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426 P R I N C I P L E S A N D P R A C T I C E O F C R I T I C A L C A R E

Extension of choroid Cranium
plexus into lateral ventricle Dura mater
Choroid plexus Arachnoid (endosteal
of third ventricle granulations layer)

Fluid
movement
Superior
sagittal sinus
Arachnoid
granulation
Dura mater
(meningeal
layer)
Cerebral Subarachnoid Arachnoid Subdural
cortex space space
Pia
mater
Superior
sagittal
Mesencephalic sinus
aqueduct
Lateral aperture B
Choroid plexus of
fourth ventricle Spinal
Median aperture cord
Arachnoid Central canal
Subarachnoid space

Dura mater

Filum
terminale

A

FIGURE 16.7 Circulation of the cerebrospinal fluid: (A) sagittal section indicating the sites of formation and routes of circulation of cerebrospinal fluid
(arrows); (B) orientation of the arachnoid granulations. 1

Cerebral Circulation do not have valves, so the blood flows freely by gravity.
1
The brain must maintain a constant flow of blood in The face and scalp veins can flow into the brain venous
order for brain activity to occur. The arterial blood flow sinuses; therefore, infection can easily be spread into the
to the brain consists of approximately 20% of the cardiac dural venous sinuses and then enter the brain. Patient
output (see Figure 16.8). Normal cerebral blood flow is position can prevent or promote venous drainage from
5
750 mL/min. The brain autoregulates blood flow over a the brain. Head turning and tilting may kink the jugular
wide range of blood pressure by vasodilation or vasocon- vein and decrease or stop venous flow from the brain,
striction of the arteries. In response to decreased arterial which will then raise the pressure inside the cranial vault.
1
flow, the circle of Willis can act as a protective mechanism Cerebral blood flow (CBF) is the cerebral perfusion pres-
by shunting blood from one side to the other or from the sure (CPP) divided by cerebrovascular resistance (CVR).
anterior to posterior portions of the brain. This compen- CVR is the amount of resistance created by the cerebral
satory mechanism delays neurological deterioration in vessels, and it is controlled by the autoregulatory mecha-
patients.
nisms of the brain. Specifically, vasoconstriction (and
The cerebral veins drain into large venous sinuses and vasospasm) will increase CVR, and vasodilation will
1
then into the right and left internal jugular veins (see decrease CVR. It is influenced by the inflow pressure
23
Figure 16.9). The venous sinuses are found within the (systole), outflow pressure (venous pressure), cross-
folds of the dura mater. The veins and sinuses of the brain sectional diameter of cerebral blood vessels, and