Now, these ATP molecules are used as energy molecules to carry out different types of processes and reactions that take place inside the cells. Now, the problem with aerobic cellular respiration is it produces a waste byproduct, namely carbon dioxide. And carbon dioxide cannot actually be used by the cell in any useful manner. And so all these trillions of cells which use these food particles to metabolize them into ATP produce these CO2 molecules and they essentially take these CO2 molecules and dump them into the blood plasma. Why? Well, because as the blood plasma circulates through our blood vessels, through our cardiovascular system, eventually the CO2 molecules will end up in the alveoli of the lungs.