The Brain's Blood Supply

Four arteries, a safety ring called the circle of Willis, and an organ that can't store fuel: how the brain is fed, why blood flow follows activity, and what goes wrong in a stroke.

The brain is about 2% of body weight but receives around 15% of the heart's output — roughly 750 millilitres of blood per minute — and uses about 20% of the body's oxygen. Unlike muscle, it has almost no fuel reserves. Interrupt its blood supply and consciousness is lost within about 10 seconds; irreversible damage begins within minutes.

Four arteries and a ring

Blood reaches the brain through two internal carotid arteries at the front of the neck and two vertebral arteries that run up through the neck vertebrae and join to form the basilar artery. At the base of the brain these connect in a ring, the circle of Willis, which can redirect blood if one supply is narrowed. (In many people the circle is incomplete.)

From the circle branch the main cerebral arteries: the anterior cerebral artery (inner surfaces of the frontal and parietal lobes — the leg area), the middle cerebral artery (the outer surface, including face, arm, and in the left hemisphere, language areas — the most common site of stroke), and the posterior cerebral artery (occipital lobe and lower temporal lobe). Veins drain into large channels called dural venous sinuses.

Blood follows activity

When a brain region becomes more active, local blood flow increases within a few seconds — neurovascular coupling, orchestrated by neurons, astrocytes and the cells around blood vessels. The increase delivers more oxygenated blood than is used, and that change in blood oxygen is what functional MRI detects. The brain also keeps its overall blood flow steady despite changes in blood pressure, a process called autoregulation.

When supply fails

A blocked artery causes an ischaemic stroke; a burst vessel a haemorrhagic stroke. Ballooning weak spots in arteries (aneurysms), often near the circle of Willis, can rupture and bleed into the space around the brain — a subarachnoid haemorrhage, often felt as a sudden "worst headache of my life". The pattern of symptoms in a stroke tells doctors which artery is affected.