I have tried and deleted to write an explanation for a 5 year old and I have failed a couple of times. went into too much detail and it made no sense.
I would encourage you to look up:
Intercostal muscles.
Have not seen them mentioned on here.
they kinda sit in between ribs and help with breathing. more you train they get stronger and this helps lungs move more air.
there is more to this and also when talking about Lung Capacity im thinking you might look at a peak flow test. but there are other tests that measure lung power/ efficiency Have a look at a FEV test, also buy a peak flow meter they are cheap and its a fun game to see how high you can score.
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