What are the tiny sacs in the lungs where gas exchange occurs called?

The tiny sacs in the lungs where gas exchange occurs are known as alveoli. These structures are crucial for the respiratory system as they provide a large surface area for the exchange of oxygen and carbon dioxide between the air and the blood. Each alveolus is surrounded by a network of capillaries, where oxygen from the inhaled air passes into the bloodstream, and carbon dioxide, a waste product of metabolism, is expelled from the blood into the alveoli to be exhaled.

Alveoli are specifically adapted for this process, featuring thin walls that allow for efficient diffusion of gases. The design of the alveoli, with their clustered arrangement resembling grapes, maximizes the surface area available for gas exchange, making them essential for proper respiratory function.

The other choices refer to related structures but do not serve the same function. For instance, bronchioles are smaller air passages that lead to the alveoli but are not the sites of gas exchange themselves. The pleura refers to the membranes surrounding the lungs that facilitate movement during breathing. Capillaries, while involved in the transport of gases, are not the sacs where the actual gas exchange takes place, as this occurs specifically in the alveoli.