Alveoli are the smallest structures in the respiratory system. They are arranged in clusters throughout the lungs at the ends of the passageways that bring air into the lungs (respiratory tree).

The tracheas and bronchial trees were opened and examined for ciliary ... and upstream is down toward the alveoli. The motive power for this upside-down stream is, of course, the cilia of the ...

a: Gross anatomy of lung and thorax. b: Microscopic anatomy of bronchial wall. C: View of terminal airway and alveoli. d: Alveolar structure.