162
CARDIOVASCULAR PHYSIOLOGY CONCEPTS
R. Internal
Carotid
L. Internal
Carotid
■ FIGURE 7.9 Major cerebral arteries perfusing the
brain. This view is of the ventral surface of the brain
and brainstem. The carotid and vertebral arteries
are the major source of cerebral blood flow and
are interconnected through the Circle of Willis and
basilar artery. Smaller branches from these vessels
perfuse different brain regions. L, left; R, right.
REGULATION OF CEREBRAL BLOOD FLOW
Like most other organs, cerebral blood flow is
determined by its perfusion pressure (arterial-
venous pressure) and its vascular resistance;
however, because the cerebral circulation is
located within a rigid cranium, changes in
intracranial pressure (ICP) can have signifi-
cant effects on cerebral perfusion (Fig. 7.10).
ICP is the pressure found in the fluid-filled
space between the rigid cranium and the brain
tissue. For example, cerebral vascular hemor-
rhage, brain edema caused by cerebral trauma,
or tumor growth can increase ICP, which can
lead to vascular compression and reduced cer-
ebral blood flow. The venous vessels are most
susceptible to compression because of their
low intravascular pressure, and thinner, com-
pliant walls. Because ICP is normally greater
than the venous pressure outside the cranium
and the venous vessels can easily collapse,
the effective perfusion pressure of the brain is
not the mean arterial pressure (MAP) minus
central venous pressure, but rather the MAP
minus the ICP. ICP normally ranges from 0
to 10 mm Hg; however, if it becomes elevated
(e.g., 20 mm Hg or greater), and especially if
there is systemic hypotension, the effective
cerebral perfusion pressure (CPP) and blood
flow can be significantly reduced.
Like
the
coronary circulation,
the
cer-
ebral blood flow is tightly coupled to oxy-
gen consumption. Therefore, cerebral blood
flow increases (active or functional hyper-
emia) when neuronal activity and oxygen
Rigid Cranium
CPP = MAP - ICP
ICP increased by:
• intracranial bleeding
MAP
• cerebral edema
• tumor
CVP
Increased ICP:
• collapses veins
• decreases effective CPP
reduces blood flow
■ FIGURE 7.10 Effects of intracranial pressure
(ICP)
on cerebral blood flow. ICP is the pressure w ithin the
rigid cranium
(gray
area of figure). Increased ICP decreases transmural pressure (inside minus outside
pressure) of blood vessels (particularly veins), which can cause vascular collapse, increased resistance,
and decreased blood flow. Therefore, the effective cerebral perfusion pressure (CPP) is mean arterial pres-
sure
(MAP)
minus ICP.
CVP,
central venous pressure.
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