The volume of plasma in the blood vessels. At the onset of a bout of intense strenuous exercise, the plasma volume decreases by as much as 20%. This reduction is due to fluid moving from the blood vessels into the surrounding tissue fluid, probably as a result of the increase in hydrostatic pressure within blood vessels and an increase in metabolites in the tissue fluid. During prolonged, strenuous exercise, further decreases of plasma volume result from sweating. As the plasma volume becomes reduced, the concentration of red blood cells increases. This causes the blood viscosity to increase and the heart has to work much harder to pump the blood around the body. Consequently, a reduction of plasma volume is likely to impair athletic performance. One of the adaptations to regular endurance training is an increase of plasma volume.
Subjects: Sports and Exercise Medicine.