Final answer:
In the respiratory system, the alveoli are the site of gas exchange, where oxygen and carbon dioxide are transferred between the lungs and the bloodstream.
Step-by-step explanation:
The site of gas exchange in the respiratory system of humans is the alveoli. The alveoli are tiny air sacs located at the end of the alveolar ducts within the lungs. Each alveolus is surrounded by a network of capillaries where oxygen from the inhaled air passes into the blood, and carbon dioxide from the blood is expelled into the exhaled air. This exchange is possible due to the thin walls of the alveoli and close proximity to the capillaries, which allow for the efficient transfer of gases.
The correct answer to the question is B.) Alveoli.