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Why don't we breathe nitrogen while it makes most of the air?

Why do we always tend to breathe oxygen, not hydrogen and nitrogen?

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up vote 10 down vote accepted

I'd argue that we do "breathe" all those gases. Air that we inhale (at sea level) is about 78% N$_2$, 20.9% O$_2$, 1% argon, and smaller percentages of CO$_2$, neon, methane, etc. So all those gases are going into the lungs with every breath in.

We take up oxygen preferentially because we have hemoglobin to bind O$_2$. When hemoglobin binds the oxygen, it upsets the balance and pulls more oxygen across the alveolar membrane. This is aided by pulmonary circulation which carries the blood away. Here's a demo of the diffusion process.

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Nitrogen dissolved in the blood and pressurized during an underwater dive can, during a return to the surface, bubble out of the blood, just like the release of pressure opening a bottle of a carbonated drink causes bubbling. In human divers, it can causes the painful and potentially lethal condition called decompression sickness or "the bends". – PlaysDice Apr 25 '14 at 17:17

Animals use oxygen as a chemical energy source because oxygen gas can react with many other compounds to form oxides, which releases energy and happen spontaneously.

Both carbon and nitrogen can be made to react with oxygen, but otherwise they are pretty inert. So of all the gasses in the air present at over a fraction of a percent, oxygen is the only one we can use for energy.

Hydrogen (and sulfer) are both possible substitutes for oxygen in the role of redox energy source, but are normally pretty small components of our environment. On another planet they might well be the basis of biometabolism.

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Nitrogen is much less reactive than oxygen. Indeed, if I haven't totally forgotten my long-ago chemistry courses, most chemical reactions involving N2 are energy-consuming. Thus you get nitrogen compounds produced by lightning, in auto engines, and other places where there's a lot of energy to spare.

Oxygen reactions, OTOH, are energy-producing. You might think instead of fire: most organic stuff will burn (if dried), but it only combines with the oxygen in the air, not the nitrogen.

PS: Indeed, many nitrogen compounds take so much energy to create that they are explosives. Ammonium nitrate, nitroglycerin, trinitrotolulene (TNT), even the potassium nitrate (saltpeter) used to make gunpowder.

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The bond in oxygen molecules is high energy, and ready to undergo an energy-yielding reaction with other molecules like sugar.

The bond in nitrogen not chemical useful to us...other organisms use energy to "fix" nitrogen to make energy rich nitrogen compounds that we can use.

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Basically when air fills our alveoli, by the process of diffusion, only oxygen in the air is taken into the blood stream while the other gases along with the waste CO2 is exhaled. So you do breathe in nitrogen, but it is exhaled as it is by the body. The whole process of the respiratory system is explained here with diagrams.

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Nitrogen and CO2 are also taken in. All gases dissolve and diffuse across the alveolar membrane – One Face Jan 6 '15 at 17:28

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