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I tested this out with my friends, and I find that after they hold their breath and can't hold it anymore, they exhale air, instead of inhaling air.

Interestingly, they all try to inhale in as much air as possible before starting to hold their breath. When I told them to exhale as much air as possible before starting to hold their breath, they inhaled air after they can't hold it anymore.

It's understandable that when one exhales then holds his breath, he needs air and thus he inhales afterwards. But when one inhales then holds his breath, shouldn't he inhale again after "using up all that air he inhaled before holding hi breath"?

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The lungs are not consuming the air as such, they are just adding a Carbon element to the O2 in it. –  Hermann Ingjaldsson Feb 22 at 18:54
    
@HermannIngjaldsson ??? I thought CO2 was the by-product of cellular respiration. –  Greek Fellows Feb 23 at 3:46
    
O2->CO2 is what the lungs do with the air you give them. –  Hermann Ingjaldsson Feb 23 at 7:32
    
@HermannIngjaldsson um.. could you be more specific? I don't think the lungs convert O2 to CO2; it should be the cells throughout the body that uses O2 and produces CO2, which is sent out from the blood into the lungs. Or do you mean the conversion of O2 to CO2 in the cells of lungs? –  Greek Fellows Feb 24 at 14:56
    
you breathe in air, which contains that necessary thing, oxygen. and you breath out that same air, just then a bit of the oxygen has been combined with a carbon atom. the cells throughout the body take o2 and give back co2, its just that the lungs take care of extracting that necessary o2 out of the air for them. –  Hermann Ingjaldsson Feb 24 at 18:11
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This is more about basic physics than biology. When you hold your breath, you normally take in one last long breath and keep it in as long as possible, Your lungs are therefore already full of gas (remember that the oxygen used by our lungs is only ~22% of the total volume of air you inhale). Therefore, when you release that breath and want to take in a new one you need to first empty your lungs out in order to refill them.

In addition, you need to have your lungs empty in order to be able to expand them and draw in more air. There is no biological reason for any of this, it is the same principle as trying to use a turkey baster:

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If you use it once and pull up some liquid into it, you will have to let air out before you can use it again. Your lungs work in the same way. Test this by exhaling all the air from your lungs and then holding your breath without inhaling. When you want to start breathing again, you'll be able to breathe directly in without needing to exhale first.

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Good question.

If you inhale on top of inhaled air this is more work. There is more dead air, air which is not as useful due to the lower concentration gradient. And we breathe more to exhale carbon dioxide than we require oxygen. Low oxygen levels only push us to breathe when oxygen levels are a good deal lower, however tiny changes in carbon dioxide causes changes in our ventilation rate almost immediately.

It is also more work as our muscles require more energy to inflate an already inflated chest. This is the case in COPD patients. Try this: take a deep breath in and now holding that air in, breathe normally on top of that air. Feel how difficult it is and how breathless you feel.

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Well the simple logical answer is that before we hold our breathe we first inhale some air inside as you should have noticed. So after holding it for some times , carbondioxide which is toxic to our body will be released after completing metabolic processes inside our body which will be pushed to our oral cavity ready to be expelled. Thats the reason why we cant hold our breathe for long and can't inhale any air since our oral cavity is already full of unnecessary carbondioxide to be exhaled.

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Welcome to the site! While you may have the beginnings of a good answer, it is trapped inside of a gigantic run-on sentence with several misspellings and run-together words that make it very difficult to determine what exactly you're trying to say. Please edit your answer to improve it, highlighting your answer to the question and not just giving a review of the mechanics of breathing, which isn't really relevant. –  MattDMo Feb 26 at 21:12
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