Respiratory bronchiole proceed into -

Ans. is 'd' i.e., Alveolar ductRespiratory tract anatomyo Nasal cavity, oral cavity, pharynx, and larynx form upper respiratory tract. Larynx opens into lower respiratory tract which starts from Trachea. Trachea bifurcates into bronchi (principal bronchi). Progressive branching of bronchi forms bronchioles. Further branching of bronchioles leads to terminal bronchioles. Terminal bronchiole emanate into respiratory bronchioles. Respiratory bronchioles proceed into the alveolar ducts, which immediately branch into alveolar sacs (alveoli).o The part of lung distal to the terminal bronchiole is called acinus, i.e., respiratory bronchiole, alveolar duct and alveolar sac. A cluster of three to five terminal bronchioles, each with its appended acinus, is usually referred to as pulmonary lobule. So, pulmonary lobules include a cluster of three to five terminal bronchioles with its respiratory bronchioles, alveolar ducts, and alveolar sacs.o Terminal bronchioles contain Clara cells which secrete a protein that lines the small air passages. Alveoli (alveolar sac) are lined by type 1 pneumocytes (95%) and type 2pneumocytes (5%). Type 2pneumocytes secrete surfactant and also helps in repair of alveoli after the destruction of type-1 pneumocytes. Pores of Kohn are present in the alveolar wall to allow passage of bacteria and exudate between adjacent alveoli.o The respiratory tract is lined by pseudostratified columnar epithelium, except vocal cord, which is lined by stratified squamous epithelium.o Terminal bronchi contain maximum smooth muscle cells relative to wall thickness.o Cartilage is found in larynx, trachea, and bronchi but not in bronchioles.