Anatomy and Physiology 2e · Anatomy of the Nervous System
Circulation and the Central Nervous System
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In 30 seconds
The brain is the body's most demanding organ: about 2% of body weight, yet commonly taught to receive ≈15–20% of cardiac output and ≈20% of oxygen. Neurons cannot store fuel and die quickly without oxygen, so the CNS needs continuous, precisely regulated blood.
Blood reaches the brain by two paired routes. The internal carotid arteries supply most of the cerebrum; the vertebral arteries merge to form the Basilar artery The vessel formed by the two vertebrals meeting Full entry →, supplying the brain stem, cerebellum, and parts of the cerebrum. At the base of the brain the systems link in a ring, the Circle of Willis The arterial ring at the brain's base linking both systems Full entry →, providing redundant pathways — if one route is blocked, blood can often detour. From the ring, three pairs of major arteries — anterior, middle, and posterior cerebral arteries — fan out to the cortex. The Blood–brain barrier Selective barrier of tight junctions + astrocyte end-feet Full entry → tightly controls which molecules leave the blood for the brain, and the choroid plexus converts blood filtrate into cerebrospinal fluid.
Why this matters
Stroke — interruption of blood flow to the brain — is a leading cause of disability and death, and circulation anatomy explains its effects: each major artery supplies a characteristic territory, so a blockage produces a recognizable deficit pattern — how clinicians begin localizing a stroke. The circle of Willis explains why some blockages cause little damage (collateral flow) while others are catastrophic. The blood–brain barrier matters for pharmacology: many drugs cannot enter the brain. Expect exam questions on artery territories, the circle's components, and CSF flow direction.
The college version
Core Concepts
Arterial supply: two systems, one brain
- Internal carotid system (anterior circulation): each internal carotid enters the skull through the carotid canal, giving off the anterior cerebral artery (ACA) and middle cerebral artery (MCA), which supply most of the cerebrum.
- Vertebrobasilar system (posterior circulation): the two vertebral arteries ascend through the cervical vertebrae, enter via the foramen magnum, and merge into the basilar artery along the front of the pons, branching to the pons and cerebellum before splitting into the posterior cerebral arteries (PCA).
The circle of Willis: the brain's backup plan
At the base of the brain, the two systems interconnect in a roughly circular anastomosis: the two anterior cerebral arteries (joined by the anterior communicating artery), the internal carotid arteries, and the posterior communicating arteries (connecting the carotids to the posterior cerebral arteries).
Because the ring connects left and right, front and back, a slowly developing blockage in one feeder vessel can often be bypassed. But the protection is incomplete: anatomy varies among people, and a sudden blockage may outpace collateral flow.
The three great cerebral arteries and their territories
- ACA: medial frontal and parietal lobes — motor and sensory areas for the leg and foot.
- MCA: lateral frontal, parietal, and temporal lobes — face and arm areas plus most language areas in the dominant hemisphere; the most common site of major stroke.
- PCA: occipital lobe (vision) and part of the temporal lobe.
The shorthand: ACA = leg, MCA = face/arm + language, PCA = vision. Commonly taught patterns; individual anatomy and collateral flow vary, so presentations must be interpreted by qualified clinicians.
Venous drainage: back to the heart
Venous drainage: cerebral veins empty into the dural venous sinuses (including the superior sagittal sinus), which converge into the internal jugular veins and also receive cerebrospinal fluid via the arachnoid granulations.
The blood–brain barrier: a selective gate
Brain capillaries differ from others: endothelial cells are joined by tight junctions and wrapped by astrocyte end-feet. Together they form the blood–brain barrier (BBB), which blocks most water-soluble molecules, toxins, and pathogens; admits needed substances (oxygen, glucose, some amino acids) via transporters; and keeps out many drugs — a major pharmacology consideration.
CSF circulation: the fluid link
The choroid plexus (a capillary network in the ventricles) produces cerebrospinal fluid, which flows lateral → third → fourth ventricles into the subarachnoid space and is reabsorbed through arachnoid granulations into the superior sagittal sinus. This one-way circulation cushions the CNS, removes waste, and stabilizes intracranial pressure.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Internal carotid supply | Vertebrobasilar supply | Carotids feed the anterior cerebrum; vertebrals feed the posterior brain |
| MCA territory | ACA territory | MCA = lateral cortex (face, arm, language); ACA = medial cortex (leg) — a classic test distinction |
| Blood–brain barrier | Blood–CSF barrier (choroid plexus) | The BBB is the main gatekeeper at brain capillaries; the choroid plexus has its own, more permeable barrier |
| Circle of Willis | "Total stroke protection" | The circle offers collateral routes but is incomplete and variable — overestimating it is a common trap |
| CSF flow | Blood flow | CSF is made in the ventricles, circulates into the subarachnoid space, and is reabsorbed into venous sinuses — not back into arteries |
| Dural venous sinus | Artery | Sinuses are venous channels draining toward the heart, not oxygenated arterial supply |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Your brain is a castle that never sleeps, and blood is its food delivery system. Two main roads bring food in — from the front (the carotids) and the back (the vertebrals) — meeting in a roundabout at the castle gate (the circle of Willis), so if one road is blocked, trucks drive around the other way. From the roundabout, three roads deliver to different neighborhoods: the top floor (legs), the main hall (face, arms, talking), and the library (seeing). The gate has a strict guard (the blood–brain barrier) that admits only approved packages, and a moat (cerebrospinal fluid) surrounds the castle, refreshed constantly and drained.
Worked example
Suppose a person suddenly cannot move the right face and arm and has trouble speaking, but the leg moves normally and vision is intact.
- Face and arm weakness points to the face-and-arm motor areas — lateral brain surface.
- Trouble speaking (left-hemisphere language areas) also lies on the lateral surface.
- The lateral frontal, parietal, and temporal lobes are the middle cerebral artery territory.
- The leg is spared because its motor area sits on the medial surface — anterior cerebral artery territory — and vision is intact because the occipital lobe (posterior cerebral artery territory) is uninvolved.
The likely story: a blockage in the left MCA or its branches — a commonly taught localization exercise, not a diagnosis; real cases vary with collateral flow, and only clinicians make treatment decisions. The reasoning — symptom pattern → vascular territory → vessel — is the skill the anatomy teaches. Reverse it: vision loss with preserved movement points to the PCA; one-legged weakness with spared arm and face points to the ACA.
Key takeaways
- Brain ≈ 2% of body weight but gets a large share of cardiac output and oxygen (commonly taught ≈ 15–20% and ≈ 20%).
- Anterior circulation: internal carotids → ACA + MCA. Posterior: vertebrals → basilar → PCA.
- Circle of Willis = anterior + posterior communicating arteries linking the systems; collateral protection is real but incomplete and variable.
- Territory shorthand: ACA = leg, MCA = face/arm + language, PCA = vision.
- Blood–brain barrier: tight junctions + astrocyte end-feet; blocks most water-soluble molecules and many drugs; selective windows in a few regions.
- CSF circuit: choroid plexus (lateral → third → fourth ventricles) → subarachnoid space → arachnoid granulations → superior sagittal sinus.
Check yourself
5 review questions from the chapter. Try each one, then open the answer.
What two paired arterial systems supply the brain, and what vessel do the vertebrals form when they merge?
Show answer
The internal carotid arteries (anterior circulation) and the vertebral arteries (posterior circulation); the vertebrals merge to form the basilar artery.
Name the components that make up the circle of Willis.
Show answer
The anterior communicating artery, the two anterior cerebral arteries, the internal carotids (linking the middle cerebral arteries), the posterior communicating arteries, and the two posterior cerebral arteries.
Which cerebral artery is associated with the leg's motor/sensory areas? With the face, arm, and language?
Show answer
The ACA supplies the medial motor/sensory areas for the leg; the MCA supplies the lateral areas for the face and arm plus most language areas in the dominant hemisphere.
What structures form the blood–brain barrier, and what does it do?
Show answer
Tight junctions between brain capillary endothelial cells, reinforced by astrocyte end-feet. They selectively control which molecules pass from blood into brain tissue — blocking most water-soluble substances and many drugs while admitting oxygen and glucose.
Trace the path of cerebrospinal fluid from production to reabsorption.
Show answer
Choroid plexus produces CSF in the lateral ventricles → third → fourth ventricle → subarachnoid space → circulates around the brain and cord → reabsorbed through arachnoid granulations into the superior sagittal sinus.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Internal carotid artery
- Paired artery carrying blood into the skull
- Basilar artery
- The vessel formed by the two vertebrals meeting
- Circle of Willis
- The arterial ring at the brain's base linking both systems
- Anterior/middle/posterior cerebral arteries
- The three paired arteries supplying the cortex
- Blood–brain barrier
- Selective barrier of tight junctions + astrocyte end-feet
- Arachnoid granulation
- Arachnoid projection into a dural sinus that reabsorbs CSF
Sources & references
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