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One‐minute oscillations in the oxygen saturation level of blood and the hemoglobin volume in calf muscular tissue were found during upright standing in humans. Spectral analyses indicated that one source of the one‐minute wave in body fluid volume change is the spontaneous constriction of blood vessels triggered by an elevation of transmural pressure when blood pooling is evoked.

To clarify how neural and humoral mechanisms operate to control cardiovascular unctions in man under weightlessness, the response of sympathetic nerve activity was observed in healthy human subjects by means of microneurographic technique with the changes of several hemodynamic parameters and hormonal responses during thermoneutral head-out water immersion. Muscle sympathetic nerve activity was markedly suppressed by head-out immersion, concomitantly with a reduction of the leg volume, an increase of the stroke volume and a reduction of total peripheral resistance. At the same time, plasma level of norepinephrine, vasopressive and antidiuretic hormones (ADH, aldosterone, renin activity, angiotensin I·II) were reduced, while vasodepressive and diuretic hormone (ANP) was markedly increased. The systemic blood pressure was maintained almost unchanged during head-out water immersion. The suppressive response of sympathetic nerve activity seemed to be age-dependent. This response was less prominent in the elderly than in young subjects. It is concluded that the suppressive response of muscle sympathetic activity plays an important role to maintain hemodynamic homeostasis under weightlessness to compensate for the cephalad fluid shift and the resultant increase of the stroke volume in cooperation with the hormonal responses.

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.