Components of the blood–air barrier in the lung are derived from which of the following sources?

E. The blood–air barrier comprises the structures through which gaseous exchange occurs between air in alveoli and blood in pulmonary capillaries. The attenuated pulmonary epithelium (type I pneumocytes) is derived from endoderm. The simple, squamous epithelium (endothelium) lining pulmonary capillaries is derived from visceral mesoderm.