What is pulmonary surfactant?

What is Pulmonary Surfactant?

Pulmonary surfactant is a lipid-protein complex secreted by type II pneumocytes that coats the alveolar surface and reduces surface tension to near-zero levels during exhalation, preventing alveolar collapse and maintaining normal lung function 1, 2.

Composition and Structure

Surfactant consists of approximately:

• 90% lipids (primarily phospholipids, with dipalmitoylphosphatidylcholine being the key saturated phospholipid)
• 10% surfactant proteins (SP-A, SP-B, SP-C, and SP-D)

The hydrophobic proteins, particularly SP-B, are essential for proper surfactant function, enabling rapid adsorption of surfactant vesicles to the air-liquid interface through a fusion-like mechanism 1, 2.

Biophysical Function

Surfactant performs two critical mechanical functions:

1. Rapid adsorption: Surfactant vesicles quickly insert into the alveolar air-liquid interface, requiring SP-B for efficient interfacial insertion 1

2. Surface tension reduction: When compressed during exhalation, the film reduces surface tension to exceptionally low levels (close to zero), which:

   • Prevents alveolar collapse at end-expiration
   • Reduces the work of breathing
   • Maintains alveolar stability throughout the respiratory cycle 1, 3

3. Barrier function: Assembles a chemical and physical barrier against inhaled pathogens 2

Clinical Significance

Surfactant deficiency or dysfunction is directly responsible for major respiratory pathologies 4, 2:

• Primary deficiency: Respiratory distress syndrome (RDS) in preterm infants due to lung immaturity
• Secondary deficiency/inactivation: Occurs in meconium aspiration syndrome, pneumonia/sepsis, pulmonary hemorrhage, and acute respiratory distress syndrome (ARDS)

The clinical impact is profound: surfactant therapy substantially reduces mortality and respiratory morbidity in preterm infants with RDS, decreasing pneumothorax, pulmonary interstitial emphysema, and chronic lung disease 4, 5.

Key Clinical Context

Understanding surfactant is essential because its absence or dysfunction represents a major cause of morbidity and mortality, particularly in neonates 4. The material's unique ability to achieve near-zero surface tension distinguishes it from other biological surfactants and explains why exogenous surfactant replacement therapy has become a life-saving intervention since the early 1990s 5, 2.