What physiological change is associated with parasympathetic stimulation of the respiratory system?

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.

Correct answer: False

Internal respiration refers to the exchange of gases between the blood and body tissues at the cellular level, where oxygen diffuses from the capillaries into the cells and carbon dioxide moves from the cells into the blood. It occurs after oxygenated blood has been transported from the lungs to systemic tissues via the circulatory system. The physical movement of air into and out of the lungs, on the other hand, is called external respiration or ventilation, which involves inspiration and expiration driven by pressure gradients. Therefore, equating internal respiration with airflow is inaccurate, making the statement false.