Surfactant is produced by: March 2010
Ans. D: Type II alveolar cells The low surface tension when the alveoli are small is due to the presence in the fluid lining the alveoli of surfactant, a lipid surface-tension-lowering agent. Surfactant is a mixture of dipalmitoylphosphatidylcholine (DPPC), other lipids, and proteins. If the surface tension is not kept low when the alveoli become smaller during expiration, they collapse in accordance with the law of Laplace. Surfactant is produced by type II alveolar epithelial cells. Typical lamellar bodies, membrane-bound organelles containing whorls of phospholipid, are formed in these cells and secreted into the alveolar lumen by exocytosis. Tubes of lipid called tubular myelin form from the extruded bodies, and the tubular myelin in turn forms the phospholipid film. Some of the protein-lipid complexes in surfactant are taken up by endocytosis in type II alveolar cells and recycled. Surfactant is impoant at bih. The fetus makes respiratory movements in utero, but the lungs remain collapsed until bih. After bih, the infant makes several strong inspiratory movements and the lungs expand. Surfactant keeps them from collapsing again. Surfactant deficiency is an impoant cause of infant respiratory distress syndrome (IRDS, also known as hyaline membrane disease), the serious pulmonary disease that develops in infants born before their surfactant system is functional. Maturation of surfactant in the lungs is accelerated by glucocoicoid hormones. Fetal and maternal coisol increase near term, and the lungs are rich in glucocoicoid receptors. Patchy atelectasis is also associated with surfactant deficiency in patients who have undergone cardiac surgery involving use of a pump oxygenator and interruption of the pulmonary circulation. In addition, surfactant deficiency may play a role in some of the abnormalities that develop following occlusion of a main bronchus, occlusion of one pulmonary aery, or long-term inhalation of 100% 0,. Cigarette smoking also decreases lung surfactant.
Alveolar macrophages
Lymphocytes in the alveoli
Type I alveolar cells
Type II alveolar cells