Atelectasis is defined as alveoli devoid of air with loss of lung volume.
If alveoli are filled with material displacing air, it is not considered atelectasis.

Resorptive
- When there is endobronchial obstruction, there is no more ventilation and air gets absorbed from alveoli. Alveoli collapse with significant loss of lung volume, i.e., lobar atelectasis from endobronchial lung cancer.
Relaxation
- Lungs are held close to the chest wall by negative pressure in the pleura. In pneumothorax or pleural effusion the negative pressure in the pleura is lost. The lung relaxes to its resting position.
Adhesive
- Surfactant is necessary for keeping the alveoli open. In ARDS and pulmonary embolism there is loss of surfactant and alveoli collapse.

Left upper lobe atelectasis secondary to obstruction from endobronchial squamous cell carcinoma.

Note the pneumothorax on right. The right lung relaxes to its resting state, "relaxation atelectasis".

Diffuse alveolar atelectasis due to loss of surfactant. Adult respiratory distress syndrome (ARDS).