Alveoli are the air sacs of the lungs. Each alveolus is composed of a single, thin layer of epithelium arranged in a hollow pouch shaped structure. Each alveolus is surrounded by a capillary bed, elastic fibers, and smooth muscle cells and neighboring alveoli are connected by alveolar pores, permitting air movement between alveoli.

The alveoli perform gas exchange, the primary function of the respiratory system, by exchanging respiratory gasses between the body and the environment. Air moves in and out of the alveoli during breathing and diffuses passively into the blood supply provided by the capillary beds surrounding each alveolus.

Figure 1: A close-up of the final portion of the bronchial tree showing the key structures of the alveoli.