of the dogfish that were higher than in expired water. Thus, both teleostsand elasmobranchs can have arterial oxygen tensions higher than thatin expired water; however, in teleosts, these are the result of a countercurrent arrangement of flows, whereas in elasmobranchs a serial multicapillary system may predominate. These high arterial oxygen tensions arealso explicable in terms of the active transport or exchange diffusionof oxygen across the gill epithelium (see the discussion on pages 104-105and 388-39 1 in the "Development of the Lung," de Reuck and Porter,eds., 1967 ). Although active transport or exchange diffusion of oxygencannot be excluded, there is no evidence for the involvement of theseprocesses in oxygen transport across the gills.
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