Alveolar Ducts

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.

• The thin segment of alveolar septum contains type I pneumocytes, a fused basal lamina is made of type I pneumocyte and capillary endothelium, and capillary endothelial cells.
• The thick segment of alveolar septum is made of type I pneumocytes, basal lamina of type I pneumocytes and capillary endothelium, connective tissue, and capillary endothelial.
• The type I pneumocytes/alveolar cells are extremely thin squamous cells, lining 95% of the surface of alveoli. They are joined to one another and to other cells by occluding junctions. The junctions form an effective barrier between the air space and the components of the wall. They’re not capable of cell division.
• The type II pneumocytes/alveolar cells are also called septal cells. They are secretory cells, cuboidal, and interspersed among the type I cells. They are the progenitors for type I cells and comprise 60% of the lining cells. They bulge into the air space and the apical cytoplasm has numerous granules. These contain phospholipids, neutral lipids, and proteins. They are secreted by exocytosis and form an alveolar lining layer (surfactant).

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).