.; 1998.
Water forms an important structural and functional constituent of the intercellular and intracelular lung tissue (e.g., Bastacky et al. 1987). Furthermore, a hydrated layer lines the air spaces of the lung (e.g., Fishman et al. 1957; Cantin et al. 1987; Chinard 1992). In the larger air spaces, the aqueous layer is comprised mostly of mucus, a glycoprotein-containing phase which is about 98% water (e.g., Sturgess 1979). The mucus forms an important source of moisture which humidifies the inhaled air, traps solid particles, and protects the ciliated epithelium. At the alveolar level, the hydrated layer occurs in form of an aqueous subphase in which proteins, carbohydrates, ions, and surfactant are dissolved. In the vertebrate lungs, where detailed investigations have been carried out, gas exchange occurs across an extracellular alveolar fluid film which lines the surface. The lining has been lucidly demonstrated by Finley et al. (1968), Weibel and Gil (1968), Kikkawa (1970), Bastacky et al. (1987, 1993), and Hook et al. (1987). In the airways, the thickness of the surface liquid lining is 20 to 150 µm (Widdicombe 1997) while on the alveolar surface, the thickness ranges from 0.1 to 0.241Im (Weibel and Gil 1968; Bastacky et al. 1993, 1995; Stephens et al. 1996). In the human lung, it has been estimated physiologically (e.g., Rennard et al. 1986) that the epithelial lining fluid (ELF) makes up 20 to 40 ml while through morphometric techniques (e.g., Untersee et al. 1971; Gorin and Steward 1979), the ELF was estimated to range from 15 to 70 ml. The alveolar fluid layer contributes significantly to the gas exchange function of the lung
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