Download Cerebral circulation

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‫االسراء اية ‪58‬‬
‫‪Dr abdelaziz Hussein, Mansoura‬‬
‫‪Faculty of Medicine‬‬
By
Dr. Abdel Aziz M. Hussein
Lecturer of Medical Physiology
Member of American Society of Physiology
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine
• It is very important and essential circulation
• Arrest of cerebral circulation for more than 5 sec
causes loss of consciousness
• Arrest for more than 3 min causes irreversible
damage of the grey mater of the cortex
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine
2 internal carotid
arteries (major
source)
2 vertebral
arteries→ form
basilar artery
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine
• These arteries unit together forming the circle of
Willis from which 6 cerebral arteries arise to supply
the brain
• No crossing of circulation from one side to the
other (because pressure is equal on both sides).
• The cerebral arteries are connected together by
pre-capillary anastomosis, but can't prevent
cerebral infarction→ functionally end arteries
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine
◊ Normal value
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.
◊ Measurement of CBF
By Kety method which is based on Fick's principle.
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine
Intrinsic
mechanisms
(autoregulation)
Extrinsic
mechanisms
1. Nervous
regulation
Change in
ABP
2. Chemical
regulation
3. Mechanical
regulation
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine
• It is the ability of brain to maintain its flow
constant despite of changes in ABP.
◊ Range:
• From 70 to 150 mmHg.
• In hypertensive patient this mechanism operates up
to 180 mmHg
◊ Time :
• It operates and restores the CBF to its normal basal
level within 1-2 minutes.
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine
1250 ml / min
CBF
1000
750
500
50
100
ABP
150
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine
200 mmHg
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine
• Mechanisms:
a) Myogenic
response
b) Metabolic
response
Smooth ms
respond to stretch
by contraction
Local changes in
brain metabolites
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine
• a) With increased ABP  stretch of the
vascular wall  smooth ms contraction 
V.C decrease of CBF back to its normal
level.
• b) With decreased ABP the opposite occurs.
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine
a) Increased ABP leads to:
• Local  O2 tension and  CO2 and H+ 
V.C of cerebral vessels   CBF back to its
normal level.
b) On the other hand decreased ABP leads to:
• Local  O2 tension and  CO2 and H+ 
V.D of cerebral vessels   CBF back to its
normal level
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine
a) Sympathetic ++:
•The cerebral blood vessels receive sympathetic
supply from the superior cervical ganglia
i) Mild to moderate sympathetic stimulation VC
effect has little effect on the CBF as it is overcomed
by the autoregulation mechanism
ii) In severe sympathetic ++ (as in ms exercise), strong
VC of large and medium sized arteries occurs→ is very
important to prevent the high pressure to reach the small
vessels→ protect them from rupture (cerebral
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine
hemorrhage)
b. Parasympathetic++ → no role in regulation of
CBF
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine
• Hypercapnia (↑CO2) and acidosis (↑H) →
produces marked VD of the cerebral vessels
and ↑ the CBF.
• When the CO2 tension ↑es 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).
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine
a) Blood viscosity → its ↓→ ↑es the CBF and
vice versa.
b)The mean cerebral arterial and venous
blood pressures:
•The CBF depends mainly on the difference ( )
the arterial and venous pressures at the brain
level, which is called the effective perfusion
pressure.
•This means that, the CBF ↑es when the arterial
pressure is ↑ed or venous pressures ↓ed, and
vice versa.
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine
c) The intracranial pressure (ICT):
-The ICT is produced mainly by the cerebrospinal fluid
(CSF) and normally about 11 mmHg.
-The effect starts to occur when the ICT rises to about
33 mmHg.
•Slight rise of ICT, compresses vessels slightly with
slight ↓ of CBF.
•Marked rise of ICT (more than 33 mmHg),
compresses the cerebral vessels with marked ↓ of CBF.
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine
c) The intracranial pressure (ICT):
•In forced expiration with straining as in cough,
defecation and labour, the mean venous pressure
↑es which ↑es ICT and the CBF is ↓ed markedly by,
1.↓ Effective perfusion pressure.
2.Compression on vessels
•This
protect the cerebral vessels from rupture
(cerebral haemorrhage).
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine
• The cerebrospinal fluid (CSF) is the fluid
which fills the ventricles of the brain and
the subarachnoid space. Its volume is
about 150 ml. It has almost the same
constituents as the brain interstitial fluid.
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine
• See before
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine
d) Acceleration forces:
i) During acceleration of the body upwards (+ve
gravity):
•Blood moves towards the feet and the ABP at the
level of the head falls.
•The venous pressure also ↓es, consequently the
ICT drops to maintain the CBF.
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine
d) Acceleration forces:
ii) During acceleration downwards (-ve gravity)
•Opposite occurs
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine
1. 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
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine
2. Glucose is the major source of energy in the brain
and sometimes amino acids during starvation.
3. 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.
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine
◊Def,
The blood brain barrier means selective permeability
of the cerebral capillaries.
◊Structure:
It is due to:
a. Absence of slit pores and tight junctions between
capillary endothelial cells
b. Lack of vesicular transport mechanism
c. End-feet of the astrocytes cells.
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine
◊Permeability:
•The blood-brain barrier is highly permeable to
water, O2, CO2; and lipid soluble substances as
alcohol and anesthetics.
•It is impermeable to plasma proteins, cholesterol or
bile pigments.
•Other substances cross the barriers with varying
degrees of difficulty.
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine
◊Function of the blood-brain barrier:
i) Protection of the brain from endogenous and
exogenous toxins
ii) Maintenance of the local environment of the
neurons in the CSF constant
iii) Preventing the escape of local
neurotransmitters into the general circulation.
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine
THANKS
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine