The validity of previosly used simplified models for the analysis of gas transfer in fish gills was tested using an integrated model which includes water flow and blood flow in counter-current arrangement. The model accounts for the resistance to diffusion of O2both in the water-blood barrier and in the interlamellar water, which is assumed to flow with a parabolic velocity profile between the secondary lamellae. The O2diffusing capacity (transfer factor) for this model (Dt) was compared to that (Dm + w) calculated from the diffusing capacity of the water-blood barrier (Dm), and from teh effective diffusive conductance of the parabolically streaming interlamellar water (Dw) as 1/Dm + w= 1/Dm+ 1/Dw. These diffusing capacities were compared with that (Dadd) calculated from Dmand diffusing capacity of a water layer of 1 4 thickness of the interlamellar space (D∼w) as 1/Dadd= 1/Dadd+ 1/D∼w. Calculations with morphometric and gas exchange parameters in the elasmobranch Scyliorhinus stellaris reveal the following features: (1) In physiological conditions, Dm + wand Dintare similar to within 10%, but Dintis always higher. (2) Dtand Dm + wincrease with increasing ventilation; Dtincreases with decreasing perfusion, while Dm + wremains constant. (3) Both Dintand Dm + wagree reasonably well with Dadd. In other anatomical and physiological conditions, particularly for relatively high Dm, Dw, and D∼wand high ventilation, greater discrepancies between Dintand Dm + wmay occur but Dm + wappears to represent a reasonable approximation of the effective O2diffusing capacity, which is best modelled as Dint. © 1986.
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