Variations in erythrocyte volume [mean corpuscular volume (MCV)] were evaluated during exercise and heat stress to determine the influence on calculated plasma volume and content changes. The results of this study on 17 men indicate that the human red blood cell can increase, decrease, or remain constant in volume during physical stress depending on the combined interactions of plasma osmolality and blood pH. Shrinking of MCV can occur when the increase in plasma osmolality is larger than 5 mosmol/kg H2O and the blood pH remains within 0.1 pH units of its resting value. Erythrocyte swelling is usually noticed with maximal exercise when the blood pH is less than 7.10, in spite of 20 mosmol/kg H20 increments in plasma osmolality. The regression equations indicate that during 30 min of exercise in a cool environment the plasma shifts calculated by either the hematocrit or the hematocrit + hemoglobin method fall within 1% of each other, but during resting heat exposure the hematocrit technique under-estimates the fluid shift by 2.5-3.0%. Application of these considerations to the calculation of plasma content changes during stress made it clear that the pattern of plasma potassium content is quite different with maximal as compared with submaximal exercise.

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Polypyrrole film electrodes are composed of macromolecular electrochemical machines, ions and water. They are considered here as a model of the intracellular matrix of ectothermic muscle cells. The oxidation/reduction responses to the working temperature in aqueous solutions were investigated herein by potential and current steps. Under potentiostatic conditions, rising temperatures stimulate deeper conformational movements of the polymeric chains leading to the exchange of more ions increasing the consumed charge, with the effect that the reaction charge responds and senses the working temperature. Under galvanostatic conditions and higher environmental thermal energies the material potential evolves at lower values during reactions consuming lower electrical energies. At any reaction time, both the consumed reaction energy and the material potential sense the working thermal conditions. Reactions involving molecular machines sense and respond to the working temperature. Similarities with energetic consumptions and sensing responses from muscles in cold‐blooded animals are discussed. A theoretical description is proposed.

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