A critical analysis of gas transferin vivo and in a blood-perfused trout head preparation.

Gas transfer and blood acid-base status in the blood-perfused trout head preparation (in vitro) were compared with intact trout (in vivo) fitted with oral membranes, dorsal aortic, ventral aortic, and opercular cannulae. Gas transfer rates were calculated from both arterial-venous blood differences and inspired-expired water differences using the Fick method. Oxygen uptake[Formula: see text] and carbon dioxide excretion[Formula: see text] were lower, while ammonia excretion[Formula: see text] was higher, in the blood-perfused head relative toin vivo rates. Gas transfer rates calculated from water were consistently greater than rates calculated from blood, the difference being greaterin vitro compared toin vivo. We conclude that the inadequacy of O2 and CO2 transfer in the blood-perfused head was not due to abnormal gill diffusive conductance, but was more likely related to the reduced magnitude of the blood-to-water O2 and CO2 diffusion gradients, low hematocrit, and decreased perfusion flow rate[Formula: see text]. Under the conditions of the present study, the blood-perfused trout head is not a suitable preparation for the study of oxygen transfer. We conclude this preparation is useful for evaluating branchial carbon dioxide or ammonia transfer only when comparable measurements or manipulations cannot be made on intact fish.