Yes, the exact structure of the blood-brain-barrier (BBB) is very complicated to describe, even in a diagram.
It took me quite a while to understand it myself, and it's an active area of research so we're still learning.
The first thing to explain is that the blood-brain-barrier actually consists of multiple layers or barriers:
- The outermost layers ("dura mater" and "arachnoid membrane/mater") enclose the brain on the outside, and do not penetrate into the interior of the brain.
- Below the arachnoid membrane is a space filled with cerebrospinal fluid (CSF). This space is called the sub-arachnoid space.
- Blood vessels that supply the brain first run through these outer layers and then penetrate into the interior of the brain.
- The inner layers of the blood-brain-barrier ("pia mater" and the "astrocyte end-feet" AKA "glia limitans") surround and follow the penetrating blood vessels into the interior of the brain. So, the further into the interior of the brain, the fewer layers in the blood-brain-barrier.
- However, between the penetrating blood vessels and the inner layers there is a space called the perivascular space. Toward the outside of the brain, this space is connected to the sub-arachnoid space and receives some cerebrospinal fluid from there.
- But as you go further into the interior of the brain, the composition of the fluid in the perivascular space changes. This inner perivascular space may be filled with immune cells and too many immune cells clumped in one spot can result in a "dilated perivascular space".
- Eventually, the fluid in this perivascular space drains out of the brain along the cranial nerves, emptying into lymph nodes. There, the fluid is processed before returning to the bloodstream.
Here are some rough diagrams that might help: