Fig. 3. The effect of mean cardiac output during surface intervals (_s, expressed as a percentage of the nominal maximum cardiac output, _max) on (A) surface intervals (t_s) and (B) dive durations (t_d). In this study _max was assumed to be 84.3 l min^-1, corresponding to a maximum heart rate of 85 beats min^-1. Simulations were conducted at three levels of hyperventilation (_n, expressed as a percentage of maximum ventilation, _max=240 l min^-1); diamonds, _n=58%; squares, _n=75%; triangles, _n=92%.