So now let's look at the cerebrospinal fluid of the brain.
In total, an adult makes about 80 to 150 mL of this fluid.
This liquid protects the brain and the spinal cord against chemical and physical injury
and it delivers oxygen, glucose, and other important substances
from the blood to the nervous tissue cells.
The cerebrospinal fluid is synthesized and found in the ventricles of the brain.
Starting with the lateral ventricles, the cerebrospinal fluid travels
from the lateral ventricles to the interventricular foramen.
Then from there, it goes to the third ventricle.
And then travels down the aqueduct of the midbrain into the fourth ventricle.
From there, it leaves out of the lateral aperture and the median aperture
to go towards the spinal cord and the central canal of the spinal cord.
Now let's take a closer look at exactly how and where the cerebrospinal fluid is made.
Choroid plexuses are found within the ventricles
and this is where the cerebrospinal fluid is going to be synthesized.
A choroid plexus is a network of capillaries that are found in the walls of the ventricles
and they are covered by ependymal cells.
These ependymal cells are what are going to make the cerebrospinal fluid.
These cells act as a filter and basically filter the blood plasma through them.
And what comes out on the other side and goes toward the brain
is going to be your cerebrospinal fluid.
Because of the tight junctions between these ependymal cells, fluid cannot leak
between the cells and must go through them
and therefore, this serves as the blood cerebrospinal fluid barrier.
So now let's look at the total flow.
I alluded to this a little bit earlier but now let's look at it a little bit closer.
So in the lateral ventricles you have the choroid plexuses.
You also have choroid plexuses in the third ventricle and the fourth ventricle as well.
The cerebrospinal fluid made in each of these is going to start first in the lateral ventricles
and travel to the third ventricle by way of the interventricular foramina.
From the third ventricle, you have the choroid plexuses that are making cerebrospinal fluid,
but you also have cerebrospinal fluid entering the fourth ventricle
by way of the aqueduct of the midbrain.
After leaving the fourth ventricle, the cerebrospinal fluid
is now going to move into the subarachnoid space of both the brain and the spinal cord.
This happens through holes known as the lateral and median apertures.
After this, the arachnoid villi of our dural venous sinuses
are going to re-absorb the cerebrospinal fluid.
Reabsorption is equal to production by the choroid plexuses
so that the pressure of the cerebrospinal fluid is constant.
Once it is reabsorbed, the cerebrospinal fluid goes into the venous blood
where it then returns to the heart and eventually goes through circulation.
And then after that, arterial blood returns back to the brain
and then we start the process all over again at the choroid plexuses.
So now that we've gone through the steps of the flow of cerebrospinal fluid,
let's look at it again using an image of the brain to track
where the cerebrospinal fluid originates and where it goes.
So starting with the choroid plexuses that are found in the right and left lateral ventricles
that is where we're gonna first make our cerebrospinal fluid.
From there, this cerebrospinal fluid is going to travel into the third ventricle
by way of the interventricular foramen.
Recall that the third ventricle also contains a choroid plexus
and it is making its own cerebrospinal fluid while also receiving cerebrospinal fluid from the lateral ventricles.
From there, the cerebrospinal fluid is going to travel by way of the cerebral aqueduct
or the aqueduct of the midbrain and then it's going to go into the fourth ventricle.
Like the lateral ventricles and the third ventricles, the fourth ventricles
also has a choroid plexus associated with it as well.
That is making its own cerebrospinal fluid while also receiving cerebrospinal fluid from those earlier ventricles.
After leaving the fourth ventricle, the cerebrospinal fluid is going into exit into the subarachnoid space
by way of the median aperture as well as the lateral aperture.
From there, it is going to travel or circulate through the subarachnoid space.
It also goes into the central canal of the spinal cord as well.
Recall that after it gets into the subarachnoid space, there are arachnoid granulation villi
which are going to reabsorb the cerebrospinal fluid and take it or return it to the venous blood.
It is returned by way of the superior sagittal sinus.
And from there, it goes to the internal jugular vein.