Dynamics of changes in the fluids of the extremities in man in different body positions

: Anthropometric measurements and occlusion venous plethysmography were used to investigate fluid inflow and outflow in the limbs of human subjects who kept normal motor activity for 4 hrs, remained in recumbency or were in the head-down position at an angle of -12 degrees and -22 degrees (to simulate effects of zero G). During these exposures diuresis, heart rate and blood pressure according to Korotkoff were also measured. In the horizontal and, to a greater extent, head-down position, when motor activity was diminished, volume blood flow velocity in the limbs decreased, i. e. blood inflow to them became smaller. Arm volume varied insignificantly since inflow and outflow were in balance whereas leg volume decreased because fluid outflow was larger than inflow. In the head-down position the tone of leg veins also declined. Thus the peripheral vascular bed developed a complex of mechanisms that were to prevent volume overload of the central bed. Fluid shifts from the legs to the central bed were made up for by the renal excretion of water in the horizontal and head-down (-12 degrees C) (-22 degrees) produced a stressful effect on compensatory mechanisms which manifested as a greater rise of diastolic pressure and bradycardia. The data obtained indicate an active involvement of the peripheral vascular bed in the adaptation to diminished motor activity in the horizontal and head-down position of human subjects.