CEREBRAL CIRCULATION

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SALIENT FEATURES REGULATION

INTRODUCTION 15% of cardiac output

10 seconds of interruption in blood flow leads to unconsciousness

Most neurologic disorders are due to vascular lesions

SALIENT FEATURES

Posterior communicating artery (part of the Circle of Willis)o Allow cross-over flow during

impairment of blood flow

http://www.youtube.com/watch?v=MPcO2ibO75o

SALIENT FEATURES brain capillaries are surrounded by the

endfeet of astrocytes closely applied to the basal lamina of

the capillaries, do not cover the entire capillary wall gaps of about 20 nm occur between

endfeet induce the tight junctions in the

capillaries

REGULATION controlled almost entirely by local metabolites exhibits autoregulation

o between arterial pressure limits of 60 and 140 mm Hg. many circulating vasoactive substances do not affect the

cerebral circulation because their large molecular size prevents them from crossing the blood-brain barrier.

REGULATION

INTRACRANIAL PRESSURE & CEREBRAL BLOOD FLOW

Intracranial pressure(ICP) rises → Cerebral vessels compressed

Change in venous pressure → a similar change in ICP Rise in venous pressure decreases cerebral blood

flow by o decreasing the effective perfusion pressure o compressing the cerebral vessels

helps to compensate for changes in arterial blood pressure at the level of the heado the body is accelerated upward, blood moves toward the feet &

arterial pressure at the level of the head decreases. o However, venous pressure also falls and intracranial pressure fallso so that the pressure on the vessels decreases and blood flow is

much less severely compromised

Monro–Kellie doctrine

CARBON DIOXIDE & HYDROGEN

CONCENTRATION most important local vasodilators

o ↑ in cerebral Pco2 o formation of carbonic acid →dissociation → H+ o ↑ in H+ concentration = ↓ in pHo causes vasodilation of the cerebral arterioleso results in an ↑ in blood flow to remove the

excess CO2.

Any other substance that ↑ the acidity of the brain tissue

↑ hydrogen ion concentration ↑ cerebral blood flow

o lactic acido pyruvic acido other acidic material formed during the course of tissue

metabolism.

IMPORTANCE

By ↑ the blood flowo Remove hydrogen ions, carbon dioxide, and other acid-

forming

Maintain constant hydrogen ion concentration in the cerebral fluids o maintain a normal, constant level of neuronal activity.

↑ hydrogen ion concentration greatly depresses neuronal activity.

OXYGEN CONCENTRATION

Oxygen deficiency (hypoxia) mechanism → vasodilation o Hypoxia-induced drop in ATP open  KATP channels on smooth muscleo  causing hyperpolarization and vasodilationo returning the brain blood flow o transport of oxygen to the cerebral tissues to near normal.

mechanism is almost exactly the same in the brain as in o coronary blood vessels, in skeletal muscle and in most other

circulatory areas of the body

ROLE OF THE SYMPATHETIC NERVOUS

SYSTEM Strong sympathetic innervation from the superior cervical sympathetic

ganglia in the neck & then into the brain along with the cerebral arteries.

supplies both the large brain arteries and the arteries that penetrate into the substance of the brain.

Autoregulation mechanism can override the nervous effects.

*mean arterial pressure rises acutely to an exceptionally high levelo the sympathetic nervous system

constricts the large- and intermediate-sized brain arteries

o prevent the high pressure from reaching smaller brain blood vessels

o prevent cerebral stroke

REFERANCE Textbook of Medical Physiology Eleventh Edition

o By Arthur C. Guyton and John E. Hall

Ganong’s Review of Medical Physiology 23rd editiono By Kim E. Barrett, Susan M. Barman, Scott Boitano and

Heddwen L. Brooks

Control of Cerebral Blood Flowo http://www.ncbi.nlm.nih.gov/books/NBK53082/

The End