The capillaries of the brain are the most likely location of the blood-brain barrier. Brain capillaries differ in several important respects from capillaries in other body locations (Fig. 24.5). For example, the endothelial cells of brain capillaries are so closely joined to each other that passage of substances cannot readily occur through the intercellular clefts between adjacent cells; furthermore, brain capillaries do not contain fenestrae (pores). Fenestrae are prominent in many capillaries, especially those in renal glomeruli and in the chorioid plexus. The ability of a drug to leave a capillary by diffusion appears to be directly related to the number of capillary pores. Compared with capillaries at other sites, brain capillaries also appear to possess very few pinocytotic vesicles, which are believed to play a role in the transport of large molecules through capillary walls.
Brain capillaries contain many more mitochondria than do other capillaries, and it is probable that the mitochondria supply energy for active transport of water-soluble nutrient substances into the brain. A large number of lipid-insoluble endogenous substances are known to be transported into the brain. These
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