• The respiratory tree is lined by ciliated pseudostratified columnar epithelium from the nose to terminal bronchioles, except for the true vocal cords, which are lined by stratified squamous epithelium. The true vocal cords do not have any lymphatics, which is why early (T1) squamous cell carcinoma (SCC) have a good prognosis

• Bronchi have discontinous cartilage and submucous glands

• Bronchioles have no cartilage or glands

• Terminal bronchioles are < 2 mm in diameter

• Alveoli are lined by Type I (95%) and Type II pneumocytes. There is no protective mucin or ciliary action in alveoli

• Type I pneumocytes are flat. They do not divide

• Type II pneumocytes are cuboidal. They synthesize surfactant and divide during repair. Eventually most of them flatten out to form Type I pneumocytes

Terminal bronchiole
Acinus
Acinus
Alveolus
Alveolus/interstitium
Alveolar capillary membrane
EM
EM
Capillary bed
Normal bronchial mucous membrane