Two important matters which concern the physiology of the brain are not disputed. The gray matter of the brain, in comparison to muscle, has a high rate of oxygen consumption, and a continuous supply of blood to the brain is essential. Interruption of the cerebral circulation produces loss of consciousness in six or eight seconds and permanent injury to the nerves in as many minutes. A priori, it would seem probable that the brain should be abundantly supplied with capillaries (the number of capillaries per unit of tissue is usually a gage of the metabolic activity of that tissue) and that blood leaving the brain should remain rich in the amount of oxygen which it carries.
It is, therefore, something of a surprise to learn that the number of capillaries in the gray matter of the brain is meager as compared
with that of muscle1 and that blood leaving
LENNOX WG. CONSTANCY OF THE CEREBRAL BLOOD FLOW. Arch NeurPsych. 1936;36(2):375-381. doi:10.1001/archneurpsyc.1936.02260080147010