The exchange takes place in the millions of alveoli in the lungs and the capillaries that envelope them.
Theories are a collection of scientific data that is generally accepted and reproducible. Theories are a result from several repeated experiments, but theories are not based on a single hypothesis. Several hypotheses are tested and appraised to become a well accepted theory. Theories may also be revised over time and can predict what may happen in the future. On the other hand, theories are not proven facts.
I believe the answer to this question is cellular respiration
