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 neuronal cell bodies lie is about four times as great as that of white matter; correspondingly, the number of capillaries 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 most of them are much less “leaky” than the blood capillaries in almost any other tissue of the body. One reason for this phenomenon is that the capillaries are supported on all sides by “glial feet,” which are small projections from the surrounding glial cells (e.g., astroglial cells) that abut against all surfaces of the capillaries and provide physical support to prevent over stretching 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 in whom high blood pressure develops, and these arterioles remain significantly constricted all the time to prevent transmission of the high pressure to the capillaries. We shall see later in the chapter that whenever these systems for protecting against transudation of fluid into the brain break down, serious brain edema ensues, which can lead rapidly to coma and death.
Cerebral “Stroke” Occurs When Cerebral Blood Vessels Are Blocked
Almost all elderly people have blockage of some small arteries in the brain, and up to 10 percent eventually have enough blockage to cause serious disturbance of brain function, a condition called a “stroke.”
Most strokes are caused by arteriosclerotic plaques that occur in one or more of the feeder arteries to the brain. The plaques can activate the clotting mechanism of the blood, causing a blood clot to occur and block blood flow in the artery, thereby leading to acute loss of brain function in a localized area.
In about one quarter of people in whom strokes develop, high blood pressure makes one of the blood vessels burst; hemorrhage then occurs, compressing the local brain tissue and further compromising its functions. The neuro logical effects of a stroke are determined by the brain area affected. One of the most common types of stroke is block age of the middle cerebral artery that supplies the midportion of one brain hemisphere. For instance, if the middle cerebral artery is blocked on the left side of the brain, the person is likely to become almost totally demented because of lost function in Wernicke’s speech comprehension area in the left cerebral hemisphere, and he or she also becomes unable to speak words because of loss of Broca’s motor area for word formation. In addition, loss of function of neural motor control areas of the left hemisphere can create spastic paralysis of most muscles on the opposite side of the body.
In a similar manner, blockage of a posterior cerebral artery will cause infarction of the occipital pole of the hemisphere on the same side as the blockage, which causes loss of vision in both eyes in the half of the retina on the same side as the stroke lesion. Especially devastating are strokes that involve the blood supply to the midbrain because this effect can block nerve conduction in major pathways between the brain and spinal cord, causing both sensory and motor abnormalities.
