You're on a roll here... that's right...
It's the alveoli, the terminal structure in the lower respiratory
tract which is involved in gaseous exchange, collapses on
expiration and expands on inspiration.
The structure may collapse permanently if the bronchus proximally
is obstructed by a lesion, which could be a tumor (lung cancer), a
foreign body (organic or non organic) or a simple tuberculous
lesion. 
Well done!
