Anatomically speaking, the respiratory parts of the lungs include the respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli. The respiratory bronchioles are branches of the terminal bronchioles, and each terminal bronchus divides into two or more respiratory bronchioles. The alveolar ducts are branches of the respiratory bronchioles, and each branch forms two to three or more alveolar ducts. The alveolar sacs are continuous with the alveolar ducts, and each alveolar duct branches to form two to three alveolar sacs, which are similar in structure to the alveolar ducts and together form the basic respiratory part of the lung. The alveoli are the terminal part of the bronchial tree and the main tissue structure that makes up the lung.