ABSTRACT
Ventilation volume was measured directly in rainbow trout using a rubber membrane attached to the mouth which separated inspired and expired water and allowed collection of the latter.
Mean ventilation volume at 8·6°C for 18 trout weighing approximately 200 g was 37±1·8 ml/min/fish. Mean ventilation rate and ventilatory stroke volume averaged 74 breaths/min and 0·5 ml/breath respectively.
Ventilation volume could be increased nearly sevenfold during moderate, shortterm hypoxia as a result of a large increase in ventilatory stroke volume and a small increase in ventilation rate.
The ratio between the flow rates of water and blood through the gills was approximately 10.
Percentage utilization of oxygen from inspired water had a mean of 46 ± 1·5% and ranged from 23 to 64%.
Artificial perfusion of the gills with water at different flow rates was achieved by tying a tube into the mouth of trout.
Perfused fish could not saturate their arterial blood with oxygen at a perfusion rate of 45 ml/min but could do so at rates ranging from 85 to 1200 ml/min.
Low arterial tensions at a perfusion rate approximating the mean
of fish with oral membranes are probably the result of a poor pattern of water flow over the gills during perfusion.Opercular movements occurred only at perfusion rates below 700 ml/min and increased in frequency as perfusion rate dropped. This ventilatory activity may have resulted from receptors sensitive either to water flow over the gills or to arterial
.As perfusion rate went up cardiac output and oxygen uptake increased. These changes were accompanied by a drop in dorsal aortic pressure which reflected vasodilation of the gills and peripheral circulation. This change in the pattern of blood flow through the gills contributed to a 50% increase in oxygen transfer factor across the gills.
At the highest perfusion rates there was no apparent impairment of gas exchange even though anatomical deadspace was probably high.
