What are the tiny sacs of lung tissue that facilitate gas exchange called?

The tiny sacs of lung tissue that facilitate gas exchange are known as alveoli. These structures are critical components of the respiratory system where the exchange of oxygen and carbon dioxide occurs. Alveoli are equipped with thin walls that allow for the efficient diffusion of gases; oxygen from the air within the alveoli can easily pass into the blood vessels, while carbon dioxide from the blood can move into the alveoli to be exhaled. This process is essential for maintaining the body's oxygen levels and removing waste gases.

In contrast, bronchi are the larger passageways that lead from the trachea into the lungs, where they branch into smaller bronchioles. Capillaries are tiny blood vessels that surround the alveoli and are involved in the exchange of gases, but they are not the structures where gas exchange actually occurs. Bronchioles are smaller branches of the bronchi and serve as airways to direct air into the alveoli, but they themselves do not perform gas exchange. Thus, alveoli are specifically designated for the gas exchange process in the lungs, making them the correct answer to the question.