Microfluidics, Lung Surfactant, and Respiratory Disorders

Abstract

The lungs are the major organs of the respiratory system and contain a branching network of airway tubes that become shorter, narrower, and more numerous as they penetrate deeper into the lung. The tracheobronchial tree comprises 3 major types of airways: cartilaginous bronchi, membranous bronchioles, and gas exchange alveolar ducts. Upper airways and terminal bronchioles act merely as conduits for the passage of gas, whereas respiratory bronchioles and alveolar ducts carry out both conducting and gas exchange functions. Airways less than 1 to 2 mm in diameter that include membranous, terminal, and respiratory bronchioles as well as alveolar ducts are known as distal airways.1 These airways have a very large total cross-sectional area and relatively low resistance to the flow of gas (~10% to 20% of the total lung resistance).2 Due to their small size and high compliance, distal airways are prone to instabilities and closure at low lung volumes (eg, at the end of expiration). To prevent these fine structures from collapse (atelectasis), secretory cells of alveolar and neighboring airway epithelium produce pulmonary surfactant that distributes at the surface of the liquid lining layer of distal lung epithelium.3,4 Functional surfactant reduces surface tension at the luminal air-liquid interface and, thus, maintains the patency of airways and alveoli at low lung volumes. The presence of hysteresis in the pressure-volume curve of the lung indeed reflects this point ( Figure 1 ): the pressure required to support a certain volume (V) of the lung units during deflation (P2) is smaller than that when lungs are inflated (P1)5. During deflation, the surfactant film is compressed and presumably a more compact layer of surfactant lipid molecules is present at the air-liquid interface, which results in a lower surface tension. Active surfactant at the luminal air-liquid …