Exposure and Disposition of Respiratory Media in Gas Exchangers

Abstract

Optimal exposure of the external respiratory medium (water/air) to blood is requisite for efficacious flux of the respiratory gases in a gas exchanger. The presentation of the respiratory media is a dynamic process that is determined by the geometry and the spatial arrangement of the conduits and structures that transport the respiratory media to and within the gas exchanger. Different designs (Fig.1) have given rise to the different presentations (Fig.3). Exposition, on the other hand, connotes a steady state in which blood is exposed to an external respiratory medium. In gas exchangers, vascularity and prominence (bulging) of the blood capillaries over the respiratory surface (Figs. 20, 48, 54, 55, 85–87, 110, 112, 115, 116, 120, 122–125, 127–133, 135–137) augment the respiratory surface area while enhancing the exposure of pulmonary capillary blood to the external respiratory medium. In the mammalian lung pulmonary capillary blood is exposed to air essentially on the two sides of the interalveolar septum (Figs. 20, 36, 84–86, 97, 110, 125); such an arrangement of the blood capillaries is called a ‘single capillary system’. In the avian lung, about 90% of the exchange the tissue comprises closely interdigitated air and blood capillaries (Maina 1989b; Maina et al. 1989a) (Figs. 6, 7, 8, 17, 57–62, 65); the blood is literally suspended in a three-dimensional space, that is, it is exposed to air virtually on all sides (Figs. 8, 57, 59, 60, 62, 126). The arrangement maximizes the respiratory surface area, promoting and explaining the relatively higher pulmonary diffusing capacity of the avian lung for O2 (Maina 1989b, 1993, 2000b, c; Maina et al. 1989a). In the African lungfish, Protopterus aethiopicus (Maina and Maloiy 1985; Maina 1987a) (Figs. 129, 137), amphibians (Figs. 123, 127, 128) and in certain reptilian lungs (Figs. 120, 122, 130, 132, 136), a double capillary arrangement, that is, where the pulmonary capillary blood is exposed to air only on one side, occurs. In the South American lungfish, Lepidosiren paradoxa (Hughes and Weibel 1976), the volume of the pulmonary capillary blood constitutes 3.5% of the total volume of the lung, whereas in mammals (e.g. mouse), the value is 14% and in birds 25% (e.g. Duncker 1974). In the suprabranchial membrane (Figs. 19, 116) and the labyrinthine organ (Figs. 19, 48, 124), the accessory respiratory organs of the catfish Clarias mossambicus (Maina and Maloiy 1986), the lung of the slug Trichotoxon copleyi (Maina 1989e) (Figs. 28, 133) and the blood vessels on the respiratory groove of the swampworm, Alma emini (Maina and Maloiy 1998; Maina et al. 1998) (Fig.41), only one layer of blood capillaries that protrude into the air space occur. In such a design, strictly speaking, neither the term ‘single capillary’ nor the term ‘double capillary’ arrangement applies. Regarding the exposure of blood to an external respiratory medium in a gas exchanger, this is perhaps the most elementary design that has ever evolved. It is, however, pertinent to note that in such designs, rather than the blood in the capillary system being suspended in air/water, it is applied on a supporting (fixed) surface. Hence, the only realizable respiratory construction is one where blood is exposed to a respiratory medium only on one side. In the accessory respiratory organs of the African catfish, Clarias mossambicus, to compensate for an inferior design, the thicknesses of the blood-gas (tissue) barrier of the suprabranchial chamber membrane (0.313 urn) and the labyrinthine organ (0.287 μn) are much smaller than that of the water-blood barrier of the gills (17.30 urn) (Maina and Maloiy 1986), a condition that promotes the diffusing capacity of the accessory respiratory organs. The exposure of blood to water in the vascular channels of the secondary lamellae, hemispherical flaps that are located on opposite sides of the filaments of fish gills (Figs. 46, 52, 54, 55, 107, 111) and the gill filaments of crustacean gills (Figs. 47, 49, 66, 121), occurs across two surfaces.