Each of the respiratory bronchioles inside a pulmonary lobule gives rise to several alveolar ducts. Protruding from the thin walls of the alveolar ducts and respiratory bronchioles are numerous cup-shaped alveoli, each measuring about 0.2 to 0.5 mm in diameter. Along the distal end of an alveolar duct, the alveoli are arranged into grape-like clusters. When two or more alveoli share a common opening to an alveolar duct, they are referred to as an alveolar sac. Small openings called alveolar pores perforate the interalveolar wall and allow air to pass between alveoli. They provide alternative routes to and from the alveoli if an obstruction occurs.
The alveoli are made up of two types of cells (or pneumocytes). About 95% of the alveolar surface consists of simple squamous epithelial type I cells, and the remaining 5% is occupied by type II cells (or septal cells). These large, rounded cells are located between the type I cells and secrete pulmonary surfactant. The complex of phospholipids and proteins in the surfactant reduces surface tension inside the alveoli, which keeps the alveolar walls from sticking together as they deflate during exhalation.
A network of capillaries and many supportive collagen and elastic fibers are found in the narrow interstitial spaces that separate the alveoli.
Numerous macrophages (or dust cells) protect the lungs from damage. They move about the air spaces and between the alveoli, where they remove (engulf) inhaled particles, foreign invaders, and other types of harmful substances.
