Gaseous exchange describes the process of bringing gas molecules into the body, while removing other gas molecules from the body, with some degree of equivalency between the two (such as one molecule in, one molecule out) in order to make the process as efficient as possible. In humans, we know the gases participating in the exchange are going to be O2 and CO2 because these gases are the fuel and waste products, respectively, of cellular respiration, which provides energy for the body in the form of ATP.
O2 and CO2 are extremely small molecules, and we have very large and thick bodies compared to these molecules; the biggest problem with this is that the deep interior of our bodies are highly insulated, and therefore it's difficult for gases to reach them. Nevertheless, our bodies use a lot of energy because of their size, so there's a lot of gas exchange that needs to take place. The body also needs to ensure that it protects itself, particularly when exposing itself to the environment during gas exchange, so it needs some kind of filtering that restricts which molecules can participate in the exchange.
Therefore, an efficient gas exchange molecule needs:
- a way to quickly distribute and collect gases throughout the body
- a large surface area to conduct a large amount of exchange
- a filter that only allows CO2 and O2 to participate in the exchange
- the ability to speed up or slow down the exchange to meet the body's demands
Part of the reason we use our lungs as our primary gas exchange organ instead of our skin is that our skin can't function both as a highly protective barrier and as a medium for gas exchange; it's too thick and heavily guarded, and while we do "breathe" through our skin a little bit, it's not enough to help the deep, insulated parts of our body participate in gas exchange, too.