Fig. 2. The effects of varying chemoreceptor thresholds on durations of dives (t_d, solid lines) and surface intervals (t_s, broken lines). (A) The effect of decreases in central chemoreceptor threshold (T_c) during surface hyperventilation (T_c, mmHg) at each of four different levels of peripheral chemoreceptor threshold (T_p, mmHg). T_c during dives was always 37.4 mmHg. T_p was constant across the dive cycle: circles, T_p=39.1 mmHg; triangles, T_p=37.4 mmHg; squares, T_p=35.7 mmHg; diamonds, T_p=34 mmHg. (B) The effect of variation in T_p on t_s and t_d at constant T_c (-5.1 mmHg). Note that increasing T_p represents decreasing peripheral chemoreceptor responsiveness to CO₂ and O₂.