The brain and spinal cord float in cerebrospinal fluid (CSF) inside three protective membranes. Knowing these layers explains the different types of brain bleeding and how a lumbar puncture works.
Three layers
From outside in, they are the tough dura mater, the thin arachnoid mater and the delicate pia mater that clings to the brain surface. CSF fills the subarachnoid space between the arachnoid and the pia.
Spaces and bleeds
An epidural hematoma, usually arterial, collects between the skull and the dura. A subdural hematoma, usually from torn veins, lies under the dura, and a subarachnoid hemorrhage, often from an aneurysm, spreads through the CSF.
Making CSF
The choroid plexus in the ventricles makes about 500 mL of CSF a day, while only about 150 mL is present at any time. It flows from the lateral ventricles through the third and fourth ventricles into the subarachnoid space.
Absorption and blockage
CSF drains into the venous sinuses through arachnoid granulations. If its flow or absorption is blocked, fluid builds up (hydrocephalus) and the pressure inside the skull rises.
| Test | Normal adult CSF | Abnormal result suggests |
|---|---|---|
| Appearance | Clear and colorless | Cloudy: infection; bloody or yellow: bleeding |
| White cells | 0 to 5 cells/µL | Meningitis or inflammation |
| Protein | About 15 to 45 mg/dL | Infection, bleeding or inflammation when high |
| Glucose | About 60 percent of blood glucose | Bacterial meningitis when low |
At the bedside
A lumbar puncture is done at L3 to L4 or L4 to L5 because the spinal cord ends above that level. Fever, headache and a stiff neck suggest meningeal irritation and need urgent assessment.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).