The brain demands a continuous blood supply, consuming much of the body’s oxygen and glucose. This nourishment is delivered through a network of blood vessels that distribute blood to distinct regions. These areas, each supplied by a particular artery, are known as vascular territories, forming a map fundamental to understanding brain function and diagnosing conditions like strokes.
The Brain’s Major Arterial Systems
The brain receives its blood supply from two primary arterial systems working in concert. The anterior circulation originates from the internal carotid arteries, which ascend through the neck. The posterior circulation is formed by the vertebral arteries, which merge to form the basilar artery at the base of the brain. These two systems converge and connect at the base of the brain, forming a ring-like structure called the Circle of Willis.
This arterial network acts as a safety mechanism, allowing for collateral blood flow if a major artery becomes narrowed or blocked. The Circle of Willis includes the anterior communicating artery, which connects the two anterior cerebral arteries, and the posterior communicating arteries, which link the internal carotid arteries to the posterior cerebral arteries. This interconnected design helps ensure that different brain regions can still receive some blood flow even if a primary supply line is compromised.
Anterior Circulation Territories
The anterior circulation supplies the front and top parts of the brain, primarily through two arteries. The middle cerebral artery (MCA), the largest cerebral artery, is a direct continuation of the internal carotid artery. It extends laterally, distributing blood to the lateral surfaces of the frontal, temporal, and parietal lobes.
The MCA territory includes cortical areas for motor and sensory control of the contralateral (opposite) face and arm. In the dominant hemisphere, the MCA also irrigates language centers like Broca’s area (speech production) and Wernicke’s area (language comprehension). Due to its extensive reach, the MCA is the artery most frequently affected by strokes.
The anterior cerebral artery (ACA) is another major branch from the internal carotid system, supplying the medial (midline) aspects of the frontal and parietal lobes. Its territory includes portions of the corpus callosum, which connects the brain hemispheres. Damage to the ACA territory often results in motor and sensory deficits affecting the contralateral leg and foot. Distinct symptoms from MCA or ACA damage provide important clues for localizing a stroke.
Posterior Circulation Territories
The posterior circulation, originating from the vertebral arteries, supplies the brainstem, cerebellum, and posterior cerebrum. The posterior cerebral artery (PCA) is the main branch of the basilar artery, formed by the vertebral arteries. The PCA’s territory includes the occipital lobe, inferior temporal lobe, and deep structures like the thalamus and parts of the midbrain.
The occipital lobe processes visual information, so a PCA stroke often leads to visual field deficits, such as homonymous hemianopia (loss of vision in one half of the visual field for both eyes). Other arteries from the basilar and vertebral arteries, including the superior cerebellar artery (SCA), anterior inferior cerebellar artery (AICA), and posterior inferior cerebellar artery (PICA), supply the cerebellum and brainstem.
These regions control many bodily functions. The brainstem regulates processes like breathing, heart rate, and consciousness, while the cerebellum coordinates movement, balance, and fine motor control. Posterior circulation strokes can lead to diverse symptoms, including dizziness, vertigo, balance problems, and disturbances in consciousness.
Clinical Implications of Territorial Strokes
When a patient presents with sudden neurological symptoms, the specific pattern of deficits can help pinpoint the exact location of the brain injury. This diagnostic process relies on the map of vascular territories.
For instance, difficulty speaking (aphasia) combined with weakness on the right face and arm suggests a stroke in the left middle cerebral artery territory. A sudden loss of vision in one half of their visual field, affecting both eyes, points to a stroke in the contralateral posterior cerebral artery territory. Isolated weakness predominantly affecting one leg, such as the left leg, indicates an issue in the right anterior cerebral artery territory. Correlating symptoms with specific arterial supply regions allows for rapid and accurate diagnosis, guiding treatment.
Watershed and Border Zone Infarcts
Beyond strokes caused by blockages in single major arteries, a distinct type of brain injury is known as a watershed or border zone infarct. These infarcts occur in the “border zones” between two adjacent major vascular territories. These areas represent the furthest reaches of arterial supply, where blood flow from two different arteries converges.
These border zones are particularly vulnerable to reductions in overall brain blood flow, such as during severe hypotension (low blood pressure) or cardiac arrest. When global blood supply diminishes, these regions at the edges of arterial supply are the first to suffer from inadequate oxygen and nutrient delivery. This is analogous to the farthest ends of an irrigation system drying out if water pressure drops.
A classic presentation of a watershed infarct is “man-in-a-barrel” syndrome, characterized by weakness in the shoulders and hips, while hands and feet retain normal strength. This pattern corresponds to watershed zones between the anterior and middle cerebral artery territories.