Carbon dioxide moves out of the tissues (cells) into the bloodstream to be exhaled in internal respiration.
The theory that is demonstrated by this experiment is natural selection.
Basically, only the strongest bacteria survived, and nature decided which of those bacteria were fit enough to get to live and reproduce in the future. Those bacteria that were "willing" and able to fight their way through a huge number of bacteria were the ones who ultimately won.
Theory because a theory is a well substantiated explanation of how a particular phenomenon works based on the set of facts which had been repeatedly confirmed through observations and experiments. A scientific theory gives reliable account of a phenomenon.
