What cell type lines the alveoli and secretes surfactant?

The alveolar cells that secrete pulmonary surfactant are type II pneumocytes. These cuboidal cells sit in the alveolar wall and release surfactant from intracellular structures called lamellar bodies into the thin fluid layer lining each alveolus.

Surfactant lowers surface tension at the air–liquid boundary, making it easier for alveoli to remain open during exhalation. Without enough surfactant, alveoli tend to collapse, impairing gas exchange. This is why inadequate surfactant is a major problem in premature infants with neonatal respiratory distress syndrome; replacement surfactant can be used as treatment.

Type I pneumocytes are a frequent source of confusion. They are extremely thin, flat cells that cover most of the alveolar surface and are specialized for rapid diffusion of oxygen and carbon dioxide, but they do not produce the main pulmonary surfactant supply. Alveolar macrophages are immune cells that remove particles and microbes, while goblet cells belong mainly to airway epithelium rather than the alveolar wall. Type II pneumocytes can also divide and help replace damaged alveolar cells.