<prompt> Which of the following is the main component of surfactant, secreted by alveolar epithelial cells?ProteinsPhospholipidsLipoproteinsNone of the above  <response>: The surface-active component of the lung extract is called pulmonary surfactant. It is a complex consisting of about 85 to 90 lipids and 10 to 15 proteins. The lipid poion is about 85 phospholipid,approximately 75 of which is phosphatidylcholine, mainly dipalmitoyl phosphatidylcholine. There are 4 specific surfactant proteins: SP-A, SP-B, SP-C, and SP-D. Surfactant is manufactured by specialized alveolar cells known as type II alveolar epithelial cells. Ref: Levitzky M.G. 2013. Chapter 2. Mechanics of Breathing. In M.G. Levitzky Ed, Pulmonary Physiology, 8e. 