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Keywords:

  • horse;
  • fluid balance;
  • chloride shift;
  • maximal exercise

Summary

Exercise at maximal intensity results in marked, but transient alterations in plasma electrolyte concentration and acid-base balance. Compartmental fluid shifts contribute to the increased haematocrit, plasma protein and electrolyte concentrations. Elevations in plasma sodium and potassium concentrations relative to chloride tend to ameliorate the impact of lactate accumulation on anion-cation balance. Lactate accumulation is associated with the development of a profound metabolic acidosis, but there are physiologically important differences between the mixed venous and arterial acid-base values. Arterial blood is characterised by a moderate hypoxaemia, marked acidosis, modest increase in the partial pressure of carbon dioxide (PCO2) and a marked decrease in bicarbonate. Mixed venous blood pH is lower than arterial and is associated with a profound increase in PCO2 with a slight increase in bicarbonate. It is estimated that over 70% of the CO2 produced during maximal exercise is transported as bicarbonate in the venous circulation. This is largely a function of the carbonic anhydrase-dependent chloride shift in which erythrocytes participate in CO2 transport from the tissues to the lungs. The chloride shift is responsible for the mixed venous-arterial differences in plasma chloride concentration. The compartmental fluid shifts as well as the alterations in plasma electrolyte concentration and acid-base balance are part of the integrated physiological response to the energy generating metabolic events associated with maximal exercise.