Cerebrospinal fluid flow

Choroid plexus

Cerebrospinal fluid is produced by a specialized tissue called the choroid plexus. Choroid plexuses are located in the walls of the lateral ventricles and in the roofs of the third and fourth ventricles. A choroid plexus shows numerous villi, via which it secretes the cerebrospinal fluid. Structurally, each villus consists of three components;

• A layer of modified ependymal cells (choroid cells), which faces the lumen of the ventricles and secretes the CSF. The cells show many apical villous projections and are tightly bound to each other via tight junctions. 
• A layer of pia mater (tela choroidea)
• A fenestrated capillary directly beneath the pia mater.

Choroid cells take up various chemicals from the underlying blood vessel which they use to actively secrete the CSF. Thus, the CSF fluid is not simply an ultrafiltrate of blood but differs from it in terms of its electrolyte, glucose and protein content. The vascular source for the  choroid plexuses differs between the lateral, third ventricle and the fourth ventricle. 

• The anterior portions of the choroid plexuses in the lateral and third ventricles are supplied by the anterior choroidal artery (a branch of the internal carotid artery). While their posterior parts are vascularized by the posterior lateral and medial choroidal arteries (branches of the posterior cerebral artery)
• The choroid plexus in the fourth ventricle is supplied by the inferior cerebellar arteries.

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Blood-brain barrier (BBB)

Besides being involved in CSF production, the layers of choroid plexus form a selectively permeable barrier called the blood-brain barrier (BBB). 
To recap, from superficial to deep, the blood-brain barrier consists of: 

• Choroid ependymal cells and their tight junctions
• Pia mater
• Endothelial cells of capillaries
• Basal membrane of endothelial capillary cells 

The function of the blood-brain barrier is to control the movement of water and solutes into the CSF, as well as from the CSF into the neural tissue.

Need some help learning about the circulation of cerebrospinal fluid? Discover why active recall is so essential to effective anatomy learning. 

Blood-­CSF barrier (BCSFB)

The blood-CSF barrier is often confused for the blood-brain barrier, while in fact it is only a part of it. The blood-CSF barrier actually refers to the tight junctions between the choroid ependymal cells, which control the passage of molecules between the underlying capillaries and cerebrospinal fluid. 

If it weren’t for the tight junctions (i.e. blood-CSF barrier), the particles such as electrolytes and blood cells that can pass through the fenestrae of the capillaries would enter the CSF and therefore disturb the electrolytic and biochemical equilibrium in the blood. This barrier is functionally important for the opposite direction as well, from the CSF to the capillaries; as it prevents large molecules (such as different kinds of drugs) from freely entering the brain blood vessels.

Arachnoid granulations

The majority of the cerebrospinal fluid (CSF) is absorbed into the venous system by the arachnoid granulations. The arachnoid granulations are the protrusions of the arachnoid mater that pierce the dura mater and protrude into the lumina of the dural venous sinuses. The core of each granulation is continuous with the subarachnoid space, thus containing the cerebrospinal fluid. The surface of each arachnoid granulation contains smaller outpouchings called arachnoid villi that increase its absorption surface. 

The CSF exits the subarachnoid space by diffusing through the walls of arachnoid granulations. The arachnoid granulations provide a valvular mechanism for the flow of CSF, which allows the inflow of CSF into the bloodstream without permitting the backflow of blood into the CSF. Normally the pressure of the CSF is higher than that of the venous system, so CSF flows through the villi and granulations into the venous blood. 

The highest number of arachnoid granulations is found in the superior sagittal and transverse sinuses. Thus, these are the sites where most of the CSF is absorbed. 

Learn everything about the arachnoid granulations with our articles, video tutorials, quizzes and labelled diagrams.

Minute channels in the cribriform plate

The second route consists of small channels that extend from the subarachnoid space through openings in the cribriform plate of the ethmoid bone. These channels traverse the cribriform plate together with the fila olfactoria of the olfactory nerve (CN I) and drain into the lymphatic channels of the nasal mucosa.

Glymphatic system

The third route is provided by the glymphatic system. It is a recently discovered system of channels that is formed by the astroglial cells around the pial arteries. Its function is to provide an entrance route for the CSF in exchange for the interstitial fluid of the brain and spinal cord. This means that small amounts of CSF enter the nervous tissue, whilst the same amount of interstitial fluid exits into the subarachnoid space in order to be eliminated through the dural venous sinuses.

Now that you've learned about the arachnoid granulations, put that knowledge to the test by taking the quiz below!