Surfactant | Anatomy2Medicine
Surfactant,Lungs

Surfactant

  • Surface Tension of Alveoli
  • a large alveolus (one with a large radius) will have a low collapsing pressure and, therefore, will require only minimal pressure to keep it open (MCQ)
  • a small alveolus (one with a small radius) will have a high collapsing pressure and require more pressure to keep it open.
  • The pressure generated by a sphere is given by the law of Laplace:

 

    • where
      • P =  Collapsing pressure on alveolus dynes/cm2  or Pressure required to keep alveolus open dynes/cm2
      • T = Surface tension dynes/cm2
      • R= Radius of the alveolus /cm2
    • Surfactant
      • How do small alveoli remain open under high collapsing pres-sures?

 

  • The answer to this question is found in surfactant

 

    • Surfactant
      • a mixture of phospholipids that line the alveoli
      • reduce their surface tension.
      • By reducing surface tension, surfactant reduces the collapsing pressure for a given radius.
      • synthesized from fatty acids by type II alveolar cells. (MCQ)
      • most important constituent is dipalmitoyl phosphatidylcholine (DPPC). (MCQ)
      • increases lung compliance (MCQ)
      • reduces the work of expanding the lungs during inspiration.
    • Mechanism by which DPPC reduces surface tension
      • phospholipid molecules has amphipathic nature
      • DPPC molecules align themselves on the alveolar surface
        • Occur with their hydrophobic portions attracted to each other
        • their hydrophilic portions repelled.
      • Intermolecular forces between the DPPC molecules break up the attracting forces between liquid molecules lining the alveoli (which had been responsible for the high surface tension).
  • Neonatal respiratory distress syndrome
  • surfactant is lacking.
  • In the developing fetus, surfactant synthesis begins as early as gestational week 24, and it is almost always present by week 35. (MCQ)
  • Infants born before gestational week 24 will never have surfactant (MCQ)
  • Infants born between weeks 24 and 35 will have uncertain surfactant status.
  • Without surfactant
    • small alveoli have increased surface tension and increased pressures, (MCQ)
    • small alveoli will collapse (atelectasis)
    • Collapsed alveoli are not ventilated and, therefore, cannot participate in gas exchange causing hypoxemia
    • lung compliance will be decreased (MCQ)
    • work of inflating the lungs during breathing will be increased.