Cerebrospinal Fluid Production and Circulation

This is Dr. Cal Shipley with a review of cerebrospinal fluid production and circulation.

Brain Anatomy

Let’s take a look at some anatomy first. The brain and spinal cord are completely surrounded by cerebral spinal fluid, also known as CSF. The CSF fluid layer acts as a mechanical shock absorber, cushioning movements of the brain in head trauma. The CSF also carries nutrients to, and waste products away from, the brain. Most of the cerebral spinal fluid is produced in a series of chambers known as the ventricles. The largest of these chambers are called the lateral ventricles.

Rotating to a side view looking from the left, we see the lateral ventricle connected to the third and fourth ventricle. The third and fourth ventricle are connected by a narrow passageway known as the aqueduct. Obstruction to flow of CSF within the aqueduct is one of the most common causes of hydrocephalus.

The brainstem represents the lower most portion of the brain and connects the brain to the spinal cord.

Here we see a close up view of the relationship between the aqueduct, the fourth ventricle and the brainstem. As can be seen here, the aqueduct actually lies within the substance of the brainstem.

Cerebrospinal Fluid Production

The majority of cerebrospinal fluid is produced within the lateral and third and fourth ventricles as a result of specialized tissue known as the choroid plexus. The functioning of the choroid plexus is yet another beautiful example of the adaptive physiology of the human body. Here we see the typical position of the choroid plexus in the lateral and third and fourth ventricles. The cerebospinal fluid is not produced by the cells of the choroid plexus, instead, CSF represents a filtrate of the blood with the filtration performed by the highly specialized cells of the choroid plexus.

The functional unit of the choroid plexus is known as a villus, or multiple, villi. The villus consists of loops of tiny capillaries separated from the ventricle by a layer of cuboidal epithelial cells. Here in a close-up view, we see the layer of cuboidal epithelial cells interposed between the capillary and the ventricle. Like all capillaries, those at the choroid plexus have a wall which is a single cell thick. This allows for ready transport of ions and molecules to and from the capillary. The cells of the cuboidal epithelium are highly specialized and are capable of generating a polarized electrical charge across their membranes.

This charge causes sodium, chloride and bicarbonate ions to migrate from the capillary plasma into the ventricle. This ionic flow creates an osmotic gradient, which draws water molecules from the capillary plasma into the ventricle. The composition of cerebrospinal fluid is mostly water, with sodium, chloride and bicarbonate ions and a smattering of other ions, as well as small amounts of amino acids, protein and glucose thrown into the mix.

The filtrate contains no cells, but under normal circumstances, small numbers of white blood cells, usually monocytes, are introduced into the cerebrospinal fluid directly from the vascular system once the CSF is within the ventricles.

The cerebrospinal fluid originates in the ventricles as a result of filtration of the plasma by the choroid plexus. It then flows or circulates through the ventricles, then surrounds the brain and spinal cord, where it is contained by layer of the meninges, known as the arachnoid.

Through a series of outpouchings on the arachnoid, known as granulations, the cerebrospinal fluid then flows into the venous sinuses of the brain, returning to the bloodstream from whence it originated.

The total volume of cerebral spinal fluid at any given moment is about 150 cc in an adult. Approximately 600 cc of cerebral spinal fluid is produced each day via blood filtration in the choroid plexus. If you do the math, 100% of the volume of cerebral spinal fluid turns over every six hours or four times a day.

Cal Shipley, M.D. copyright 2020