Functional Design of the Human Lung for Gas Exchange
At the end of a deep breath, about 80% of the lung volume is air, 10% is blood, and only the remaining 10% is tissue. Because this small mass of tissue is spread over an enormous area – nearly the size of a tennis court – the tissue framework of the lung must be extraordinarily delicate. It is indeed remarkable that the substance of the lung manages to maintain its integrity in the face of the multitude of insults that inevitably accompany a lifetime of exposure to ambient air and the complex necessity of keeping air and blood in intimate contact, but separate, for the sake of gas exchange.
Part of this success is undoubtedly attributable to the unique design of the lung, which ensures mechanical stability as well as nearly optimal conditions for the performance of the lung's primary function: to supply the blood with an adequate amount of oxygen even when the body's demands for oxygen are particularly high, as during heavy work.
At total lung capacity, the lung fills the entire chest cavity and can reach a volume, in the adult human, of some 5 to 6 L, largely depending on body size. Upon expiration, the lung retracts, most conspicuously from the lower parts of the pleural cavity, the posterior bottom edge of the lung moving upward by some 4 to 6 cm. This preferential lifting of the bottom edge is caused by retraction of the tissue throughout the entire lung, the surfaces of which are freely movable within the thoracic cavity.
The structural background for this mobility of a healthy lung is the formation, during morphogenesis, of a serosal space that is lined on the interior of the chest wall and on the lung surface by a serosa, the parietal and visceral pleurae, respectively (Fig. 2-1). However, this serosal space is minimal, since the visceral pleura is closely apposed to the parietal pleura, with only a thin film of serous fluid intercalated as a lubricant between the two surfaces.1 Both pleural surfaces are lined by a squamous epithelial layer, often called mesothelium (due to its mesodermal origin), whose surface is richly endowed with long microvilli. The apical microvilli increase the surface area available, suggesting that pleural mesothelial cells are capable of participating in active transserosal transport of solutes. The total volume of pleural fluid is about 15 to 20 mL, with approximately 1700 cells/mm3 (75% macrophages, 23% lymphocytes, 1% mesothelial cells). The volume and composition of the pleural fluid have to be tightly controlled to ensure an efficient mechanical coupling between chest wall and lung. Pleural fluid originates from pleural capillaries through microvascular filtration. Drainage occurs partly via lymphatic stomata in the parietal pleura. Transcytosis through mesothelial cells in both directions ...