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. By having millions of small alveoli, the surface area for this gas exchange is greatly increased. If we had two large alveoli instead, the surface area would be drastically reduced, limiting the amount of gas that could be exchanged at any given time.

2. Efficiency: The small size of the alveoli allows for a more efficient exchange of gases. Oxygen and carbon dioxide move across the thin walls of the alveoli and into the bloodstream (or from the bloodstream into the alveoli) by diffusion. The shorter distance across the small alveoli allows this diffusion to happen more quickly and efficiently than it would across a larger space.

3. Protection: The structure of the alveoli also provides some protection against damage. If one alveolus is damaged or blocked, there are still millions of others that can function. If we had only two large alveoli and one was damaged, half of our lung capacity could be compromised.

4. Volume Control: The millions of small alveoli allow for finer control of lung volume. They can expand and contract as needed during breathing, allowing the volume of the lungs to be precisely controlled. Two large alveoli would not allow for this level of control.

In conclusion, the structure of the lungs, with millions of small alveoli, is a great example of form following function in biology. It allows for efficient gas exchange, protection against damage, and precise control of lung volume.