Haemorrheology and long term exercise.

In general, the small number of haemorrheological studies concerning acute changes in long term exercise reveals remarkable small effects, given the substantial changes of pertinent cardiorespiratory parameters. To a large extent, this appears to result from careful maintenance of extra- and intravascular water balance and adequate control of electrolytes. Haemorrheological alterations during long term exercise depend on haematological parameters, especially haematocrit and plasma protein levels, which are acutely changed. During exercise, only small deviations of haematocrit from resting values are seen in the short term. This is explained by the usually constant plasma volume which depends on the amount of fluid intake during exercise. In contrast, the exercise-induced elevation of total intravascular protein content significantly increases the levels of plasma proteins, with the exception of fibrinogen. Although this leads to an increase of plasma viscosity, the absence of substantial haematocrit changes accounts for the remarkably small alterations, if any, of blood viscosity which are observed during long term exercise. Endurance training causes haemodilution by expansion of plasma volume, thus resulting in a reduction of blood and plasma viscosity. Red cell deformability is variable with training as well as during exercise. This appears to be related to methodological problems of measurement as well as to the type of endurance exercise performed. Physiological considerations suggest that the importance, for aerobic work capacity, of haemorrheological changes occurring in long term (submaximal) exercise may be limited. By contrast, this may be different during maximal exercise activity, when the entire cardiovascular reserve has been fully recruited.