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