Functional Aspects of Lung Structure as Related to Interaction with Particles
The organization of the lung, that is, the arrangement of the composing structures, differs from that of other human organs as, at the end of a normal breath, the lung consists of about 80% air, 10% blood, and only 10% tissue. In general, the latter is composed of structures lining the airspaces and the blood vessels and, in between, interstitial structures providing mechanical and metabolic features of lung function. In addition, the lung contains a second circulation that provides blood to the tissue itself, a neuroendocrine system, an immune system, and a covering layer at the “outer” surface of the lung-the visceral pleura. Functionally, the pulmonary tissue needs not only to be strong enough to separate air and blood effectively but also to provide a large surface area and a thin tissue barrier for gas diffusion between air and blood. To meet these functional demands, the lung possesses an air-conducting zone (airways) and a gasexchange region (alveolar region), the two of which differ significantly with respect to their qualitative composition and quantitative contribution to lung structure. Therefore, despite its small volume, pulmonary tissue has a high degree of complexity with over 40 different cell types and highly specialized noncellular components.