little protein; all constituents of CSF are reabsorbed, including small molecules,
proteins and microorganisms. There is a higher concentration of most molecules
in the brain than in the CSF, this creates a chemical gradient between the two
compartments. CSF allows for distribution of neuroactive substances, and is the
“sink” that collects wastes produced by the brain: the main ones being C02, lactate
and excess hydrogen ions (H+). It also serves as a heat sink.
The specific gravity of the cerebrospinal fluid is 1.003, as compared to 1.028
for the blood-plasma. Cerebrospinal Fluid CSF is higher in salt and lower in
protein than blood plasma. CSF is a clear, colorless gel containing small amounts
of protein, glucose, potassium, silica and relatively large amounts of sodium
chloride. CFS passes through ventricles and into the fourth ventricle from which
it escapes into the subarachnoid space through the median and lateral apertures.
From there it circulates via hydrostatic pressure through the subarachnoid cisterns
at the base of the brain, and then is directed up over the hemispheres and down
around the spinal cord, flowing down to about the second sacral vertebrae. After
it is reabsorbed into venus sinus blood via arachnoid villi; note that arachnoid villi
become hypertrophied and calcified with age (arachnoid granulations). Arachnoid
villi are small protrusions of the arachnoid (the thin second layer covering the
brain) through the dura (the thick outer layer). They protrude into the venous
sinuses of the brain, and allow cerebrospinal fluid (CSF) to exit the brain, and
enter the blood stream. The arachnoid villi act as one-way valves. Normally the
pressure of the CSF is higher than that of the venous system, so CSF flows through
the villi and granulations into the blood. It has been suggested that the endothelial
cells of the venous sinus create vacuoles of CSF, which move through the cell and
out into the blood.
The CSF is moved under the influence of hydrostatic pressure generated
by its continuous production and its circulation allows for homeostasis of the
environment that surrounds the brain. CSF movement allows arterial expansion
and contraction by acting like a spring, which prevents wide changes in intracranial
blood flow. It is expected that the brain tissue and the CSF would have the same
hydrostatic pressure in any part of the brain. The cerebrospinal fluid fills the cavity
of the ventricles and the subarachnoid spaces. The subarachnoid space extends
caudally around the spinal cord and ends in lumbar-sacral dural sac where it
surrounds the cauda equina. The lining of the tube is composed of ependymal cells
and cili, the beating of which is required for normal CSF flow. Cilliated cells are
common throughout the respiratory and genital tracts and also in the tympani-the
cartilaginous and bony margins of auditory tube, Eustachian tube connecting the
back of the nose to the middle ear and ventricals of the brain.
The fluid is made at the rate of 21ml/hr is completely changed every 6-7 hours.
It is believed that CSF takes one to two hours to reach the basal cisterns, 3 to 4
hours to reach the sylvian fissure and 10 to 12 hours to spread over the cerebral
subarachnoid space. By 24 hours it started to be cleared into the superior sagittal
sinus. As much as the brain tissue is protected by a blood brain barrier from changes
outside the central nervous system, the CSF has the same protection and does