Pulmonary Surfactant

Pulmonary surfactant is a lipid-protein mixture secreted by alveolar type II cells that forms a film at the alveolar air-liquid interface. By lowering surface tension, it stabilises alveoli against collapse, reduces the work of breathing, and through its associated proteins it also contributes to innate lung defence.

Definition

Pulmonary surfactant is a surface-active complex of phospholipids (predominantly dipalmitoylphosphatidylcholine) and surfactant proteins (SP-A, SP-B, SP-C, SP-D), produced by alveolar type II epithelial cells, that lowers surface tension at the alveolar air-liquid interface.

Scope

This topic covers what surfactant is made of, how reducing surface tension keeps alveoli open and stable across breathing, the cells that produce and recycle it, and the host-defence role of its hydrophilic proteins. It is reference-educational physiology and frames why surfactant deficiency or dysfunction matters without giving treatment instructions.

Core questions

• How does lowering surface tension prevent alveolar collapse and stabilise alveoli of different sizes?
• What are the lipid and protein components of surfactant and what does each contribute?
• How is surfactant produced, secreted and recycled by type II pneumocytes?
• How do surfactant proteins participate in innate immune defence?

Key concepts

• Surface tension and the Laplace relationship
• Dipalmitoylphosphatidylcholine (DPPC)
• Surfactant proteins SP-A, SP-B, SP-C, SP-D
• Alveolar type II cells
• Alveolar stability and compliance
• Collectins and innate defense

Mechanisms

Type II pneumocytes synthesise surfactant and store it in lamellar bodies, secreting it onto the alveolar surface where it spreads as a film. Phospholipids, chiefly DPPC, pack at the air-liquid interface and lower surface tension; because surface tension generates an inward (collapsing) pressure that, by the Laplace relationship, is greater in smaller alveoli, reducing it preferentially stabilises small alveoli and prevents them from emptying into larger ones. The hydrophobic proteins SP-B and SP-C promote rapid adsorption and spreading of the film, while the hydrophilic collectins SP-A and SP-D bind microbial surfaces and modulate immune cell function, linking surfactant to host defence. Surfactant components are continually taken up, recycled and degraded to maintain a functional pool.

Clinical relevance

Surfactant physiology explains alveolar stability and the increased work of breathing when surfactant is deficient or dysfunctional, and it underlies the concept of neonatal respiratory distress associated with surfactant immaturity. This is descriptive background to such conditions; it is not a dosing or treatment guide.

Evidence & guidelines

Surfactant composition and function are established in physiology reviews and textbooks; clinical management of surfactant-related conditions sits with the relevant clinical entities and their guidelines.

History

The role of surface tension in lung mechanics was articulated in the 1950s, with John Clements among those characterising the surface-active lining, and surfactant deficiency was subsequently connected to neonatal respiratory distress. Later work identified the surfactant proteins and extended understanding of surfactant from a purely mechanical agent to a contributor to innate immunity.

Key figures

• John Clements
• Mary Ellen Avery
• Jo Rae Wright
• Erika Crouch

Seminal works

• clements-1997
• wright-2005
• crouch-2001

Frequently asked questions

Why does the lung need surfactant?

Surfactant lowers the surface tension at the alveolar air-liquid interface, which prevents alveolar collapse, stabilises alveoli of different sizes and reduces the work needed to inflate the lung.

Does surfactant do anything besides reduce surface tension?

Yes. Its hydrophilic proteins SP-A and SP-D recognise microbes and modulate immune cells, so surfactant also contributes to innate defence of the lung.