1. 1.

    The capabilities of the oxygen transport system of rainbow trout in supplying the increased oxygen demands in exercise, in a water tunnel at 9–10.5 °C, have been investigated by increasing the velocity of water flow, with a 1 h period between increments, up to the maximum swimming speed (critical velocity, Ucrit).

  2. 2.

    At Ucrit, NO2 was elevated above the resting level by 7.5 times. The logarithm of NOO2 was linearly related to the swimming speed expressed as a proportion of Ucrit. Vg increased in almost direct proportion to the increase in NOO2.

  3. 3.

    Heart rate rose slightly at half Ucrit and reached a maximum, 1.6 times the resting rate, as Ucrit was approached. Ventral and dorsal aortic mean blood pressures rose by 60% and 20% respectively at Ucrit while their pulse pressures doubled. Central venous pressure was virtually unchanged.

  4. 4.

    Pa, Oa, O2 fell slightly during exercise but Ca, Oa, O2 was unaffected. On the other hand Pv, Ov, O2 halved and Cv, Ov, O2 fell from 3.17 (S.E. = 0.3) to 0.6(S.E. = 0.7) mmol/1. Cardiac output increased by about 3 times resting values.

  5. 5.

    The results are discussed and an attempt is made to estimate the maximum capabilities of the components of the oxygen transport system in sustained exercise.

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