In contrast to the bronchial tree, the alveolar tree is the respiratory part of the lungs, and their terminal portions (called the alveoli) form the lung parenchyma. The alveoli are also the site where the gas exchange occurs. The origin structure of the alveolar tree is the terminal bronchiole of the bronchial tree.

All structures of the alveolar tree form the pulmonary acinus located distally from the terminal bronchiole. The pulmonary acini give the alveolar tree the appearance of a bunch of grapes.

The alveolar tree is composed of the respiratory bronchioles that divide into alveolar ducts. And the alveolar ducts further end with the alveolar sacs containing alveoli. The walls of the alveolar sacs and ducts are composed of alveoli. However, the walls of the respiratory bronchioles usually also contain a few alveoli.

Respiratory bronchiole - the transition structure between the bronchial and alveolar trees.
Alveolar ducts - tiny end ducts of the alveolar tree that connect the respiratory bronchioles to the alveolar sacs. They are composed of the alveolar sacs and alveoli.
Alveolar sacs - sac-like outpouching (outpocketings) composed of alveoli clusters.
Alveoli - specialized blind-ending and sac-like terminal structures of the respiratory system that give a spongy appearance to the lungs. They are the structural and functional units of the respiratory system that function as the primary gas exchange site between the inhaled air and blood within pulmonary capillaries. Each lung contains approximately 300 - 400 million alveoli.

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Alveolar tree