Which term refers to the air sac in the lung?

The term that refers to the air sac in the lung is "alveolus." Alveoli are tiny, balloon-like structures located at the ends of the bronchioles in the lungs, and they play a crucial role in the respiratory system. Their primary function is to facilitate gas exchange; they provide a large surface area for oxygen to diffuse into the blood and for carbon dioxide to diffuse out. Each alveolus is surrounded by a network of capillaries, allowing for efficient transfer of gases between the air and the bloodstream. This structure is vital for maintaining proper respiratory function and ensuring that oxygen reaches the body’s tissues while removing carbon dioxide effectively.

The other terms, though related to the respiratory system, refer to different structures: bronchioles are the small air passages that lead to the alveoli, the trachea is the windpipe that carries air to and from the lungs, and capillaries are tiny blood vessels that facilitate the exchange of gases and nutrients between blood and tissues but are not air sacs.