Makindo Medical Notes

Introduction

• The brain receives its blood supply from two main paired arteries: the internal carotid arteries and the vertebral arteries.
• These arteries form a circular anastomosis at the base of the brain known as the Circle of Willis.
• From the Circle of Willis, major arteries branch out to supply different regions of the brain.

Collateral Circulation and Significance

• The Circle of Willis plays a critical role in collateral circulation, ensuring that if one major artery is blocked or narrowed, blood flow can still reach the affected areas through alternative routes.
• This is particularly important in cases of stroke or vascular disease.

Primary Arteries

• Internal Carotid Arteries (ICA): The ICAs arise from the common carotid arteries and supply the anterior circulation of the brain, including parts of the frontal, parietal, and temporal lobes.
• Vertebral Arteries: The vertebral arteries arise from the subclavian arteries and converge to form the basilar artery. They supply the posterior circulation, which includes the brainstem, cerebellum, and occipital lobes.

Circle of Willis

The Circle of Willis is a ring-like arterial structure located at the base of the brain, providing collateral blood flow between the anterior and posterior circulations. It consists of the following components:

• Anterior Cerebral Arteries (ACA): These arteries supply the medial portions of the frontal and parietal lobes. They arise from the internal carotid arteries and are connected by the anterior communicating artery.
• Anterior Communicating Artery: A small artery that connects the two anterior cerebral arteries and allows communication between the left and right side of the brain.
• Middle Cerebral Arteries (MCA): Branching from the internal carotid arteries, the MCA supplies the lateral portions of the frontal, temporal, and parietal lobes. It is the most commonly affected artery in strokes.
• Posterior Cerebral Arteries (PCA): These arise from the basilar artery and supply the occipital lobes, inferior temporal lobes, thalamus, and parts of the brainstem.
• Posterior Communicating Arteries: These arteries connect the posterior cerebral arteries to the internal carotid arteries, forming part of the Circle of Willis and ensuring collateral circulation.
• Basilar Artery: Formed by the union of the two vertebral arteries, the basilar artery supplies the brainstem, cerebellum, and parts of the occipital and temporal lobes.

Carotid supplies Anterior Circulation

The anterior circulation of the brain is primarily supplied by the internal carotid arteries, which give rise to several branches:

• Ophthalmic Artery: Supplies the eyes and orbital structures.
• Anterior Cerebral Artery: Supplies the medial surfaces of the cerebral hemispheres, particularly the frontal and parietal lobes, as well as parts of the corpus callosum and cingulate gyrus.
• Middle Cerebral Artery: Supplies the lateral aspects of the frontal, temporal, and parietal lobes. It also supplies critical areas involved in motor and sensory function, language, and cognition.

Vertebral Supplies Posterior Circulation

The posterior circulation is primarily supplied by the vertebral and basilar arteries, providing blood to the brainstem, cerebellum, and occipital lobes:

• Vertebral Arteries: These arteries supply the spinal cord, brainstem, and posterior parts of the cerebellum before converging to form the basilar artery.
• Basilar Artery: This artery runs along the ventral surface of the brainstem, giving rise to branches that supply the cerebellum, pons, and midbrain.
• Posterior Cerebral Arteries: These arteries branch from the basilar artery to supply the occipital lobes, parts of the temporal lobes, and the thalamus.

Arterial

Venous Drainage is to the internal jugular vein

The venous drainage of the brain occurs through a network of veins and dural venous sinuses. The major sinuses include:

• Superior Sagittal Sinus: Runs along the top of the falx cerebri and drains into the transverse sinus.
• Inferior Sagittal Sinus: Runs along the lower edge of the falx cerebri and drains into the straight sinus.
• Transverse Sinuses: Run laterally from the confluence of sinuses and drain into the sigmoid sinuses.
• Sigmoid Sinuses: These sinuses curve downward to drain into the internal jugular veins, which carry blood back to the heart.

Syndromes