M04.03.14 Cerebral circulation

Cerebral circulation refers to the movement of blood through the network of cerebral arteries and veins supplying the brain. Proper cerebral circulation is essential for delivering oxygen and nutrients to the brain and removing metabolic waste products. This note provides an overview of the anatomical, physiological, and regulatory aspects of cerebral circulation, along with key points for medical students.

Anatomy of Cerebral Circulation

Arterial Supply

• Internal Carotid Arteries: These arteries bifurcate into the anterior and middle cerebral arteries.
• Vertebral Arteries: These merge to form the basilar artery, which subsequently bifurcates into the posterior cerebral arteries.
• Circle of Willis: A ring-like arterial structure providing collateral blood flow between the anterior and posterior circulations.

Venous Drainage

• Superficial Venous System: Includes veins such as the superior sagittal sinus, which drains superficial cortical regions.
• Deep Venous System: Includes the internal cerebral veins, draining deeper structures like the basal ganglia and thalamus.

Physiology of Cerebral Circulation

Blood-Brain Barrier (BBB)

• Structure: Composed of endothelial cells, astrocyte end-feet, and pericytes.
• Function: Regulates the passage of substances from the blood to the brain, protecting neural tissue from toxins and pathogens.

Cerebral Blood Flow (CBF)

• Autoregulation: Maintains constant CBF despite fluctuations in systemic blood pressure.
• Mechanisms: Myogenic, metabolic, and neurogenic responses adjust vessel diameter.

Regulation of Cerebral Circulation

Autoregulation

• Range: Effective between mean arterial pressures of 60-150 mmHg.
• Mechanism: Involves smooth muscle contraction/relaxation in response to changes in transmural pressure.

Metabolic Factors

• Carbon Dioxide (CO2): Increased CO2 (hypercapnia) leads to vasodilation.
• Oxygen (O2): Decreased O2 (hypoxia) causes vasodilation to increase blood flow.

Neural Control

• Sympathetic Nervous System: Generally causes vasoconstriction.
• Parasympathetic Nervous System: Limited direct effect on cerebral vessels but can influence overall cardiovascular status.

Clinical Relevance

Stroke

• Ischemic Stroke: Caused by blockage in cerebral arteries.
• Hemorrhagic Stroke: Caused by rupture of cerebral vessels.

Transient Ischemic Attack (TIA)

• Description: Temporary blockage of blood flow to the brain.
• Significance: Often a warning sign for potential future strokes.

Key Points to Remember

• The Circle of Willis provides collateral circulation, which is crucial for maintaining cerebral perfusion in case of arterial blockage.
• Autoregulation is vital for maintaining stable CBF despite changes in systemic blood pressure.
• The Blood-Brain Barrier is essential for protecting the brain from harmful substances while allowing necessary nutrients to pass through.
• CO2 levels are a major regulator of cerebral vessel diameter, highlighting the importance of respiratory function in cerebral circulation.
• Clinical conditions such as stroke and TIA are directly related to disruptions in cerebral circulation and require immediate medical attention.

Bibliography

1. Adams, R.D., Victor, M., & Ropper, A.H. (2001). Principles of Neurology. 7th ed. McGraw-Hill.
2. Moore, K.L., Dalley, A.F., & Agur, A.M.R. (2017). Clinically Oriented Anatomy. 8th ed. Lippincott Williams & Wilkins.
3. Guyton, A.C., & Hall, J.E. (2011). Textbook of Medical Physiology. 12th ed. Saunders Elsevier.
4. Sacco, R.L., Kasner, S.E., Broderick, J.P., et al. (2013). An updated definition of stroke for the 21st century. Stroke, 44(7), 2064-2089.
5. Koehler, R.C., Roman, R.J., & Harder, D.R. (2009). Astrocytes and the regulation of cerebral blood flow. Trends in Neurosciences, 32(3), 160-169.