Physiological responses characterizing the early adaptation to weightlessness were studied in five normal men. Supplementary data on central venous pressure (CVP) were obtained in three additional subjects. Zero gravity was simulated by a 24-h period of head-down tilt at 5°. Tilt produced a central fluid shift. Orthostatic tolerance and exercise capacity were reduced posttilt. These changes were similar to those observed during and after space flight and support the validity of the experimental model. CVP increased transiently from 5.6 to a peak of 8.5 cmH2O (P < 0.02). Control levels for CVP were approached at 90 min, at that time the echocardiographic left ventricular end-diastolic diameter reached a maximum (4.7 cm, control 3.9 cm, P < 0.05). There were no changes in arterial pressure, cardiac output, or left ventricular contractile state. Urine flow was 1.98 ml/min-1 during the initial 8 h compared to 1.36 during the final 16 h (P < 0.05). Blood volume decreased by 0.5 liter (P < 0.05). Plasma renin activity, aldosterone, and antidiuretic hormone were depressed initially but returned to base line within 24 h. Plasma electrolytes remained unchanged. The results suggest that hemodynamic adaptation occurs rapidly and is essentially accomplished by 6 h. Adaptation includes a diuresis and reduction in blood volume.
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