Tuesday, July 15, 2008

Beats the heck out of me

My youngest son asked me a question today. At first glance, it seemed to be a very simple question.

Do you breathe out the same amount of air that you breathe in?

I explained that the volume of individual breaths varies, but that my first thought was that average inhalation must be the same as average exhalation -- because if it wasn't you'd either end up with a vacuum or overinflated. Then I remembered that you actually absorb molecules from the air and return others in the process of respiration. I'm not sure that the quantity is measurable, but I'm not so sure that you exhale the same amount over the long haul as you inhale. So I told him that.

His response?

Why don't you blog about it and see what other people think.

He'll nag me if you don't comment. Facts would be nice, but we'll settle for opinions if that's all you've got.


Camilla said...

So at the cellular level, aerobic respiration is:
sugar (C6H12O6) + 6O2 => 6CO2 + 6H2O + energy.

So you should be exchanging oxygen molecule for molecule with carbon dioxide. What's coming out is heavier, but has the same volume, because gasses take up the same volume per number of molecules.

As far as water goes, you should be excreting water vapor with every breath, probably in substantial quantities.

Your lungs must trap and dispose of random particulate matter from the air, and volatile byproducts of your metabolism come out, but that's presumably all smallish quantities.

Dr. Momentum said...

You need to define "more" better -- are you talking about volume or mass? Your post implies both separately in different parts.

I'll leave volume aside and assume mass is the metric you're going for. If so, I'll wager a guess that the heaviest portion of your breath is water vapor. If that is correct then it partly depends on the relative humidity.

Assuming a relatively dry air, you're breathing out a good deal of water, so your expiration is likely a lot heavier than your inspiration.

And you are probably breathing most of the time in an environment of humidity that is lower than your breath's water vapor content.

The mass of oxygen you're body is using is probably much lighter than the water you exhale.

This is my speculation, but I have noticed differences between air-pump-inflated balloons and breath-inflated balloons. Breath-inflated often has water condensate inside. Try both methods and place the balloons in the freezer or fridge to encourage the water to come out of vapor, see what collects in the bottom.

Judy said...

dr. momentum and camilla,
Both of you brought up interesting points. I originally thought of volume, but mass is an easier subject to address. Given the increased humidity of expired air, The mass should nearly always be greater than the inspired are.

I thought of another factor, which is heat. Except in very hot climates, the air you exhale will be warmer than the air you inhale. Even without the addition of water vapor, the volume should increase.

I'd love to do the balloon experiment. If I can find some clear non-latex balloons, I'll give it a try.

Dr. Momentum said...

Some sort of airtight bag might substitute for a balloon.