Summary

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, U_crit).

2. 2. At U_crit, N_O2 was elevated above the resting level by 7.5 times. The logarithm of N_OO2 was linearly related to the swimming speed expressed as a proportion of U_crit. V_g increased in almost direct proportion to the increase in N_OO2.

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

4. 4. P_{a, Oa, O2} fell slightly during exercise but C_{a, Oa, O2} was unaffected. On the other hand P_{v, Ov, O2} halved and C_{v, 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.