The document summarizes the microscopic structure and function of the respiratory system. It describes the nasal cavity, larynx, trachea, lungs and their histological components. The respiratory system consists of conducting passages that deliver air to the gas exchange surfaces in the lungs. The lungs are made up of bronchioles that divide into alveolar ducts and hundreds of millions of alveoli where oxygen and carbon dioxide exchange occurs through a thin epithelial barrier.