Fig. 2. The effects of stepwise increases in water P_CO2 (Pw_CO2) followed by the rapid lowering of Pw_CO2 (A) on cardiovascular variables in tambaqui Colossoma macropomum including (B) arterial blood pressure (PDA; N=10-11), (C) systemic vascular resistance (R_s; N=9-10), (D) cardiac output (b; N=8-10), (E) heart rate (fH; N=9-11) and (F) cardiac stroke volume (VS; N=9-10). Data for Pa_CO2 are replotted from Fig. 1 in A for ease of comparison. Pre1 and Pre2 refer to pre-exposure values compiled 2 min and immediately, respectively, before the onset of hypercarbia; the numbers 7, 16 and 26 denote values compiled for the final 2 min of the three discrete steps of increasing hypercarbia; and the data plotted to the right of the break in the x-axis represent values compiled for 2 min intervals after the initiation of rapid washout of CO₂ at 0 min. The data are presented as means ± 1 S.E.M. For stepwise increases in Pw_CO2, values that do not share a letter are significantly different from one another (one-way RM-ANOVA; P values: B, 0.386; C, 0.892; D, 0.144; E and F, <0.001). For the rapid washout of water CO₂, asterisks indicate values that are significantly different from the value at time=0 min (one-way RM-ANOVA, P values: B, 0.444; C, 0.002; D-F, <0.001).