Although the brain constitutes only 2% of body weight, it receives approximately 15% of the blood supply and consumes nearly 20% of the total oxygen and glucose available to the body. In order to supply these essential nutrients for brain function, there must be a consistent and rapid blood supply to the brain in order that the brain cells may function. This is supplied by the cerebral arteries derived from the internal carotid arteries which branch over the surface of the brain and send smaller branches into the deeper subcortical structures. The capillaries are highly branched and it has been calculated that every nerve cell is no more than 40-50 ^m from a capillary.
Because of the unique dependence of the brain on oxygen and glucose to enable the extensive oxidative metabolism to take place within the nerve cells, it is essential that the composition of nutrients, electrolytes, etc. in the fluid surrounding the brain cells is controlled. The composition of the blood varies to some extent according to the composition of the diet and therefore a mechanism has evolved to ensure that the composition of the extracellular fluid surrounding the brain cells is constant. The extracellular fluid is in equilibrium with the cerebrospinal fluid which fills the four ventricles of the brain, and covers the surface of the brain and the spinal cord.
Cerebrospinal fluid is formed from the blood and may be considered as an ultra filtrate of plasma. Thus cerebrospinal fluid contains most of the electrolytes and low molecular weight nutrients but is low in protein. It is formed from a network of capillaries in the ventricles termed the choroid plexus but the cerebral capillaries also contribute to the production of cerebrospinal fluid. The extracellular fluid and the cerebrospinal fluid compartment is separated from the blood by the blood-brain barrier. This is a barrier formed by tight junctions that exist between the endothelial cells lining the capillaries and the epithelial cells at the choroid plexus. Such a barrier prevents the influx of large molecular weight molecules but enables small molecular weight substances such as glucose, amino acids, fatty acids, electrolytes, etc., which are essential for normal brain function, to enter. In addition to the structural nature of the blood-brain barrier, there also exist specific transport sites that assist the transport of glucose and essential amino acids into the extracellular fluid. Thus the blood-brain barrier has both a structural and a metabolic role to play in maintaining homeostasis.
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