Lung capillaries and air-blood barrier (rat)

Electron microscopy. Two neighboring capillaries (C) and the alveolar spaces (A). The endothelial cell (1) of the upper capillary contains an organelle-rich cytoplasm, a centriole (2), contractile filaments (↓) and electron-dense membrane-bound granules, the so-called Weibel-Palade bodies. These bodies contain among others von Willebrand factor. The air-blood barrier (thin-walled area) is indicated by (↔) and formed by endothelial cell cytoplasm--common basal lamina−alveolar cell type I. Note the numerous pinocytotic vesicles in the endothelial cell of the capillary. Thin-walled areas are most favourable to gas exchange and alternate with thick-walled areas (not shown here) consisting of supporting fibers, extracellular matrix and cells of the alveolar framework that separate the alveolar epithelium from the capillaries.