Cerebral Circulation Physiology Notes

Cerebral Circulation Introduction

Brain tissues need adequate blood supply continuously. Stoppage of blood flow for 5 seconds leads to unconsciousness, and for 5 minutes leads to irreparable damage to the brain cells.

Cerebral Vessels And Normal Cerebral Blood Flow

The brain receives blood from the basilar artery and internal carotid artery. The branches from these arteries form a circle of Willis. The venous drainage is by sinuses, which open into the internal jugular vein.

Normally, the brain receives 750-800 mL of blood per minute. It is about 15-16% of total cardiac output and about 50-55 mL/100 grams of brain tissue per minute.

Read And Learn More: Medical Physiology Notes

Measurement Of Cerebral Blood Flow

1. Ketty And Schmidt’S Nitrous Oxide Method of Cerebral Blood Flow

• It is an indirect method to measure the blood flow to the brain. It is based on Fick’s principle. Nitrous oxide is used as an indicator substance in this method.
• The subject is asked to inhale nitrous oxide at a low concentration, which is less than the amount required for anesthesia.
• After inhalation of the gas for about 10 minutes, the amount of nitrous oxide retained in the brain tissues becomes equal to the amount of nitrous oxide present in cerebral venous blood.
• Now, the concentration of nitrous oxide is determined in the arterial blood and cerebral venous blood, and, the cerebral blood flow is calculated by the formula:
• Cerebral blood = Amount of N2Q taken by brain Arteriovenous difference of N20

2. By Using Radioactive Substances of Cerebral Blood Flow

• This method is used to determine the amount of blood flow to different regions of the cerebral cortex.
• The radioactive substance is injected into the carotid artery. By measuring the radioactivity in the brain tissues using radioactive detectors (scintillation counter), the blood flowing through each area of the brain is determined.
• The advantage of this method is that the blood flow to about 250 areas of the cerebral cortex can be measured by using many radioactive detectors.
• Radioactive xenon and 2-deoxy glucose are the commonly used radioactive substances to measure cerebral blood flow.

3. By Computerized Axial Tomography (CAT) of Cerebral Blood Flow

• Computerized axial tomography (CT or CAT) scanning was introduced in the 1970s.
• Tomography scanning is a process that combines many two-dimensional X-ray images to generate cross-sectional pictures of different organs or regions of the body.
• Advancements in technology resulted in a combination of many three-dimensional X-ray images of body structures and organs including the brain, CT scan of the brain is useful to determine brain damage and local changes in cerebral blood flow while the subject performs a task.

4. By Positron Emission Tomography (Pet) of Cerebral Blood Flow

• A positron emission tomography (PET) scanner is a type of computerized tomography machine.
• A short-lived radioactive substance called radionuclide combined with sugar is injected into the patient.
• The radionuclide emits positrons (antiparticle or antimatter counterpart of electron).
• The positron emissions from radionuclides are detected by rotating the PET scanner around the patient’s head.
• PET is used to study blood volume, oxygen consumption, pH, glucose utilization, blood flow, and the activity of receptors in brain cells.

5. By Magnetic Resonance Imaging (MRI) of Cerebral Blood Flow

• Magnetic resonance imaging (MRI) is a different type of imaging technique. It involves the polarization of hydrogen atoms in the soft tissues by using a large magnet and detecting the resonant signals (summation of the spinning energies within the living cells) from the tissues.
• Since the images are very clear, this technique is useful for scanning soft tissues, brain, spinal cord, abdomen, joints, and malignant tissues. MRI is also used to measure blood flow to the organs such as the brain.
• The measurement of blood flow to a part or area of the organ is called functional magnetic resonance imaging (-MRI).

Regulation Of Cerebral Blood Flow

Cerebral circulation is regulated by three factors:

1. Autoregulation
2. Chemical factors
3. Neural factors

1.  Cerebral Blood Flow Autoregulation

Like any other vital organ brain also regulates its own blood flow by means of autoregulation.

However, autoregulation in the brain has got its own limitations. It depends upon:

• Effective perfusion pressure
• Cerebral vascular resistance.
• Cerebral blood flow is directly proportional to the balance between effective perfusion pressure and vascular resistance in the brain.

Effective Perfusion Pressure

• Effective perfusion pressure is the balance between arterial blood pressure and venous pressure across the organ divided by resistance.
• Since venous pressure is zero in the brain, mean arterial blood pressure plays an important role in regulating cerebral blood flow.
• Autoregulation is possible in the brain if the mean arterial pressure is within the range of 60 mm Hg and 140 mm Hg. Autoregulation fails beyond this range.

Cerebral Vascular Resistance

• Resistance to blood flow in the brain is offered by intracranial pressure, cerebrospinal fluid (pressure and viscosity of blood. When the vascular resistance is more, the blood flow to the brain is less.
• Intracranial pressure and cerebrospinal fluid pressure
• The increase in the intracranial pressure or the pressure exerted by the cerebrospinal fluid (CSF) compresses the cerebral blood vessels and decreases blood flow.
• These pressures are elevated in conditions like a head injury. However, severe ischemic effects are avoided by some protective reflexes such as the Cushing reflex.

Cerebral Blood Flow Cushing reflex

• The Cushing reflex is a protective reflex that helps save the brain tissues from ischemic effects during periods of reduced cerebral blood flow. It is also called Cusnng reaction, response, or phenomenon.
• The increase in the intracranial pressure or increase in CSF pressure compresses the cerebral blood vessels and decreases the blood flow. However, the blood flow is decreased only for a short period.
• It is restored immediately by means of the Cushing reflex. When cerebral blood flow decreases by the compression of cerebral arteries, cerebral ischemia develops.
• Compression of blood vessels decreases the blood flow to the vasomotor center also.
• The local hypoxia and hypercapnia activate the vasomotor center resulting in peripheral vasoconstriction and a rise in arterial pressure.
• The increased arterial pressure helps to restore the cerebral blood flow. Thus, the Cushing reflex plays the most important role in maintaining cerebral blood flow.
• The Cushing reflex operates only when the rise in arterial blood pressure is proportional to an increase in intracranial pressure.
• When the increase in intracranial pressure is very high and if it exceeds the arterial blood pressure, this protective mechanism fails. And the cerebral ischemia becomes severe leading to irreversible damage of the brain tissues.

Cerebral Blood Flow Monro-Kellie doctrine

According to the Monro-Kellie doctrine or principle, though the cerebral arteries are compressed by the increased intracranial pressure or the cerebrospinal fluid pressure, the volume of the brain tissue is not affected. It is because the brain tissue is not compressible.

Cerebral Blood Flow Viscosity

• An increase in the viscosity of blood as in polycythemia increases the cerebral vascular resistance and blood flow decreases.
• When viscosity decreases as in the case of anemia, the resistance is decreased, and blood flow increases.
• Thus, the cerebral blood flow is inversely proportional to the viscosity of blood.

2. Cerebral Blood Flow Chemical Factors

The chemical factors, which increase cerebral blood flow are:

1. Decreased oxygen tension
2. Increased carbon dioxide tension
3. Increased hydrogen ion concentration.

Carbon dioxide is the most important factor as it causes dilatation of cerebral blood vessels leading to an increase in blood flow.

• A moderate increase in carbon dioxide tension does not alter the blood flow due to autoregulation.
• When arterial partial pressure of carbon dioxide rises above 45 mm Hg, cerebral blood flow increases.
• Carbon dioxide combines with water to form carbonic acid, which dissociates into bicarbonate ions and hydrogen ions.
• The hydrogen ion causes the dilatation of blood vessels in the brain.
• Hypoxia increases cerebral blood flow by vasodilatation.

3. Cerebral Blood Flow Nervous Factors

• The blood vessels of the brain are supplied by sympathetic vasoconstrictor fibers.
• But, these fibers do not play any role in regulating cerebral blood flow under normal conditions.
• In pathological conditions like hypertension, the sympathetic nerves cause constriction of cerebral blood vessels leading to a reduction in blood flow.
• It prevents cerebral vascular hemorrhage and cerebral stroke.

Applied Physiology – Stroke

Stroke Definition

• Stroke is the sudden death of neurons in localized areas of the brain due to inadequate blood supply.
• It is characterized by reversible or irreversible paralysis with other symptoms. A stroke is also called a cardiovascular accident (CVA) or brain attack.

Stroke Types

Stroke is classified into two types:

• Ischemic stroke occurs due to interruption of blood flow to a part of the brain by thrombus or atherosclerotic embolus
• Hemorrhagic stroke develops by the rupture of a blood vessel in the brain and spilling of blood into the surrounding areas.

Stroke Causes

Most common factors (risk factors) which lead to stroke:

1. Heart disease
2. Hypertension
3. High cholesterol in the blood
4. High blood sugar – diabetes mellitus
5. Heavy smoking
6. Heavy alcohol consumption.

Stroke Symptoms

Symptoms of stroke depend upon the area of the brain that is damaged. Generally, stroke causes dizziness, loss of consciousness, coma, or death.

Other features of stroke are:

1. Weakness
2. Numbness or paralysis particularly on one side of the body
3. Impairment of speech
4. Emotional difficulties
5. Loss of coordination
6. Loss of memory.