A. Anatomy of the blood-brain barrier
It is the barrier between cerebral capillary blood and the CSF. CSF fills the ventricles and the subarachnoid space.
It consists of the endothelial cells of the cerebral capillaries and the choroid plexus epithelium.
B. Formation of CSF by the choroid plexus epithelium
Lipid-soluble substances (C02 and 02) and H20 freely cross the blood-brain barrier and equilibrate between blood and CSF.
Other substances are transported by carriers in the choroid plexus epithelium. They may be secreted from blood into the CSF or absorbed from the CSF into blood.
Protein and cholesterol are excluded from the CSF because of their large molecular size. The composition of CSF is approximately the same as that of the interstitial fluid of the brain but differs significantly from blood.
CSF can be sampled with a lumbar puncture.
C. Functions of the blood brain barrier
- It maintains a constant environment for neurons in the CNS and protects the brain from endogenous or exogenous toxins.
- It prevents the escape of neurotransmitters from their functional sites in the CNS into the general circulation.
- Drugs penetrate the blood-brain barrier to varying degrees. For example, nonionized (lipid-soluble) drugs cross more readily than ionized (water-soluble) drugs.
Inflammation, irradiation, and tumors may destroy the blood-brain barrier and permit entry into the brain of substances that are usually excluded (e.g., antibiotics, radiolabeled markers).