Cerebral circulation

Cerebrovascular System

Cerebral vascular territories

Cerebral vascular territories midline

The ophthalmic artery and its branches.

The anterior and posterior circulations meet at the Circle of Willis, pictured here, which rests at the top of the brainstem

Cerebral Circulation

Cerebral circulation refers to the movement of blood through the network of cerebral arteries and veins that supply the brain. It plays a critical role in delivering essential nutrients and oxygen to the brain while removing metabolic waste products.

Overview

Adults typically have a cerebral blood flow rate of 750 milliliters per minute, accounting for approximately 15% of cardiac output. The arteries deliver essential elements like oxygenated blood, glucose, and other nutrients to the brain, whereas the veins remove waste such as carbon dioxide and lactic acid.

The brain's sensitivity to disruptions in its blood supply has led to the evolution of various safeguards, such as autoregulation of its blood vessels. Failure of these mechanisms can result in conditions like stroke. Rapid accelerations can also disrupt cerebral circulation, leading to severe conditions that could be life-threatening.

The provided overview focuses on an idealized human cerebral circulation, though the pattern and naming conventions might vary between species.

Blood Supply

Cortical Areas and Arterial Blood Supply

The brain's blood supply is typically divided into anterior and posterior segments, each relating to different sets of arteries supplying the brain:

• Internal carotid arteries: Supply the anterior portion of the brain.
• Vertebral arteries: Supply the brainstem and the posterior part of the brain.

These anterior and posterior circulations connect through bilateral posterior communicating arteries, forming the Circle of Willis. This circle acts as a backup, ensuring continuous blood flow even if one artery becomes blocked.

Anterior Cerebral Circulation

The anterior cerebral circulation delivers blood to the front part of the brain and includes:

• Internal carotid arteries
• Anterior cerebral artery (ACA)
• Anterior communicating artery
• Middle cerebral artery (MCA)

Posterior Cerebral Circulation

The posterior cerebral circulation provides blood to the back section of the brain. The main arteries include:

• Vertebral arteries
• Posterior inferior cerebellar artery (PICA)
• Basilar artery
• Anterior inferior cerebellar artery (AICA)
• Pontine branches
• Superior cerebellar artery (SCA)
• Posterior cerebral artery (PCA)
• Posterior communicating artery

Venous Drainage

The brain's venous drainage system comprises:

• Superficial system: Made up of dural venous sinuses located on the brain's surface.
• Deep system: Consists of traditional veins inside the brain's deeper structures.

Physiology

Cerebral blood flow (CBF) denotes the blood volume supplied to the brain over a given time. This blood flow is meticulously regulated to ensure the brain's metabolic needs are met while preventing conditions like hyperemia or ischemia, both of which can be harmful to the brain tissue. Several factors, such as blood viscosity, dilation of blood vessels, and cerebral perfusion pressure (CPP), influence CBF.

CPP is the pressure gradient driving blood flow to the brain, and is calculated using the formula:

\[ \text{CBF} = \frac{\text{CPP}}{\text{CVR}} \]

Where CVR represents cerebrovascular resistance. CPP and CVR control is multifaceted, involving metabolic control, pressure autoregulation, chemical control, and neural control.

Role of Intracranial Pressure

Increased intracranial pressure (ICP) can reduce blood flow to brain cells, leading to inadequate perfusion or elevated pressure within the skull.

Imaging

Techniques like arterial spin labelling and positron emission tomography help in measuring CBF. These are also vital for determining regional CBF within specific brain areas.

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