Respiratory bronchiole proceed into:

Bronchioles have a diameter smaller than 1 mm and lack cartilage and glands within their walls. Goblet cells (and cilia) decrease in number and almost negligible at the levels of bronchioles (small lumen). Hyaline cartilage also is almost non-existent at the levels of bronchioles. Epithelium gradually changes from pseudostratified columnar to simple columnar to cuboidal to squamous. Alveolus is lined by type-I pneumocyte (simple squamous epithelium) for respiratory gas exchange. Type- II pneumocyte is a cuboidal cell for surfactant secretion. Bronchial arteries (branches of descending thoracic aorta) supply till the level of respiratory bronchiole. They perfuse the proximal air conducting pathways including tertiary and terminal bronchioles and reach till the beginning of respiratory unit. Pulmonary arteries alone vascularize the further distal pathways, including alveolar ducts and the alveoli.