Lungs are made up of air-filled sacs, the alveoli. They do not collapse even after forceful expiration because of :-    Residual volumeInspiratory reserve volumeTidal volumeExpiratory reserve volume

Lungs are made up of air-filled sacs called alveoli. These alveoli do not collapse even after forceful expiration due to several factors, including:

1. Residual volume: This is the volume of air that remains in the lungs after a maximal expiration. It acts as a cushion, keeping the alveoli open and preventing them from collapsing completely.

2. Inspiratory reserve volume: This is the additional volume of air that can be inhaled after a normal tidal volume inhalation. While it does not directly prevent alveolar collapse, it helps maintain lung expansion and overall lung function.

3. Tidal volume: This is the volume of air that is normally inhaled and exhaled during quiet breathing. It helps maintain the exchange of oxygen and carbon dioxide in the alveoli, which is essential for proper lung function.

4. Expiratory reserve volume: This is the additional volume of air that can be exhaled after a normal tidal volume exhalation. While it does not directly prevent alveolar collapse, it helps maintain lung elasticity and prevent excessive air trapping.

In summary, the combination of residual volume, inspiratory reserve volume, tidal volume, and expiratory reserve volume work together to keep the alveoli open and prevent their collapse during forceful expiration.