We inhale O2 and exhale CO2, so does that mean we just add carbon and exhale the same oxygen we just inhaled? Why cant we just exhale carbon?


We inhale O2 and exhale CO2, so does that mean we just add carbon and exhale the same oxygen we just inhaled? Why cant we just exhale carbon?

In: Biology

The oxidation of hydrocarbons provides energy for other chemical reactions essential to live. If you just exhaled sugar, for example, you wouldn’t have been able to utilize it for energy after consuming it.

In other words, Oxygen is used to burn carbon, and CO2 is the result of burning carbon compounds against oxygen for energy.

Only a sith deals in absolutes.

Our bodies are not 100% efficient.
We do not use up all the oxygen in each breath of air that we take into our lungs. What we breath out, is not just the waste that our body has ejected from our lungs after use, but the unused breathe we just took in as well.

Air is made up of more than just O2 as well.

When we breath in oxygen, it diffuses across the membranes in the lungs into the bloodstream and then gets transported to different parts of your body and eventually to individual cells. The cells take oxygen and react it with glucose (a type of sugar) to create energy + water + carbon dioxide (CO2). The energy is used to power cellular functions, the water eventually gets excreted as urine or sweat, and the carbon dioxide get transported back through the bloodstream to your lungs, which you then exhale.

I’m a chemist, not a biologist, but I’m 100% certain there are no processes in the body that creates pure carbon (examples of pure carbon = diamond, charcoal, graphene). Pure carbon is a solid (especially in the conditions encountered in the human body) and so even if the body could produce it, it couldn’t be expelled through breathing.

There’s no such thing as gaseous carbon at room temperature and atmospheric pressure. If you want to expel carbon as a gas, you’ll need to find a form of carbon that will be a gas in these conditions, and is also exothermic (generates energy) to produce. Carbon dioxide is simply the best option to do that.

You also seem to be under the impression that we are “wasting” the oxygen we take in by expeling it back out as CO2. The CO2 isn’t being created purely to cart carbon away, it’s just the unusable “ash” left over from using that oxygen to make energy. It has oxygen *in it*, sure, but you can’t use that oxygen anymore. It’s spent.

No. Our cells make carbon dioxide, not carbon as a byproduct of metabolism (Carbon is a solid at room temperature, and anyway, it takes a lot of energy to create carbon dioxide from oxygen and carbon. You can just mix them up, it takes a chemical reaction).

Our lungs don’t attach carbon to the oxygen we just inhaled, they exchange fresh oxygen for carbon dioxide that was carried to our lungs via our bloodstream.

So, let’s use an example to give another viewpoint.

When you have a lovely campfire of burning wood, it releases a lot of heat: This is energy of the wood being released by reactions with oxygen. In fact, a lot of the reactions with oxygen are pretty much the same, just at different speeds.

Now, we also need energy to do things, but we can’t exactly put a campfire inside us to do that. That’s why we ‘slow-burn’ various chemical compounds to give us energy. Oxygen happens to be abundant and very reactive, so it’s available and pretty good at what it does. Just as a campfire is not just adding carbon to oxygen (well it is, but there’s more to it), we are basically burning something like sugar to give us the energy.


A bit more detailed explanation involving entropy:
Imagine you have a bunch of perfectly organized rocks in a line. Then you shake up whatever your rocks are on, and they’re not organized anymore. To organize the rocks again, you need to spend energy.

So, what’s very organized in chemical terms? Well, oxygen for one, and the sugar we burn for food. So, something has spent energy to organize these things, and then we combine them to recover that energy. Simple, right?
(This is a gross oversimplification that only helps explain the process and there’s a lot of details I’ve left out so please don’t quote that for anything but ELI5 purposes)

Breathing in O2 is exactly like intake of an engine, it goes inside where the fuel is and its needed to burn the fuel to produce energy. Then that engine needs to exhaust the byproduct of this “burning” i.e. CO2.
Both our intake and exhaust are through the same hole though.
Also it makes no sense for engine to just exhaust out specifically carbon atoms from its fuel. CO2 is the byproduct of “burning” of the any carbon(organic) based fuel, “burning” for energy is the main point.

We inhale O2 and send it to the cells via blood. Cells use it to carry out some chemical reactions along with the carbon we have eaten – as food (which is also bought to the cells by blood only). The CO2 is a gaseous byproduct (along with other byproducts) of that reaction and needs to be expelled out. Blood carries this CO2 to the lungs and exchange it with the fresh O2 from outside – in the lungs. Lungs are just an exchange area – where two items are swapped. Other byproducts are carried by blood to other organs for disposal – mainly kidneys.

There are obviously practical difficulties of exhaling diamonds or graphite like others have pointed out but this explanation kinda misses the point. Your body doesn’t just have to ‘get rid of carbon somehow’. Just like a fire, your body needs the oxygen in its own right to burn your food and this is which gives us the much-needed energy to live. The global reaction is essentially identical to the one of a fire, although of course the mechanics in our body are much more controlled and complex in order to harvest the energy of this ‘burning’ reaction as good as possible: the food/wood gives its electrons to oxygen, so the food/wood becomes CO2 being depleted of electrons, and there is part of the oxygen getting electrons and becoming water. So yes, besides CO2, the other essential product of fire is water, which may be a bit counter-intuitive.

In a fire part of the oxygen atoms will interact and end up in the CO2 quite directly, but in your body this does not happen directly: the oxygen molecules incorporated in your food while it is getting burned there comes from water molecules which may just as well be the water you drink as the water you produced from the O2 you breathed in, anyways, the net reaction will be the same as in a fire, and the process of burning food will also produce excess water which you will have to breathe out in the form of water vapor. On a cold day you can see how much water vapor you actually breathe out, of which by the way our not very good friend the coronavirus makes good use.