The alveoli, which constitute the majority of the lung tissue, are thin-walled sacs which are enveloped by a rich network of capillaries. Depending upon the location of the section, the airway epithelium can range from a tall, pseudostratified columnar type to a simple, cuboidal, non-ciliated form in the smallest airways. As the respiratory tree diminishes in size, supporting cartilage is gradually replaced by smooth muscle.
Cartilage is completely absent beyond the tertiary bronchi.