CSF and the blood–brain barrier !

Concept
Exam priority: medium

The brain floats in a clear fluid made inside its cavities, and a tight wall of vessel cells chooses what reaches the neurons.

Updated 2026-11-01

Also called: Cerebrospinal fluid, Circumventricular organs

See it in 3D: Choroid plexus · Lateral ventricles · Third ventricle · Cerebral aqueduct · Fourth ventricle · Superior sagittal sinus

Key points

Flow: lateral ventricles → interventricular foramina → third ventricle → cerebral aqueduct → fourth ventricle → median and lateral apertures → subarachnoid space → arachnoid granulations → superior sagittal sinus.
Volume ≈150 mL, production ≈500 mL/day: CSF turns over about 3–4 times a day.
Compared with plasma, CSF has very little protein, glucose about 60% of the plasma value, less K⁺ and slightly more Cl⁻.
Non-communicating (obstructive) hydrocephalus: a block inside the ventricular system (aqueductal stenosis, a neurocysticercosis cyst). Communicating: poor absorption (after meningitis or subarachnoid hemorrhage).
The barrier is missing in the circumventricular organs (area postrema, OVLT, median eminence, posterior pituitary) so they can sample blood.
Bacterial meningitis: many neutrophils, high protein, low glucose; viral meningitis: lymphocytes and normal glucose.

What it is

Cerebrospinal fluid (CSF) is secreted mainly by the choroid plexus, about 500 mL a day. It fills the ventricles and the subarachnoid space (about 150 mL at any moment) and drains into the venous sinuses through the arachnoid granulations. It cushions the brain, reduces its effective weight and helps clear solutes. The blood–brain barrier, made of capillary endothelial cells joined by tight junctions and supported by astrocytes and pericytes, lets lipid-soluble molecules and those with carriers through and keeps most proteins, many drugs and sudden ion swings out.

Reference values (adult)

Parameter	Value	Note
CSF volume	≈150 mL
CSF production	≈500 mL/day	≈0.35 mL/min
Opening pressure	≈7–20 cmH₂O	lateral decubitus
Glucose	≈45–80 mg/dL	≈60% of plasma glucose
Protein	≈15–45 mg/dL
Cells	≤5 lymphocytes/µL

Approximate values for a healthy adult at rest; each laboratory sets its own ranges.

CSF patterns

Condition	Cells	Glucose	Protein
Normal	≤5 lymphocytes/µL	≈60% of plasma	Low
Bacterial meningitis	High, neutrophils	Low	High
Viral meningitis	Moderate, lymphocytes	Normal	Normal or slightly high
Tuberculous or fungal meningitis	Moderate, lymphocytes	Low	High
Subarachnoid hemorrhage	Red cells; xanthochromia after hours	Normal	High

Clinical notes

Neurocysticercosis (Taenia solium), common in Colombia and Latin America, is a leading cause of acquired epilepsy and can block CSF flow and cause hydrocephalus.
In infants with hydrocephalus the head grows and the fontanelle bulges, because the sutures are still open.
Inflamed meninges make the barrier leaky, so some antibiotics that normally enter the CSF poorly reach useful levels during meningitis.
Post-lumbar-puncture headache: a CSF leak lets the brain sag and pull on pain-sensitive structures; it is worse upright and eases lying down.

Test yourself

Think of the answer, then tap the question to see it.

Why does aqueductal stenosis enlarge the lateral and third ventricles but not the fourth?

CSF made in the lateral and third ventricles cannot pass the aqueduct, so it builds up upstream; the fourth ventricle lies downstream of the block.

Why is CSF glucose low in bacterial meningitis?

Bacteria and neutrophils consume glucose, and inflamed meninges impair its carrier-mediated transport (GLUT1) from blood into CSF.

Why can the area postrema detect toxins in blood and trigger vomiting while most of the brain cannot?

It is a circumventricular organ with fenestrated capillaries and no tight barrier, so blood-borne substances reach its neurons directly.

Why do O₂, CO₂ and ethanol enter the brain easily while glucose needs a transporter?

Tight junctions close the route between cells, so solutes must cross endothelial membranes. Lipid-soluble molecules diffuse through them; polar ones such as glucose depend on carriers like GLUT1.

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See it in 3D

The structures of Durgical's 3D anatomy where it happens.

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Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: 13.3 Circulation and the Central Nervous System (OpenStax (Rice University), 2022); Neuroanatomy, Ventricular System (StatPearls (NCBI Bookshelf), 2023); Neuroanatomy, Choroid Plexus (StatPearls (NCBI Bookshelf), 2023); Hydrocephalus (StatPearls (NCBI Bookshelf), 2023); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 62). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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Representative model, for educational purposes only.