Updated: 6/5/2018

Important Lung Products

Topic
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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
 

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