[The brain and arterial hypertension. 1. Physiologic and physiopathologic aspects]

The cerebral blood flow is maintained at a constant level in spite of blood pressure variations, because of a very performing auto-regulation phenomenon: vasoconstriction in case of elevated blood pressure, vasodilatation in case of blood pressure drop. The upper limit of the mean blood pressure beyond which nothing can prevent the cerebral blood flow from increasing, ranges between 150 and 170 mmHg. The lower limit, from which the dilatation of the cerebral blood vessels is not able to prevent the decreased cerebral blood flow, has been set between 50 and 70 mmHg. The auto-regulation of the cerebral blood flow is influenced by numerous factors: hypercapnia, hypoxia and ischemia complete inhibit it, making the cerebral blood flow directly related to the blood pressure; the sympathetic stimulation shifts the entire curve toward the right, because of the constriction of the pia-mater arteries, followed with a reactional vasodilatation of the small cerebral vessels. In the course of chronic arterial hypertension, the same shift of the auto-regulation curve toward high pressures is observed, related to structural alterations of the vascular wall; this make them more prone to constriction than dilatation: when these alterations are reversible, antihypertensive treatment may sometimes bring the auto-regulation curve in its initial position.