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The reactivity of cortex, putamen and white matter to changes in arterial CO2 tension has been assessed. There was no significant difference between the CO2 reactivity obtained for the three tissues which were in the region of 2% to 3.5% increase over basal blood flow per mm Hg PCO2 increase. The data suggested a somewhat greater reactivity for white matter than for gray matter, though differences were not significant. Excellent autoregulation to altered perfusion pressure, induced either by hemorrhage or by raising the intracranial pressure with cisternal infusion, was found in gray matter of cortex and putamen and in white matter. With reduction of perfusion pressure by both techniques, it appeared that zero blood flow could be more readily induced in white matter than in gray matter. Autoregulatory curves to change in perfusion pressure obtained by either method seemed identical, suggesting that the mechanisms involved in the maintenance of cerebral blood flow in the face of reduced arterial pressure or rising intracranial pressure are the same. Hyperemia was observed in all three areas of the cerebral circulation examined following the restoration of perfusion pressure after a period of reduction. No significant differences in the degree of hyperemia induced by similar stimuli were observed in these three sites.

Physiological Responses of Local Areas of the Cerebral Circulation in Experimental Primates Determined by the Method of Hydrogen Clearance