Alveolar ducts are elongated air passages that have almost no walls and have only alveoli as their peripheral boundary. Rings of smooth muscle are present in the knob-like park of the interalveolar septa. The end in alveolar sacs are spaces surrounded by clusters of alveoli. Each alveolus is surrounded by a network of capillaries that brings blood into close proximity to inhaled air. The tissue between adjacent alveoli is alveolar septum or septal wall. The septum is vascularized with the richest capillary networks in the body. It consists of fibroblasts and sparse extracellular matrix (ECM). The ECM contains elastic, which allows alveoli to expand with inspiration and contract passively with expiration, and reticular fibers, which prevents both the collapse and excessive distention of the alveoli. The air-blood barrier is made of thin and thick segments.
Alveolar macrophages are derived from monocytes, and function in the connective tissue of the septum and in the air space of the alveolus. In the air space they scavenge the surface to remove inhaled particulate matter (ex. Dust and pollen), hence why they’re called dust cells. Other macrophages remain in septal connective tissue and stay for life filled with debris. Alveolar macrophages phagocytose red blood cells that may enter the alveoli in heart failure. Lungs of smokers at autopsy show many alveolar and septal macrophages filled with carbon particles. They also engulf infectious organisms (ex. Tubercle bacilli).