The effect of graded doses of mannitol upon renal blood flow, plasma volume, blood volume, hematocrit, urine output, serum osmolality and arterial blood pressure was studied in 12 dogs. Those animals receiving a priming dose of fluid before mannitol was given exhibited strikingly greater increases in plasma volume than did the unprimed animals; however, their renal blood flows were essentially the same. This evidence suggests that mannitol does not exert its effect on renal blood flow directly through increase of plasma volume. Increased urine output persisted long after renal blood flow returned to normal; this suggests that mannitol's diuretic effect is not entirely dependent upon changes in renal blood flow.
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