Factors affecting 3H2O transfer capacity of isolated perfused trout gills

Tritiated water (3H2O) transfer capacity (PdA) and vascular impedance (Zg) of isolated trout (Salmo gairdneri) gills perfused with or without epinephrine were studied under conditions of altered osmotic gradients, fluid stirring, perfusion rate (Qa), and efferent pressure (Pe). Transfer capacity was unaffected by osmotic gradients, indicating that 3H2O moves across gills by diffusion and that PdA is independent of hydraulic water movement. Fluid stirring increased PdA asymptotically, suggesting that boundary layers significantly affect diffusion across isolated gills. Transfer capacity was directly related to Qa; Zg was inversely related to Qa. The PdA results can be explained in terms of lamellar recruitment and the distribution of flow between secondary lamellae. Increased Qa reduced Zg due to recruitment and distension of gill vessels. Elevated Pe decreased Zg and PdA. The effects of Pe on Zg resulted from distension of the gill vasculature and increased venous drainage, whereas the effects of Pe on PdA can be explained by changes in the distribution of perfusion between secondary lamellae.