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Here's a table from Guyton and Hall Textbook of Medical Physiology showing the partial pressures of gases in different types of air:

Table 40-1

We can see that as the partial pressures of some gases decrease or increase, the partial pressures of the other gases increase or decrease, respectively. Let's focus on nitrogen. The PN2 in humidified air is lower than in atmospheric air because the PH2O in humidified air is higher (I'm not sure if the word "because" is correct here, but I guess it is). The PN2 in alveolar air is higher than in humidified air because the PO2 in alveolar air is lower.

Since partial pressure is proportional to concentration, I believe a gas should go somewhere for its partial pressure to decrease or come from somewhere for its partial pressure to increase.

Where do nitrogen molecules go or come from in the two cases I described? As the PN2 in humidified air decreases, where do the nitrogen molecules that cause this decrease go? As the PN2 in alveolar air increases, where do the additional nitrogen molecules that cause this increase come from?

I'm asking only about nitrogen because I think the reasons should be similar for the other gases (i.e., the PO2 in humidified air and atmospheric air) or are clear (i.e., diffusion into and from the blood for O2 and CO2).

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    $\begingroup$ You can't just look at nitrogen and how its concentration changes. Their concentrations all rely on each other. Nitrogen is inert; in your examples it doesn't go anywhere. The partial pressure of N decreases in humidified air because more water is in the air, making the percent nitrogen fall. The N in inspired and expired air is basically unchanged. We add a bit of moisture to exhaled air. $\endgroup$
    – anongoodnurse
    Jan 24, 2022 at 18:42
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    $\begingroup$ You can get an answer in the accompanying text in Guyton & Hall that describes your table. It says: "Because the total pressure in the alveoli cannot rise to more than the atmospheric pressure (760 mm Hg at sea level), this water vapor simply dilutes all the other gases in the inspired air. Table 40-1 also shows that humidification of the air dilutes the oxygen partial pressure at sea level from an average of 159 mm Hg in atmospheric air to 149 mm Hg in the humidified air, and it dilutes the nitrogen partial pressure from 597 to 563 mm Hg." $\endgroup$
    – Domen
    Jan 24, 2022 at 19:06
  • $\begingroup$ @anongoodnurse If nitrogen didn't go anywhere, wouldn't that increase the total pressure? For example, imagine a container that contains only gaseous nitrogen. This nitrogen creates some pressure. If we added some water molecules, these molecules would too create some pressure. If we didn't remove some of the nitrogen, I think the total pressure would increase, since it would be a sum of the pressures created by the nitrogen and water. $\endgroup$
    – athlonusm
    Jan 24, 2022 at 19:24
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    $\begingroup$ @Domen I think you should write that as an answer. $\endgroup$
    – Bryan Krause ♦
    Jan 24, 2022 at 20:35
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    $\begingroup$ @athlonusm - This isn't as much about physics as about straight math, or if you must, algebra. For example, if you have a 900 cc of nitrogen in a big floppy balloon (so, 100%) and you add 100 cc of water. The amount of nitrogen doesn't change, i.e. the N didn't go anywhere, but its concentration dropped to 90%, and H20 goes from 0% to 10%. (<-- that's a tree). You're looking at a tree, when you should be looking at the forest (all the components and how they relate to each other causing percentages of each to change when one component changes.) $\endgroup$
    – anongoodnurse
    Jan 25, 2022 at 16:10

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