re over 700 million alveoli present in the human lungs, representing a total surface area of 70-90m2. The wall of each alveolus is only 0.0001mm thick (0.1mm). On its outside is a dense network of blood capillaries, all of which originate from the pulmonary artery and rejoin to form the pulmonary vein. Lining each alveolus is moist squamous epithelium. This consists of very thin, flattened cells, reducing the distance over which diffusion must occur. Collagen and elastic fibres are al
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