Abstract
Abstract Recent research demonstrated that the combination of exercise and heat stress has a major impact on metabolic and cellular processes. However no detailed evaluation is available on the effects of ambient temperature on the main determinants of blood rheology at rest and in response to exercise. To ascertain the possible effects of environmental temperature on blood rheology, ten normal healthy subjects (VO2max 46.5 ml.kg.min-1) performed two exercise trials, in random order, at approximately 60% VO2max for 45-min. Subjects were asked to follow their normal diet, and to avoid alcohol ingestion and caffeine consumption and to undertake no physical activity for the previous 24-h. One of the test trials was conducted in thermoneutral environment (20±1 0C), while the other was performed inside an environmental chamber in hot dry condition (36±1 0C). On each test trial, venous blood samples were removed; with no stasis; at rest; following exercise and at the end of 30-min recovery. Whole blood was measured for lactate, haematocrit (Hct), hemoglobin (Hb), plasma viscosity (PV), and plasma fibrinogen concentration (Fb). Plasma volume changes were estimated from Hct and Hb readings. Plasma volume decreased significantly (P<0.05) after exercise. Exercise was followed by a significant (P<0.05) reduction in plasma volume in both test trials. Although exercise in the heat exacerbated the reduction in plasma volume (P<0.05), it had no significant effect (P>0.05) on blood lactate concentration following exercise. The ANOVA and post-hoc analyses showed that when post exercise raw data were not adjusted for plasma volume changes, a significant increase (P<0.05) in plasma viscosity and plasma fibrinogen was found in both thermoneutral and hot trials. In contrast, when the raw data post exercise for plasma viscosity and plasma fibrinogen were adjusted for plasma volume changes and the mean values were re-calculated and statistically compared; no significant (P>0.05) difference between rest and post exercise was demonstrated in both the thermoneutral and hot trials. Thirty minutes following exercise in thermoneutral and hot conditions, all the main determinants of blood rheology returned to the pre-exercise level. It is concluded that vigorous exercise alters blood rheological profile, and heat in addition to exercise did not affect these responses more that exercise alone. Furthermore, the mechanism responsible for the increase in all measured blood rheological variables in response to exercise is plasma shifts from intravascular to the extravascular spaces rather than plasma volume loss.