Alveolar epithelium composition

Despite the fact that most of the alveolar surface is composited of alveolar epithelium, three primary types of cells are present in the alveoli type I alveolar cells, type II alveolar cells, and alveolar macrophages. Type I alveolar cells are also referred to as squamous pulmonary epithelial cells and are the continuous lining of the alveolar sac. Type II alveolar cells are also referred to as septal cells. Type II alveolar cells secrete the alveolar fluid that is necessary to keep the surface moist and to maintain surface tension of the alveolar fluid surface tension is necessary to keep the alveoli from collapsing. Alveolar fluid is a suitable environment for proteins when compared to the low pH and high protease levels associated with the intestine... 

Inhaled air travels along the conductive airways until it reaches the very distal respiratory part formed by the respiratory airways, which are terminated by the sack like compartments of the alveoli. The surface of these air-filled compartments is formed by epithelial cell layers, the alveoli by the alveolar epithelium, respiratory airways by the respiratory epithelium, and conducting airways by the bronchial epithelium (Fig. 1). According to its localization along the airways the epithelial cell layers differs with regard to its cellular composition, epithelial barrier, and transport function. 

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