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One-minute wave in body fluid volume change enhanced by postural sway during upright standing.

To analyze how the 1-min oscillation in postural sway and 1-min wave in body fluid volume change contribute to human circulatory homeostasis, several levels of body circumference, foot pressure center, electromyograms, and volumes of the leg, abdomen, and thorax were measured during upright standing for 40 min in 20 healthy young men. Spectral analyses of these parameters revealed that a 1-min rhythm is found in all parameters and that the 1-min wave in body fluid volume changes in the lower leg, which occur in fluid pooling caused by gravity, propagate upward. Muscle pumping in the lower leg triggered by the postural sway was found to increase the power of this 1-min wave. A quantitative analysis of body circumferences disclosed that the 1-min wave in body fluid volume change compensates for gravitational downward fluid shift, with the volume of 6.3 +/- 5.0 ml/cycle at the heart level. We concluded that a coupling mechanism between the 1-min oscillation in postural sway and the upward propagation of 1-min wave in body fluid volume change contributes to maintain systemic blood pressure during upright standing in humans.