by Karima Roumila
As a follow up to the previous article on the anatomy and physiology of the brain and of Brain Awareness Week activities, we would like to continue our learning and explore the terms we so often hear when dealing with hydrocephalus. These definitions are taken from our various educational publications, which are all found here on our website.
Cerebral Spinal Fluid (CSF) is produced within the cavities of the brain that are called ventricles. Below is a drawing of the ventricles. As you look at the drawing, imagine the ventricles as chambers filled with fluid. There are four in all: the two lateral ventricles, the third ventricle and the fourth ventricle. As you can see, the ventricles are interconnected by narrow passageways. Your neurosurgeon can learn valuable information about the patient’s condition by closely monitoring the size and shape of these ventricles.
Cerebrospinal Fluid Circulation and Absorption
CSF is formed within the ventricles by small, delicate tufts of specialized tissue called the choroid plexus. The solid arrows in the drawing: cerebrospinal fluid (CSF) circulatory Pathway, show the major pathway of CSF flow. Beginning in the lateral ventricles, CSF flows through
two passageways into the third ventricle. From the third ventricle it flows down a long, narrow passageway (the aqueduct of Sylvius) into the fourth ventricle. From the fourth ventricle it passes through three small openings (foramina) into the subarachnoid space surrounding the brain and spinal cord. CSF is absorbed through blood vessels over the surface of the brain back into the bloodstream. Some absorption also occurs through the lymphatic system. Once in the bloodstream, it is carried away and filtered by our kidneys and liver in the same way as are our other body fluids.
The ventricular system is the major pathway for the flow of CSF. CSF also flows directly from the ventricles into the brain tissue surrounding them. This is shown by the broken arrows. Here the CSF passes through the spaces between the cells to where it eventually enters the subarachnoid space. It is believed that the brain tissue does not absorb any CSF, but simply provides another pathway for the fluid moving to the subarachnoid space. Some small amounts of CSF are also absorbed into lymphatic channels along the membranes covering the nerves (nerve sheaths) as they leave the brain stem and spinal cord .
Our bodies produce approximately a pint (500 ml) of CSF daily, continuously replacing CSF as it is absorbed. Under normal conditions there is a delicate balance between the amount of CSF that is produced and the rate at which it is absorbed. Hydrocephalus occurs when this balance is disrupted. Although there are many factors that can disrupt this balance, the most common is a blockage, or obstruction, somewhere along the circulatory pathway of CSF. The obstruction may develop from a variety of causes, such as brain tumors, cysts, scarring and infection.