What are the smallest structures in the lung responsible for gas exchange?

The smallest structures in the lung responsible for gas exchange are the alveoli. Alveoli are tiny, balloon-like sacs located at the end of the respiratory tree. Their primary function is to facilitate the exchange of oxygen and carbon dioxide between the air we breathe and the blood.

The walls of the alveoli are extremely thin, composed of a single layer of epithelial cells that allow for efficient diffusion of gases. Each alveolus is surrounded by a network of capillaries, which are small blood vessels where gas exchange occurs. When air enters the alveoli, oxygen passes through their walls and into the blood in the capillaries, while carbon dioxide from the blood diffuses into the alveoli to be exhaled.

This structure is vital for respiration, as it maximizes the surface area available for gas exchange, making it a crucial aspect of the respiratory system. The other options, though related to the respiratory system, do not play a direct role in the process of gas exchange within the lungs.