Study topic · about 8 minutes
Cerebrospinal fluid, the ventricles and the meninges
Cerebrospinal fluid (CSF) is a clear, watery fluid that fills the brain's four ventricles and bathes the brain and spinal cord. The choroid plexus makes about half a liter of it every day. It flows through the ventricles, out around the brain and spinal cord, and back into the blood. Three layers of tissue, the meninges, wrap the brain and spinal cord and hold the fluid in.
Key points
- An adult has about 150 milliliters of CSF at any moment but makes about 500 milliliters a day, so it is replaced several times daily.
- CSF cushions the brain and lets it float: a brain of about 1.2 to 1.4 kilograms weighs only about 50 grams in it.
- CSF flows from the lateral ventricles to the third ventricle, through the cerebral aqueduct to the fourth ventricle, then out around the brain.
- The three meninges, from the outside in, are the dura, arachnoid and pia mater, and CSF flows between the arachnoid and the pia.
- The blood-brain barrier, made of tightly sealed capillary walls, keeps most germs and many medicines out of the brain.
- Too much CSF causes hydrocephalus, in which the ventricles swell and press on the brain.
What is cerebrospinal fluid?
CSF is a clear, colorless fluid made from blood plasma. It is mostly water, with salts, some glucose, very little protein and almost no cells. An adult has about 150 milliliters of it at any moment, around two-thirds of a cup. The body makes about 500 milliliters a day, so the whole supply is replaced several times a day.
- Cushioning and buoyancy. The brain floats in CSF. A brain of about 1.2 to 1.4 kilograms weighs only about 50 grams in the fluid. That keeps it from crushing its own nerves and blood vessels, and the fluid also softens knocks to the head.
- Removing waste. As CSF flows and drains away, it carries off waste products from the brain. Research since 2012 suggests that fluid also washes through the brain along its blood vessels, most actively during deep sleep. This route, the glymphatic system, is still being worked out.
- Keeping the chemistry steady. CSF keeps a stable mix of salts and nutrients around brain cells, which they need to send signals properly.
Where CSF is made: the choroid plexus
The choroid plexus is a frilly, blood-rich tissue that hangs inside all four ventricles, with its largest parts in the lateral ventricles. Its tiny blood vessels are leaky, but a single layer of special cells covers them. These cells pull water, salts and chosen nutrients out of the blood and release them as CSF.
The covering cells are sealed to each other by tight junctions, so nothing slips between them. This seal is the blood-CSF barrier, and it keeps the fluid's recipe under strict control. The choroid plexus makes most of the CSF, about 80 percent by the classic textbook figure. The rest seeps in from the brain tissue itself.
The four ventricles and the path of CSF
The ventricles are four connected, fluid-filled spaces inside the brain. They formed from the hollow inside of the neural tube, the embryo's first version of the brain and spinal cord. Pushed along by the pulse of the arteries, CSF moves through them in one direction:
- The two lateral ventricles, one C-shaped chamber in each hemisphere, where most of the fluid is made.
- The interventricular foramina (foramina of Monro), two small holes that lead into the third ventricle.
- The third ventricle, a thin slit between the left and right halves of the thalamus.
- The cerebral aqueduct (aqueduct of Sylvius), a narrow canal through the midbrain.
- The fourth ventricle, a tent-shaped space between the brainstem in front and the cerebellum behind.
- Out through three openings, the two foramina of Luschka at the sides and the foramen of Magendie in the middle, into the subarachnoid space.
- Around the brain and down around the spinal cord. A little also trickles into the thin central canal of the spinal cord.
- Back into the blood through the arachnoid granulations, small pouches of the arachnoid that poke into large veins in the dura, the dural venous sinuses.
For exams, CSF returns to the blood through the arachnoid granulations. Newer research shows that much of it also drains along nerves and through lymphatic vessels in the dura. Scientists still debate how the work is shared.
The meninges and where bleeds happen
Three layers of tissue, the meninges, wrap the brain and spinal cord. The outer dura mater ("tough mother") is thick, tough and fixed to the inside of the skull. The middle arachnoid mater is thin, with a web of fine threads beneath it that gives it its spider-like name. The inner pia mater ("tender mother") is a delicate film that clings to every fold of the brain.
CSF fills the subarachnoid space, between the arachnoid and the pia, where the large arteries of the brain also run. In the skull, the other two gaps are normally closed: the epidural space outside the dura, and the subdural space just under it. A bleed can force either one open.
| Bleed | Where | Usual cause | Typical picture |
|---|---|---|---|
| Epidural hematoma | Between the skull and the dura | A skull fracture that tears an artery, often the middle meningeal artery near the temple | The person may seem fine for a while (a "lucid interval"), then gets worse fast. On a CT scan it looks like a lens. |
| Subdural hematoma | Between the dura and the arachnoid | Torn bridging veins, which cross this gap on their way to the venous sinuses | Often an older adult after a fall. It can build up slowly, over days or weeks. On a CT scan it looks like a crescent. |
| Subarachnoid hemorrhage | In the subarachnoid space, mixing with the CSF | A head injury, or a burst aneurysm (a weak bulge in an artery) | A sudden, explosive headache, which people often call "the worst headache of my life". |
The blood-brain barrier
In most of the body, the walls of the smallest blood vessels, the capillaries, have small gaps that let many substances leak out. In the brain, the cells of the capillary wall are sealed together by tight junctions, so there are no gaps. Star-shaped support cells called astrocytes wrap the capillaries with their "end feet" and help build and keep up this seal. Together, this is the blood-brain barrier.
| Substance | Gets in? | Why |
|---|---|---|
| Oxygen and carbon dioxide | Yes, freely | Small gases pass straight through cell membranes |
| Alcohol, caffeine and nicotine | Yes, easily | They are small and dissolve in fat, so they slip through membranes |
| Glucose | Yes, with help | Special transporters carry it across |
| Most bacteria and other germs | No | They are far too large, and the sealed wall leaves no gaps |
| Large proteins, such as antibodies | Mostly no | They are too large to cross |
| Many medicines | Often no | They are too large, carry an electric charge or do not dissolve in fat, or pumps push them back out |
This is why some medicines cannot reach the brain. For example, dopamine itself cannot cross. Parkinson's disease is treated instead with levodopa, a building block that crosses and is turned into dopamine in the brain. Read more in drugs and the brain.
A few small spots have a leaky barrier on purpose, so the brain can sample the blood. One of them, the area postrema at the bottom of the fourth ventricle, detects toxins and can trigger vomiting.
Lumbar puncture, hydrocephalus and meningitis
Lumbar puncture (spinal tap)
To test CSF, doctors take a small sample from the lower back. A thin needle goes between two vertebrae, usually around the L3 to L5 level in adults. The spinal cord itself ends higher up, around the L1 to L2 level. Below that, the fluid-filled sac holds only loose nerve roots, the cauda equina ("horse's tail"). The needle can reach the fluid there without touching the cord.
Normal CSF is clear and colorless. Cloudy fluid suggests an infection, and blood or a yellow tint (xanthochromia) can point to bleeding into the subarachnoid space.
Hydrocephalus
The name comes from the Greek for "water head". Too much CSF builds up, and the ventricles swell and press on the brain around them.
| Type | What goes wrong | Example |
|---|---|---|
| Non-communicating (obstructive) | Something blocks the flow inside the ventricles, so fluid backs up behind the block | In aqueductal stenosis, a narrowed cerebral aqueduct, the lateral and third ventricles swell but the fourth does not |
| Communicating | The ventricles stay open to each other, but the fluid is not soaked back into the blood | Scarred arachnoid granulations after meningitis or bleeding |
| Normal pressure hydrocephalus | A slow, communicating type, mostly in older adults; the pressure of the fluid is usually normal | An older adult with trouble walking, bladder leaks and memory problems |
Normal pressure hydrocephalus matters because it can look like dementia, yet unlike most dementias it can sometimes be treated. Surgeons often place a shunt, a thin tube that drains extra fluid to the belly. Walking problems often come first. Large ventricles do not always mean too much fluid, though. When brain tissue shrinks, after a stroke or in Alzheimer's disease, the ventricles grow to fill the space. This hydrocephalus ex vacuo is not true hydrocephalus.
Raised pressure can also make the optic disc at the back of the eye swell. Doctors can see this sign, called papilledema, through the pupil. The optic disc is where the optic nerve leaves the eye: see the visual pathway.
Meningitis
Inflammation of the meninges, usually from an infection, is called meningitis. Viruses cause most cases, which tend to be milder. Bacterial meningitis is less common but more dangerous, and it is a medical emergency. Typical signs are fever, headache, a stiff neck and discomfort in bright light, and a lumbar puncture shows the cause.
This page is for learning only. A sudden, severe headache, a fever with a stiff neck, or confusion after a head injury needs emergency care right away.
Memory tricks
PADPia, Arachnoid, Dura: the meninges PAD the brain.
Luschka lateral, Magendie medialThe two foramina of Luschka are at the sides, and the single foramen of Magendie is in the middle.
Wet, wobbly and wackyWet: loss of bladder control. Wobbly: trouble walking. Wacky: memory and thinking problems.
Quick quiz
5 questions. Your answers are saved and come back in your daily review.
Which structure makes most of the cerebrospinal fluid?
Show the answer
Choroid plexus. The choroid plexus, inside all four ventricles, makes most of the roughly 500 milliliters produced each day. The arachnoid granulations do the opposite: they return CSF to the blood.
Choroid plexus. The choroid plexus, inside all four ventricles, makes most of the roughly 500 milliliters produced each day. The arachnoid granulations do the opposite: they return CSF to the blood.
A baby is born with a narrowed cerebral aqueduct. Which ventricles swell?
Show the answer
The lateral and third ventricles. Fluid backs up above the block, so the lateral ventricles and third ventricle swell while the fourth stays normal. This is non-communicating hydrocephalus.
The lateral and third ventricles. Fluid backs up above the block, so the lateral ventricles and third ventricle swell while the fourth stays normal. This is non-communicating hydrocephalus.
An older adult falls and, over the next weeks, becomes confused. A scan shows a crescent-shaped bleed over the brain. What is it most likely to be?
Show the answer
Subdural hematoma. Bridging veins cross the gap between the dura and the arachnoid. When they tear, blood collects there slowly, in a crescent: a subdural hematoma.
Subdural hematoma. Bridging veins cross the gap between the dura and the arachnoid. When they tear, blood collects there slowly, in a crescent: a subdural hematoma.
What mainly seals the blood-brain barrier?
Show the answer
Tight junctions between the cells of the capillary walls. Brain capillary cells are sealed together by tight junctions, with help from astrocytes, so most substances must be carried across on purpose.
Tight junctions between the cells of the capillary walls. Brain capillary cells are sealed together by tight junctions, with help from astrocytes, so most substances must be carried across on purpose.
An older adult has trouble walking, loses bladder control and grows forgetful. Scans show large ventricles. What is the likely cause?
Show the answer
Normal pressure hydrocephalus. "Wet, wobbly and wacky" is the classic trio of normal pressure hydrocephalus, a buildup of CSF that can sometimes be treated.
Normal pressure hydrocephalus. "Wet, wobbly and wacky" is the classic trio of normal pressure hydrocephalus, a buildup of CSF that can sometimes be treated.
These parts now come back in your daily review, just before you would forget them.
Practice these parts in 3DIn these study guides
- Anatomy and physiologyVentricles, fluid and blood supply (section 13.3)
- NursingCerebrospinal fluid and the ventricles
- Medical schoolVentricles and cerebrospinal fluid
Take it further in 3D
- Wiring and fluidWhite matter cables and the ventricles. A short guided lesson in the 3D app.
Sources
- Anatomy and Physiology 2e, 13.3 Circulation and the Central Nervous System, OpenStax
- Physiology, Cerebral Spinal Fluid, StatPearls Publishing (NCBI Bookshelf)
- Hydrocephalus, National Institute of Neurological Disorders and Stroke (NIH)
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