The subarachnoid space is a fluid-filled gap between two layers of tissue that surround your brain and spinal cord. It holds about 125 ml of cerebrospinal fluid (CSF), roughly half a cup, which cushions your brain and helps keep it nourished. The remaining 25 ml of CSF sits inside the brain’s internal chambers (ventricles), bringing the total to about 150 ml in the average adult.
Where It Sits in the Meninges
Your brain and spinal cord are wrapped in three protective layers called the meninges. From outermost to innermost, they are the dura mater, the arachnoid mater, and the pia mater. The subarachnoid space is the gap between the middle layer (arachnoid) and the innermost layer (pia), which clings directly to the surface of the brain and spinal cord.
Tiny web-like strands called arachnoid trabeculae stretch across the space, connecting the two layers like scaffolding. These strands help hold the structure together while still allowing fluid to flow freely. The space also extends as sleeve-like coverings around the cranial and spinal nerves, ending where the arachnoid and pia fuse with the outer covering of each nerve.
What the Space Contains
The subarachnoid space is not just a passive pocket of fluid. It contains three key things: cerebrospinal fluid, major blood vessels, and cisterns (enlarged pools where the space widens). Many of the brain’s largest arteries travel through this space before diving into brain tissue, including branches of the carotid and vertebral arteries that form the ring of vessels at the base of the brain.
Several cranial nerves also pass through the subarachnoid space on their way to the structures they serve. The optic nerves, facial nerves, and nerves controlling eye movement all travel through specific regions of this space, bathed in cerebrospinal fluid.
Cisterns: Where the Space Widens
The subarachnoid space is not uniform in thickness. Where the brain’s surface curves away from the overlying membranes, the space widens into pockets called cisterns. These cisterns are clinically and surgically important because they house major blood vessels and nerves in predictable locations.
The largest is the cisterna magna, located beneath the cerebellum and behind the brainstem. Others include the Sylvian cistern, which sits in the groove between the temporal and frontal lobes and carries the middle cerebral artery, and the pontine cistern, which lies in front of the pons and receives fresh CSF from the fourth ventricle. There are at least seven named cisterns, each containing a distinct set of arteries and cranial nerves.
How It Protects the Brain
Your brain weighs roughly 1,500 grams (about 3.3 pounds) but is structurally soft. The cerebrospinal fluid in the subarachnoid space provides buoyancy that offsets much of that weight, reducing the downward pull on delicate brain tissue and the nerve roots exiting from it. Without this buoyant support, the brain’s own mass would compress and damage its lower structures.
The fluid also acts as a shock absorber. When your head accelerates or decelerates suddenly, such as during a fall or a car accident, the CSF cushions the brain against the inside of the skull. This does not eliminate injury risk entirely, but it significantly reduces the mechanical force transmitted to brain tissue during everyday movement and minor impacts.
Waste Clearance and Nutrient Delivery
Beyond cushioning, the cerebrospinal fluid circulating through the subarachnoid space helps regulate the brain’s chemical environment. It delivers nutrients, maintains stable pressure inside the skull, and removes metabolic waste products that brain cells generate.
Researchers have recently described a system called the glymphatic system, which functions as the brain’s version of the body’s lymphatic drainage network. CSF from the subarachnoid space flows along channels surrounding blood vessels that enter the brain, flushing out waste proteins and other byproducts. A newly identified fourth meningeal layer, called the subarachnoid lymphatic-like membrane (SLYM), appears to play a role in organizing this flow. This waste-clearance function is particularly active during sleep.
Normal Pressure Inside the Space
The fluid in the subarachnoid space exerts a measurable pressure, typically between 6 and 25 cmH₂O in adults, with an average around 18 cmH₂O. Some healthy people can have pressures as high as 30 cmH₂O without symptoms, so a single pressure reading always needs to be interpreted alongside other clinical findings. Persistently high or low pressure can signal conditions like hydrocephalus (fluid buildup) or a CSF leak.
Why Lumbar Punctures Target This Space
A lumbar puncture, commonly called a spinal tap, is a procedure that collects CSF directly from the subarachnoid space. The needle is inserted between the L3-L4 or L4-L5 vertebrae in the lower back. This site is chosen because the spinal cord itself ends higher up, around the T12-L1 level, so inserting the needle below that point avoids any risk of contacting the cord. At this level, only the dangling nerve roots of the cauda equina float in the CSF, and they tend to drift aside when a needle passes through.
Doctors use the landmark of an imaginary line between the tops of the hip bones to locate the L4 vertebra by feel. CSF collected this way can be tested for infection, inflammation, bleeding, and abnormal proteins, making the subarachnoid space a direct diagnostic window into the central nervous system.
Subarachnoid Hemorrhage
Because major arteries run through the subarachnoid space, bleeding here is a medical emergency. A subarachnoid hemorrhage occurs when blood leaks into the space, most commonly from a ruptured brain aneurysm, which is a weakened, ballooned section of an artery wall. Head injuries from falls, car accidents, or violence are another common cause. Less frequently, tangled clusters of abnormal blood vessels (arteriovenous malformations) or inflamed blood vessel walls (vasculitis) can trigger bleeding.
The hallmark symptom is a sudden, explosive headache that people often describe as the worst of their life. It reaches peak intensity within seconds, not minutes. Other symptoms can include nausea, vomiting, a stiff neck, vision changes, and brief loss of consciousness. This pattern of instant, severe headache is distinct from other headache types and warrants immediate emergency evaluation.