Final answer:
The alveoli are where the majority of gas exchange between the circulatory and respiratory systems occurs, due to their thin walls and direct contact with capillaries. The correct answer is option A
Step-by-step explanation:
The majority of gas exchange between the circulatory and respiratory systems occurs in the alveoli. The alveoli are thin-walled structures that are in direct contact with the capillaries. In the complex structure of the respiratory system, air passes through the trachea and the bronchi before reaching the bronchioles and eventually the alveolar ducts.
The alveolar sacs, which contain clusters of alveoli, are the sites where oxygen is absorbed into the blood and carbon dioxide is expelled from the blood. This efficient exchange is facilitated by the large surface area of the alveoli and their proximity to the capillaries. While other structures like the trachea, bronchi, and bronchioles are involved in air conduction and preparatory phases, the alveoli are where actual gas exchange takes place.