Surfactant

Describe the properties, production and regulation of, surfactant and relate these to its role in influencing respiratory mechanics

Surface Tension

• Surface tension describes the tendency of a fluid to minimise its surface area
• It is related to the attraction between particles in the fluid relative to particles outside the fluid
• Surface tension is why:
  • Water scattered on a surface forms rounded droplets
  • Why multiple droplets will tend to coalesce into a single larger droplet

• This relationship is described by Laplace's Law
  $P = {2T\Theta \over r}$, where:
  • $P$ is pressure
  • $T$ is wall thickness
  • $\Theta$ is surface tension
  • $r$ is radius

• Alveoli obey Laplace's Law
• High surface tension causes three problems with alveoli
  • Compliance falls when the alveolus is empty
    As the radius falls, the pressure required to open it (at a given surface tension) will be increased. This increases work of breathing.
  • Smaller alveoli will preferentially empty into bigger alveoli
    Smaller alveoli require greater transmural pressures to remain inflated. This causes smaller alveoli to empty into larger ones.
  • Fluid transudation
    Surface tension draws fluid from interstitial spaces and contributes to pulmonary oedema.

• Overall, high surface tension is detrimental to the lungs

Surfactant

• Surfactant is a substance which substantially reduces work of breathing by reducing alveolar surface tension
• Surfactant is produced by type II alveolar cells in response to lung inflation and respiration
• It is composed of:
  • 85% phospholipid
  • 5% neutral lipid
  • 10% protein
• Surfactant is amphipathic
  Each component has a hydrophobic and hydrophilic end.
  • This causes the molecules to orient themselves along the air-liquid interface, disrupting the attractive bonds between water molecules
  • Surface tension is reduced in proportion to the concentration of molecules
• The concentration of surfactant changes throughout the respiratory cycle
  • During expiration alveoli collapse
    The decrease in alveolar radius is offset by the increase in surfactant concentration, so the fall in radius is mitigated by the drop in surface tension.

References

1. CICM September/November 2012
2. Lumb A. Nunn's Applied Respiratory Physiology. 7th Edition. Elsevier. 2010.