Introduction

Surfactant proteins secreted by the lungs play a critical role in respiratory physiology. Pulmonary surfactant is a complex mixture of lipids and proteins that lines the alveoli, preventing their collapse and facilitating efficient gas exchange.

Primary Function of Lung Surfactant Proteins

• The major function of surfactant proteins (SP-A, SP-B, SP-C, and SP-D) is to reduce surface tension at the air-liquid interface within alveoli.

• This reduction of surface tension prevents alveolar collapse (atelectasis), especially at the end of expiration when alveoli shrink.

• Surfactant increases lung compliance, making it easier for lungs to expand during inspiration.

Other Functions (Briefly)

• Certain surfactant proteins (SP-A and SP-D) have immune functions and help in host defense by binding pathogens and facilitating their clearance.

• Surfactant also helps maintain dryness of airways by reducing fluid accumulation in alveolar spaces.

Why Other Options Are Less Relevant

• (1) Do not allow growth of bacteria: While surfactant proteins have immune roles, this is not their major function.

• (2) Stops entry of dust particles: Airway filtration is primarily a function of nasal passages and mucociliary escalator, not surfactant.

• (4) Helps in absorption of oxygen: Surfactant facilitates lung inflation but does not directly absorb oxygen.

Summary Table

Conclusion

The major function of surfactant protein secreted by the lungs is:

(3) Keeps alveoli inflated