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Journal of Experimental Biology 92,125-141 (1981)
Published by Company of Biologists 1981

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

MICHÈLE G. WHEATLY ¹ and E. W. TAYLOR ¹

¹ Department of Zoology and Comparative Physiology, University of Birmingham, Birmingham B15 2TT, U.K.

1. Male and female crayfish, Austropotamobius pallipes (Lereboullet) maintained a constant rate of O₂ uptake (M_oo2) during a progressive reduction in ambient O₂ tension down to a critical tension (P_c) of 40 mmHg at15 °C.

2. Heart rate (f_H) slowed progressively during hypoxia but blood flow was maintained by an increase in cardiac stroke volume.

3. At a P_I, _oo2, of 50; mmHg the rate of ventilation (V_w) had increased to 2.8 times the normoxic value. This was effected by a doubling in respiratory frequency (f_R) 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 (E_w), and there is evidence that the ability of the respiratory surface to transfer O₈ (T_OO2) improved during hypoxia.

4. The hyperventilation enhanced CO₂ elimination so that CO₂ tensions were halved and respiratory alkalosis occurred in the haemolymph.

5. 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 (E_b) remained high in moderately hypoxic water (P_OO2 63 ± 1 mmHg).

6. At P_I, _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, E_b was reduced. As a consequence of these changes M_OO2 fell.

7. 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.

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