In school, I have learnt that the deoxygenated blood brought in to the Alveoli by Pulmonary Arteries contains Carbon Dioxide. And that Carbon Dioxide present in the blood diffuses into the Alveoli and squeezed out with the help of Diaphragm.

Then, why doesn't the Nitrogen present in the Alveoli gets diffused to the blood? My guess was that the blood vessels don't allow anything other than CO2 and O2 to pass through.

But I recently saw a video on KhanAcademy where Sal said that some amount of Nitrogen does get mixed with blood. Is Sal right? If yes, then why doesn't the whole Nitrogen present in the Alveoli gets diffused from there (where it is present in high concentration) to blood (where it is present in low concentration)?

To summarize,

  • Why doesn't the inhaled-Nitrogen present in the Alveoli diffuses into the blood present in Pulmonary Veins, similar to how CO2 present in blood gets diffused out to the Alveoli?
  • 3
    $\begingroup$ It does get diffused, your blood has plenty of N2. Actually, the arterial pN2 is higher than the pO2, making N2 the most abundant gas in your blood. $\endgroup$
    – user24284
    Commented May 27, 2017 at 7:27
  • $\begingroup$ @GerardoFurtado So, Sal Khan was wrong. I guess you should put that as an answer so that I can accept it. $\endgroup$
    – Arjun
    Commented May 27, 2017 at 7:29
  • $\begingroup$ He's technically correct, if he said "some" amount. $\endgroup$
    – user24284
    Commented May 27, 2017 at 7:30
  • $\begingroup$ @GerardoFurtado How? Why only some? $\endgroup$
    – Arjun
    Commented May 27, 2017 at 7:31
  • $\begingroup$ Because technically some is indefinite, it can be anything from a very little amount to almost 100%. Well, I was about to post, but someone already did it. $\endgroup$
    – user24284
    Commented May 27, 2017 at 7:38

2 Answers 2


I like it that nitrogen came to your mind ;)

I guess in school they didn't tell you about the nitrogen at all, for the matter of simplification, because it is of no function in the body and the $O_{2}$ and $CO_{2}$ are the important gasses for biological function of breathing.

However, all of these small gas molecules are very small and generally the biological membrane is very permeable to them. That's why the cell cannot decide to let $O_{2}$ and $CO_{2}$ in and $N_{2}$ not as you suggest.

(However, the cell is able to control the permeability of these gases to some content, based on the level of cholesterol in its membrane$^1$)

The process of gas exchange is generally passive. In alveolar membrane the gas flows from the site of its higher concentration (in case of gases we talk rather about partial pressure) to the site of lower concentration. The speed of this process depends mostly on the difference in partial pressures and if you let the blood "interact" with the air in alveoles for long enough, partial pressure of all small gas molecules would come almost to equilibrium. (This happens under physiological conditions for all of the three mentioned gases)

Khan academy is right too actually. This is because as $N_{2}$ isn't either produced or consumed in the body, its partial pressure in the venous blood is similar to that in the alveolar air, and there is no force to cause its movement between these two compartments in general.

Partial pressures of major gases in different body compartments

In the table you can see, that the partial pressure of nitrogen in the blood is the same as in the alveolar gas.

Picture source

  • $\begingroup$ "the gas flows from the site of its lower concentration to the site of higher concentration" This is physically impossible. $\endgroup$
    – user24284
    Commented May 27, 2017 at 7:39
  • $\begingroup$ @GerardoFurtado thanks, random mistake. corrected $\endgroup$
    – mpribis
    Commented May 27, 2017 at 7:44

Nitrogen is not a metabolic gas, its partial pressure is very high. No compound of blood can react with Nitrogen hence its diffusion is not possible.
its diffusion is only possible when a person is suddenly lifted from depth of ocean, such diffusion causes bubbles in blood vessels leading to death due to disease called KESHAN THIS IS WHY WARNINGS ARE GIVEN PRIOR TO SCUBE DIVING

  • $\begingroup$ Please add some references explaining how you got this answer $\endgroup$
    – L.B.
    Commented Feb 9, 2018 at 18:45

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