CSF is found in the cerebral ventricles and cisterns and in the subarachnoid space surrounding the brain and spinal cord. Its major function is to protect the central nervous system (CNS) against trauma.
Most of the CSF is formed by the choroid plexuses of the cerebral (mainly lateral) ventricles. Smaller amounts are formed directly by the ven-tricles’ ependymal cell linings, and yet smaller quantities are formed from fluid leaking into the perivascular spaces surrounding cerebral vessels (blood–brain barrier leakage). In adults, normal total CSF production is about 21 mL/hr (500 mL/d), yet total CSF volume is only about 150 mL. CSF flows from the lateral ventricles through the intraventric-ular foramina (of Monro) into the third ventricle, through the cerebral aqueduct (of Sylvius) into the fourth ventricle, and through the median aperture of the fourth ventricle (foramen of Magendie) and the lateral apertures of the fourth ventricle (foram-ina of Luschka) into the cerebellomedullary cistern (cisterna magna) (Figure26–4). From the cerebello-medullary cistern, CSF enters the subarachnoid space, circulating around the brain and spinal cord before being absorbed in arachnoid granulations over the cerebral hemispheres.CSF formation involves active secretion of sodium in the choroid plexuses. The resulting fluid is isotonic with plasma despite lower potassium, bicarbonate, and glucose concentrations. Its protein content is limited to the very small amounts that leak into perivascular fluid. Carbonic anhydrase inhibitors (acetazolamide), corticosteroids, spirono-lactone, furosemide, isoflurane, and vasoconstric-tors decrease CSF production.
Absorption of CSF involves the translocation of fluid from the arachnoid granulations into the cere-bral venous sinuses. Smaller amounts are absorbed at nerve root sleeves and by meningeal lymphatics.
Because the brain and spinal cord lack lymphat-ics, absorption of CSF is also the principal means by which perivascular and interstitial protein is returned to the blood.