The acinus is composed of which of the following structures?

• A. Respiratory bronchioles, alveolar ducts, alveolar sacs
• B. Primary bronchi, lobar bronchi, segmental bronchi
• C. Trachea, carina, segmental bronchi
• D. Bronchioles, alveoli, pleura

Answer: (A)

The acinus is the functional unit of the lung involved in gas exchange and is primarily composed of respiratory bronchioles, alveolar ducts, and alveolar sacs. This structure is critical in the respiratory process as it is where oxygen and carbon dioxide are exchanged between the air and blood.

Respiratory bronchioles lead to alveolar ducts, which then open into clusters of alveolar sacs. The alveolar sacs contain numerous alveoli, which are tiny air sacs surrounded by capillaries—this is where the gas exchange occurs. This anatomical arrangement facilitates efficient respiration due to the large surface area provided by the multitude of alveoli.

In contrast, the other options consist of structures that do not make up the acinus. Primary bronchi, lobar bronchi, and segmental bronchi are involved in conducting air but do not participate directly in gas exchange; they are part of the larger airway system. The trachea, carina, and segmental bronchi also serve similar conducting functions but are not associated with the acinar structure. Lastly, while bronchioles and alveoli are part of the lung structure, including the pleura does not align with the definition of an acinus, as pleura are the membranes surrounding the