When our teacher was talking about the gas exchange inside the alveoli, he mentioned the flow of air and the flow of blood was going in opposite directions so that there always would be a concentration difference.

Because of this the gas exchange between the alveolar space and capillary continues and reaches ~99%.

I could not understand how the air would always go in the same direction so I researched the topic. All resources I found state that when the air enters the alveolus there is a pressure gradient for both oxygen and carbon dioxide between the capillary and the alveolar space.

But I found nothing mentioning the opposite direction of flow for air/blood.

So does that mean gas exchange for oxygen and carbon dioxide stops when diffusion equilibrium is reached?

(I'm not a native English speaker so I hope this question makes sense.)

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    $\begingroup$ I think that is about fishes, and most aquatic creatures. They exchange gases from their gills. Most advanced animals have blood vessels in their gills that flow on the opposite direction to the water that is flowing through the gills. $\endgroup$
    – cc12amu
    May 8, 2022 at 15:38
  • $\begingroup$ Welcome to Biology.SE. Please take the tour and consult the help center starting with How to Ask for details on using this site, which promotes self-learning with some expert help. See also the "homework" policy. Please edit this down to a single question and tell us where you've looked for answers, what you do know about the topic, and where exactly you still have questions. ——— In particular this seems like the sort of question your teacher could best answer ... $\endgroup$
    – tyersome
    May 8, 2022 at 17:26
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    $\begingroup$ oxygen will go into the blood and co2 will go into the alveola never the oposite. $\endgroup$ May 9, 2022 at 3:04
  • $\begingroup$ @trondhansen Bingo. I'm sure that's what the tutor meant. $\endgroup$ May 9, 2022 at 3:08
  • $\begingroup$ @cc12amu This seems to be the case. I dont think my teacher would get it wrong. Probably my sleep-deprivated brain is recalling the whole thing very differently, thanks for the answer. $\endgroup$
    – Quasoir
    May 10, 2022 at 18:17

2 Answers 2


The air in the alveoli has a higher oxygen concentration than the blood, and the blood has a higher carbon dioxide concentration than the air. Due to this concentration gradient, the carbon dioxide will diffuse from the blood to the air, and the oxygen will diffuse from the air to the blood. My guess is that this is what your teacher referred to as "opposite flow".

However, you are right in noting that this flow will eventually reach equilibrium, and gas exchange will cease between the air and the blood. This is where the role of the lungs come in. The lungs constantly exchange carbon dioxide rich air for oxygen rich air, and hence the gas exchange continues happening. Cellular respiration also converts the oxygen rich blood to carbon dioxide rich blood.


Difference in the Partial pressures of Oxygen and Carbon Dioxide in the alveoli and blood capilariess respectively cause respiration.

  • In alveolar blood, PO2 is low while PCO2 is high, whereas in atmosphere its vice versa, so it diffuses out to the atmosphere.

  • In capillary blood, PO2 is high and PCO2 is low, whereas in the cell its the opposite, so it diffuses into the cell.

Even if the diffusion equillibrium is reached, the negative pressure gradient of the lungs make sure that there is a constant exchange of gases.

You are good at observing that bro. I didnt even this of that when i studied respiration.


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