The Air-Blood Barrier
Pulmonary alveoli are the terminal sac-like extensions of the distal airways that are specialized for gas exchange. The alveolar surface is covered by very thin epithelial cells (type I cells) that, together with the underlying capillary endothelium, comprise the air-blood barrier, across which gas exchange occurs. Injury to epithelial cells (arising from genetic mutations or environmental insults) can lead to influx of inflammatory cells, fibroblast proliferation and deposition of extracellular matrix in the alveolar wall. Progressive thickening of alveolar walls, a condition referred to as interstitial lung disease (ILD), results in decreased gas exchange and, ultimately, death.
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Cross section of an individual alveolus. Arrows indicate the air-blood barrier composed of a type I epithelial cell, the underlying basal lamina and an endothelial cell. |  Click to enlarge.
Histological sections of normal lung tissue (left panel) and lung tissue from a patient with interstitial lung disease (right panel).Note the thickness of the air-blood barrier (arrows). |
