Alveolus in lung (dog)

Electron microscopy. (A) indicate alveolar space; (C) indicate capillary with erythrocyte. Drawing-pin represents endothelium. (1) type I alveolar cell; (↓) indicate thin cytoplasm of pneumocyte I. (2) type II alveolar cell. (3) alveolar macrophage. (*) are spots of elastin, intermingled with collagen. (4) interstitial myofibroblast. (↔) thin-walled area of the air-blood barrier. Thin-walled areas are most favourable to gas exchange and alternate with thick-walled areas (thick arrow, ◄▬►) consisting of supporting fibers, extracellular matrix and cells of the alveolar framework that separate the alveolar epithelium from the capillaries.