Reduces surface tension, increases compliance, stabilises alveoli and keeps them dry
Mnemonic
“Surfactant stops small alveoli collapsing into big ones”, by reducing surface tension more where the alveolus is smallest.
Produced by type II pneumocytes from about 24 weeks, adequate by 34 to 36 weeks, and composed mainly of dipalmitoylphosphatidylcholine.
Its functions follow from the law of Laplace, P = 2T/r:
- Increases compliance, so less work is needed to breathe
- Prevents alveolar collapse at end expiration, maintaining FRC
- Equalises pressures between alveoli of different sizes, preventing the small from emptying into the large
- Keeps alveoli dry, by opposing the surface tension that would otherwise draw fluid in
“Without surfactant, every breath is the first breath”, which is exactly the picture in neonatal respiratory distress syndrome: stiff lungs, atelectasis, ground glass appearance and air bronchograms.
Antenatal corticosteroids accelerate production, which is why they are given when preterm delivery is anticipated.
Expansion
Secreted by type II pneumocytes from about 24 weeks, adequate by about 35 weeks. Its main component is dipalmitoylphosphatidylcholine.
Functions:
- Reduces surface tension, so increases compliance and reduces the work of breathing
- Stabilises alveoli of different sizes
- Prevents alveolar collapse at end-expiration
- Keeps alveoli dry, by reducing the surface tension forces that would otherwise suck fluid in from capillaries
The stabilising role depends on the law of Laplace: pressure equals two times tension divided by radius. Without surfactant, a small alveolus would generate a higher pressure than a large one and empty into it. Surfactant molecules become more concentrated as an alveolus shrinks, lowering tension disproportionately in small alveoli and equalising pressures.
Deficiency causes neonatal respiratory distress syndrome, with stiff lungs, atelectasis and hypoxaemia; antenatal corticosteroids accelerate production.