We have the endothelium of the blood capillary. This is the blood plasma and this is the red blood cell. So let's suppose I move my hand back and forth. 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.