The meninges are three separate membranes, layered together, which serve to encase the brain and spinal cord. The dura is the toughest, outermost layer, and is closely attached to the inside of the skull. The middle layer, the arachnoid, is important in the normal flow of the cerebrospinal fluid (CSF), a lubricating fluid which bathes both the brain and the spinal cord. The innermost layer, the pia, helps direct brain blood vessels into the brain. The space between the arachnoid and the pia contains CSF, which serves to help insulate the brain from trauma. Through this space course many blood vessels.
CSF, produced within specialized chambers deep inside the brain, flows over the surface of the brain and spinal cord. This fluid serves to cushion these relatively delicate structures, as well as supplying important nutrients for brain cells. CSF is reabsorbed by blood vessels which are located within the meninges.
Because the brain is enclosed in the hard, bony case of the skull, any disease process which produces swelling will ultimately prove destructive to the brain. The skull cannot expand at all, so when swollen brain tissue pushes up against the skull's hard bone, the brain tissue becomes damaged and may ultimately die. Furthermore, swelling on the right side of the brain will not only cause pressure and damage to that side of the brain, but by taking up precious space within the tight confines of the skull, the left side of the brain will also be pushed up against the hard surface of the skull, causing reciprocal damage to that side of the brain as well.
The cells of the brain require a very well-regulated environment for optimal function. Careful balance of oxygen, carbon dioxide, glucose (sugar), sodium, calcium, potassium, and other substances must be maintained in order to avoid damage to the relatively unforgiving brain tissue.
The cells lining the brain's capillaries (tiny blood vessels) are specifically designed to prevent many substances from passing into brain tissue. This is commonly referred to as the blood-brain barrier. The blood-brain barrier prevents various toxins (substances which could be poisonous to brain tissue), as well as many agents of infection, from crossing from the blood stream into the brain tissue. While this barrier obviously is an important protective feature for the brain, it also serves to complicate therapy in the case of an infection, by making it difficult for medications to pass out of the blood and into the brain tissue where the infection resides.
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