Alveoleepitelet

Alveoleepitelet, or the alveolar epithelium, is the epithelial lining of the alveoli in the lungs. It forms the thin gas-exchange surface that separates inhaled air from the pulmonary capillary blood.

The alveolar epithelium consists mainly of two cell types: type I pneumocytes, extremely flattened cells that cover most of the alveolar surface and form the primary gas-diffusion barrier; and type II pneumocytes, cuboidal cells that secrete pulmonary surfactant and act as progenitors that can proliferate and differentiate into type I cells after injury.

Type II cells also produce surfactant proteins SP-A, SP-B, SP-C, and SP-D, which contribute to innate immunity and surface activity. The alveolar walls contain a network of capillaries and alveolar macrophages in the airspace, together forming the alveolar-capillary barrier, a very thin interface that facilitates diffusion of oxygen and carbon dioxide.

Developmentally, type II cells appear during late gestation and differentiate into type I cells after injury; alveolarization continues after birth in humans, expanding the surface area available for gas exchange.

Clinical relevance includes injury to the alveolar epithelium, which can lead to edema and impaired gas exchange, as seen in acute respiratory distress syndrome. Defects in surfactant production or function contribute to neonatal respiratory distress and can play a role in various lung injuries and diseases.