Cerebrospinal fluid (CSF) is a clear, colorless fluid formed within the ventricles of the brain. The choroid plexus (of the lateral, third and fourth ventricles) produces about 70% of the CSF by ultrafiltration and secretion. The ependymal lining of the ventricles and cerebral subarachnoid space produce the remainder of the CSF total volume. About 500 mL of CSF is formed per day, although only 90–150 mL is present in the system at any one time. Reabsorption of CSF occurs at the arachnoid villi.
CSF circulates slowly from the ventricular system into the space surrounding the brain and spinal cord and serves as a hydraulic shock absorber, diffusing external forces to the skull that might otherwise cause severe injury. The CSF also helps to regulate intracranial pressure (ICP), supply nutrients to the nervous tissues, and remove waste products. The chemical composition of CSF does not resemble an ultrafiltrate of plasma. Certain chemicals in the CSF are regulated by specific transport systems (e.g., K+, Ca2+, Mg2+), whereas other substances (e.g., glucose, urea, creatinine) diffuse freely. Proteins enter the CSF by passive diffusion at a rate dependent on the plasma-to-CSF concentration gradient. The term blood–brain barrier is used to represent the control and filtration of blood plasma components (e.g., restriction of protein diffusion from blood into brain tissue) to the CSF and then to the brain capillaries. The ratio of increased albumin in CSF to blood serum is always caused by blood–brain barrier dysfunction because albumin is found extensively in blood. A decreased CSF flow rate is due to decreased production or restriction or blockage of flow.
Most CSF constituents are present in the same or lower concentrations as in the blood plasma, except for chloride concentrations, which are usually higher (Table 5.1). Disease, however, can cause elements ordinarily restrained by the blood–brain barrier to enter the spinal fluid. Erythrocytes and leukocytes can enter the CSF from the rupture of blood vessels or from meningeal reaction to irritation. Bilirubin can be found in the spinal fluid after intracranial hemorrhage. In such cases, the arachnoid granulations and the nerve root sheaths will reabsorb the bloody fluid. Normal CSF pressure will consequently be maintained by the reabsorption of CSF in amounts equal to its production. Blockage causes an increase in the amount of CSF, resulting in hydrocephalus or increased ICP. Of the many factors that regulate the level of CSF pressure, venous pressure is the most important because the reabsorbed fluid ultimately drains into the venous system.
Despite the continuous production (~0.2–0.7 mL/min) and reabsorption of CSF and the exchange of substances between the CSF and the blood plasma, considerable pooling occurs in the lumbar sac. The lumbar sac, located at L4–L5, is the usual site used for puncture to obtain CSF specimens because damage to the nervous system is less likely to occur in this area. In infants, the spinal cord is situated more caudally than in adults (L3–L4 until 9 months of age, when the cord ascends to L1–L2); therefore, a low lumbar puncture should be made in these patients.