Cerebral Blood Flow regulation

Learning Objectives

By the end of this lectures all students should be able to:

• Describe the cerebral blood flow auto regulation that protect the brain from changes in arterial pressure.

• Describe the effect of carbondioxide, hydrogen ions and oxygen on cerebral blood flow.

Cerebral Blood Flow

• Normal blood flow through

the brain of the adult

person averages 50 to 65

milliliters per 100 grams of

brain tissue per minute.

• 15 % of the resting cardiac

output.

Regulation of Cerebral Blood Flow

Three metabolic factors have potent effects in

controlling cerebral blood flow :

1. Carbon dioxide concentration

2. Hydrogen ion con centration

3. Oxygen concentration

Cerebral Blood Flow

Excess Carbon Dioxide & Hydrogen Ions:

• Increase Carbon dioxide concentration in arterial blood, perfusing the brain greatly increases cerebral blood flow. 70 per cent increase in arterial PCO2 approximately doubles

cerebral blood flow.

• Carbon dioxide combines first with Water in the body fluids

to form Carbonic acid

• Subsequent dissociation of this carbonic acid gives rise to

Hydrogen ions.

• The Hydrogen ions then cause vasodilatation of the cerebral

vessels. THUS INCREASING THE CBF.

Substances Increasing H-ion Conc.

• Lactic acid.

• Pyruvic acid.

• Acidic material formed during the course of

tissue metabolism.

Importance of Cerebral Blood Flow Control by Carbon Dioxide

and Hydrogen Ions.

• H- ion concentration greatly depresses neuronal

activity. But, it also increase the blood flow which

inturn carry the following away from the brain:

• Hydrogen ions.

• Carbon dioxide.

• Acid forming substances.

Loss Of Carbon Dioxide

• Loss of carbon dioxide removes carbonic acid and other acids from the tissues.

• H-ion conc. comes back to normal.

• This inturn , maintain the normal neuronal activity.

Increased CO2 and H+ ion concentration and oxygen defeciency cause vasodilationdepressed neuronal activity

CBF and CO2

• Carbon dioxide causes cerebral vasodilation. As the arterial tension of CO2 rises, CBV and CBF increases and when it is reduced vasoconstriction is induced.

Oxygen Deficiency as a Regulator of Cerebral Blood Flow

• Rate of utilization of oxygen by the brain tissue under normal conditions is almost exactly 3.5 (± 0.2) milliliters of oxygen per 100 grams of brain tissue per minute except, for intense period of brain activity.

• Decrease/ insufficient blood flow & Oxygen to brain

• Stimulates Oxygen Deficiency mechanism

• Vasodilation , restoring the blood flow & oxygen supply to brain.

Importance

• Value of P02 is 35 to 40 mm of Hg for normal brain

activity.

• Value below 30 mm Hg immediately begins to increase

cerebral blood flow by causing vasodilation.

• P02 levels below 20 mm Hg. lead to diminished cerebral

neuronal activity and derangement of mental capability.

Increasing CBF-Hyperemia

• Low arterial oxygen tension has profound effects on cerebral blood flow. When it falls below 50 mmHg (6.7 kPa), there is a rapid increase in CBF and arterial blood volume

Measurement of Cerebral Blood Flow, and Effect of Brain Activity

on the Flow

• A method has been developed to record blood flow in as many as 256 isolated segments of the human cerebral cortex simultaneously.

• A radioactive substance, such as radioactive xenon, is injected into the carotid artery

• Then the radioactivity of each segment of the cortex is recorded as the radioactive substance passes through the brain tissue.

• For this purpose, 256 small radioactive scintillation detectors are pressed against the surface of the cortex.

• The rapidity of rise and decay of radioactivity in each tissue segment is a direct measure of the rate of blood flow through that segment.

Events leading To Alteration In Cerebral Blood Flow:

• Making a fist of the hand.

• Reading a book.

• Epileptic attacks.

• Directing intense light into the eyes for sometime.

Autoregulation of Cerebral Blood Flow With Arterial Pressure Changes

Autoregulation

• CBF is maintained at a constant level in normal brain in the face of the usual fluctuations in blood pressure by the process of autoregulation.

• Normally autoregulation maintains a constant blood flow between CPP 50 mmHg and 150 mmHg.

• Cerebral blood flow is "autoregulated" extremely well between arterial pressure limits of 60 and 140 mm Hg.

• Below 60 mm of Hg of arterial pressure the cerebral pressure become severely decreased.

Metabolic Autoregulation

• Arterioles dilate in response to potent chemicals that are by-products of metabolism such as lactic acid, carbon dioxide and pyruvic acid.

• CO2 is a potent vasodilator • Increased CO2/decreased BP --> vasodilation • Decreased CO2/increased BP -->vasoconstriction

Neurogenic Autoregulation• Autonomic system and neurochemical

control of CBF in general is a minor control.•Metabolic autoregulation is most

important being majorly in control of CBF.

Pressure autoregulation

• Changes in cerebral perfusion pressure will be followed by changes in cerebral blood flow unless diameter regulation takes place.

• This type of autoregulation is termed pressure autoregulation and is the type of autoregulation referred to as autoregulation after head injury.

• The limits of pressure autoregulation range from 40 to 150 mm of mercury of perfusion pressure.

• Beyond these limits, vessel caliber follows flow passively leading to collapse of vessels at low pressure and forced dilatation or pressure breakthrough at high pressures.

Role of the Sympathetic Nervous System In Controlling Cerebral Blood Flow

• Sympathetic innervation of cerebral circulatory system passes upward from the superior cervical sympathetic ganglia in the neck and then into the brain along with the cerebral arteries.

Sympathatic innervation supply to the following:• Large brain arteries.• Arteries which penetrate the substance of the

brain.• Autoregulation mechanism is not effected by

the nervous supply so, transection or stimulation of it does not much effect the cerebral blood flow. Autoregulation is effected only slightly by the neural system.

Clinical Importance Of Sympathatic Supply:

• In response to increased mean arterial pressure due to excerise or other states of excessive circulatory activity.

Sympathatic innervation causes vasoconstriction

Cerebral Microcirculation

• The number of blood

capillaries in the brain is

greatest because of its

increase metabolic needs. • The overall metabolic rate

of the brain gray matter where the neuronal cell bodies lie is about four times as great as that of white matter

Characteristics Of Brain Capillaries

• Gray matter in the brain is the portion

containing the neuronal cells it has the

following features:

• Metabolic rate is high.

• Increase number of capilllaries.

• Rate of blood flow is also high.

• Capillaries are less leaky.

Clinical Importance

Walls Of The Small Arterioles:

• Arterioles leading to the brain capillaries

become greatly thickened in high blood

pressure, and these arterioles remain

significantly constricted all the time to prevent

transmission of the high pressure to the

capillaries.

• Capillaries are supported on all sides by "glial

feet," which are small projections from these

surrounding glial cells.

• These projections protect and provide physical

support to prevent overstretching of the

capillaries in case of high capillary blood

pressure.

Cerebral Stroke

• It is a condition in which there is blockage of

arteries of the brain that cause serious

disturbance of brain function it is called

"stroke."

Types Of Stroke

Ischemic Stroke Hemorrhagic Stroke

Ischemic Stroke:

It occurs due to interruption of blood flow to a part of brain by thrombus or artherosclerotic embolus.

Hemorrhagic Stroke

• It develops by the rupture of a blood vessel in the brain and spilling of blood into the surrounding area. This spillage causes pressure effects in the surrounding structures and thus depressing their respective functions.

Clinical Features Of Stroke

• Hemorrhage occurs , compressing the local brain tissue and further compressing its functions.

• The neurological effects of stroke are determined by the area of the brain affected.

Risk Factors for Stroke

• Heart Disease

• Hypertension

• High Cholesterol

• Diabetes

• Excessive smoking

• Heavy Consumption of Alcohol.

Most Common Arteries Effected In Stroke:

• Middle Cerebral Artery.

• Posterior Cerebral Artery.