Lungs are made up of air-filled sacs, the alveoli. They

Lungs are made up of air-filled sacs, the alveoli. They do not collapse even after forceful expiration, because of:

1. Inspiratory Reserve Volume
2. Tidal Volume
3. Expiratory Reserve Volume
4. Residual Volume

Volume of air present in lungs after forceful expiration as residual volume prevents the collapsing of alveoli.