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Electron micrographs of pulmonary capillaries in chicken (A) and dog lung (B). Note that the blood-gas barrier in the bird is much thinner and much more uniform in thickness than in the mammal. EPI, epithelium; F, fibrils of type I collagen; FB, fibroblasts. [A is modified from Maina (18); B is modified from Weibel (34).]
