Cascades to Coast

When you breathe air and breathe it out, how does it turn into carbon dioxide?

The air that we breathe in goes into our lungs. From the lungs, the arteries absorb the oxygen and carry it to the cells in the body. Each cell uses this oxygen to "burn" fuel - just like we burn wood - to generate energy. And, just like when we burn wood, burning the fuel in the cell uses up O2 and produces CO2. The carbon dioxide (and remaining oxygen) is collected by veins and taken back to the lungs, where it is breathed out. The air we breathe in has 21% O2 and 0.04% CO2. The air we breathe out has about 17% O2 and 4% CO2.