c 61 cerebral blood flow

8/6/2019 C 61 Cerebral Blood Flow

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C 61Cerebral Blood Flow

By

Prof. Dr. Abdul Majid

MBBS, M.Phil, FCPS


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Cerebral Blood Flow

Blood flow to the brain is supplied by

four large arteries-two carotid & two

vertebral arteries-which merge toform a circle of Willis at the base of

the brain. The arteries arising from

the circle of Willis travel along the

brain surface & give rise to pialarteries, which branch out into


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smaller vessels called penetrating

arteries & arterioles. The penetrating

vessels are separated slightly fromthe brain tissue by an extension of

the subarachnoid space called the

Virchow-Robin space. Finally

arterioles give rise to capillaries.


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Normal rate of cerebral blood

flow:

Normal blood flow through the brainof the adult person averages 50 to

65ml/100g/min or 750 to 900ml/min

or 15% of the resting cardiac output.


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Regulation of cerebral bloodflow:

As in other tissues of the body it ishighly related to metabolism of thetissue. Several metabolic factorsare believed to contribute to

cerebral blood flow regulation;1. Carbon dioxide concentration.

2. Hydrogen ion concentration .


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3. Oxygen concentration.

4. Substances released from

astrocytes.Role of CO2:

Increase in CO2 in the arterial

blood increases cerebral blood flowby combining first with water in the

body fluids to form carbonic acid,


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with subsequent dissociation of this

acid to form hydrogen ions. The

hydrogen ions then causevasodilatation of the cerebral

vessels. Other substances that

increase the acidity of the brain

tissue include lactic acid, pyruvic &any acidic material formed during the

course of tissue metabolism.


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Increased hydrogen ion

concentration greatly depresses

neuronal activity. Therefore, it is

fortunate, that increased hydrogen

ion concentration increases cerebral

blood flow also.


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Role of oxygen deficiency:

Whenever, there is decreased blood

flow to the brain, decreased oxygensupply immediately causes

vasodilatation.

Role of substances released from

astrocytes:

Astrocytes surround the blood

vessels of the CNS.


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Astrocytes are star shaped non-

neuronal cells that support & protect

as well as provide nutrition. The

substances released from astrocytes

may be nitric oxide, metabolites of

arachidonic acid, potassium ions &

adenosine.


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Role of auto regulation:

During normal daily activities, arterial

pressure can fluctuate widely, risingto high levels during states of

excitement or strenuous activity &

falling to low levels during sleep.

However, cerebral blood flow is autoregulated b/w arterial pressure limits


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of 60 & 140mmHg. (mean ABP) in

normal persons. In hypertensive

cases upper limit is 160 to

180mmHg. But, ifABP decreases

below 60mmHg, cerebral blood flow

becomes severely decreased.


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Role of sympathetic nervous

system:

It prevents vascular hemorrhagesinto the brain by constriction of large

& intermediate sized brain arteries.


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Cerebral stroke occurs when

cerebral blood vesicles are block

or ruptured:

Almost all elderly people have

blockage of some small arteries in

the brain.

Most strokes are caused by

arteriosclerotic plaques that occur

in one are more of the feeder

arteries to the brain.


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The plaques can activate the clotting

mechanism of the blood, causing the

blood clot to occur & block blood

flow in the artery, there by leading to

acute loss of brain function in a

localized area.


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Cerebrospinal Fluid System

The cerebrospinal fluid volume is

150ml & it is formed at the rate of

500 to 550ml/day.

This fluid is present in the ventricles

of the brain, in the cisterns around

the out side of the brain, & in the

subarachnoid space around both thebrain & the spinal cord.


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All these chambers are connected

with one another & the pressure of

the fluid is maintained at a constant

level. Average pressure is 130mm of

water, ranging from 70 to 180mm of

water.

Functions:

A major function of the CSF is to

cushion the brain with in its solid

vault.


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The brain & CSF have about the

same specific gravity so the brain

floats in the fluid. Therefore, if a blow

to the head is not to intense, moves

the brain simultaneously with the

skull, causing no one portion of the

brain to the contorted by the blow.


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Countercoup:

When a blow to the head is

extremely severe, it may notdamage the brain on the side of the

head where blow is struck but on the

opposite side. This phenomenon is

called countercoup.

If contusion occurs on the same side

as the impact injury, it is called coup

injury.


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Formation, flow & absorption of

CSF:

About 2/

3rd

or more of this fluidoriginates as secretion from the

choroid plexuses in the four

ventricles, mainly in the two lateral

ventricles.


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Additional small amounts of fluid are

secreted by ependymal surfaces of

all the ventricles & by the

arachnoidal membranes.

A small amount comes from the

brain itself through the prevescular

spaces that surround the bloodvessels passing through the brain.


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Concentration of Various Substances

in Human CSF & Plasma


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Flow:

The fluid secreted in the lateralventricles passes first into the 3rd

ventricle.

It flows down along the aqueductsylvius into the 4th ventricle.

Finally it passes out of the 4thventricle through three smallopenings, two lateral foramina ofLuschka & a midline foramen of

Magendie,


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entering the cisterna magna, a fluid

space behind the medulla & beneath

the cerebellum.

The cisterna magna is continuous

with the subarachnoid space that

surrounds the entire brain & spinal

cord.


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Than the fluid moves upwards from

the cisterna magna through the

subarachnoid spaces surrounding

the cerebellum. From here it is

absorbed by arachnidal villi which

project into large sagital sinus &

other venous sinuses of thecerebellum. These villi act as valves.


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In the brain parevescular spaces act aslymphatics.

Causes of high CSF pressure:

1. Brain tumor.2. Hemorrhage or infection that occurs in

the cranial vault.

3. Decrease number of arachnoid villi or

abnormal absorptive properties ofarachnoid villi.


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Measurement of CSF pressure:

The person lies exactly horizontally

on his or her side so that the fluidpressure in the spinal canal is

equal to the pressure in the cranial

vault. A spinal needle is than

inserted into the lumber spinalcanal blow the lower end of the


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cord & the needle is connected to a

vertical glass tube that is open to the

air at its stop. The spinal fluid is

allowed to rise in the tube as high as

it will. If it rises to a level 136mm

above the level of the needle, the

pressure is set to be 136mm ofwater pressure


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or dividing this by 13.6, which is the

specific gravity of mercury, about

10mmHg pressure.

Raised CSF pressure causes edema

of optic disc called papilledema.

Hydrocephalus:

Hydrocephalus mean excess water

in the cranial vault.


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Types:

1. Communicated type (External

Hydrocephalus).

2. Non-communicated type (Internal

Hydrocephalus).

Communicated type: The

communicated type ofhydrocephalus is usually caused


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by blockage of fluid flow in the

subarachnoid spaces around the

basal regions of the brain or by

blockage of the arachnoid villi.

Non-communicated type: The

non-communicated type is usually

caused by block in the aqueduct ofsylvius or by the blockage of

foramina of Magendie & Luschka .


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Blood CSF & blood brain barriers:

The concentration of several

important constituents of the CSFare not the same as in ECF

elsewhere in the body. Many large

molecular substances hardly pass at

all from the blood into the CSF areinto interstitial fluids of the brain,

even though these same substances

pass readily into


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the usual interstitial fluids of the

body. Therefore, it is said that

barriers, called blood CSF barrier &

blood brain barrier exist b/w the

blood & the CSF & brain fluid

respectively.

In generally the blood CSF & bloodbrain barriers are highly permeable

to water, carbon dioxide, oxygen,


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& most lipid soluble substances such

as sodium, chloride, and potassium;

and almost totally impermeable to

plasma proteins and most non-lipid-

soluble large organic molecules.

Brain edema:

one of the most serious

complications of abnormal cerebral


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fluid dynamics is the development of

brain edema. Because the brain is

encased in a solid cranial vault,

accumulation of extra edema fluid

compresses blood vessels, often

causing seriously decreased blood

flow & destruction of brain tissue.


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Causes:

The usual cause of brain edema is

either greatly increase capillary

pressure or damage to capillary wallthat makes a wall leaky to fluid, a

common cause is serious blow to

the head leading to brainconcussion, in which the brain


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tissues & capillaries are traumatized

& capillary fluid leaks into the

traumatized tissues. Positive

feedback mechanism sets in leading

to further decreased blood flow,

dilatation of arterioles & damage to

sodium potassium pumps & edemaof cells.

c 61 cerebral blood flow

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