Important Lung Products

Surfactant

• Alveolar Surface Tension
  • surface tension generates a pressure that tends to collapse alveolus
  • law of Laplace
    • P = 2T/r
      • P = collapsing pressure on alveolus = pressure required to keep alveolus open
      • T = surface tension
      • r = radius of alveolus
    • pressure tending to collapse alveolus is directly proportional to surface tension generated by molecules of liquid lining alveolus (gas-liquid interface)
    • pressure tending to collapse alveolus is indirectly proportional to alveolar radius
      • large alveolus (large radius) has a low collapsing pressure
      • small alveolus (small radius) has a high collapsing pressure
      • surface tension → tendency for alveolar collapse on expiration
        • expiration → ↓ alveolar radius
• Surfactant
  • a mixture of phospholipids that line alveoli and reduce alveolar surface tension
    • consists primarily of dipalmitoyl phosphatidylcholine (DPPC), or lecithin
  • synthesized by type II pneumocytes
  • ↓ alveolar surface tension → ↓ collapsing pressure on alveolus
    • small alveolus remains open
  • ↑ lung compliance → ↓ work of expanding lungs during inspiration
    • without surfactant, small alveolus has a tendency to collapse (atelectasis)
• Neonatal Respiratory Distress Syndrome (RDS)
  • caused by a deficiency or absence of surfactant 
    • ↑ surface tension → ↑ collapsing pressure on alveolus → alveolar collapse
      • → ↓ V/Q
        • collapsed alveoli are not ventilated and do not participate in gas exchange
        • → hypoxemia
    • ↓ lung compliance → ↑ work of expanding lungs during inspiration
• difficulty reinflating lungs

Additional Important Lung Products

• Prostaglandins
• Histamine
  • binds receptors on smooth muscle cells
    • induces vasodilation, bronchoconstriction
• Kallikrein
  • catalyzes proteolytic cleavage of kininogens in kinin synthesis
    • inactive kininogens → active kinins (e.g., bradykinin)
  • e.g., bradykinin binds receptors on endothelial cells
    • induces vasodilation, bronchoconstriction
• Angiotensin-Converting Enzyme (ACE)
  • catalyzes angiotensin I → angiotensin II
    • angiotensin II induces vasoconstriction
  • acts as a kininase in kinin degradation
    • ACE inactivates bradykinin
    • ACE inhibitors → ↑ bradykinin → cough, angioedema