13.3 Circulation and the Central Nervous System

The central nervous system relies on a complex network of blood vessels to function. Internal carotid and vertebral arteries supply the brain, while spinal arteries nourish the spinal cord. The Circle of Willis provides crucial collateral circulation, ensuring a backup blood supply.

Cerebrospinal fluid, produced in the choroid plexuses, circulates through the brain's ventricles and the spinal cord's central canal. It cushions the brain, removes waste, and maintains homeostasis. Disruptions in blood flow or CSF dynamics can lead to serious conditions like stroke.

Circulatory Supply to the Central Nervous System

Blood Vessels of the Central Nervous System

The brain receives arterial blood from two paired sources: the internal carotid arteries (supplying the front and top of the brain) and the vertebral arteries (supplying the back and bottom). These two systems connect at the base of the brain to form the Circle of Willis, which acts as a safety net if one vessel becomes blocked.

Anterior circulation (internal carotid system):

• Each internal carotid artery branches into the anterior cerebral artery, the middle cerebral artery, and the anterior communicating artery
• Together, these supply most of the cerebral hemispheres

Posterior circulation (vertebrobasilar system):

• The two vertebral arteries merge to form the basilar artery
• The basilar artery branches into the posterior cerebral arteries and posterior communicating arteries, supplying the brainstem, cerebellum, and occipital lobes

Circle of Willis:

• This is an arterial anastomosis at the base of the brain, formed by the anterior cerebral, middle cerebral, posterior cerebral, anterior communicating, and posterior communicating arteries
• Its purpose is collateral circulation: if one feeding artery is narrowed or blocked, blood can still reach the affected region through alternate routes

Venous drainage of the brain:

• Blood drains from the brain into dural venous sinuses, which are channels formed between layers of the dura mater
• Major sinuses include the superior sagittal, inferior sagittal, straight, transverse, and sigmoid sinuses
• These sinuses ultimately empty into the internal jugular veins, which carry blood back toward the heart

Spinal cord blood supply:

• The anterior spinal artery supplies the anterior two-thirds of the spinal cord
• The posterior spinal arteries (a pair) supply the posterior one-third
• Venous drainage occurs through the anterior and posterior spinal veins, which empty into the internal vertebral venous plexus

Regulation of Cerebral Blood Flow

The brain accounts for only about 2% of body weight but receives roughly 15–20% of cardiac output. Three mechanisms keep that supply tightly controlled:

• Cerebral autoregulation maintains a relatively constant blood flow to the brain even when systemic blood pressure fluctuates. This works through the dilation and constriction of cerebral arterioles in response to pressure changes.
• Neurovascular coupling matches local blood flow to neural activity. When a brain region becomes more active, nearby blood vessels dilate to deliver more oxygen and glucose to meet the increased metabolic demand.
• The blood-brain barrier (BBB) is formed by tight junctions between the endothelial cells of brain capillaries. It selectively controls which substances pass from the bloodstream into brain tissue, blocking most pathogens and large molecules while allowing essential nutrients like glucose and oxygen through.

Ventricular System and Cerebrospinal Fluid

Components of the Brain Ventricular System

The ventricular system is a series of interconnected, fluid-filled cavities within the brain. Understanding the anatomy here is really about following the path that CSF takes from where it's made to where it's absorbed.

• Lateral ventricles (one in each cerebral hemisphere) are the largest ventricles. Each connects to the third ventricle through the interventricular foramen (foramen of Monro).
• Third ventricle is a narrow, midline cavity situated in the diencephalon, between the thalamus and hypothalamus. It connects inferiorly to the fourth ventricle through the cerebral aqueduct (aqueduct of Sylvius).
• Fourth ventricle is a diamond-shaped cavity located between the pons/medulla posteriorly and the cerebellum anteriorly. CSF exits the ventricular system here through three openings:
  • Two lateral apertures (foramina of Luschka)
  • One median aperture (foramen of Magendie)
  • These openings allow CSF to flow into the subarachnoid space
• Central canal is a narrow channel running through the center of the spinal cord, continuous with the fourth ventricle

Cerebrospinal Fluid Dynamics

Production: CSF is produced primarily by the choroid plexuses, highly vascularized structures found in each ventricle. Choroid plexus cells actively transport ions and water from the blood into the ventricular space. Ependymal cells lining the ventricles contribute a smaller amount of CSF as well. The body produces roughly 500 mL of CSF per day, though only about 150 mL circulates at any given time.

Circulation follows a unidirectional path:

1. CSF is produced in the lateral ventricles
2. Flows through the interventricular foramina into the third ventricle
3. Passes through the cerebral aqueduct into the fourth ventricle
4. Exits through the lateral and median apertures into the subarachnoid space surrounding the brain and spinal cord
5. Gets reabsorbed into the venous blood via arachnoid villi (also called arachnoid granulations), which are small projections of the arachnoid mater that protrude into the dural venous sinuses, especially the superior sagittal sinus

Functions of CSF:

• Mechanical protection: CSF cushions the brain within the skull. It reduces the brain's effective weight from about 1,400 g to roughly 25 g through buoyancy, which dramatically reduces the risk of the brain striking the skull during sudden movements.
• Chemical protection: CSF helps remove metabolic waste products and toxins from the CNS
• Homeostasis: CSF helps regulate intracranial pressure and provides a stable chemical environment for neurons

The glymphatic system is a recently described waste-clearance pathway. It uses CSF flowing along perivascular channels to exchange with interstitial fluid deep in the brain, flushing out metabolic waste (including proteins like amyloid-beta). This system is most active during sleep.

Circulatory Disruptions and Stroke

Circulatory Disruptions and Stroke Occurrence

A stroke occurs when blood supply to part of the brain is interrupted, causing brain cells to lose oxygen and begin dying within minutes. There are two major categories.

Ischemic stroke (~87% of all strokes): caused by a blockage in a cerebral blood vessel.

• Thrombotic stroke: a blood clot (thrombus) forms directly within a cerebral artery, usually at the site of atherosclerotic plaque buildup or endothelial damage
• Embolic stroke: a clot (embolus) forms somewhere else in the body (commonly the heart or carotid arteries), breaks free, travels through the bloodstream, and lodges in a smaller cerebral artery

Hemorrhagic stroke: caused by a ruptured blood vessel, leading to bleeding that compresses surrounding brain tissue and raises intracranial pressure.

• Intracerebral hemorrhage: bleeding directly within the brain tissue, most often linked to chronic hypertension or cerebral amyloid angiopathy
• Subarachnoid hemorrhage: bleeding into the subarachnoid space, frequently caused by a ruptured cerebral aneurysm

Risk factors for stroke:

• Hypertension damages vessel walls over time and is the single greatest modifiable risk factor for both ischemic and hemorrhagic stroke
• Atherosclerosis narrows arterial lumens with fatty plaques, increasing the chance of thrombosis or embolism
• Atrial fibrillation causes blood to pool in the heart chambers, promoting clot formation and raising embolic stroke risk
• Diabetes mellitus accelerates vascular damage and promotes atherosclerosis
• Smoking injures blood vessel endothelium, raises blood pressure, and promotes plaque formation
• Age and family history are non-modifiable risk factors; stroke risk increases with age, and a family history may indicate genetic predisposition