Pulmonary Surfactant Mechanisms in Respiratory Pathophysiology

Pulmonary surfactant is a complex lipid–protein mixture that coats the alveolar surface, reducing surface tension and preventing alveolar collapse at end-expiration. Its principal lipid component, dipalmitoylphosphatidylcholine (DPPC), is complemented by minor anionic phospholipids such as phosphatidylglycerol and surfactant proteins SP-A, SP-B, SP-C and SP-D. These constituents are synthesised and stored in lamellar bodies within type II pneumocytes and secreted into the alveolar lining fluid, where they form a monolayer at the air–liquid interface. The monolayer lowers surface tension, enhances lung compliance and contributes to innate immune defence by modulating macrophage activity and binding pathogens. In respiratory distress syndromes – whether neonatal, acute or chronic – quantitative deficits or qualitative alterations of surfactant lipids and proteins lead to elevated surface tension, alveolar instability, impaired gas exchange and heightened inflammation. Disruption may arise from phospholipid inhibition by plasma proteins, oxidative damage, enzymatic degradation or mechanical injury. Restoration of surfactant function is therefore central to therapies for neonatal respiratory distress, acute respiratory distress syndrome and chronic obstructive pulmonary disease, as well as a potential target in emerging conditions such as vaping-associated lung injury and viral pneumonia.

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Technical terms

Pulmonary surfactant: A lipid–protein complex secreted by type II pneumocytes that reduces alveolar surface tension and supports lung compliance.

Dipalmitoylphosphatidylcholine (DPPC): The major surface-active phospholipid in surfactant, responsible for low end-expiratory surface tension.

Surfactant proteins (SP-A/B/C/D): Amphipathic proteins that facilitate film formation, lipid spreading and host defence against pathogens.

Lamellar bodies: Intracellular organelles in type II pneumocytes that store surfactant lipids and proteins before secretion.

Surface tension: The force at the air–liquid interface in alveoli that opposes lung expansion; reduced by surfactant to ease breathing.

Bronchoscopic lavage: A delivery method for instilling therapeutic surfactant directly into distal airways via a bronchoscope.

References

1. Structure and function of lamellar bodies, lipid-protein complexes involved in storage and secretion of cellular lipids.. Journal of Lipid Research (1991).
2. Anionic Pulmonary Surfactant Phospholipids Inhibit Inflammatory Responses from Alveolar Macrophages and U937 Cells by Binding the Lipopolysaccharide-interacting Proteins CD14 and MD-2* ♦. Journal of Biological Chemistry (2009).
3. Exposure to Aldehyde Cherry e‑Liquid Flavoring and Its Vaping Byproduct Disrupt Pulmonary Surfactant Biophysical Function. Environmental Science and Technology (2024).
4. Decreased surfactant lipids correlate with lung function in chronic obstructive pulmonary disease (COPD). PLOS ONE (2020).
5. Surfactant therapy for COVID-19 related ARDS: a retrospective case–control pilot study. Respiratory Research (2021).
6. Phospholipid regulation of innate immunity and respiratory viral infection. Journal of Biological Chemistry (2019).