I am a high school student and I am a little confused in the concept of breathing:

My confusion is that, when we talk of exchange of gases we say that gases always diffuse from their high "PARTIAL PRESSURE" to their "LOW PARTIAL PRESSURE" and we know that partial pressure of oxygen in our alveoli is about 104mmHg and in our blood vessels it is about 95mmHg ,so we can see that this statement sounds true.

But partial pressure of oxygen in atmospheric air is about 159mmHg and in alveoli it is about 104mmHg then why the air that we exhale also have about 16% of oxygen in it? I think this statement is not correct oxygen don't have brain it will always move from the higher PRESSURE OF TOTAL MIXTURE TOWARDS THE LOWER PRESSURE OF TOTAL MIXTURE, e.g assume a balloon filled with different gases in it and total pressure of gases inside the balloon in much higher than the surrounding, but balloon has very less amount of Oxygen in it as compared to other gases so, its partial pressure is less then the surrounding so of course when you let it go all the air from balloon rushes out even oxygen, and its seems logical as oxygen doesn't know how is it different from other gases , how only oxygen can left inside the balloon?

I think beside this In exchange of gases we would talk in terms of partial pressure because we have to talk only about oxygen and carbon-dioxide ,not all gases, so just to represent their concentrations we talk in terms of partial pressures? But saying this statement that gases always diffuse from their high partial pressure to their low partial pressure this is wrong ,please correct me If I am going wrong somewhere and please give a logical explanation to it.


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    $\begingroup$ welcome to se.biology. I tried editing your question to make it more clear, but I struggled a bit with the second paragraph. I think you will get better answers if you try and condense the second paragraph into something more clear and focused. Adding more paragraphs, punctuation and only asking a single question might help. $\endgroup$
    – user438383
    Sep 30, 2020 at 13:39
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    $\begingroup$ A piece of advice in composing questions. Do what I generally do, which is write it in a word processor document first, edit it, and run an English spelling and grammar check. And English is my native language. And write short sentences. $\endgroup$
    – David
    Sep 30, 2020 at 15:55

1 Answer 1


You seem to be confused about diffusion versus bulk flow of fluid (air/gas).

In the lungs, there is diffusion between blood and the gas in the lungs. There is sufficient surface area that these equilibrate fairly quickly, and the partial pressure of oxygen in blood leaving the lungs is very similar to the gas in the lungs. Partial pressures are the best way to understand and explain diffusion of gases.

When you breathe out, a fraction of the bulk air moves out and mixes with outside gas. The partial pressures don't matter for this part, however, all that matters is that you have a big opening (as far as a gas is concerned) through the mouth/nose. Then again, when you breathe in, a fraction of bulk external air moves in and mixes with gas that remained in the lungs (you never completely breathe it out). This mixture will initially have more oxygen in it than the mixture that was exhaled, but it again quickly equilibrates with the blood via diffusion.

If you were to breathe in and wait long enough, eventually the partial pressure of oxygen in the blood would drop (used up in metabolism), and the concentration in the lung gas would also drop to match. However, you would lose consciousness far far before this would ever approach "zero".

The lungs are not a machine to pull all the oxygen molecules out of the air and provide it to the body. They are a gas-exchange surface to provide a massive surface area to help make the internal oxygen concentration as close as is possible to the external oxygen concentration. It never quite gets there, of course, but it gets relatively close.


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