If a person inhaled exactly one liter, and then exhaled exactly one liter?

Question

would his (or her) weight change?

Alternate way of asking: If we could isolate the one liter inhaled and the one liter exhaled what would be the diferrence in their weight?

I came across some info the other day that said (if I remember right) that CO2 is denser than Oxygen. So I am thinking that the exhaled liter would weigh a little more than the inhaled liter and hence the body would have lost a miniscule amount.

I am sure this might be more complex with the Krebs cycle and all that but it would seem to me that what I am asking could be tested or perhaps even have an answer right now.......I hope

Thanks for the great answers so far. I understand the water vapor angle but that is not of concern to me. Just as sweating results in weight loss but you get thirsty and put it right back in.

I thought of this angle of questioning a long time ago and originally asked it in the form of charting a person's weight while he lay on a very sensitive scale.

Perhaps my question could be "What is the actual mechanism of weight loss?"
Excluding sweating and even bodily wastes. I used to envision laying on such a scale for a week (a mind experiment of course) with zero cals in. I think a body at rest uses about 900 cals a day which to me is about 1/4 pound of fat so after a week, my guess mis you would be down about 2 pounds.


The oxygen/CO2 angle still seems significant to me. If a person breathes a liter 20 times a minute for 24 hours and air at one pound/13 cubic ft, that comes out to 35 pounds a day, 8 pounds is O2.
But how much of the O2 ends up as CO2?

Answer 1

Yes the person's weight would change. This is because the number of molecules of gas in a given volume does not change. Since 1 mole of any gas occupies 22.4 liters, 1 liter of gas is equal to 1/22.4 moles. For pure oxygen gas (O2) this is equal to 1.429 g. For pure carbon dioxide gas (CO2) this is equal to 1.969 g. So a person inhaling pure oxygen and exhaling pure carbon dioxide would get lighter by 0.54 g.

However, there are some factors that complicate matters. First, the composition of air going in and coming out are not pure oxygen and carbon dioxide. Depending on where and when you do the experiment, there could be more water in the surrounding air (humidity) making that air weight different, or your blood could have different levels of other dissolved gases, giving it a different composition, and so on.

Second, you have to consider that the weight of air changes as pressure and temperature change. Both the temperature and the pressure of our lungs is different than the surrounding environment, so when you say "exhaled exactly one liter," you have to get more specific. That is, are we measuring that one liter in the lungs , right as it's coming out, or a few moments after it has come out and cooled down (or heated up depending on where you do the experiment)?

I should also note that the Krebs cycle doesn't really apply here. Because Krebs happens inside of cells, it has no bearing on the measurement of gases that we inhale and exhale. I guess it could matter if you considered isotopes of oxygen and carbon, but that's a whole other issue altogether. But even then, isotopes are pretty evenly spread out so their weights are averaged. Anyway, I digress.

The answer to your question is "yes" in a very controlled and highly ideal situation and "maybe" in reality, depending on who, what, when, and where.

Answer 2

Miniscule is the right word. CO2 molecules have a mass of 12+16+16=44 amu. O2 molecules have a mass of 16+16=32 amu. So on the basis of an even exchange of oxygen and carbon dioxide molecules, that would be so.

I think the loss of water vapor would be more significant. Depending on the humidity of the air, of course, we breathe out more water vapor than we breathe in. So we should be losing some small amount of weight in that way.

Answer 3

This is an extremely interesting question!! Let's look at respiration:

C6H12O6 (glucose) + 6O2 (oxygen) → 6CO2 (carbon dioxide) + 6H2O (water)

OK, we are just interested in the gas here
6O2 (g) → 6CO2 (g). For every oxygen molecule that gets consumed 1 CO2 gets formed. In other words we breath out 1 carbon molecule in addition to the oxygen which stays the same.

So, in the simple sense of of how I understand your question, exhaled air contains more mass than inhaled air.

Answer 4

Yes; one would grow slightly heavier. The inhaled air is not 100% saturated with water; the exhaled air is, so is less dense. Offsetting this is the fact that the exhaled air contains increased carbon dioxide, which is more dense than air; the two effects counteract each other to a degree. It follows that if the experiment were conducted in a steam room, with essentially 100% humidity, one would grow slightly lighter.

Answer 5

Actually, the volume exhaled may be different from the volume inhaled and depends on diet (fats vs carbs) being metabolized. Different volumes would imply different moisture in and out as well as different chemistry and different weights for different volumes. It is more complex as you suspect. Activity level also plays a role.

Answer 6

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Answer 7

I believe that is correct. The person would have a net carbon loss, and therefore, a net weight loss.