Studies on the fish gill microclimate.

An isolated and perfused fish gill filament preparation has been developed to facilitate investigations on the fish gill microclimate. Measurements have been made of the concentration of sodium at the lamellar surface and in the mixed-exit water, and of the maximum interlamellar velocity. When the external water has a sodium concentration of 0.10 mmol.l-1, water at the gill surface has a mean sodium concentration of 0.146 mmol.l-1. Mixed-exit water, 50 microns behind the perfused filament, has a mean sodium concentration of 0.118 mmol.l-1. When the water flowing over the filament has a maximum velocity of 18 cm.s-1, the maximum velocity between secondary lamellae is 1.5 cm.s-1. These measurements have been used to support the development of a velocity-modified diffusion model which describes ion movement in the interlamellar space. The analysis and observations on the preparation suggest that substantial unstirred layers do not exist at the lamellar surfaces in this preparation. Simple diffusion to and from the tissue surfaces, in conjunction with parabolic velocity profiles within the interlamellar spaces adequately explain net sodium movements.