Bloodbrain barrier

Ionic equilibrium is maintained across the cerebral vascular endothelium preserving cerebral volume homeostasis. The major functions of the blood-brain barrier are as follows (Rissal 1994):

1. protection of the brain from the blood milieu via tight junctions which prevent uncontrolled passage of molecules;

2. specialized active transport of selected molecules;

3. metabolism or modification of substances.

The ability of solute to cross the blood-brain barrier depends on both properties of the barrier and properties of the solute. Exchange across the barrier is easy for small lipid-soluble uncharged molecules and difficult for large polar substances. Most metabolic substrates (glucose and amino acids) are charged and polar, and so specialized carrier systems exist to enable their transport across the blood-brain barrier. Facilitated diffusion allows equilibrium down concentration gradients between plasma and brain, and energy-dependent transport mechanisms exist against electrical or concentration gradients.

Water passage follows osmotic forces across the intact blood-brain barrier since tight junctions resist hydrostatic forces. If the blood-brain barrier is disrupted around areas of tumor, infection, or trauma, this osmotic gradient is lost and hydrostatic egress of protein-rich fluid provokes vasogenic cerebral edema. Increased permeability to ions and larger molecules such as albumin forms the basis of tests examining the integrity of the blood-brain barrier.

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