Figure 1. Comparison between the specialized capillaries of the CNS and those of other tissues.

(A) Capillaries of the CNS are continuous capillaries. The endothelial cells have tight junctions and are covered by astrocytic end feet. Small lipophilic molecules can passively pass through the BBB. Furthermore, certain molecules can actively pass the BBB using the carrier-mediated cell membrane transport system. (B) Capillaries of other tissues are often fenestrated, moderately porous capillaries (renal glomeruli, intestinal villi, and in most glands) or sinusoid, highly porous capillaries (in the liver, bone marrow, lymphoid tissues, and some endocrine organs). These ultramicroscopic pores are responsible for the majority of exchange between the blood and tissue. Furthermore, capillary exchange is executed by transcytosis, a process in which the endothelial cell encloses material of the bloodstream in an invagination of the cell membrane to form a vesicle, then moves the vesicle across the endothelium cell to eject the material at the tissue side.