1. Eleven normal subjects were studied before and after removal of 15 to 20% of their blood volume within 35 minutes. 2. This amount of blood loss did not produce conspicuous effects upon any of the usually measured circulatory or metabolic parameters. 3. The results suggest that the splanchnic circulation functions as an important blood reservoir in man, that it can be preferentially depleted of blood by a mechanism which does not automatically increase vascular resistance, and that the ability of our subjects to tolerate blood loss was attributable in large part to this response.

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Twenty-four measurements of cerebral blood flow (CBF) were made with Kr85 in 13 normal unpremedicated male subjects during halothane anesthesia. Mean end-tidal halothane concentration was 0.98%. Ve was maintained constant in each subject at about 12 liters/min and inhaled co2 concentration was varied so that PaCOCO2 ranged between 19–56 mm Hg. Mean cerebral perfusion pressure varied from 41.0 to 76.4 mm Hg and tended to be lower at PaCOCO2 levels >47 mm Hg. CBF varied linearly with PaCOCO2 under the conditions of this study. The response of the cerebral circulation to change in PaCOCO2 during halothane anesthesia is compared with changes observed by others in awake man and in anesthetized animals. The shape of the PaCOCO2 response curve was observed to be altered by the vasodilatation which occurred during halothane anesthesia. The Kr85 method of calculating CBF is superior to the nitrous oxide technique because of krypton's lower blood solubility and more rapid equilibration. cerebral blood flow; general anesthesia; carbon dioxide Submitted on December 23, 1963