Oxygen exchange and vascular resistance in the totally perfused rainbow trout

A whole trout preparation (Salmo gairdneri) externally ventilated with water and internally perfused with artificial medium via a cardiac pump is described for the study of O₂ exchange and vascular resistance. As cardiac output (Q) was raised, ventral and dorsal aortic pressures increased while branchial (Rg) and systemic (Rs) vascular resistances fell, reflecting considerable passive distensibility. Arterial oxygenation was negative at low Q's due to significant internal O₂ demand by the gill tissue, but increased to zero or positive values at intermediate Q's, and eventually declined at high Q's because of transit time limitation. O₂ uptake from the ventilatory flow rose with increasing Q. Epinephrine (10^-5 M) decreased Rg, increased Rs, and enhanced arterial oxygenation. Artificial elevation of dorsal aortic pressure decreased Rg but did not affect arterial oxygenation. A 10-fold elevation of ventilatory flow increased arterial oxygenation but did not alter Rg or Rs. Endogenous metabolism of branchial tissue accounted for 11.7% of resting O.2 uptake in vivo, and comprised an internal component taking O₂ from perfusion flow and an external component drawing O₂ from ventilatory flow.