Physiology of cerebral circulation in human.ppt

cerebral arteries and specific features

2. •It is very important and essentialcirculation
•Arrest of cerebral circulation for more than 5 sec
causes loss of consciousness
•Arrest for more than 3 min causes irreversible
damageof the grey mater of the cortex
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine

3. 2 internal carotid
arteries (major
source)
2 vertebral
arteries→ form
basilar artery
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine

4. Dr abdelaziz Hussein, Mansoura
Faculty of Medicine

5. Dr abdelaziz Hussein, Mansoura
Faculty of Medicine

6. •These arteries unit together forming the circle of
Willisfrom which 6 cerebral arteries arise to supply
the brain
•No crossing of circulationfrom one side to the
other (because pressure is equal on both sides).
•The cerebral arteriesare connected together by
pre-capillary anastomosis, but can't prevent
cerebral infarction→ functionally end arteries
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine

7. ◊Normal value
By Kety method which is based on Fick's principle.
•In normal adult the brain weights 1400 gm.
•It receives 750 ml blood/min (14%of COP).
•In children CBF is doublethe adult value and it
falls to the adult level at puberty.
◊Measurement of CBF
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine

8. Dr abdelaziz Hussein, Mansoura
Faculty of Medicine

9. Intrinsic
mechanisms
(autoregulation)
Change in
ABP
Extrinsic
mechanisms
1. Nervous
regulation
2. Chemical
regulation
3. Mechanical
regulation
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine

10. Dr abdelaziz Hussein, Mansoura
Faculty of Medicine

11. •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

12. CBF
ABP
500 750 1000 1250 ml / min
50 100 150 200 mmHg
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine

13. Dr abdelaziz Hussein, Mansoura
Faculty of Medicine

14. •Mechanisms:
a) Myogenic
response
Smooth ms
respond to stretch
by contraction
b) Metabolic
response
Local changes in
brain metabolites
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine

15. •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

16. a) Increased ABP leads to:
•Local O
2tension and CO
2and H
+

V.C of cerebral vesselsCBF back to its
normal level.
b) On the other hand decreased ABP leads to:
•Local O
2tension and CO
2and H
+

V.D of cerebral vesselsCBF back to its
normal level
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine

17. Dr abdelaziz Hussein, Mansoura
Faculty of Medicine

18. 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 autoregulationmechanism
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
hemorrhage)
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine

19. b. Parasympathetic++→ no role in regulation of
CBF
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine

20. Dr abdelaziz Hussein, Mansoura
Faculty of Medicine

21. •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

22. Dr abdelaziz Hussein, Mansoura
Faculty of Medicine

23. a) Blood viscosity→ its ↓→ ↑esthe CBF and
vice versa.
b)The mean cerebral arterial and venous
blood pressures:
•The CBF depends mainly on the difference ( )
the arterial and venous pressuresat the brain
level, which is called the effective perfusion
pressure.
•This means that, the CBF ↑eswhen the arterial
pressure is ↑edor venous pressures ↓ed, and
vice versa. Dr abdelaziz Hussein, Mansoura
Faculty of Medicine

24. Dr abdelaziz Hussein, Mansoura
Faculty of Medicine

25. 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

26. Dr abdelaziz Hussein, Mansoura
Faculty of Medicine

27. 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

28. •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

29. •See before
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine

30. d) Acceleration forces:
i) During acceleration of the body upwards(+ve
gravity):
•Blood moves towards the feet and the ABPat 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

31. d) Acceleration forces:
ii) During acceleration downwards(-ve gravity)
•Opposite occurs
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine

32. 1. Cerebral circulationis enclosed in a solid skull, so
the brain tissue, blood and CSF volumes are kept
constant at any time.
•Brain tissue and CSF are incompressiblewhile the
blood vessels are compressible.
•So ↑ ICT affects mainly blood vessels and ↓CBF
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine

33. 2. Glucose is the major source of energyin the brain
and sometimes amino acids during starvation.
3. Brain is very sensitiveto 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

34. Dr abdelaziz Hussein, Mansoura
Faculty of Medicine

35. ◊Def,
The blood brain barrier means selective permeability
of the cerebral capillaries.
◊Structure:
It is due to:
a.Absence of slit poresand tight junctions between
capillary endothelial cells
b.Lack of vesicular transportmechanism
c.End-feet of the astrocytes cells.
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine

36. Dr abdelaziz Hussein, Mansoura
Faculty of Medicine

37. ◊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

38. Dr abdelaziz Hussein, Mansoura
Faculty of Medicine

39. ◊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

40. THANKS
Dr abdelaziz Hussein, Mansoura
Faculty of Medicine