What is considered the functional unit of the lung?

• A. Bronchi
• B. Alveolus
• C. Capillary
• D. Trachea

Answer: (B)

The alveolus is considered the functional unit of the lung because it is the primary site of gas exchange. Each alveolus is a tiny, balloon-like structure surrounded by a network of capillaries, which allows oxygen to diffuse into the bloodstream and carbon dioxide to be expelled from it. Their structure, being thin-walled and numerous, maximizes the surface area available for this exchange process, making them crucial for effective respiration.

The alveolar walls are lined with a lipid layer that reduces surface tension, assisting in maintaining their openness and ensuring efficient gas exchange under varying pressures during breathing. This design supports the necessary functions that facilitate the transfer of respiratory gases between the lungs and the blood, allowing oxygen to enter the body and carbon dioxide to be removed. Other structures like the bronchi, capillaries, and trachea play supporting roles in breathing and pulmonary function but do not serve as the primary site for gas exchange.