Internal respiration is the physical movement of air into and out of the lungs.

Correct answer: False

Terminal bronchioles represent the smallest airways in the conducting zone of the respiratory system and do not directly open into alveoli. Instead, they transition into respiratory bronchioles, which are the initial structures of the respiratory zone capable of gas exchange. The respiratory bronchioles then branch into alveolar ducts, which terminate in clusters of alveoli where the majority of gas exchange occurs. This anatomical progression ensures that air is conducted efficiently through the bronchiolar tree while maintaining separation between the conducting and respiratory zones. Therefore, stating that terminal bronchioles directly enter the alveoli is inaccurate and does not reflect the true structural organization of the lungs.

A. Bone: Bone provides rigid structural support in the skeletal system but is not present in the respiratory tract. Alveolar ducts are soft, flexible structures and do not contain bone tissue, making this choice anatomically incorrect.

B. Fibrous tissue: Fibrous connective tissue contributes to the structural framework and elasticity of the lungs, but alveolar ducts specifically rely on smooth muscle to regulate airway diameter. Fibrous tissue alone does not allow the dynamic changes needed during respiration.

C. Smooth muscle: Alveolar ducts are composed of smooth muscle and connective tissue, which allows them to adjust diameter during ventilation. This smooth muscle facilitates airflow regulation to the alveolar sacs, aids in maintaining airway patency, and contributes to the fine control of gas distribution in clusters of alveoli.

D. Cartilage: Cartilage provides rigidity in larger airways such as the trachea and bronchi, but it is absent in the alveolar ducts. The flexibility of alveolar ducts is necessary for effective alveolar expansion and gas exchange, which cartilage would restrict.