Ion Transport Across the Choroid Plexus

Abstract

The cerebrospinal fluid (CSF) fills the ventricles, canals, and spaces surrounding the central nervous system (CNS). The major functions of the CSF are to provide physical protection and support for the brain and spinal cord, and to control the chemical environment of the neurones and glia of the CNS. There is free communication between the CSF and the extracellular spaces of the brain parenchyma, and so the composition of the fluid bathing the elements of the CNS is close to the composition of the CSF. In man the volume of the CSF is about 140 ml and the fluid turns over about every five hours, i. e. about 600 ml of CSF are secreted each day. Although the CSF resembles an ultrafil-trate of plasma, its composition remains remarkably constant, despite wide fluctuations in the composition of plasma. This is due to the so-called blood-brain and blood-CSF barriers that restrict the movement of solutes between the blood and brain, and the active transport of solutes between the blood and the fluid compartments of the CNS. Homeostatic mechanisms are known to maintain the potassium, hydrogen, magnesium, calcium, amino acid, and catecholamine CSF concentrations within narrow limits. Disturbances of these mechanisms have profound effects on the function of the CNS, e. g. elevation of the glycine concentration of the CSF produces hypothermia, hypotension, and motor incoordination. For a recent review of the physiology and pathology of the cerebrospinal fluid consult the monograph by Rapoport (1976).