Question

lo 4.2. what are the alveoli?
tiny hollow tubes in the lungs.
microscopic capillaries in the blood vessels.
small air sacs located at the end of the bronchioles.
glands that produce mucus in the respiratory tract.
valves that regulate blood flow in the circulatory system.

Explanation:

Alveoli are small air - filled sacs in the lungs. They are located at the end of the bronchioles. Their main function is gas exchange (oxygen and carbon dioxide exchange between the air and the blood).

• Tiny hollow tubes in the lungs could refer to bronchioles or other small tubes but not alveoli.
• Microscopic capillaries are in blood vessels, not the structure of alveoli.
• Glands that produce mucus are not alveoli.
• Valves regulating blood flow are part of the circulatory system, not related to alveoli.

Answer:

Small air sacs located at the end of the bronchioles.