Which structure is directly involved in gas exchange? - Trachea - Pharynx - Alveoli - Bronchi

The structure directly involved in gas exchange is the Alveoli.

Extra: The alveoli are tiny air sacs within our lungs where the exchange of oxygen and carbon dioxide takes place. They are located at the end of the bronchial tubes. When you inhale, oxygen travels down the trachea, through the bronchi, and eventually reaches the alveoli. The oxygen then passes through the alveolar walls into the blood in the capillaries. At the same time, carbon dioxide, which is a waste product of metabolism, passes from the blood into the alveoli so that it can be exhaled. This process occurs due to the differences in partial pressures of oxygen and carbon dioxide in the alveoli and the blood, a concept known as diffusion. The structure of the alveoli maximizes their surface area to volume ratio, which is critical for effective gas exchange. This feature, along with a thin barrier between the alveolar air space and the capillaries, allows for rapid gas exchange that sustains life.