The Blood Supply
The Blood Supply Medial View
The Blood Supply Lateral View
transports oxygen and other nutrients necessary for the
health of neurons, so a constant flow of blood to the brain
must be maintained.
According to Love
and Webb,1992, the brain uses approximately twenty percent
of the body's blood and needs twenty-five percent of the
body's oxygen supply to function optimally. Blood flow in a
healthy person is 54 milliliters per 1000 grams of brain
weight per minute. There are 740 milliliters of blood
circulating in the brain every minute. 3.3 milliliters of
oxygen are used per minute by every 1000 grams of brain
tissue. This means that approximately 46 milliliters of
oxygen are used by the entire brain in one minute. During
sleep, blood flow to the brain is increased, but the rate of
oxygen consumption remains the same.
main artery of the body is called the aorta. It supplies blood to all
parts of the body with the exception of the lungs. The
aorta ascends from the heart and forms an
arch, from which arise two subclavian arteries. Each
subclavian has two main branches, the common
carotid and the vertebral. Both of these carry blood
to the brain.
carotid divides into an external carotid
artery, which supplies blood to the face
and an internal carotid
artery which supplies the brain with blood.
carotid is a fairly straight artery, so it is not prone to
blockages due to the build up of cholesterol. Even if a
blockage does occur, it would obviously not cause a stroke
as this artery does not carry blood to the brain.
carotid artery ascends along one side of the neck.
They pass behind the ear into the temporal lobe and enter
the subarachnoid space. Then, they run posteriorly to the
medial end of the fissure of Sylvius where they bifurcate
into two main branches, the anterior cerebral
artery and the middle cerebral
As the internal
carotids have many twists and turns, there are many places
where plaque can build up, causing a blockage. Such
blockages can be identified by sonogram (non-invasive), or
by angiograms (invasive). Also, a sound called a
bruit can sometimes be heard via
stethoscope when a blockage exists.
cerebral artery goes above the optic chiasm to the
medial surface of the cerebral hemispheres. It arches around
the genu (horn) of the corpus callosum (FitzGerald, 1996).
It supplies blood to the medial cortex, including the medial
aspect of the motor strip and the sensory strip. This means
that damage to the anterior cerebral artery can cause
sensory and motor impairment in the lower body. For example,
a patient who has had a stroke affecting this artery may be
incontinent or have unilateral paralysis from the hips on down.
cerebral artery also delivers blood to some parts of the
frontal lobe and corpus striatum. So a blockage in this
artery can affect cognition and cause motoric problems due
to damage to fibers in the internal capsule or to the basal
The other main
branch of the internal carotids is the
cerebral artery. This large artery has tree-like
branches that bring blood to the entire lateral aspect of
each hemisphere. This means that this artery supplies blood
to the cortical areas involved in speech, swallowing and
language, including the lateral motor strip, lateral sensory
strip, Broca's area, Wernicke's area, Heschl's gyrus, and
the angular gyrus. In addition, it provides most of the
blood supply to the corpus striatum.
If a patient has a
blockage in the middle cerebral artery, it is probable that
s/he will have aphasia. S/he will probably also
have impaired cognition and
corticohyposthesia, or numbness, on the opposite side
of the body. Problems with hearing and the sense of smell
may also result from damage to this artery because it
supplies the lateral surface of the temporal lobe.
branches of the middle cerebral are the medial and lateral
striata arteries. The striata supply the basal ganglia,
internal capsule, and thalamus (FitzGerald, 1996). Because
they are the main blood supply to the internal capsule, they
are called by some the arteries of
stroke. When something happens to these
arteries, the bottleneck of fibers within the internal
capsule can be damaged, causing many disabilities. The
striata are very thin arteries and blood pressure within it
high. For this reason, they are considered by many to be
more vulnerable to hemorrhages than to blockages, although
FitzGerald says that occlusion of one of these areteries is
the major cause of of classical stroke where pyramidal tract
damage results in contralateral hemiplegia.
which arise from the internal carotid arteries include the
anterior communicating artery
the posterior communicating
artery joins the anterior cerebral arteries
of each hemisphere together.
arteries join the middle cerebral arteries to
the posterior cerebral arteries, which are part of the
basilar artery system.
of the vertebral arteries
ascend through the spinal column and
enter the brain through the magnum foramen. Once in the
brain, they continue to ascend, traveling beside the brain
stem. At the lower border of the pons the two vertebral
arteries join together to form the basilar
artery or vertebro-basilar
arteries and the basilar are straight arteries and therefore
not as subject to blockages due to the build up of
cholesterol as are the internal carotids.
inferior cerebellar not only supply the cerebellum but take
blood to the lateral medulla. Anterior and posterior spinal
arteries the ventral and dorsal medulla, respectively
(FitzGerald 1996). The three arteries are branches of the
The side of the
pons and the cerebellum receive blood from the anterior
inferior cerebellar artery and the superior cerebellar
artery. These arteries are branches of the basilar. The
anterior inferior cerebellar artery also has a branch, the
labyrinthine artery, that supplies the inner ear. The
basilar also gives off about twelve pontine arteries that
supply the medial pons (FitzGerald, 1996).
At the superior
border of the pons, the basilar artery divides to form the
two posterior cerebral
Before the basilar
artery divides, several other arteries arise from it. These
include the anterior, inferior, and posterior cerebellar
arteries as well as pontine
branches. So, the cerebellum and pons are
supplied by branches of the basilar.
arteries supply the part of the brain found
in the posterior fossa of the skull, including the medial
area of the occipital lobes and the inferior aspects of the
temporal lobes. They also supply the midbrain and deliver
blood to the thalamus and some other subcortical structures.
Blockages in this artery can affect the sense of smell, and
cause cranial nerve damage, as well as visual problems,
including visual agnosia, hemianopsia and alexia.
arteries, which arise both from the divisions
of the internal carotid arteries and from the basilar
system, supply blood to the choriod plexuses and also to the
hippocampus. Blockages in these arteries can affect the
production of cerebrospinal fluid and can also cause memory
problems. Below is a graphic image of the circle of Willis. Open the link to visualize some of the vascular problems that can occur.
Circle of Willis
The Circle of Willis
Arteriosus is the main arterial anastomatic
of the brain. According to Bhatnagar and Andy, 1995,
anastomosis occurs when blood vessels
bring blood to one spot from which it is then redistributed.
The Circle of Willis is a point where the blood carried by
the two internal carotids and the basilar system comes
together and then is redistributed by the anterior, middle,
and posterior cerebral arteries.
cerebral arteries of the two hemispheres are joined together
by the anterior communicating artery. The middle cerebral
arteries are linked to the posterior cerebral arteries by
the posterior communicating arteries. This
anastamosis or communication between
arteries make collateral circulation which Love and Webb, 1995,
define as "the flow of blood through an alternate route" (p.
40) possible. This is a safety mechanism, allowing brain
areas to continue receiving adequate blood supply even when
there is a blockage somewhere in an arterial system. The
blood streams of the internal carotid system and the basilar
system meet in the posterior communicating arteries. If
there are no problems in either system, the pressure of the
streams will be equal and they will not mix. However, if
there is a blockage in one of them blood will flow from the
intact artery to the damaged one, preventing a cerebral
As long as the
Circle of Willis can maintain blood pressure at fifty
percent of normal, no infarction or death of tissue will
occur in an area where a blockage exists. If collateral
circulation is good, no permanent effects may result from a
adjustment time is required before collateral circulation
can reach a level that supports normal functioning; the
communicating arteries will enlarge as blood flow through
them increases. In such cases, a transient ischemic
attack may occur, meaning that parts of the
brain are temporarily deprived of oxygen.
Some people lack
one of the communicating arteries that form the Circle of
Willis. In this case, if a blockage develops, collateral
circulation will be impeded and the collateral blood
supply will be compromised, causing brain
damage to occur.
There are some
watershed areas in the brain located
at the ends of the vascular systems. Problems with blood
supply are particularly likely to occur here, especially in
those who have hardening of the arteries. Blockages in the
water shed areas can cause transcortical
Factors Affecting the Blood Supply to the Brain
or High Blood Pressure
Abnormally low blood pressure can
cause brain damage. This may occur as a result of surgical
shock which involves blood pressure as low as 70 milliliters
per kilogram of tissue.
Hypertension, or blood pressure that
remains high regardless of activity level, can cause
arteries to narrow over time.
Cerebrovascular resistance makes
collateral circulation more difficult. It can be caused by
an arterial spasm (remember that arteries are lined with
muscles). Another potential cause of resistance is increased
viscosity of the blood, which can result from leukemia or
from high levels of tri-glycerides in the blood, or other
factors such as increased red blood cells, often seen in
chronic obstructive pulmonary disease (COPD). Increased
cerebrospinal fluid pressure can also lead to high levels of
is hardening of the arteries which often occurs with old age
but can also happen in young people.
An occluded artery
may cause a stroke due to one of the extraneural factors listed above which
can compromise the overall integrity of the cerebrovascular
system. If an individual does not have one of these problems
and has a sufficient number of communicating arteries, a
blockage may not have a significant effect on blood supply
throughout the brain.
substances present in the blood supply are unable to pass
through the meninges into the cells of the central nervous
system. The blood brain barrier includes two components, the
blood/cerebrospinal fluid barrier and the arachnoid barrier
Cerebrospinal fluid is a filtrate of
blood by the choroid plexuses (capillary networks) of the
ventricles which are formed by fusion of the pia mater and
the ependyma (ventricular lining). In the course of this
process, not all components of blood are allowed to enter
the brain. According to Webster, 1999, only clear plasma
passes through, leaving blood cells behind.
barrier layer is a part of the arachnoid meningeal layer. It
is formed by tight junctions between the
of cerebral capillaries in the arachnoid mater.
Glucose diffuses across the
blood-brain barrier through a process that is like selective