In the adult, the average intracranial volume is 1,700 mL; the volume of the brain is from 1,200 to 1,400 mL, CSF volume ranges from 70 to 160 mL (mean: 104 mL), and that of blood is approximately 150 mL. In addition, the spinal subarachnoid space contains 10 to 25 mL of CSF. Thus, at most, the CSF occupies less than 10 percent of the intracranial and intraspinal spaces. The proportion of CSF in the ventricles, and cerebral cisternae and sulci in the subarachnoid spaces varies with age. These variations have been plotted in CT scans by Meese and coworkers; the distance between the caudate nuclei at the anterior horns gradually widens by approximately 1.0 to 1.5 cm during an adults lifetime, and the width of the third ventricle increases from 3 to 6 mm by age 60 years.
Formation of CSF
The introduction of the ventriculocisternal perfusion technique by Pappenheimer and colleagues over 50 years ago made possible measurements of the rates of formation and absorption of the CSF. They established that the average rate of CSF formation is 21 to 22 mL/h (0.35 mL/min), or approximately 500 mL/d; thus the volume of CSF as a whole is renewed 4 or 5 times daily.
The choroid plexuses, located in the floor of the lateral, third, and fourth ventricles, are the main sites of CSF formation (some CSF is formed by the meninges even after the choroid plexuses are removed). The thin-walled vessels of the plexuses allow passive diffusion of substances from the blood plasma into the extracellular space surrounding choroid cells. The choroidal epithelial cells, like other secretory epithelia, contain organelles, indicating their capacity for an energy-dependent secretory function, that is, active transport. The blood vessels in the subependymal regions and the pia also contribute to the CSF, and some substances enter the CSF as readily from the meninges as from the choroid plexuses. Thus, electrolytes and glucose equilibrate with the CSF at all points in the ventricular and subarachnoid spaces. The transport of sodium, the main cation of the CSF, is accomplished by the action of a sodium-potassium-ion exchange pump at the apical surface of the choroid plexus cells, the energy for which is supplied by adenosine triphosphate (ATP); drugs that inhibit the ATP system therefore reduce CSF formation (Cutler and Spertell). Electrolytes enter the ventricles somewhat more readily than they enter the subarachnoid space (water does the opposite). It is also known that the penetration of certain drugs and metabolites into the CSF (and brain) is in direct relation to their lipid solubility. Ionized compounds, such as hexoses and amino acids, being relatively insoluble in lipids, enter the CSF slowly unless facilitated by a membrane transport system. This type of facilitated (carrier) diffusion is stereospecific; that is, the carrier (a specific protein or proteolipid) binds only with a solute having a specific configuration, and conducts it across the membrane, where it is released into the CSF, and intercellular fluid.
Passive diffusion gradients appear to determine the entry of serum electrolytes and some small proteins into the CSF, and also the exchanges of CO2. Water and sodium diffuse as readily from blood to CSF and intercellular spaces as in the reverse direction. This explains the rapid effects on CSF sodium and water of intravenously injected hypotonic and hypertonic fluids.