Does ambient temperature affect exercise-induced changes in the main determinants of blood rheology?

To ascertain the effects of environmental temperature on blood rheology, ten subjects performed two exercise trials, in random order, at approximately 60% VO(2) max for 45-min. One trial was conducted in thermoneutral environment (20 ± 1°C), while the other was performed in hot dry condition (36 ± 1°C). Venous blood was removed at rest; following exercise and recovery. Blood was measured for lactate, haematocrit, and hemoglobin, while plasma was measured for viscosity, and fibrinogen. Plasma volume changes were estimated from Hct and Hb readings. Exercise was followed by a significant (P < 0.05) reduction in plasma volume in both test trials. Lactate increased significantly (P < 0.05) following exercise with no difference being observed between trials. When post exercise raw data were not adjusted for plasma volume changes, a significant increase (P < 0.05) in plasma viscosity (PV) and plasma fibrinogen (Fb) was found with no difference between thermoneutral and hot trials. When the raw data post exercise for PV and Fb were adjusted for plasma volume changes, no significant difference between rest and post exercise was demonstrated. Rheological variables returned to the pre-exercise level at the end of recovery. In conclusion, vigorous exercise transiently increased PV and Fb, and the added heat stress did not affect these responses more than exercise alone. The mechanism responsible for the increase in PV and Fb in response to vigorous exercise appears to be related to plasma shifts from intravascular to the extravascular spaces rather than plasma volume loss.