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It has long been known that when a man stands motionless, urine flow (1-13) and the excretion of chloride (2, 3, 5, 7, 11) and of sodium (5, 13) decrease markedly. Simultaneously, the plasma contracts in volume and increases in concentration (8, 10, 14-17), while the blood flow through the kidneys (9, 10, 18, 19) and other abdominal viscera (20) diminishes, and the rate of glomerular filtration of plasma and its contained sodium is reduced (6, 9, 10, 13, 18, 21). There is some evidence that release of the posterior pituitary antidiuretic hormone may be responsible for the retention of water (11) but the mechanism of the retention of sodium and the stimulus or stimuli initiating it remain obscure. Some renal physiologists would place greatest emphasis on the reduction in glomerular filtration rate which occurs during standing, and the resulting decreased load of sodium presented to the tubules (5, 22, 23). It is not likely that an increase in the renal venous pressure (24, 25) plays an important role in the antidiuresis of quiet standing, because when a subject is tilted from the supine to the erect position, pressure in the inferior vena cava at the level of the kidneys does not rise to the level reported by Blake and associates (25) to cause a reduction in the urine flow, and the renal arterio-venous pressure gradient does not change materially (26).

STUDIES OF THE ANTIDIURESIS OF QUIET STANDING: THE IMPORTANCE OF CHANGES IN PLASMA VOLUME AND GLOMERULAR FILTRATION RATE 1