Four major arteries supply blood to your brain: two internal carotid arteries at the front and two vertebral arteries at the back. Together, these deliver roughly 50 milliliters of blood per 100 grams of brain tissue every minute, a substantial share of your heart’s total output despite the brain weighing only about 3 pounds. These four arteries connect at the base of the brain in a ring-shaped structure that provides backup circulation if one route becomes blocked.
The Two Front Arteries: Internal Carotids
The internal carotid arteries run up each side of your neck and enter the skull near your temples. They are responsible for the majority of the brain’s blood supply, feeding the large frontal, parietal, and temporal lobes where most of your thinking, movement, sensation, and language processing happen.
Once inside the skull, each internal carotid splits into two major branches. The middle cerebral artery is the larger of the two and supplies the outer surfaces of the brain, including the motor and sensory areas that control your face and arms. It also feeds Broca’s area (which helps you produce speech) and Wernicke’s area (which helps you understand language), both located on the dominant side of the brain, typically the left. The anterior cerebral artery curves over the top of the brain and supplies the strip of cortex responsible for sensation and movement in your legs.
Before splitting, each internal carotid also gives off a branch called the ophthalmic artery, which supplies your eyes, the optic nerve, and some muscles of the forehead and face. This is why severe carotid artery disease can sometimes cause sudden vision loss on one side.
The Two Back Arteries: Vertebrals and Basilar
The vertebral arteries take a more protected route, threading upward through small openings in the neck vertebrae before entering the skull through the opening at the base of the spine. Once inside, the two vertebral arteries merge into a single basilar artery that runs along the front of the brainstem.
This vertebral-basilar system supplies structures you depend on for basic survival and coordination: the brainstem (which controls breathing, heart rate, and consciousness), the cerebellum (which coordinates balance and fine movement), the thalamus (a relay station for nearly all sensory information), and the occipital lobes at the back of the brain (where visual processing occurs). The basilar artery ends by splitting into the two posterior cerebral arteries, which feed the occipital lobes and the inner surfaces of the temporal lobes. A blockage here can cause sudden vision loss, vertigo, difficulty swallowing, or loss of consciousness, depending on exactly which branch is affected.
The Circle of Willis: Built-In Backup
At the base of the brain, the front and back systems connect through a ring of arteries called the circle of Willis. Short communicating arteries link the anterior cerebral arteries to each other and connect the internal carotid system to the posterior cerebral arteries on each side. This creates a pentagon-shaped loop that can reroute blood if one of the four main supply arteries narrows or becomes blocked.
Think of it as a traffic roundabout. If one highway into the roundabout is closed, blood can still reach any exit through the remaining roads. In practice, though, this backup system is less reliable than it sounds. Anatomical variations in the circle of Willis are extremely common. Up to 21% of people have a structural variation in the front communicating artery alone, and many people have one or more segments that are unusually small or even absent. When the circle is incomplete, the brain has less ability to compensate during a blockage, which raises stroke risk.
Small Perforating Arteries Feed Deep Structures
The major arteries at the brain’s surface send tiny branches straight down into the deeper tissue. Among the most important are the lenticulostriate arteries, small perforating vessels that branch off the middle cerebral artery and supply the basal ganglia and internal capsule, deep structures involved in movement control and the relay of signals between the brain and body. The putamen, a key part of the basal ganglia, is mainly fed by the outer (lateral) group of these tiny arteries.
These small vessels are clinically significant because they are common sites for two dangerous events. High blood pressure can weaken their walls over time, leading to deep brain hemorrhages. They can also become blocked, causing small “lacunar” strokes that affect movement, sensation, or coordination in very specific ways. Because these arteries are end vessels with no backup connections, even a tiny blockage can cause permanent damage to the tissue they supply.
How the Brain Controls Its Own Blood Flow
Your brain has a built-in pressure regulation system called autoregulation. Within a certain range of blood pressure, the brain’s small arteries automatically widen or narrow to keep blood flow steady. The classic teaching placed this safe range at a mean arterial pressure of roughly 60 to 150 mmHg, but more recent research suggests the effective range is narrower than previously believed, and the brain is better at compensating when pressure rises than when it drops. This means sudden drops in blood pressure, from dehydration, blood loss, or standing up too quickly, can reduce brain blood flow faster than you might expect.
The Blood-Brain Barrier
Brain capillaries are structurally different from capillaries elsewhere in your body. The cells lining brain capillaries are sealed together by tight junctions that block most substances from slipping between them. Unlike capillaries in your muscles or gut, brain capillaries have no small openings (fenestrations) and very little active transport of fluid across their walls. Supporting cells wrap around the outside of these capillaries, reinforcing the seal. The result is a highly selective filter, the blood-brain barrier, that allows oxygen, glucose, and certain small molecules through while keeping out most toxins, bacteria, and large proteins circulating in your blood.
This barrier is essential for stable brain function, but it also makes treating brain diseases difficult because many medications cannot cross it.
How Blood Leaves the Brain
After delivering oxygen, blood drains into a network of veins that empty into channels called dural venous sinuses, rigid passages formed between layers of the tough membrane surrounding the brain. The main drainage path runs from the top of the brain through the superior sagittal sinus, which flows backward to a junction called the confluence of sinuses near the back of the skull. From there, blood moves through the transverse sinuses along the sides of the skull, curves down through the sigmoid sinuses, and exits through the internal jugular veins in the neck. These jugular veins carry the blood back toward the heart via the superior vena cava, completing the circuit.
What Happens When Supply Is Disrupted
Because each artery feeds a specific territory, the symptoms of a blockage point directly to which vessel is affected. A middle cerebral artery blockage typically causes weakness and numbness on one side of the face and arm, often with difficulty speaking. An anterior cerebral artery blockage tends to affect the leg more than the arm. Posterior cerebral artery blockages cause vision loss in one half of the visual field and can impair memory if the temporal lobe is involved.
Narrowing of the carotid arteries, called carotid stenosis, is one of the most common threats to brain blood supply. Stenosis greater than 70% of the artery’s diameter is classified as severe and significantly increases stroke risk. Moderate narrowing may not cause symptoms on its own but can progress over time, especially with uncontrolled high blood pressure, smoking, or high cholesterol. Vertebral artery narrowing is less common but can be particularly dangerous because the brainstem structures it supplies control consciousness and vital functions like breathing.