Ventilation and Its Effect on “Infinite Pool” Exchangers

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Abstract
SYNOPSIS. Unlike internal exchange surfaces, the skin contacts an “infinite pool” of air or water with which exchange of gases, water, ions, and other solutes may occur. Even though the “infinite pool” may be well mixed, an unstirred diffusion boundary layer is always present about the skin and may constitute a significant resistance to exchange. The thickness of the diffusion boundary layer (as approximated by the fluid dynamic boundary layer) is related to the flow of the respiratory medium, viscosity and density of the medium, and the morphology of the exchange surface. Oxygen microelectrode studies suggest that, in most circumstances, the diffusion boundary layer in water is at least as thick as the blood-respiratory medium distance in amphibian skin. Accordingly, the movement of water about the skin {i.e., skin ventilation) should have pronounced effects on cutaneous exchange, especially at low “free stream” velocities. Mounting physiological evidence suggests that: (1) skin ventilation can augment cutaneous gas exchange; and (2) some vertebrates actively ventilate their skins, especially in aquatic hypoxia. The ubiquity and significance of diffusion boundary layers are central to a general understanding of cutaneous exchange and all surface-mediated exchange processes.

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