why does the alveoli only absorb oxygen when we breathe in air and not also nitrogen or any other gases?

  • Composition of the air that we breathe in consists of many gases.
  • Nitrogen - 78%, Oxygen - 21%, Carbon dioxide - .03 - .04%, Hydrogen - traces, noble gases - traces. Nitrogen and other gases have no effect on the workings of our body and so it is emitted again when we breathe out. The only gas in the air which is used by human body is oxygen.


  • Alveoli are microscopic air sacs. They are the primary sites of exchange of gases. Gas exchange also occurs in lungs and tissues by diffusion.
  • Atmospheric air contains little CO2, but blood flowing in pulmonary capillaries has a higher concentration of CO2.
  • CO2 diffuses from higher concentration in blood across walls of alveolar capillaries to lower concentration in air in alveoli.
  • Blood coming into pulmonary capillaries is oxygen poor and alveolar air is oxygen rich.
  • Oxygen diffuses from higher concentration in alveoli across walls of alveolar capillaries to lower concentration in blood.

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Partial pressure plays a major role here. The partial pressure of oxygen in air is greater than in alveoli thus it diffuses into alveoli. It also takes in nitrogen but it is not used by the tissues so it just passes as it is. But it would be wrong to say that we breathe in only oxygen. Nitrogen is also present in the lungs in a considerable amount infact it also prevents the alveoli from collapsing. Opposite is the case of CO2, here the partial pressure of CO2 in lungs is more than in air so it goes out.

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