Alveoli and Alveolar Ducts

Alveolar ducts are microscopic tubes lined with simple squamous epithelium that branch from respiratory bronchioles and conduct air into the respiratory zone. These ducts terminate in grape-like structures called alveolar sacs, which contain individual alveoli where gas exchange occurs. The thin epithelial lining of alveolar ducts facilitates efficient gas diffusion.

Type I pneumocytes are squamous epithelial cells covering approximately 97 percent of the alveolar surface, enabling gas exchange across their thin 25 nm thickness. Type II pneumocytes are cuboidal cells that secrete pulmonary surfactant, a phospholipid and protein mixture that reduces surface tension and prevents alveolar collapse during exhalation.

Alveolar macrophages, also called dust cells, are immune phagocytic cells located in alveolar walls that patrol and remove inhaled pathogens, microscopic dust, and debris. These cells are essential for maintaining lung health by preventing harmful particles from accumulating in the respiratory zone and compromising gas exchange.

The respiratory membrane, formed by the combination of the alveolar epithelium and capillary endothelium, measures approximately 0.5 micrometers in thickness. This extraordinary thinness facilitates simple diffusion of gases, allowing oxygen to be absorbed into the blood while carbon dioxide is released into alveolar air through physical principles governing gas exchange.

Alveolar pores are small openings that interconnect neighboring alveoli, ensuring uniform air pressure distribution across all alveoli and throughout the lungs. This interconnection allows air to flow between adjacent alveoli, preventing pressure imbalances and enabling efficient ventilation of all respiratory structures during breathing.

Pulmonary surfactant is a phospholipid and protein mixture secreted by type II pneumocytes that coats the alveolar surface. It decreases surface tension of the fluid lining alveoli, preventing alveolar collapse during exhalation and maintaining the structural integrity of the approximately 150 million alveoli in each lung.

Each alveolus is surrounded by an extensive capillary network with encircling elastic fibers that stabilize alveolar positions and facilitate gas exchange. The intimate contact between capillaries and the thin alveolar epithelium creates the respiratory membrane, enabling efficient oxygen uptake and carbon dioxide release during external and internal respiration.