# How much gas is exchanged in one human breath?

When we breathe, our lungs absorb a portion of the oxygen in the air, and replace it with some amount of carbon dioxide and water vapor. Typically, how much $O_2$ (in grams, milliliters, or moles for instance) is absorbed and how much $CO_2$ and $H_2O$ are released in one breath of a healthy adult?

Of course, the exact amounts will vary from person to person and based on how deeply the person is breathing, lung health, etc. I'm just looking for a ballpark figure.

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Just as a side note, it's a common error to believe that you're just breathing out CO2 and H2O. You're also breathing out a fair amount of oxygen as well. –  Jeremy Jan 2 '13 at 22:07

According to Wikipedia

"In a healthy, young adult, tidal volume is approximately 500 ml per inspiration..."

(tidal volume is the volume inspired/expired)

Using this figure, together with values for gas composition also taken from Wikipedia, I estimate that in each breath we take in 18 mg O2 (1.1 mmol) and we release 36 mg of CO2 (1.2 mmol) plus 20 mg H2O (1.1 mmol). These are, as you say, ballpark figures.

Sample calculation:

O2 inspired = 21% by volume; O2 expired = 16% by volume

O2 change = 5% by volume = 5*500/100 = 25 mL

1 mole gas = 22.4 L; 1 mmol gas = 22.4 mL

O2 change = 25/22.4 mmol = 1.1 mmol

MW O2 = 16

O2 change = 17.6 mg

The relative values are reassuringly close to what you might predict from the textbook equation for oxidation of carbohydrate: C6H12O6 + 6O2 -> 6CO2 + 6H2O

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Thanks! I guess all the figures I needed were on Wikipedia, but thanks for putting 'em together for me. Fun fact: you lose about 1.5 pounds of weight per day just to breathing (since the CO2 and H2O are heavier than the O2; assuming 20K breaths/day). –  Nathan Reed Jan 2 '13 at 23:40
@NathanReed, interesting point, but you have your numbers wrong. H2O is not heavier than O2. H2O is a single molecule of Oxygen with a couple of extra protons (H+). It will be lighter than O2. –  terdon Feb 7 '13 at 3:19
@terdon I meant that the CO2 and H2O combined are heavier than the O2. –  Nathan Reed Feb 7 '13 at 4:32
The textbook equation for respiration doesn't always hold. The molar ratios really depend on what is being metabolized by the organism, and that can depend on a number of factors including health and activity. For a more precise ratio you need to consider the respiratory quotient - see en.wikipedia.org/wiki/Respiratory_quotient –  docscience Nov 17 '14 at 2:35