1. The rate of oxygen uptake by rainbow trout does not alter during progressive deoxygenation of the environment. Blood lactate, however, shows a significant increase, indicating an increase in anaerobic metabolism during hypoxia.

2. The percentage utilization of oxygen from the water decreased from 55% to approximately 20% during hypoxia and was associated with a 13-fold increase in ventilation volume.

3. The arterial blood of a trout resting in aerated water was 95-100% saturated: the Po2 was 122 mm. Hg and the Pco2 was 1-1.5 mm. Hg.

4. The venous blood of a resting trout was 70% saturated, had a Po2 of 35 mm. Hg and a Pco2, of 2.5 mm. Hg.

5. During hypoxia the percentage saturation of the arterial blood decreased to 37% and that of the venous blood to 3%. The P o2 in the arterial blood was 10 mm. Hg; in the venous blood, 6 mm. Hg. The Pco2 in the arterial blood was 3.5-4 mm. Hg; in the venous blood, 4.5-5.00 mm. Hg respectively.

6. The ability of the fish to withstand hypoxia was related to the oxygen capacity of the blood, which was on average 9 vol.%. The red blood cells swelled during hypoxia, the hematocrit increased, but the red cell count did not alter.

7. Blood pH was 7.7, falling to 7.4 during hypoxia.

8. Cardiac output did not change during progressive hypoxia in the water, stroke volume increased to offset the decrease in heart rate.

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