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If the partial pressure of oxygen in the atmosphere is 159 mmHg and that inside human alveoli is 104 mmHg, and if these values are fixed or don't change very much, then why does exhaled air have oxygen in it? Or when we compress our lungs this value changes?

Oxygen should not move from a higher partial pressure to a lower partial pressure no matter what other gases are exhaled because when we inhale, the pressure inside the lungs becomes 1-3mm Hg lower than the atmosphere; then air starts to enter and at a point when it stops entering this is the moment when the total pressure outside and inside becomes equal, and oxygen and other gases diffuse in the blood and carbon dioxide in the alveoli; which is why the partial pressure in the alveoli is 104 mmHg. But when we exhale the lungs start to compress and thus pressure inside the lungs increases and air moves outwards. But how is it possible that only the partial pressure of oxygen doesn't change and the pressure of all other gases inside the lungs increases?

It should also increase because if our expired air has oxygen in it then it should also increase?

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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
    Commented 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
    Commented Sep 30, 2020 at 15:55

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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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