The Effect of Progressive Hypoxia on Heart Rate, Ventilation, Respiratory Gas Exchange and Acid-Base Status in the Crayfish Austropotamobius Pallipes

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Abstract
Male and female crayfish, Austropotamobius pallipes (Lereboullet) maintained a constant rate of O2 uptake (Moo2) during a progressive reduction in ambient O2 tension down to a critical tension (Pc) of 40 mmHg at15 °C. Heart rate (fH) slowed progressively during hypoxia but blood flow was maintained by an increase in cardiac stroke volume. At a PI, oo2, of 50; mmHg the rate of ventilation (Vw) had increased to 2.8 times the normoxic value. This was effected by a doubling in respiratory frequency (fR) accompanied by increases in both the mean amplitude of the pressure pulse and the mean hydrostatic pressure recorded in the branchial chambers. The effectiveness of removal of O2 from the water ventilating the gills was maintained (Ew), and there is evidence that the ability of the respiratory surface to transfer O8 (TOO2) improved during hypoxia. The hyperventilation enhanced CO2 elimination so that CO2 tensions were halved and respiratory alkalosis occurred in the haemolymph. This alkalosis had the effect of increasing the affinity of the blood pigmentfor O2, reducing the half saturation pressure (P50) from 8 to 4.5 mmHg. Thus despite reductions in pre- and post branchial O2 tensions by 50% the arteriovenous O2 content difference was maintained and the effectiveness of removal of O2 into the blood (Eb) remained high in moderately hypoxic water (POO2 63 ± 1 mmHg). At PI, OO2, levels below 40 mmHg, when crayfish were denied access to air, they were unable to sustain the hyperventilation. Haemolymph O2 content and the a - v O2 content difference decreased, and as the haemolymph was no longer saturated with O2 on its passage through the gills, Eb was reduced. As a consequence of these changes MOO2 fell. Under these conditions a partial switch to anaerobic metabolism occurred, with lactic acid accumulating in the haemolymph. The resultant metabolic acidosis partially off set the respiratory alkalosis.