Final answer:
The respiratory portion of the lower respiratory tract includes the lungs, bronchioles, and alveoli, where gas exchange takes place facilitated by the extensive network of capillaries.
Step-by-step explanation:
The respiratory portion of the lower respiratory tract includes three main structures that are responsible for gas exchange. These are the lungs, the alveoli contained within them, and the bronchioles that lead to these alveoli. The trachea and bronchi are involved in conducting air to the lungs but do not participate in the gas exchange. The alveoli are the tiny air sacs where gas exchange between the air and blood takes place, facilitated by the extensive capillaries around them. Human lungs contain approximately 400 million alveoli, highlighting the extensive area for gas exchange. This critical respiratory zone includes the terminal bronchioles and alveoli, as they are directly involved in the exchange of oxygen and carbon dioxide.