Definition / Explanation: $(1)$ Alveoli

(N/A) Alveoli are the primary sites of gaseous exchange in the lungs. Each alveolus forms a bag-like or cavity-like structure,which is surrounded by a network of very thin pulmonary capillaries. The wall of the alveolus is composed of a single layer of squamous epithelial cells,which facilitates the diffusion of gases ($O_2$ and $CO_2$) between the air and the blood.