The lung itself has elastic tissue incorporated within it. And this is... What are the diseases like emphysema where you lose that elasticity? You take a breath in and the airways open up really nicely, but they don't snap back. Instead of a balloon filling a balloon up and letting it go, it will empty itself. It's almost like if you did it with a paper bag, it just stays full. Emphysemaics would suffer from an overinflated lung which can cause them to be kind of barrel-chested.