Hemoglobin, hematocrit, and calculated plasma volume changes induced by a short, supramaximal task.

The influence of a short, supramaximal task of 30 s duration on plasma volume (PV) was tested in 14 female physical education students. Hemoglobin (Hb), hematocrit (Hct), and osmolality (Osm) were tested pre- and post-exercise. Performance of the exercise test, consisting of 30 s all-out pedaling against a load of 4.4 J/kg body weight per pedal revolution, brought about a significant elevation in the measured variables. The changes in Hct, Hb, and Osm were 3.14%, 0.92g X dl-1, and 9.58 mOs/kg-1, respectively. The changes in PV, calculated from the increase in Hct alone and from changes in Hct and Hb, was found to be -10.4% and -11.2%, respectively. It is suggested that very short periods of maximal exercise can bring about some 10% - 15% drop in PV. Mechanisms for this decrease are not clear. It could result from changes in tissue osmolality, filtration pressure, or opening up of collapsed capillaries.