Propose why our lungs contain millions of small alveoli instead of two large, lung-sized alveoli.

The reason our lungs contain millions of small alveoli instead of two large, lung-sized alveoli is primarily due to the need for maximizing the surface area for gas exchange.

1. Surface Area: The primary function of the lungs is to exchange gases - oxygen and carbon dioxide - between our body and the environment. This gas exchange happens in the alveoli. If we had two large alveoli, the surface area available for this gas exchange would be significantly less than what is available with millions of small alveoli.

2. Efficiency: With millions of small alveoli, the distance that gases must diffuse across is very small, which makes the process of gas exchange much more efficient.

3. Protection: Having millions of small alveoli also provides a level of protection. If one alveolus gets damaged or blocked, there are still many others that can function. If we had only two large alveoli and one got damaged, it would significantly impact our ability to breathe.

4. Volume Control: Smaller alveoli also allow for better control of lung volume. They can expand and contract as needed, allowing the lungs to more efficiently move air in and out.

In summary, the structure of our lungs, with millions of small alveoli, is a great example of how form follows function in biology. The structure allows for maximum efficiency in the process of gas exchange, which is vital for our survival.