Alveolar duct

Each bronchiole divides into two or more respiratory bronchioles, with scattered alveoli, and each of these again divides into several alveolar ducts, with a greater number of alveoli connected with them. Each alveolar duct is connected with a variable number of irregularly spherical spaces, which also possess alveoli, the atria. With each atrium a variable number (2–5) of alveolar sacs are connected which bear on all parts of their circumference alveoli or air sacs. (Miller.)