Respiratory, and some cardiovascular and metabolic, variables were measured in tufted ducks swimming at different velocities. There were no substantial changes in any of the measured variables up to a swimming speed of 0.5 m s−1. Above this speed there were progressive increases in heart rate, oxygen uptake and respiratory minute volume. At the middle of the maximum speed range (0.8 m s−1) these variables were 1.7 times, 2.7 times, and 3.4 times their resting values respectively. There was, therefore, an excessive increase in ventilation (hyperventilation), compared with the extra demand for oxygen, and this was evident as a significant decrease in oxygen extraction and as a significant fall in PCOCO2 in arterial blood.

The possible causes of the hyperventilation are not obvious as there was no hyperthermia and no change in pH of the arterial blood; a 2.3 times increase in lactic acid was balanced by the reduction in PCOCO2. There was some evidence of locomotorrespiratory coupling at the highest swimming speeds (leg beat frequency: respiratory frequency, 6:1), which appeared to constrain any further rise in respiratory frequency. At the highest swimming speed tidal volume, for the first time, increased above the resting value and the level of hyperventilation was increased. Hyperventilation may, therefore, serve to maintain arterial pH in the face of a metabolic acidosis.

Arterial blood pressure, POO2 and haematocrit did not change during swimming. There was a doubling in the level of plasma adrenalin with little change in noradrenalin. The possible effects of these increases are discussed.

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