388
UNIT THREE
C
erebrospinal fluid (CSF) is secreted and
reabsorbed continuously, so the fluid
pressure in the ventricles remains rela-
tively constant. However, infection, a tumor, or
a blood clot can interfere with the ±
uid’s circula-
tion, increasing pressure in the ventricles (intra-
cranial pressure or ICP). This can collapse cerebral
blood vessels, retarding blood ±
ow. Brain tissues
forced against the skull may be injured.
A
lumbar puncture
(spinal tap) measures CSF
pressure. A physician inserts a ²
ne, hollow needle
into the subarachnoid space between the third
and fourth or between the fourth and ²
fth lumbar
vertebrae—below the end of the spinal cord (²
g.
11A). An instrument called a
manometer
measures
the pressure of the fluid, usually about 130 mil-
limeters of water (10 millimeters of mercury). At
the same time, samples of CSF may be withdrawn
and tested for abnormal constituents. Red blood
cells in the CSF, for example, may indicate a hemor-
rhage in the central nervous system (CNS).
A temporary drain inserted into the suba-
rachnoid space between the fourth and ²
fth lum-
bar vertebrae can relieve pressure. In a fetus or
infant whose cranial sutures have not yet united,
increasing ICP may enlarge the cranium, a condi-
tion called
hydrocephalus,
or “water on the brain”
g. 11B). A shunt to relieve hydrocephalus drains
fluid away from the cranial cavity and into the
digestive tract, where it is either reabsorbed into
the blood or excreted.
11.1
CLINICAL APPLICATION
Cerebrospinal Fluid Pressure
Skin
Spinal cord
Conus
medullaris
Arachnoid mater
Sacrum
Coccyx
Filum terminale
Third lumbar
vertebra
Dura mater
Subarachnoid
space
(a)
Ventricles
Ventricles
(b)
FIGURE 11A
A lumbar puncture is performed
by inserting a ²
ne needle, usually below the fourth
lumbar vertebra, and withdrawing a sample of CSF
from the subarachnoid space. (For clarity, spinal
nerves are not shown.)
FIGURE 11B
CT scans of the human brain. (
a
) Normal
ventricles. (
b
) Ventricles enlarged by accumulated ±
uid.
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