Cerebrospinal fluid

Embedded within the brain are four ventricles or chambers that form a continuous fluid-filled system. In the roof of each of these ventricles is a network of capillaries referred to as the choroid plexus. It is from the choroid plexuses of the two lateral ventricles (one in each cerebral hemisphere) that cerebrospinal fluid (CSF) is primarily derived. Due to the presence of the blood-brain barrier, the selective transport processes of the choroid plexus determine the composition of the CSF. Therefore, the composition of the CSF is markedly different from the composition of the plasma. However, the CSF is in equilibrium with the interstitial fluid of the brain and contributes to the maintenance of a consistent chemical environment for neurons, which serves to optimize their function.

The CSF flows through the ventricles, downward through the central canal of the spinal cord, and then upward toward the brain through the subarachnoid space that completely surrounds the brain and spinal cord. As the CSF flows over the superior surface of the brain, it leaves the subarach-noid space and is absorbed into the venous system. Although CSF is actively secreted at a rate of 500 ml/day, the volume of this fluid in the system is approximately 140 ml. Therefore, the entire volume of CSF is turned over three to four times per day.

The one-way flow of the CSF and the constant turnover facilitate its important function of removing potentially harmful brain metabolites. The CSF also protects the brain from impact by serving as a shock-absorbing system that lies between the brain and its bony capsule. Finally, because the brain and the CSF have about the same specific gravity, the brain floats in this fluid. This reduces the effective weight of the brain from 1400 g to less than 50 g and prevents compression of neurons on the inferior surface of the brain.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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