The relationships between PCO2 and pH were determined in cell-free undiluted haemolymph of the arachnids Eurypelma californicum, Pandinus imperator and Cupiennius salei. The pH/bicarbonate diagrams and the CO2 equilibrium curves were calculated, using the Henderson­Hasselbalch equation, for haemolymph sampled at rest and during recovery from exercise. The calculations of solubility (alphaCO2) and dissociation constant (pK"') were based on additional ion concentration measurements. Blood gas analyses corroborate these results: after locomotor activity, there is a metabolic acidosis linked to the accumulation of lactate in the haemolymph. The concentration of bicarbonate in the haemolymph of resting individuals is quite different in the three species and is related to the extent of post-exercise bicarbonate depletion. During early recovery, buffering in the haemolymph strongly depends upon CO2 release. Potassium and magnesium concentrations in the haemolymph increase after exercise. During cold-acclimation (to 10 °C), there is a metabolic acidosis in the tarantula's haemolymph that is linked to the accumulation of acetate.

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