The level of hydrogen ions in the blood (itself an indicator of how much co2 there is in our blood) drives the diaphragm and the muscles in the thorax (chest) to expand and contract, adjusting the volume and therefore the air pressure inside your lungs, thus air will enter or leave based on the internal air pressure. If someone loses that drive they hyperventilate, so we give them a bag to breathe with, they then consume co2 and that drives their breathing again. People assume oxygen levels drive breathing, but it is actually the level of co2 in the blood. (unlesss you have copd).
Oxygen molecules (made in stars) will go via diffusion from low concentrstion to high concentration, just as all things do.
So oxygen will enter the blood (low concentration) from the air (high concentration) and likewise for the exact opposite reason, CO2 leaves the blood (high concentration) for the air (lower concentration).
Our lungs have a region of sacs called alveoli, in their lining that interact with the putside air and the body's blood vessels carrying blood along the lining of the lungs. This is where the transfer called diffusion happens.
Breathing is very mechanical, it happens in various animals, in various ways... it is all explained and done so without invoking a gap for 'spirit' whatever that is by your own definition, understanding, interpretation.
There is no mystery or magic. Where exactly in this process is the spirit and why call it the spirit instead of Oxygen atoms, made in a star? I assume you don't think spirit is made in stars? Why is oxygen spirit, why not helium, gold, carbon? You must admit, what we once weee told as believers has been left wanting by evidence!
I am confident I know where this comes from, you are trying to make genesis fit what you can see around you, what has been learned by science.When god breathed spirit into Adam.... maybe consider that your evidence suggests this is untrue, but was how ancient peoples assumed life began when creating genesis stories in their cultures. It is not difficult to see how they came to such conclusions in the ignorance of humankind 4000+ years ago.
Because sadly, there is no gap to squeeze spirit in. We understand the process well.
Also, if you now retreat 'spirit' to the initiation of life, please remember we have created synthetic life in a labratory. From scratch! Do you believe god breathed life into the biological machines we have made as humans? We have them create fuel from waste products by changing their DNA by human design. As with all living things, they respire, just as we breathe. When the synthetic life is constructed it simply starts functioning, like a water mill being built and automatically coming to life when the last piece is added. Does this not make you question what you thought life was at least?
P.s. I have not once said doing this equates us with god or a creator, we are only altering what already existed, not creating life from 'nothing'. Nobody claims that.
Before you google diffusion, though it is,very interesting, it is based on the forces of physics and happens in all sorts of contexts. You may have heard of diffusion with water, osmosis. When you pour a cordial into a glass of water, the cordial juice seeks the least concentrated regions of water and likewise the water seeks the least water concentrated regions of the cordial, as seen when the drinks are mixed together.. just to save a conversation where diffusion may be called spirit by some. Diffusion is all about pressure gradients.
The reason oxygen is used snd co2 is released, is no accident. Just as your car needs oxygen and fuel, so does our cells, and also just as with your car, our cells release c02 and water as a waste product. It is internal combustion of another form.
