The alveolus and air-blood barrier in the lung (rat)

Electron microscopy. The alveolus (1) is lined by a thin extension (2) of the alveolar epithelial cell type I (2), the pneumocyte I and the thin endothelium (3) of the capillary filled with erythrocytes (4) and blood platelets (5). The thin-walled air-blood barrier (↔) consists of the transition from endothelium → a common single basal lamina → pneumocyte I. Note that oxygen diffusion also takes place at the other transitions, the so-called thick-walled areas. Thin-walled areas are most favourable to gas exchange and alternate with thick-walled areas (thick line, ▬) consisting of supporting fibers, extracellular matrix and cells of the alveolar framework that separate the alveolar epithelium from the capillaries.