Final answer:
The exchange of O2 and CO2 between blood and the lungs occurs in the alveoli. These small air sacs are lined with capillaries that facilitate the diffusion of gases due to concentration gradients.
Step-by-step explanation:
The site of the exchange of O2 (oxygen) and CO2 (carbon dioxide) between blood and air-filled spaces in the lungs is the alveoli. When we inhale, air follows a specific pathway: starting at the nasal cavity, it moves to the larynx, then the trachea, into the bronchi, down to the bronchioles, and finally reaches the alveoli. The alveoli are small, grape-like clusters surrounded by networks of thin-walled pulmonary capillaries. As air enters the alveoli, because of the higher concentration of oxygen in the inhaled air compared to that in the blood of the pulmonary capillaries, oxygen diffuses from the alveoli into the blood. Similarly, carbon dioxide, being more concentrated in the blood, diffuses from the blood into the alveoli to be expelled from the body during exhalation. The alveolar ducts are connected to alveolar sacs where this gas exchange occurs.