How do your lungs get "fitter"?

Factual Questions
1

When you begin a running or exercise program after being lazy for a while (not that I would know what that’s like or anything;) ), the first few times are brutal. You can really feel it in your chest, but as you get more accustomed to the exercise, the breathing seems so much less laboured.

Why is this? Do your lungs get more efficient at exchanging oxygen? Can your body create more red blood cells to carry oxygen? Do the rib muscles that control your breathing get stronger and have more stamina?

(I don’t have asthma or any related illness, I’m just a 20-something who tends to let exercise lapse for awhile.)

2

As I understand it, yes. Your lungs get greater capacity as you use them more. The more capacity, the less work your heart has to do to pump that oxygen around your body. Or something like that.

3

The exact concept you’re looking for is VO2 max. I don’t exactly understand it myself, but it’s generally regarded as one of the best measures of cardiovascular fitness, if not the best.