Surfactant Alveolar Surface Tension surface tension generates a pressure that tends to collapse alveolus law of Laplace P = 2T/r login to view 3 more bullets 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 login to view 4 more bullets 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 login to view 3 more bullets ↓ lung compliance → ↑ work of expanding lungs during inspiration login to view 1 more bullet 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