Physiology and autoregulation of cerebral circulation
neurology
,
notes
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physiology
Cerebral circulation is very important and essential circulation
Why because if there is arrest of cerebral circulation
for more than 5 seconds it causes loss of consciousness
Arrest for more than 3 min causes irreversible damage of the
grey mater of the cortex of brain
2 internal carotid arteries (major source)
2 vertebral arteries join to form basilar artery
These arteries unit together forming the circle of Willis
from which 6 cerebral arteries arise to supply the brain
There is no crossing of circulation from one side to the
other as pressure is equal on both sides
functionally these
vessels are called end arteries .The cerebral arteries are connected together
by pre-capillary anastomosis, but can't prevent cerebral infarction
Normal values
In normal adult the brain weights 1400 gm.
It receives 750 ml blood/min (14%of COP).
In children CBF is double the adult value and it falls to
the adult level at puberty.
Factors involved in regulation of CBF
Intrinsic and extrinsic
mechanism is involvedin cerebral autoregulation
Intrinsic mechanisms
Change in arterial blood pressure ( Autoregulation of CBF)
Extrinsic mechanisms are the following
- Nervous regulation
- Chemical regulation
- Mechanical regulation
Autoregulation of CBF
This is the ability
of brain to maintain its flow constant despite of changes in ABP.
Range of cerebral blood flow
Range of cerebral blood flow varies from 70 to 150 mmHg.
In hypertensive patient this mechanism operates up to a
blood pressure level of 180 mmHg
It operates and restores the CBF to its normal basal level
within 1-2 minutes of derangement
Mechanism of autoregulation of cerebral blood flow
1.Myogenic response
It is produced by the smooth musles response to stretch by
contraction
Myogenic Mechanism
- With increased ABP stretch of the vascular wall result in smooth musles contraction vasoconstriction and decrease of CBF back to its normal level.
- With reduction in ABP the opposite occurs.
2. Metabolic response is the local changes in brain
metabolites
- Increased ABP leads to:Local increase in O2 tension and reduction of CO2 and H+(hydrogen ion) will lead to cerebral vasoconstriction and CBF back to its normal level.
- On the other hand decreased ABP leads to: Local decrease in O2 tension and increase in CO2 and H+ (hydrogen) will result in vasodilatation of cerebral vessels and bring the CBF back to its normal level
It include both sympathetic and parasympathetic regulation
a) Sympathetic regulation:
The cerebral blood vessels receive sympathetic supply from
the superior cervical ganglia
ii) In severe sympathetic stimulation (as in moderat to
severe exercise) there is strong vasoconstriction
of large and medium sized arteries and is very important to prevent the high pressure
to reach the small cerebral vessels and protect them from rupture (cerebral
hemorrhage)
It has no role in regulation of CBF
Hypercapnia (↑CO2) and acidosis (↑H) result in marked vasodilatation of the
cerebral vessels and ↑ the CBF.
When the CO2 tension increase in the blood, it crosses the
blood-brain barrier and combines with H2O to form H2CO3 which dissociates to
HCO3 & H → H causes dilatation of the cerebral vessels (CO2 has no direct
VD effect).
Blood viscosity- when
there is a reduction in blood viscosity
it increase the CBF and vice versa.
The mean cerebral arterial and venous blood pressures:The CBF depends mainly on the difference between the
arterial and venous pressures at the brain level, which is called the effective
perfusion pressure.This means that, the CBF increaseses when the arterial
pressure is ↑ed or venous pressures ↓ed, and vice versa.
The intracranial pressure (ICT):The ICT is produced mainly by the cerebrospinal fluid (CSF)
and normally this is about 11 mmHg.The effect starts to occur when the ICT rises to about 33
mmHg.Slight rise of ICT results in compression of cerebral vessels slightly with slight reduction of CBF.Marked rise of ICT (more than 33 mmHg), compresses the
cerebral vessels with marked reduction of CBF
The intracranial pressure (ICT):In forced expiration with straining as in cough, defecation
and labour, the mean venous pressure increases which increased ICT and the
CBF is decreased markedly by,decrease in effective perfusion pressure.Compression on vesselsThis protect the
cerebral vessels from rupture (cerebral haemorrhage).
Cushing Reflex
Cushing reflex is a physiological nervous system response to
increased intracranial pressure (ICP) that results in Cushing's triad of
increased blood pressure, irregular breathing, and bradycardia
Acceleration forces:
During acceleration of the body upwards against gravity blood moves towards the feet and the
ABP at the level of the head falls. The
venous pressure also decreases, consequently the ICT drops to maintain the CBF.
During acceleration downwards (-ve gravity) Opposite occurs
The key characteristics of cerebral circulation
- Cerebral circulation is enclosed in a solid skull, so the brain tissue, blood and CSF volumes are kept constant at any time.
- Brain tissue and CSF are incompressible while the blood vessels are compressible.So ↑ ICT affects mainly blood vessels and ↓CBF
- Glucose is the major source of energy in the brain and sometimes amino acids during starvation.
- Brain is very sensitive to hypoxia and hypoglycemia however, hypoxia is more serious:
- Loss of consciousness if hypoxia is more than 5 sec.
- Irreversible tissue damage if hypoxia is more than 3 min.
