F3x: Describe the properties, production and regulation of, surfactant and relate these to its role in influencing respiratory mechanics

Definition

Surfactant = A lipoprotein complex formed by T2 pneumocytes which ↓surface tension at liquid-gas interface

Composition

  • 90% lipids
    • 80% DPPC (DIPALMITOYL PHOSPHATIDYL CHOLINE)
  • 4% plasma proteins
  • 2% carbohydrates
  • 4% surfactant proteins

Synthesis

  • T2 pneumocytes make 2 things:

a) Surfactant

b)T1 pneumocytes

  • Synthesis is RAPID
  • t ½ 12hr = high turnover
  • ↓synthesis with hypoxia, acidosis, ↓temp
  • Surfactant is mostly recycled by T2 pneumocytes

Surface Tension

  • When there is liquid-gas interface, the liquid molecules are strongly attracted to each other
  • This force is KA SURFACE TENSION
  • Force created by ST leads to ↓area of interface
  • ∴gas pressure inside a bubble is always greater surrounding P
  • Because surface inside bubble is under constant tension
  • The pressure is predicted by Law of LaPlace:
P
  • ∴smaller bubble (↓V) = ↑P & tendency to collapse into bigger bubbles (alveoli)

Action of Surfactant

  • DPPC is an amphipathic molecule
  • Lines up @ gas-liquid interface

→ Hydrophilic facing liquid

→ Hydrophobic facing gas

  • ↓force of attraction b/w H2O molecules ∴↓ST
  • Effects ∝ to [surfactant]
  • ∴in EXPRN when alveolar SA ↓& surfactant molecules are close together, there is strong opposition to ST
  • In INSP as alveolar size ↑, DPPC molecules are further apart ∴effect is less

NB: Deep breathing is essential to dispensing surfactant equally → ∴without deep breathing → limits surfactant → atelectasis → ↑WoB

Effects of Surfactant

Alveolar Stability

  • Because ↓ST ∴↓ alveolar collapse

Elastic Recoil & Hysteresis

  • ST contributes to 70% of Elastic Recoil
  • ST is greatest at v. high lung vol & INSP
  • This is the most important contributor to HYSTERESIS

Prevents Pulmonary Oedema

  • ST sucks fluid from pulm caps → alveoli
  • By ↓ST = ↓alveolar transudation

↓ WoB

  • Keeps alveoli open & on favourite part of compliance curve

Immune Role

  • Lipid component has some antioxidant activity