Cerebral circulation

cardiac

1. Cerebral Circulation Maria idrees; PT

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

3. CEREBRAL VESSELS AND NORMAL CEREBRAL BLOOD FLOW Brain receives blood from the basilar artery and internal carotid artery. Branches of these arteries form circle of Willis. Venous drainage is by sinuses, which open into internal jugular vein. Normally, brain receives 750 to 800 mL of blood per minute. It is about 15% to 16% of total cardiac output and about 50 to 55 mL /100 g of brain tissue per minute.

4. REGULATION OF CEREBRAL BLOOD FLOW Cerebral circulation is regulated by three factors: Autoregulation Chemical factors Neural factors.

5. AUTOREGULATION Like any other vital organ, brain also regulates its own blood flow by means of autoregulation . However, the autoregulation in brain has got its own limitations. It depends upon: i . Effective perfusion pressure ii. Cerebral vascular resistance.

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

7. ii. Cerebral Vascular Resistance When the vascular resistance is more, the blood flow to the brain is less. Resistance to blood flow in brain is offered by intracranial pressure, cerebrospinal fluid pressure and viscosity of blood. Intracranial pressure and cerebrospinal fluid pressure 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 head injury. However, severe ischemic effects are avoided by some protective reflexes such as Cushing reflex.

8. Cushing reflex

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

10. Viscosity 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.

11. CHEMICAL FACTORS Chemical factors which increase the cerebral blood flow: Decreased oxygen tension Increased carbon dioxide tension Increased hydrogen ion concentration

12. Carbon dioxide is the most important factor, as it causes dilatation of cerebral blood vessels, leading to 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, the cerebral blood flow increases. Carbon dioxide combines with water to form carbonic acid, which dissociates into bicarbonate ions and hydrogen ion. The hydrogen ion causes dilatation of blood vessels in brain. Hypoxia increases cerebral blood flow by vasodilatation.

13. NERVOUS FACTORS Cerebral blood vessels 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 reduction in blood flow. It prevents cerebral vascular hemorrhage and cerebral stroke.