  Erythrocytes and vesicles shed by erythrocytes from human and rat blood were
collected and analyzed after temperature was elevated by physical exercise or
by exposure to external heat. The images of erythrocytes and vesicles were
analyzed by the light microscopy system with spatial resolution of better than
90 nm. The samples were observed in an aqueous environment and required no
freezing, dehydration, staining, shadowing, marking or any other manipulation.
Temperature elevation, whether passive or through exercise, resulted in
significant concentration increase of structurally transformed erythrocytes
(echinocytes) and vesicles in blood. At temperature of 37 oC, mean vesicle
concentrations and diameters in human and rat blood were (1.50+-0.35)x10^6 and
(1.4+-0.2)x10^6 vesicles/{\mu}L, and 0.365+-0.065 and 0.436+-0.03 {\mu}m,
respectively. It was estimated that 80% of all vesicles found in human blood
are smaller than 0.4 {\mu}m. Thermodynamic analysis of experimental and
literature data showed that erythrocyte transformation, vesicle release and
other associated processes are driven by entropy with enthalpy-entropy
compensation. It is suggested that physical state of hydrated cell membrane is
responsible for the compensation. The increase of vesicle number related to
elevated temperatures may be indicative of the heat stress level and serve as
diagnostic of erythrocyte stability and human performance.
