So as I'm moving my arm, what's taking place is the muscle cells are producing ATP molecules in a process we call aerobic cellular respiration, which uses up oxygen and produces carbon dioxide and ATP molecules. Now, the ATP molecules are used as the energy source. And the carbon dioxide molecules, because they cannot be used in any useful way, they have to be released by those cells of the exercising tissue. So we have these non polar carbon dioxide molecules. And because these carbon dioxide molecules are non polar, they don't have any charge. They can easily move across the cell membrane of the cell.