2. INTRODUCTION
• Brain tissues need adequate blood supply
continuously. Stoppage of blood flow to brain
for 5 seconds leads to unconsciousness and
for 5 minutes leads to irreparable damage to
the brain cells.
3. CEREBRAL VESSELS AND NORMAL
CEREBRAL BLOOD FLOW
• Brain receives blood from the basilar artery
and internal carotid artery. Branches of these
arteries form circle of Willis. Venous drainage
is by sinuses, which open into internal jugular
vein.
• Normally, brain receives 750 to 800 mL of
blood per minute. It is about 15% to 16% of
total cardiac output and about 50 to 55
mL/100 g of brain tissue per minute.
4. REGULATION OF CEREBRAL BLOOD
FLOW
• Cerebral circulation is regulated by three
factors:
1. Autoregulation
2. Chemical factors
3. Neural factors.
5. AUTOREGULATION
• Like any other vital organ, brain also regulates
its own blood flow by means of
autoregulation.
• However, the autoregulation in brain has got
its own limitations.
• It depends upon:
i. Effective perfusion pressure
ii. Cerebral vascular resistance.
6. i. Effective Perfusion Pressure
• Effective perfusion pressure is the balance
between the mean arterial blood pressure and
venous pressure across the organ, divided by
resistance.
• Since venous pressure is zero in brain, mean
arterial blood pressure plays an important role in
regulating cerebral blood flow.
• Autoregulation is possible in brain if the mean
arterial pressure is within the range of 60 mm Hg
and 140 mm Hg.
• Autoregulation fails beyond this range on either
side.
7. ii. Cerebral Vascular Resistance
• When the vascular resistance is more, the blood
flow to the brain is less. Resistance to blood flow
in brain is offered by intracranial pressure,
cerebrospinal fluid pressure and viscosity of blood.
Intracranial pressure and cerebrospinal fluid
pressure
• Increase in the intracranial pressure or the
pressure exerted by the cerebrospinal fluid (CSF)
compresses the cerebral blood vessels and
decreases blood flow. These pressures are
elevated in conditions like head injury. However,
severe ischemic effects are avoided by some
protective reflexes such as Cushing reflex.
9. Monro-Kellie doctrine
• According to Monro-Kellie doctrine or
principle, though the cerebral arteries are
compressed by increased intracranial pressure
or cerebrospinal fluid pressure, the volume of
brain tissue is not affected. It is because the
brain tissue is not compressible.
10. Viscosity
• Increase in the viscosity of blood as in
polycythemia, increases the cerebral vascular
resistance and blood flow decreases. When
viscosity decreases as in the case of anemia,
the resistance is decreased and blood flow
increases. Thus, the cerebral blood flow is
inversely proportional to the viscosity of
blood.
11. CHEMICAL FACTORS
• Chemical factors which increase the cerebral
blood flow:
i. Decreased oxygen tension
ii. Increased carbon dioxide tension
iii. Increased hydrogen ion concentration
12. • Carbon dioxide is the most important factor, as it
causes dilatation of cerebral blood vessels,
leading to increase in blood flow.
• A moderate increase in carbon dioxide tension
does not alter the blood flow due to
autoregulation.
• When arterial partial pressure of carbon dioxide
rises above 45 mm Hg, the cerebral blood flow
increases. Carbon dioxide combines with water to
form carbonic acid, which dissociates into
bicarbonate ions and hydrogen ion.
• The hydrogen ion causes dilatation of blood
vessels in brain. Hypoxia increases cerebral blood
flow by vasodilatation.
13. NERVOUS FACTORS
• Cerebral blood vessels are supplied by
sympathetic vasoconstrictor fibers. But, these
fibers do not play any role in regulating
cerebral blood flow under normal conditions.
• In pathological conditions like hypertension,
the sympathetic nerves cause constriction of
cerebral blood vessels, leading to reduction in
blood flow. It prevents cerebral vascular
hemorrhage and cerebral stroke.
