fluid is a clear body fluid occupying the subarachnoid space and the brain ventricular system around and inside the brain and spinal cord. It acts as a cushion or buffer for the cortex, providing basic mechanical and immunological protection to the brain inside the skull. It is produced by modified ependymal cells of the choroid plexus found in all components of the ventricular system except for the cerebral aqueduct and the posterior and anterior horns of the lateral ventricles. CSF flows from the lateral ventricle to the third ventricle through the interventricular foramen (also called the foramen of Monro). The third ventricle and fourth ventricle are connected to each other by the cerebral aqueduct (also called the aqueduct of Sylvius). CSF then flows into the subarachnoid space through the foramina of Luschka (there are two of these) and the foramen of Magendie (only one of these).
There is approximately 150 mL of CSF circulating around the brain, in the ventricles and around the spinal cord. The CSF is replaced every 8 hours. Absorption of the CSF into the bloodstream takes place in the superior sagittal sinus through structures called arachnoid villi. When the CSF pressure is greater than the venous pressure, CSF will flow into the bloodstream. However, the arachnoid villi act as ‘one‐way valves’: if the CSF pressure is less than the venous pressure, the arachnoid villi will not let blood pass into the ventricular system.
Clinical practice point
Due to the brain’s many important roles, damage to any of its lobes from injuries, illnesses or chronic conditions such as brain tumour can cause major losses in brain function. A space‐occupying lesion of the brain is usually due to malignancy but it can be caused by other pathologies such as an abscess or haematoma. The effect of a tumour may be local, due to focal brain damage, and the presentation can provide an indication of the location of the lesion but not its cause. There may be more general symptoms related to raised intracranial pressure or seizures, behavioural changes or false localising signs. While large lesions in some regions, for example the frontal lobe, may be relatively silent, a small lesion in the dominant hemisphere may impact severely on speech, for example.
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