40 60 80 100 120 140 160 180 200 Mean arterial blood pressure (mm Hg)
k Chronic hypertension. This condition is associated with a rightward shift in the arterial pressure range over which autoregulation of cerebral blood flow occurs (upper panel) because, for any given arterial pressure, resistance vessels of the brain have smaller-than-normal diameters (lower panel). As a consequence, people with hypertension can tolerate high arterial pressures that would cause vascular damage in healthy people. However, they risk reduced blood flow and brain hypoxia at low arterial pressures that are easily tolerated by healthy people.
or more connective tissue, or both. The drawback to such adaptation is partial loss of the ability to dilate and regulate blood flow at low arterial pressures. This loss occurs because the passive structural properties of the resistance vessels restrict the vessel diameter at subnormal pressures and, in doing so, increase resistance. In fact, the lower pressure limit of constant blood flow (autoregulation) can be almost as high as the normal mean arterial pressure (see Fig. 17.2). This can be problematic if the arterial blood pressure is rapidly lowered to normal in a person whose vasculature has adapted to hypertension. The person may faint from inadequate brain blood flow, even though the arterial pressure is in the normal range. Fortunately, a gradual reduction in arterial pressure over weeks or months returns autoregulation to a more normal pressure range.
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