Meningitis

With meningitis, mening- refers to the meninges which are three protective membranes that cover the brain and spinal cord, and -itis refers to inflammation; so meningitis is an inflammation of the meninges.

More specifically, it refers to the inflammation of the two inner layers which are called the leptomeninges.

The outer layer of the meninges is the dura mater, the middle layer is the arachnoid mater, and the inner layer is the pia mater.

These last two, the arachnoid and pia maters, are the leptomeninges.

Between the leptomeninges there’s the subarachnoid space, which houses cerebrospinal fluid, or CSF.

CSF is a clear, watery liquid which is pumped around the spinal cord and brain, cushioning them from impact and bathing them in nutrients.

In one microliter or cubic millimeter, there are normally a few white blood cells, up to 5.

If we look at a bigger sample, like say a decilitre, then around 70% of those will be lymphocytes, 30% monocytes, and just a few polymorphonuclear cells -- PMNs -- like neutrophils.

That same volume will contain some proteins, as well, about 15-50 mg as well as some glucose, about 45-100 mg, which is close to two thirds of the glucose we’d find in the same volume of blood.

The CSF is held under a little bit of pressure, below 200 mm of H2O, which is just under 15 mm of mercury -- which is less than a fifth of the mean arterial pressure.

Now at any given moment, there’s about 150 ml of CSF in the body.

This is constantly replenished, with around 500 ml of new CSF produced everyday and the excess, or 500 minus 150 mL or 350 mL, is absorbed into the blood.

But for any nutrients to enter and leave the CSF, and the brain itself for the matter, they have to go through the tightly regulated by the blood-brain barrier.

The blood brain barrier is the special name given to the blood vessels in the brain. That’s because the endothelial cells in the blood vessels are so tightly-bound to one another that they prevent leakage and only allow certain molecules to slip through them.