Separation of Respiratory Media in Gas Exchangers

Abstract

The thickness of the tissue partitioning between water/air and blood in part sets the conductance, that is, the diffusing capacity (Do₂) of a gas exchanger. Furthermore, Do₂ is determined by factors such as the respiratory surface area, the volume of the pulmonary capillary blood, the prevailing partial pressure gradient and the permeability of the tissue compartments that O₂ diffuses across (Fick’s law). The Do₂ correlates directly with the surface area (S) and the permeation constants (K) and inversely with the thickness of the tissue barrier: Do₂ =S Kt^−l. In the vertebrate air breathers, the thickness of the blood-egas (tissue) barrier generally decreases from amphibian, reptilian, mammalian to avian lungs. Owing to the thick blood-gas/water (tissue) barrier, which measures between 20 and 50 urn, cutaneous gas exchange is characterized by limitations of diffusion (e.g. Malvin 1988). Increased vascularity (Fig.115) and perfusion may promote O₂ uptake, particularly during hypoxic episodes (e.g. Malvin and Hlastala 1986). In fish gills, a thick water-blood barrier (Hughes and Morgan 1973) (Figs. 54, 55) and the presence of an unstirred boundary water layer over the secondary lamellae may curtail O₂ transfer, producing significant diffusion limitations (e.g. Randall and Daxboeck 1984). Although generally thicker than the blood-gas (tissue) barrier of the lungs, the water-blood barrier in the gills of certain species of fish may be as thin as 0.2 μm in some parts (e.g. Hughes and Morgan 1973; Maina 1991). In mammals (e.g. Gehr et al. 1981) birds (e.g. Maina 1989b; Maina et al. 1989a) and even fish (e.g. Hughes and Morgan 1973), the thicknesses of the tissue barriers of the gas exchangers correlate with the metabolic needs as well as the environment in which they live. The tuna, Katsuwonis pelamis, one of the most energetic fish, has a water-eblood barrier that is only 0.6 μm thick (e.g. Hughes and Morgan 1973), and Oreochromis alcalicus grahami, a fish that lives in the hot, highly alkaline Lake Magadi of Kenya that is virtually hypoxic at night, has a barrier that is as thin as 0.2 μm (e.g. Maina et al. 1996b).

The organism is a compromise. The result of natural selection is adequacy and not perfection. Bennett (1987)