Ventilation volume (<img src="http:/img/fbpe/rbbio/v60n2/a08vg.gif" alt="a08vg.gif (265 bytes)" align="absmiddle"> – mlH2O.min–1 ), respiratory frequency (fR – breaths.min–1) and tidal volume (V T – mlH2O.breath–1 ) were measured in a group of Piaractus mesopotamicus (650.4 ± 204.7 g; n = 10) during normoxia and in response to graded hypoxia. The fR was maintained constant, around 100 breaths.min–1, from normoxia until the O2 tension of the inspired water (PiO2) of 53 mmHg, below which it increased progressively, reaching maximum values (157.6 ± 6.3 breaths.min–1) at 10 mmHg. The V T rose from 1.8 ± 0.1 to 6.0 ± 0.5 and 5.7 ± 0.4 mlH2O. breath–1 in the PiO2 of 16 and 10 mmHg, respectively. The <img src="http:/img/fbpe/rbbio/v60n2/a08vg.gif" alt="a08vg.gif (265 bytes)" align="absmiddle"> increased from 169.3 11.0 (normoxia) to 940.1 ± 85.6 mlH2O. min–1 at the PiO2 of 16 mmHg, below which it also tended to decrease. A second group of fish (29 to 1510.0 g, n = 34) was used for the evaluation of allometric relationships concerning ventilation and dimensions of the buccal and opercular cavities. At maximum <img src="http:/img/fbpe/rbbio/v60n2/a08vg.gif" alt="a08vg.gif (265 bytes)" align="absmiddle">, the V T corresponded to 93.2 ± 2.4% of the buccal volume and 94.9 2.3% of the opercular volume, suggesting that the <img src="http:/img/fbpe/rbbio/v60n2/a08vg.gif" alt="a08vg.gif (265 bytes)" align="absmiddle"> of P. mesopotamicus is limited by the volumes of buccal and opercular cavities in severe hypoxia.
hypoxia; normoxia; ventilatory parameters; buccal and opercular volumes; Piaractus mesopotamicus
