Plasma Volume Changes Related to Posture and Exercise

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Abstract
Plasma volume shifts were investigated in 4 male subjects who remained in either the upright or low-sit posture on a bicycle ergometer during 60 min of rest, 45 min of exercise and 45 min of recovery. Rest in the upright and low-sit posture induced plasma volume decreases of 14 and 6.9%, respectively. When postural effects were partialed out, exercise in the upright posture resulted in minimal alterations in plasma volume. Work in the low-sit posture was characterized by hemoconcentration during the early phases of pedalling, followed by a slight increase in plasma volume which remained stable for the duration of the exercise period. During recovery, plasma volumes returned rapidly to the pre-exercise levels associated with each seated posture. When analyzing fluid shifts, posture and time should be rigorously controlled so as to differentiate between the effects of posture and exercise.