Sodium and water excretion and renal hemodynamics during lower body negative pressure

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Abstract
Zero gravity conditions such as occur in orbital space flight are known to produce significant losses of body fluid and electrolyte. Lower body negative pressure (LBNP) applied to the supine subject has been suggested as a possible preventive measure. The present study demonstrated that 60 mm Hg LBNP applied for 1 hr produced moderate declines in glomerular filtration rate, renal plasma flow, and tubular reabsorption of sodium, with marked falls in rate of urine flow, free water clearance, and sodium excretion. Although antidiuretic hormone and salt-retaining hormones may have played a role in the responses seen, the changes which occurred in sodium and water excretion appear explainable primarily on the basis of diminished glomerular filtration rate. It is concluded that LBNP is a potent stimulus to retention of salt and water and therefore has a potentially valuable place in maintaining or restoring plasma volume during prolonged weightlessness.