Lect. 19
The cerebral circulation
Objectives:
1. Explain the mechanisms by which blood flow to the brain is
regulated.
2. The cerebral circulation is maintained in response to changes in
arterial blood pressure. (Explain how).
Physiologic anatomy:
The brain is supplied by the internal carotid and vertebral arteries, which
form the circle of Willis. From each side of the circle arise three cerebral
arteries; anterior, middle and posterior. They run along the convex
surface of the cerebral hemispheres to supply the cerebral cortex and send
deep branches to supply the subcortical structures. With the exception of
the circle of Willis, there is no anastomosis between the intracranial
arteries, but some anastomoses exist between the smaller arterioles. Still,
these anastomoses are inadequate to nourish the brain tissue when an
arterial branch is occluded. That is why the cerebral arteries are
considered functionally as end-arteries. The superficial and deep veins
drain the cerebral blood into the large venous sinuses which exist
between the folds of the dura mater. The venous sinuses are prevented
from collapse by the tough structure of the dura and the attachment of
their walls to the bones of the skull. The cerebral venous blood is drained
from the sinuses by the jugular veins, mainly the internal jugular in man.
In contrast to the arterial supply, the cerebral venous system contains
plenty of anastomoses between the superficial and deep veins, and
between the intra- and extracranial veins. That is why occlusion of the
internal jugular veins does not arrest the cerebral venous return, even if it
is bilateral.
The brain is highly vulnerable to damage by hypoxia or ischemia. This is
because of three factors:
1. The high metabolic rate of the brain (7kcal/kg/hr) compared with
that
of the whole body (1 kcal/kg/hr).
2. The metabolic reactions of the brain are all aerobic.
3. The lack of significant energy stores in the brain. Glucose is the
main
metabolic substrate of the brain, yet the glycogen content of the
brain
(1.6 g/kg) meets its metabolic needs for only two minutes.
Autoregulation of the cerebral blood flow:
A sudden rise in the arterial blood pressure (ABP) leads to transient
increase in the cerebral blood flow. If the rise in pressure is maintained,
autoregulation mechanisms operate to restore the cerebral blood flow
back to its normal basal level within 1-2 minutes. The cerebral blood flow
is autoregulated in the blood pressure range of 70-140 mmHg in normal
persons or up to 180 mmHg in hypertensive persons. The cerebral blood
flow is regulated in response to a rise in ABP by the following
mechanisms:
1. Myogenic vasoconstrictor response; caused by the increased
tension in the vascular wall.
2. Metabolic vasoconstriction response; caused by washing out the
vasodilator metabolites released by brain metabolism.
3. Neurosympathetic response; sympathetic stimulation constricts the
cerebral blood vessels.
A fall in the ABP leads to the opposite mechanisms with a resultant
vasodilation to maintain a constant blood flow rate.
Control of the cerebral blood flow:
Three main control mechanisms regulate and adjust the cerebral blood
flow:
Nervous control:
The cerebral blood vessels receive sympathetic nerve supply from the
cervical division of the sympathetic nervous system. It constricts the large
and intermediate arteries during sympathetic activity. Under ordinary
conditions, the vasoconstrictor effect of the sympathetic nerves on
cerebral vessels is overridden by the autoregulation mechanisms.
However, sympathetic cerebral vasoconstriction is strong and is very
important in the following conditions:
In severe muscular exercise when arterial blood pressure rises to
very high levels. Vasoconstriction of the large and intermediate
vessels protects the small vessels and prevents their rupture.
After rupture of a small cerebral vessel, e.g. cerebral stroke,
subdural hematoma or brain tumour. Sympathetic reflexes cause
severe constriction of the large arterial supply to limit the
intracranial bleeding.
Metabolic control:
The blood flow to the brain is regulated mainly by its own metabolism.
The cerebral vessels are characterized by being extremely sensitive to
hypoxia, hypercapnia and acidosis, which produce marked vasodilation of
the cerebral vessels and increase the cerebral blood flow. Hypercapnia
increases the H
2
CO
3
and H
+
levels in blood. CO
2
has no direct vasodilator
effect. It is the H
+
ion produced by the hypercapnia which dilates the
vessels.
Physical control: (by the intracranial pressure)
The intracranial cavity has a fixed volume because it is enclosed in the
rigid bones of the skull. It contains the brain, whose volume is
approximately 1500 mL, plus 75 mL of blood and 75 mL of cerebrospinal
fluid (CSF). Because the brain tissue and fluids are incompressible, the
total volume of the blood, the CSF and the brain is constant at any time. It
follows that:
1. Any rise in the intracranial pressure compresses the cerebral
vessels and reduces the cerebral blood flow. A drop in the
intracranial pressure expands (dilates) the vessels and increases the
cerebral blood flow.
2. Any change in the venous pressure immediately causes a similar
change in the intracranial pressure which influences the cerebral
blood flow.
The brain is very richly supplied with blood. In a normal adult male, the
brain weighs about 1.5 kg (2% of body weight) and receives about 750
mL of blood/min (14% of the cardiac output). The cerebral blood flow is
not uniform in all parts of the brain. The average blood flow in the gray
matter is about six times that of the white matter. The largest blood flow
per gram is in the inferior colliculus of the midbrain (1.8 mL/g/min),
followed by the sensorimotor cortex (1.4 mL/g/min). The least blood flow
is in cerebral white matter (0.2 mL/g/min).
VARIATIONS IN THE TOTAL AND REGIONAL CEREBRAL
BLOOD FLOW
Total cerebral blood flow increases in hypoxia, hypercapnia and acidosis.
It decreases during deep, quiet, slow-wave sleep.
Regional blood flow in the brain varies during different physiological or
pathological conditions. The following are examples:
Quiet thinking during rest increases the blood flow in the prefrontal
association area.
Voluntary clinching of the right hand increases the blood flow in
the hand area of the left sensorimotor cortex.
Looking at a luminous object increases the blood flow in the visual
occipital cortex.
In an epileptic focus, the blood flow increases during the epileptic
seizure but decreases in other pans of the brain. In between
seizures, the blood flow decreases in the epileptic focus but
remains normal in other parts.
In manic depressive patients, there is a general decrease in the
cortical blood flow when the patients are depressed.
On standing from the supine position, the arterial pressure in the head
area drops to 65 mmHg. The cerebral blood flow decreases by 20%, but
the O
2
supply remains unaffected due to increased O
2
extraction
coefficient. If standing is accompanied with muscular exercise, cerebral
blood flow is maintained at the basal level (750 mL/min).
For example, if the body is accelerated upwards, gravitational forces act
toward the feet (positive gravity), i.e. blood moves toward the feet. The
arterial pressure at the level of the head decreases which tends to decrease
the cerebral blood flow. This is compensated to a large extent by the drop
in venous pressure which decreases the intracranial pressure→ less
compression of the blood vessels→ vasodilation → increase in the blood
flow back toward the normal level.
On acceleration downwards (negative gravity), the opposite reactions
occur to prevent marked increase in the cerebral blood flow.
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