صدق الله العظيم الاسراء 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

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

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 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 double the adult value and it falls to the adult level at puberty. ◊ Measurement of CBF Dr abdelaziz Hussein, Mansoura Faculty of Medicine

Intrinsic mechanisms (autoregulation) Change in ABP Extrinsic mechanisms 1. Nervous regulation 2. Chemical regulation 3. Mechanical regulation 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

CBF ABP ml / min mmHg Dr abdelaziz Hussein, Mansoura Faculty of Medicine

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

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  O 2 tension and  CO 2 and H +  V.C of cerebral vessels   CBF back to its normal level. b) On the other hand decreased ABP leads to: Local  O 2 tension and  CO 2 and H +  V.D of cerebral vessels   CBF back to its normal level 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 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 ++ 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

b. Parasympathetic++ → no role in regulation of CBF 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

a) Blood viscosity 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

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

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 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 ii) During acceleration downwards (-ve gravity) Opposite occurs Dr abdelaziz Hussein, Mansoura Faculty of Medicine

1. Cerebral circulation 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 2. Glucose is the major source of energy in the brain and sometimes amino acids during starvation. 3. Brain is very sensitive 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

◊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

◊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

◊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