Studies using radioisotopic tracer techniques confirm that the main constituents of the CSF (see Table 2-2) are in dynamic equilibrium with the blood. Similarly, CSF in the ventricles and subarachnoid spaces is in equilibrium with the intercellular fluid of the brain, spinal cord, and olfactory and optic nerves. Certain structures and physiologic mechanisms that maintain this equilibrium are subsumed under the term blood–brain barrier, which is used to designate all of the interfaces between blood, brain, spinal cord and CSF. The site of the barrier varies for the different plasma constituents. One is the endothelium of the choroidal and brain capillaries; another is the plasma membrane and adventitia (Rouget cells) of these vessels; a third is the pericapillary foot processes of astrocytes. Large molecules, such as albumin, are prevented from entry by the capillary endothelium, and this is the barrier also for such molecules as are bound to album in, for example, aniline dyes (trypan blue), bilirubin, and many drugs. Other smaller molecules are blocked from entering the brain by active mechanisms at the capillary plasma membrane or astrocytes.
The substances formed in the nervous system during its metabolic activity diffuse rapidly into the CSF. Thus, the CSF has, as mentioned, a “sink action,” to use Davson’s term, by which the products of brain metabolism are removed into the bloodstream as CSF is absorbed.
Harvey Cushing aptly termed the CSF the “third circulation,” comparable to that of blood and lymph. From its principal site of formation in the choroid plexus of the lateral ventricles, CSF flows downward sequentially through the third ventricle, aqueduct, fourth ventricle, and foramina of Magendie (medially) and Luschka (laterally) at the base of the medulla, to the perimedullary and perispinal subarachnoid spaces, thence around the brainstem and rostrally to the basal and ambient cisterns, through the tentorial aperture, and finally to the lateral and superior surfaces of the cerebral hemispheres, where most of it is absorbed (Fig. 29-1). The pressure in the CSF compartment is highest in the ventricles and diminishes successively along the subarachnoid pathways. Arterial pulsations of the choroid plexuses help drive the fluid centrifugally from the ventricular system.
The spinal fluid is in contact everywhere with the extracellular fluid of the brain and spinal cord, but the extent of bulk flow of the CSF through the brain parenchyma is small under normal conditions. The periventricular tissue offer considerable resistance to the entrance of CSF and, although the pressure difference between the ventricles and the subarachnoid spaces over the convexity of the hemispheres (transmantle pressure) is above zero, the open ventricular–foraminal–subarachnoid pathway directs the bulk of CSF flow in this direction. Only if this conduit is obstructed does the transmantle pressure rise, compressing the periventricular tissues and leading to ventricular enlargement, that is, hydrocephalus, and to transependymal flow of CSF.
Absorption of CSF is mainly through the arachnoid villi. These are microscopic excrescences of arachnoid membrane that penetrate the dura and protrude into the superior sagittal sinus and other venous structures. Multiple villi are aggregated in these locations to form the pacchionian granulations or bodies, some of them large enough to indent the inner table of the calvarium. These may increase in size with aging, and large ones (3.0 to 9.5 mm in diameter) may rarely herniate through bony defects and erode the skull, providing a portal of entry to organisms from the ear that lead to meningitis (Samuels et al).