Fig. 6. (A-C,E-G) The effects of external injections of CO₂-enriched water (black circles) or CO₂-free acidified water (white circles) on heart rate (fH; A-C; N=16-17 for CO₂-enriched water and N=7 for acidified water) and systemic vascular resistance (R_s; E-G; N=14 for CO₂-enriched water and N=6 for acidified water) in tambaqui Colossoma macropomum. The shaded areas represent the 20 s period of injection. (D,H) The peak changes in fH (fH; D) and R_s (R_s; H) for injections of air-equilibrated water and water equilibrated with 1, 3, 5 or 10% CO₂ in air. Values are means ± 1 S.E.M. An asterisk denotes a statistically significant difference from the initial pre-injection value (one-way RM-ANOVA; P<0.001 for all CO₂ injections; P=0.016 for injection of acidified water on fH in C). For peak responses, groups that do not share a letter are significantly different from one another (one-way RM-ANOVA; P<0.001 for both).