Physiology of Cerebrospinal Fluid Circulation: Amphibians, Mammals, and Hydrocephalus

Abstract

The chemical environment of the brain is buffered from external influences by the blood—brain barrier at the brain capillary endothelium, the blood—cerebrospinal fluid (CSF) barrier at the choroid plexus epithelium and by a barrier membrane, the arachnoid, which envelops the brain and CSF. The CSF is secreted by the choroid plexuses which are sited inside the cerebral ventricles (two lateral ventricles, third and fourth ventricles). The composition of the CSF is closely regulated, having a lower K⁺ and Ca²⁺ concentration and a higher Na⁺ and Mg²⁺ concentration than would be expected from a simple plasma ultrafiltrate. Proteins and nonelectrolytes such as urea, glucose and amino acids are also low in CSF. One function of the CSF, together with the blood—brain barrier, therefore, is to maintain brain ion homeostasis and provide the correct chemical environment for neuronal function. A second function is to provide buoyancy for the brain by maintaining a hydrostatic pressure on both its internal and external aspects. Thus the human brain weighs approximately 1300 g in air but only 50 g in water and the CSF is assumed to provide protection from injury. A third, less convincing, function of the CSF may be to provide a flow pathway for the transportation of waste products and pharmacologically active substances. In support of this, many secretory/sensory circumventricular organs are situated in specialised ependyma adjacent to the CSF. However, because CSF turnover is slow and measured in hours rather than minutes, only very local transportation would have a rapid response time.