About the brain

The brain is the most complex organ in the body. It coordinates movement, sensation, language, memory, and thought, while also regulating unconscious functions such as breathing, heart rate, and sleep. Although it makes up only about 2% of body weight, the brain uses around 20% of the body’s oxygen and energy supply.

To meet these demands, the brain relies on a dense and highly specialised network of blood vessels. These arteries and veins are arranged to deliver oxygen-rich blood to different regions of the brain and to remove waste products efficiently. Even short interruptions in this blood supply can cause damage to brain cells, which is why vascular problems such as stroke, aneurysms, or intracranial haemorrhage can have rapid and severe consequences.

The surface of the brain is folded into ridges (gyri) and grooves (sulci), which allow a large surface area to fit within the skull. Different regions are specialised:

• The cerebral hemispheres control higher functions like speech, movement, sensation, reasoning, and memory.
• The cerebellum, at the back of the brain, fine-tunes balance and coordination.
• The brainstem, at the base, connects the brain to the spinal cord and regulates vital life functions.

All of these regions depend on a delicate balance between structure and circulation. The protective layers (meninges), the fluid spaces (ventricles and subarachnoid space), and the branching arterial system together form an environment that supports healthy brain activity but is vulnerable to disruption when disease affects the vessels.

Brain Coverings and Fluid

• Meninges – Three thin layers of tissue that cover and protect the brain and spinal cord (dura, arachnoid, and pia mater).
• Subarachnoid space – The space between these layers where clear fluid flows.
• Cerebrospinal fluid (CSF) – A cushioning fluid that surrounds the brain and spinal cord, helping to protect them from injury.
• Ventricles – Fluid-filled spaces inside the brain where CSF is made and circulated.

Blood Supply to the Brain

The brain receives blood through four main arteries in the neck:

• Carotid arteries (front of the neck) – Carry blood to the front and middle parts of the brain.
• Vertebral arteries (back of the neck) – Join together to form the basilar artery, supplying the back of the brain.

Together, these arteries form a ring at the base of the brain called the Circle of Willis, which helps provide an alternative route for blood flow if one vessel is narrowed or blocked.

Intracranial Arteries – The Brain’s Blood Highways

Once blood reaches the base of the brain it travels along smaller arteries that spread across the brain’s surface and deep into its tissues. These are known as the intracranial arteries.

The main groups are:

• Anterior cerebral arteries (ACA)

  • Branch from the carotid arteries.
  • Supply blood to the front and inner parts of the brain, including areas that control movement and sensation of the legs.

• Middle cerebral arteries (MCA)

  • The largest branches of the carotid arteries.
  • Provide blood to much of the outer surface of the brain, especially areas involved in speech, movement, sensation of the face and arms, and vision.
  • Blockage here is the most common cause of major stroke.

• Posterior cerebral arteries (PCA)

  • Arise from the back circulation (vertebral–basilar system).
  • Supply the back of the brain, especially the occipital lobes, which control vision, and parts of the temporal lobes, which help with memory.

• Basilar artery

  • Formed by the joining of the two vertebral arteries.
  • Supplies the brainstem and cerebellum—vital for balance, coordination, and life-sustaining functions like breathing.

These arteries are linked together by small connecting vessels, the anterior and posterior communicating arteries, that form the Circle of Willis. This network helps protect the brain: if one pathway becomes narrowed or blocked, blood can sometimes still reach the affected area through alternative routes.