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 · Lateral ventricles · Third ventricle · Fourth ventricle · Choroid plexus
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.
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.
See it in 3D
The structures of Durgical's 3D anatomy where it is found.
Related
- Gray and white matter, cerebellar cortex, meninges
- Glial cells: astrocytes, oligodendrocytes, microglia, Schwann cells
- Blood vessels: artery, vein and capillary layers
- The neuron and synapse
Related conditions
Microorganisms
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 (OpenStax (Rice University), 2022); Hydrocephalus: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026).
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Representative model, for educational purposes only.