[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Histology](https://durgical.com/en/histology/) › [Nervous tissue](https://durgical.com/en/histology/#a-nervous) › Blood–brain barrier, choroid plexus and cerebrospinal fluid

[Histology](https://durgical.com/en/histology/) · [Nervous tissue](https://durgical.com/en/histology/#a-nervous)

# Blood–brain barrier, choroid plexus and cerebrospinal fluid

- Topic

The brain guards what enters it from the blood, and it floats in a clear fluid made by tufts of vessels inside its cavities.

Updated 2026-11-01

**See it in 3D:** [Cerebral aqueduct](https://durgical.com/en/anatomy/aqueduct-of-midbrain/) · [Lateral ventricles](https://durgical.com/en/anatomy/lateral-ventricle/) · [Third ventricle](https://durgical.com/en/anatomy/third-ventricle/) · [Fourth ventricle](https://durgical.com/en/anatomy/fourth-ventricle/) · [Choroid plexus](https://durgical.com/en/anatomy/choroid-plexus/)

## Key points

- Barrier = endothelial tight junctions + basement membrane + astrocyte end feet.
- Choroid plexus: fenestrated capillaries, but the epithelium's tight junctions form the blood–CSF barrier.
- Flow: lateral ventricles → interventricular foramina → third ventricle → cerebral aqueduct → fourth ventricle → apertures → subarachnoid space → arachnoid granulations.
- Area postrema has no barrier: the vomiting centre can sense toxins in blood.
- Bacterial meningitis CSF: neutrophils, high protein, low glucose.

## What it is

The blood–brain barrier is built by brain capillary endothelium sealed with tight junctions, its basement membrane, pericytes and the end feet of astrocytes. The choroid plexus, a fold of pia and capillaries covered by modified ependymal cells, makes cerebrospinal fluid (CSF); here the barrier moves to the epithelium (the blood–CSF barrier). CSF flows through the ventricles into the subarachnoid space and returns to venous blood through arachnoid granulations; blocking that path causes hydrocephalus.

## Key facts

- Circumventricular organs lack the barrier so they can sense or secrete into blood: area postrema, posterior pituitary, median eminence, pineal gland, OVLT and subfornical organ.
- Small lipid-soluble molecules, oxygen and carbon dioxide cross freely; glucose and amino acids need carriers.
- Normal CSF is clear with very few cells, less protein than plasma and about two thirds of the plasma glucose.

## Structure

- Brain capillaries: continuous, non-fenestrated endothelium with tight junctions, few pinocytic vesicles and many mitochondria.
- Astrocyte end feet cover almost all of the capillary surface; pericytes share the basement membrane.
- Choroid plexus: fenestrated capillaries in a pial core, covered by a simple cuboidal epithelium with microvilli, joined by tight junctions.
- Ependyma lines the ventricles: cuboidal to columnar cells with cilia and microvilli.

## Step by step

- The choroid plexus of the lateral, third and fourth ventricles secretes CSF.
- From each lateral ventricle, CSF passes through an interventricular foramen to the third ventricle.
- It runs through the cerebral aqueduct to the fourth ventricle.
- It leaves through the median and two lateral apertures into the subarachnoid space around brain and spinal cord.
- Arachnoid granulations return it to the dural venous sinuses with its wastes; about half a litre is made daily, so it turns over several times.

## Clinical notes

- Non-communicating hydrocephalus: flow blocked inside the system, classically aqueductal stenosis or a tumour near the fourth ventricle.
- Communicating hydrocephalus: absorption fails at the granulations, for example after meningitis or subarachnoid haemorrhage.
- Bacterial meningitis: inflamed meninges loosen the barrier; CSF shows neutrophils, high protein and low glucose.
- Vasogenic oedema around tumours and abscesses reflects a broken barrier and enhances with contrast on imaging; in Colombia neurocysticercosis is a frequent cause of such lesions and of hydrocephalus.

## Memory aid

Think of a river from source to sea: two lakes (lateral ventricles), a pond (third), a narrow canal (aqueduct), a delta (fourth) and drains back to the sea (venous sinuses).

## Test yourself

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

## Which cells seal the blood–brain barrier, and with what junctions?

Brain capillary endothelial cells, joined by tight junctions; the basement membrane and astrocyte end feet complete it.

## Where is the barrier located in the choroid plexus?

In its epithelium: the capillaries are fenestrated, so tight junctions between the cuboidal choroid cells form the blood–CSF barrier.

## Narrowing of which channel classically causes non-communicating hydrocephalus?

The cerebral aqueduct, between the third and fourth ventricles.

## Why can the area postrema trigger vomiting in response to blood-borne toxins?

It is a circumventricular organ with fenestrated capillaries and no blood–brain barrier.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/histology/blood-brain-barrier-choroid-plexus-and-cerebrospinal-fluid/)

## See it in 3D

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

- [Cerebral aqueduct](https://durgical.com/en/anatomy/aqueduct-of-midbrain/)
- [Lateral ventricles](https://durgical.com/en/anatomy/lateral-ventricle/)
- [Third ventricle](https://durgical.com/en/anatomy/third-ventricle/)
- [Fourth ventricle](https://durgical.com/en/anatomy/fourth-ventricle/)
- [Choroid plexus](https://durgical.com/en/anatomy/choroid-plexus/)

## Related

- [Gray and white matter, cerebellar cortex, meninges](https://durgical.com/en/histology/gray-and-white-matter-cerebellar-cortex-meninges/)
- [Glial cells: astrocytes, oligodendrocytes, microglia, Schwann cells](https://durgical.com/en/histology/glial-cells-astrocytes-oligodendrocytes-microglia-schwann-cells/)
- [Blood vessels: artery, vein and capillary layers](https://durgical.com/en/histology/blood-vessels-artery-vein-and-capillary-layers/)
- [The neuron and synapse](https://durgical.com/en/histology/the-neuron-and-synapse/)

## Related conditions

- [Hydrocephalus](https://durgical.com/en/conditions/hydrocephalus/)
- [Bacterial meningitis](https://durgical.com/en/conditions/bacterial-meningitis/)
- [Multiple sclerosis](https://durgical.com/en/conditions/multiple-sclerosis/)
- [Neurocysticercosis](https://durgical.com/en/conditions/neurocysticercosis/)

## Microorganisms

- [Neisseria meningitidis](https://durgical.com/en/microbiology/neisseria-meningitidis/)
- [Streptococcus pneumoniae](https://durgical.com/en/microbiology/streptococcus-pneumoniae/)

[← All of histology](https://durgical.com/en/histology/)

Educational content on normal microscopic structure and its clinical relevance; no substitute for studying slides or for a health professional's advice.

Sources: [13.3 Circulation and the Central Nervous System](https://openstax.org/books/anatomy-and-physiology-2e/pages/13-3-circulation-and-the-central-nervous-system) (OpenStax (Rice University), 2022); [Hydrocephalus: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/001571.htm) (MedlinePlus, US National Library of Medicine, 2026).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/histology/blood-brain-barrier-choroid-plexus-and-cerebrospinal-fluid/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/histology/blood-brain-barrier-choroid-plexus-and-cerebrospinal-fluid/