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The movement of any material from blood to brain involves not only its passage across the blood-brain barrier (BBB) but also its delivery to the capillary beds by blood. Within the blood flowing into the capillaries, the material can be carried by plasma water, plasma proteins, and blood cells. The partitioning of the material among these intravascular compartments, the dynamics of exchange among them, and the relative flows of each within the capillary system affects uptake. Influx across the BBB is a function of the capillary surface area (S) and permeability coefficient (P), both of which vary among brain areas. This joint dependency is clearly shown by the physiological expression of “capillary permeability,” the permeability-surface area (PS) product. The P of the PS product differs among materials and depends on variables such as the substance’s lipid solubility, molecular size, diffusion coefficient, extent of metabolism within the endothelial cell, and interaction with transporters that facilitate flux in one or the other or both directions across the BBB. |
