920
U N I T 1 0
Nervous System
U N D E R S T A N D I N G
Intracranial Pressure
Nonexpandable skull
Brain
tissue
Cerebrospinal
fluid
Blood
The intracranial pressure (ICP) is the pressure within the intracranial cavity. It
is determined by (1) the pressure-volume relationships among the brain tissue,
cerebrospinal fluid (CSF), and blood in the intracranial cavity; (2) the Monro-Kellie
hypothesis, which relates to reciprocal changes among the intracranial volumes;
and (3) the compliance of the brain and its ability to buffer changes in intracranial
volume.
Intracranial Volumes and
Pressure.
The ICP represents the
pressure exerted by the essentially
incompressible tissue and fluid vol-
umes of the three compartments
contained within the rigid confines
of the skull—the brain tissue and
interstitial fluid (80%), the blood
(10%), and the CSF (10%).
1
B
A
Monro-Kellie Hypothesis.
Nor
mally, a reciprocal relationship exists
among the three intracranial vol-
umes such that the ICP is maintained
within normal limits. Because these
volumes are practically incompress-
ible, a change in one component must
be balanced by an almost equal and
opposite effect in one or both of the
remaining components. This is known
as the
Monro-Kellie hypothesis.
Of the three intracranial volumes, the
fluid in the CSF compartment is the
most easily displaced. The CSF
(A)
can be displaced from the ventricles
and cerebral subarachnoid space to
the spinal subarachnoid space, and
it can also undergo increased absorp-
tion or decreased production. Because
most of the blood in the cranial cav-
ity is contained in the low-pressure
venous system, venous compression
(B)
serves as a means of displacing
blood volume.
2
