Pressures in the pial arterial microcirculation of the cat during changes in systemic arterial blood pressure.

Dynamic arterial blood pressures were recorded by micropuncture of 99 cortical surface arteries and arterioles in cats anesthetized with pentobarbital or chloralose and urediane, using the Wiederhielm servocontrolled micropipette pressure system incorporating pipette tips less than 0.5/A in diameter. During these measurements, systemic blood pressure was varied by supplemental intravenous administration of barbiturate, by intravenous administration of norepinephrine, or by arterial hemorrhage and reinfusion. Pial arterial blood pressure varied systematically with the changes in systemic blood pressure, forming a basis for analysis and synthesis of the results. Measurements in individual vessels indicated that the range of pressures and of pressure responses of the cortical surface vessels was variable with both vessel size and prevailing end-expiratory gas CO2 content. Pooled data indicated that 398 of the loss in pressure head occurred in arteries upstream from the largest (300/x or more in diameter) surface arteries, 21% between these arteries and surface arterioles 50^/. or less in diameter, and 40% downstream from the surface arterioles at a systemic blood pressure of 50 mm Hg. At a systemic blood pressure of 180 mm Hg the upstream loss decreased to 33%, loss across the pial vessels decreased to 15%, and the downstream loss increased to 52%. Thus, the results indicated that the cerebrovascular response to changes in systemic blood pressure occurred predominantly downstream from the surface arterial vasculature.