The lungs function as a pump, expanding and contracting to bring air in and expel air out, and contain millions of alveoli that increase surface area for gas exchange.

“Now, what most people don't realize is that the lungs are not just too big bags of air. Your lungs are actually too big bags of air that inside of them have hundreds of millions of little sacs that are called the alveoli of the lungs. And by having those hundreds of millions of little sacs, you increase the surface area of the lungs. And by increasing the surface area, you allow more oxygen to pass from the air in your lungs into the bloodstream than if you didn't have those sacs. And you allow more carbon dioxide to move from the bloodstream into those sacs of the lungs, and then when you exhale, the carbon dioxide can be removed.”