I was reading my textbook and it says that breathing involves inhaling and exhaling but it doesn't say whether they happen simultaneously or sequentially.

Also is my overall understanding of breathing competent? When you breathe in air, air ends up in the lungs passing through the nasal passages (heated, moistened, cleaned) through the pharynx, through the glottis through the larynx, through the trachea and into the bronchi and then into the bronchioles. Once air gets to the lungs, Is their already carbon dioxide in the lungs waiting to be expelled or will carbon dioxide diffuse into the alveoli and into the lungs when oxygen diffuses into the blood. And after this the carbon dioxide will be expelled?

  • $\begingroup$ My understanding is that gas exchange is a simple diffusion process in the alveoli level. When there's a concentration difference, it happens. It happens for every molecule simultaneously. $\endgroup$
    – Memming
    Jan 12, 2014 at 22:40
  • 5
    $\begingroup$ I think you should: 1) change the title to reflect your question and 2) edit the body and decide whether you are asking about inhaling & exhaling (which are sequential) or gas exchange in alveoli (which is simultaneous). $\endgroup$
    – Th334
    Jan 15, 2014 at 0:20

1 Answer 1


Inhalation and exhalation happen sequentially as Herman stated in the comments.

Yes, your general understanding of inhalation is correct. After the air gets into the lungs, the oxygen is diffused into the capillaries covering the alveoli. The now oxygenated blood travels back to the heart to be circulated throughout the body. As this blood enters capillaries in other regions in the body, the oxygen diffuses out of them and into the surrounding cells. Carbon dioxide (CO2) produced by the cells as a waste product then diffuses into the blood vessels. The CO2-rich blood is then circulated back to the heart to be pumped back to the lungs. This blood flows into the capillaries surrounding the alveoli, and diffuses out. The exchange of carbon dioxide and oxygen occurs simultaneously. So, as oxygen is diffused from the lungs into the bloodstream, CO2 in the bloodstream is diffused out into the lungs.

Here are diagrams to depict what is described above.

This image depicts the exchange of CO2 and O2 in the lungs. This diagram gives an overview of how these gases travel through the body as I described.


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