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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 '14 at 18:54
    
@HermannIngjaldsson ??? I thought CO2 was the by-product of cellular respiration. – Greek Fellows Feb 23 '14 at 3:46
    
O2->CO2 is what the lungs do with the air you give them. – Hermann Ingjaldsson Feb 23 '14 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 '14 at 14:56
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@HermannIngjaldsson I think that's a very limited and confusing way to describe it, especially the first two comments. Your statements suggest that C is added to O2 in the lungs which is not the case. The lungs are purely a gas exchange organ in this sense. O2 is brought in, CO2 kicked out, following concentration gradients. The air you breath in is not the same as you breath out. – Nathan Dec 15 '15 at 16:05
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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:

                                  enter image description here

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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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"?

Before we hold our breathe we first inhale some air inside.

So, after holding it for some times, carbondioxide (CO2) which can be toxic to our body will be released after completing metabolic processes inside our body which will be pushed to our thorax, ready to be expelled.

The partial pressure of this gas (pCO2), is the primary fast (minutes or hours) trigger for breath; where partial pressure of Oxygen (pO2) is a slow trigger (hours or days). This explains why patients with severe re-acutized cronic obstructive pulmonary disease (COPD) stop spontaneous breathing, even if their pO2 falls, because of augmented pCO2.

The pCO2 is sensed in the human body in the Central Nervous System (CNS), precisely in the ponto-bulbar centers of breathing. The pO2 is sensed at periferical strucrures as the glomi aorticus and carotideus.

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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 '14 at 21:12

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