**2.3.1 Introduction**

The cerebrospinal fluid (CSF) fills the subarachnoid space between the arachnoid membranes and the pia mater called leptomeninges which protect the Central Nervous System -CNS- (encephalon and spinal cord). Seventy percent of the CSF is formed in secretory structures called choroid plexi and the remaining 30% is produced from the cerebral capillaries (Carpenter, 1985).

Although the composition of CSF is similar to a plasma filtrate, there are differences which indicate that the CSF formed is produced both by a process of filtration and by active secretion, an osmotic balance being observed between CSF and plasma. A similarity can be seen in the composition of CSF and the extracellular liquid of the nervous system, indicating an easy interchange between both compartments. The CSF and in the cerebral interstice are separated from the blood circulation by the hematoencephalic and hematocephalorachideal barriers which prevent the free passing of substances. This is why they are considered to be functional elements of protection of the nerve cells (Nolte, 1994). In comparison with plasma, the CSF contains a greater concentration of Na, Cl and Mg and a lower one of glucose, proteins, amino acids, uric acid, K, bicarbonate, Ca and phosphate (Guyton, 1990). These differences indicate that the CSF is produced by a mechanism of active secretion and varies according to the location of CSF extraction with regard to the structures it bathes.

Water passes from the stroma to the CSF following the concentration gradient produced by the ATPase-dependent carrier proteins, Cl, Ca and Mg (Nolte, 1994). Cellular metabolytes also enter the extracellular liquid from neurones and glial cells.

The CSF maintains an appropriate chemical environment for neurotransmission and removes metabolic products and substances which are harmful for the CNS.
