (e.g. with balloon) than by just holding it?

If I take a balloon and blow some air into it, then let it go back into my lungs, repeatedly, I can do it longer than it takes to get bored, but when just holding my breath, a minute is already very hard.

One could think, it should not matter, as it is the same amount of air/oxygen in both cases.

So I am curious, what does happen in the lungs that makes the difference? I guess it has to have to do with CO2 somehow being less concentrated in my lungs when circulating the air? If yes, why is it so?
gpt
I don't know the precise values, but some portion of the air you inhale never makes it to the aveoli; it's called dead-air space. this is why dogs can pant so much and not hyperventilate- the fresh air is only moving in the mouth and some way into the trachea. 

so when you take one breath and hold it, some portion arrives at the aveoli to exchange gasses, but the remainder is just sitting in the trachea, bronchi, and broncheoli without exchanging gasses. the air in the aveoli gains more and more carbon dioxide (CO2 levels, not oxygen, is the primary drive to breathe), and eventually the CO2 levels in the blood start to rise.

By breathing into the balloon, the aveoli are cleared of this stale air. You now distribute your CO2 into the whole volume of air, instead of it building up right next to your bloodstream, (in the aveoli).