1 — Alveolar saccule; 2 — Respiratory bronchiole; 3 — Terminal bronchiole; 4 — Segmental bronchus; 5 — Cartilaginous plate; 6 — Intrasegmental bronchi; 7 — Bronchiole; 8 — Respiratory bronchioles; 9 — Elastice fibres; 10 — Pulmonary alveoli; 11 — Interalveolar septa;
12 — Alveolar foramen; 13 = 3 + 10 + 11 + 12 — Pulmonary acinus