As is true for almost all other tissues of the body, the number of blood capillaries in the brain is greatest where the metabolic needs are greatest. The overall metabolic rate of the brain gray matter where the neu-ronal cell bodies lie is about four times as great as that of white matter; correspondingly, the number of capil-laries and rate of blood flow are also about four times as great in the gray matter.
An important structural characteristic of the brain capillaries is that they are much less “leaky” than the blood capillaries in almost any other tissue of the body. One reason for this is that the capillaries are supported on all sides by “glial feet,” which are small projections from the surrounding glial cells that abut against all sur-faces of the capillaries and provide physical support to prevent overstretching of the capillaries in case of high capillary blood pressure.
The walls of the small arterioles leading to the brain capillaries become greatly thickened in people who develop high blood pressure, and these arterioles remain significantly constricted all the time to prevent transmission of the high pressure to the capillaries.
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