I know that lungs are extended and compacted by muscles to create partial vacuum to breath in, or to create elevated pressure to breath out. But one thing seems not right to me...

As far as I know, air comes through bronchi airways (like pipes) then branch like twigs and end up in tiny berry-like alveoli where gas actually exchanged... That's all clear, but I can't connect this 2 things together.

It seems to me, that bronchi + alveoli is a closed system. So how by creating partial vacuum inside lungs themselves (like in bags) makes air come into alveoli if it's a closed system and the air pressure outside alveoli does not has access to outside air at all (as it seems to me).

enter image description here

I hope this image will make it a little bit clearer... So does bronchi have some kind of openings in them to let pressure inside "bags" connect to outside world or does alveoli walls are so thin, that it actually allows gas to flow freely? What am I missing here?

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    $\begingroup$ this may help your understanding. youtube.com/watch?v=H62wTF9vKPQ one video is worth a thousand words.I recommend turning the sound off. $\endgroup$
    – John
    Mar 26, 2020 at 20:22
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    $\begingroup$ Why do you say the "bronchi + alveoli is a closed system"? The bronchi are open to the atmosphere through the mouth and nose. $\endgroup$
    – mgkrebbs
    Mar 26, 2020 at 20:59
  • $\begingroup$ I would require to your approx age. Are you high school student? $\endgroup$ Mar 27, 2020 at 7:01
  • 1
    $\begingroup$ No, I'm 27 y.o. $\endgroup$ Mar 28, 2020 at 10:45

1 Answer 1


(Note: Since the question language isnt clear; it just seems to me the
1. The purple-blue "Wall" (technically the plurae; containing plural space not shown) is being refered as the bag.
2. The ruptures are drawing artefacts. There arenot such ruptures.
3. I assume you know that the lumen of the broncheoles is continuous with the lumen of the alveoli ("grapes" in your analogy). I would request you to clarify the language of the question.)

"So how by creating partial vacuum inside lungs themselves (like in bags) makes air come into alveoli"

Up to best of my understanding; atmospheric air has its own pressure, and own ability to expand. So when chest muscles expand and let the lungs to expand (probably you refer it with analogy of "bag") gives space to the alveoli to expand, the atmospheric air shall come through the broncheole and fill the alveoli to expand them until and unless a certain equilibrium appear.


The "bag" you mentioned (Purple-blue mark?) isnt like an empty bag; it can transfer the pressure to alveolar wall. Because the "bag" is filled with compact alveolar wall (imagine your "bag" (blue) as a ruber balloon and remaining lung tissue as a mass of sponge tightly filled in that baloon) and there are some tissue fluids too. so an improvised analogy would be like this:


Note the springs i have drawn are not real spring. I hypothetically drawn to mean the wall is able to rtansfer the push and pull to alveoli.

The realistic histological section will help to clear some confusion I hope.

enter image description here (source: http://medcell.med.yale.edu/histology/respiratory_system_lab/respiratory_bronchioles.php)


So does bronchi have some kind of openings in them to let pressure inside "bags" connect to outside

If there were rupture in bronchi or alveoli; then opposite is expected. it should leak the air to freely move to the "bag" you said, and would destroy the way to develop pressure difference and will cover up the alveoli and will crumble them (imagine the video that user @John has shared; if the red balloon is ruptured, you will be no more be able to inflate the baloon the way they are inflating.

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    $\begingroup$ I see... So the actual partial vacuum/increased pressure are actually created in the alveoli themselves, and lungs walls are just there to compress/decompress alveoli themselves? $\endgroup$ Mar 27, 2020 at 5:07
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    $\begingroup$ intuitively Seems so. But i am not sure whether alveoli has a significant elasticity to maintain their fluffy (enlarged) state by their own, but that too may be an effect. $\endgroup$ Mar 27, 2020 at 5:30
  • $\begingroup$ Hm, that was a really good video. Didn't though about pressure differences in closed system that has and elastic open system inside of it... So, seems like this closed "bags" have constant pressure of about 1 atmosphere, right? So deep under water it would be crushed, but in outer space, where there is no pressure, it would potentially burst (or it's walls can actually withstand outward 1 atm)? And that's why on high mountains its so hard or even impossible to breathe? Cos alveoli are crushed by reliantly high internal "bag" pressure, and increasing it's size does not produce enough vacuum... $\endgroup$ Mar 28, 2020 at 11:01
  • $\begingroup$ ... to let low outside pressure to extend alveoli? So, it's just purely this internal gas pressure of sealed "bag" that affects alveoli, not a mechanical push on them from the "bags" walls? Cos in this case (if it where only mechanical) it would be easy to breathe on high mountains. Can you confirm my understanding of this matter? P.S. Oh, and I see on that image that terminal bronchi and alveoli themselves transform into squamous type cells (like in top of skin) to be elastic, right? $\endgroup$ Mar 28, 2020 at 11:05
  • $\begingroup$ Sorry for idiotic typos, today is no my day, in terms of writing stuff... $\endgroup$ Mar 28, 2020 at 11:12

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